5 Skull Base and Temporal Bone(Table 5.1 – Table 5.4)


5 Skull Base and Temporal Bone(Table 5.1 – Table 5.4)

Table 5.1 Skull base apertures and their content




Cribriform plate

Medial floor of anterior cranial fossa

Olfactory nerve (CN I)

Ethmoid arteries

Optic canal

Lesser wing of sphenoid bone

Optic nerve (CN II)

Ophthalmic artery

Subarachnoid space, CSF, dura by optic nerve

Superior orbital fissure

Between lesser and greater sphenoid wings

Oculomotor nerve (CN III)

Trochlear nerve (CN IV)

Ophthalmic division of trigeminal nerve (CN V1)

Abducens nerve (CN VI)

Superior ophthalmic vein

Inferior orbital fissure

Between body of maxilla and greater wing of sphenoid

Infraorbital artery, vein, and nerve

Foramen rotundum

Medial cranial fossa floor inferior to the superior orbital fissure

Maxillary division of trigeminal nerve (CN V2)

Emissary veins

Artery of foramen rotundum

Foramen ovale

Floor of middle cranial fossa lateral to sella turcica

Mandibular division of trigeminal nerve (CN V3)

Accessory meningeal branch of maxillary artery Emissary veins from cavernous sinus to pterygoid plexus

Foramen spinosum

Posterolateral to foramen ovale

Middle meningeal artery

Recurrent meningeal branch of the mandibular nerve (CN V3)

Foramen lacerum

Between sphenoid body and greater wing of sphenoid bone at petrous apex

Meningeal branches of ascending pharyngeal artery

Vidian (pterygoid) canal

In sphenoid bone inferomedial to foramen rotundum

Vidian artery

Vidian nerve

Carotid canal

Within petrous temporal bone

Internal carotid artery

Sympathetic plexus

Jugular foramen

Posterolateral to carotid canal, between petrous temporal bone and occipital bone

Pars nervosa (anteromedial)

Inferior petrosal sinus

Glossopharyngeal nerve (CN IX)

Jacobson nerve

Pars vascularis (posterolateral)

Internal jugular vein

Vagus nerve (CN X)

Accessory nerve (CN XI)

Arnold nerve

Small meningeal branches of ascending pharyngeal and occipital arteries

Stylomastoid foramen

Behind styloid process

Facial nerve (CN VII)

Hypoglossal canal

Base of occipital condyles

Hypoglossal nerve (CN XII)

Foramen magnum

Floor of posterior fossa

Medulla and its meninges

Spinal segment of accessory nerve (CN XI)

Vertebral arteries and veins

Anterior and posterior spinal arteries

Table 5.2 Skull base lesions


CT Findings


Temporal bone lesions

See Tables 5.3 to 5.7


Intrinsic lesions of the skull base


A lesion is judged to be intrinsic to the skull base if the volume of the mass is centered in the plane of the skull base.

Congenital/developmental lesions

Primary cholesteatoma (epidermoid cyst)

Intraosseous congenital cholesteatomas are bone-expanding and bone-destructive lesions. They appear as low-density, unenhanced masses. Cholesteatoma that contains compacted squamous debris may appear heterogeneous. In rare cases, calcifications may be seen, either within or at the periphery of the cholesteatoma.

Squamous epithelial rest of embryonal origin. Occurs in the temporal bone (i.e., petrous apex, middle ear/mastoid), other skull bones, within the meninges or brain, or any other part of the body. Within the skull base, this lesion is referred to as congenital cholesteatoma; when found in the cisterns, the term epidermoid cyst is applied.


Heterogeneous, mixed density mass due to variable amounts of CSF and brain tissue extending through a defect in the base of skull, contiguous with intracranial brain parenchyma. The apertura is smooth and defined by a rim of cortical bone. The instillation of a low dose of intrathecal contrast before CT may aid in distinguishing a simple meningocele from an encephalocele (although MRI is the best modality for confirming the presence of brain tissue in a cephalocele).

Occipital cephaloceles consist of cervico-occipital, low occipital (involving the foramen magnum), and high occipital lesions (above the intact foramen magnum). Frontoethmoidal cephaloceles are subdivided into frontonasal, nasoethmoidal, and naso-orbital cephaloceles. Transethmoidal, sphenoethmoidal, transsphenoidal, spheno-orbital, and sphenomaxillary encephaloceles are types of basal cephaloceles.

A cephalocele is the protrusion of intracranial contents, including meninges and brain matter, through a defect in the skull base. Cephaloceles may be congenital or acquired secondary to surgery or trauma or due to spontaneous causes. Cephaloceles are most commonly found in the midline, at the occiput, skull base, or vertex. Frontoethmoidal (sincipital) encephaloceles occur as extranasal masses. Associated abnormalities are callosal hypogenesis, interhemispheric lipomas, neuronal migration anomalies, colloid cysts, midline craniofacial dysraphisms, hypertelorism, microcephaly, microphthalmos, and hydrocephalus.

Inflammatory/infectious conditions


Poorly defined areas of osteolysis in contiguity with the focus of infection. Intracranial extension may lead to cavernous sinus thrombosis, meningitis, epidural or subdural empyemas, cerebritis, and cerebral abscess formation.

Skull base osteomyelitis is uncommon, and most cases arise from contiguous spread of ear infections. Skull base osteomyelitis infrequently complicates sinonasal infection. Odontogenic cellulitis and abscess may spread into the suprazygomatic and nasopharyngeal masticator spaces, causing osteomyelitis of the skull base.

It typically occurs in a diabetic or immunosuppressed patient incompletely treated for necrotizing otitis externa. Pseudomonas aeruginosa is the usual pathogen. Less frequently, Aspergillus, Salmonella, Staphylococcus, Mycobacterium tuberculosis, or Mucormycosis is implicated.

Benign neoplasms


Fig. 5.1

Fig. 5.2

Meningiomas usually grow as an extra-axial, sessile, or globose, well-marginated bulky mass, dural-based at an obtuse angle. En plaque meningiomas grow as a flattened plate or sheet, especially at the sphenoid ridge, or less commonly at the superior or posterior surfaces of petrous bone. The rare intraosseus meningioma is usually sclerotic, occasionally lytic, and can mimic fibrous dysplasia or Paget disease. Most meningiomas are homogeneously hyperdense, some isodense, and a few hypodense compared with gray matter. Marked enhancement is typical. Cysts, necrosis, and hemorrhage appear as hypodense, nonenhancing areas; < 20% reveal psammomatous, nodular, or rimlike calcifications. Bony remodeling and hyperostosis may be evident. Invasion through the skull base may be present, either through natural foramina or by bone destruction.

Meningiomas represent 15% to 20% of intracranial neoplasms, have a peak incidence of 60 y, and affect predominantly female patients (F:M = 3:1). Meningiomas in childhood are rare and frequently associated with neurofibromatosis 2. Thirty-three percent arise along the dura of the skull base (sphenoid wing, sellar, and parasellar area, olfactory groove) and posterior fossa (clivus, petrous bone, foramen magnum, and jugular foramen). Because of the relatively slow tumor growth, the symptoms are often minimal and may include headache, anosmia, visual disturbance, or other cranial nerve palsies.

Malignant neoplasms


Fig. 5.3

Fig. 5.4

Focal, multifocal, or diffuse involvement of the skull base with osteolytic, osteoblastic, or mixed-type lesions. The extraosseous soft tissue extension is usually small.

Metastatic tumors are the most common malignancy of the skull base resulting from direct extension or hematogenous spread. Breast and lung, kidney, prostate, uterus, and colon carcinoma and head and neck malignancies frequently involve the skull base, especially in the late stage of tumor evolution. Multiple lesions are common. Primary neoplasms are often known at time of presentation. In children, leukemia, neuroblastoma, Wilms tumor, and Ewing sarcoma are the most common primary sites.

Malignant lymphoma

Invasive central skull base lesion, with homogeneously enhancing soft tissue density. Permeation of the tumor through the bone with preserved cortical outlines, with tumor present on both sides of the skull base and infiltration of the tumor along the dural surfaces, similar to “dural tail” without hyperostotic reaction, may be present.

Occurs usually in patients with systemic non-Hodgkin lymphoma and patients with AIDS. Primary non-Hodgkin lymphoma of the skull base without any nodal or lymphatic lesion is a very rare condition and with a different clinical presentation from other extranodal sinonasal and nasopharyngeal lymphoma: cranial nerve palsy, including ophthalmoplegia, visual loss, and hearing loss are most common presenting symptoms.


Solitary intraosseous expansile osteolytic lesion with scalloped, poorly marginated, nonsclerotic margins. The soft tissue mass is usually homogeneous, iso- to mildly hyperdense relative to brain, with mild to moderate homogeneous enhancement. No tumoral calcification, but peripherally displaced osseous fragments may be seen. May have both exocranial and endocranial extraosseous soft tissue components. Multiple myeloma presents similarly to hematogenous metastases. It can have lytic lesions, which may also demonstrate diffuse osteopenia, and rarely sclerosis.

Intraosseous plasmacytoma rarely manifests as primary skull base tumor. Especially sphenoid body, clivus, periorbital, and petrous temporal bone may be affected. Most present in the fifth to ninth decade with male predilection. Local pain, headache, and cranial neuropathies are most common symptoms. Progression to multiple myeloma is common.


Fig. 5.5

Expansile, multilobulated, well-circumscribed midline mass involving the clivus. Chordomas cause lytic bone destruction without sclerosis. Bony sequestra within the tumor mass are frequently seen. The soft tissue mass is iso- to hypodense relative to the brain, with low to moderate contrast enhancement. The enhancement pattern is inhomogeneous secondary to areas of cystic necrosis and/or myxoid material. Expanding tumor invades or displaces cavernous sinus and sella superiorly, jugular foramen and petrous apex laterally, basilar artery and brainstem posteriorly, basisphenoid, sphenoid, and ethmoid sinuses anteriorly, nasopharynx anteroinferiorly, jugular foramen and foramen magnum posteroinferiorly.

Thirty-five percent of all chordomas arise in the skull base around spheno-occipital synchondrosis (50% are sacrococcygeal, 15% arise from vertebral bodies). Other rare locations are sellar region, sphenoid sinus, nasopharynx, maxilla, paranasal sinuses, and intradural. Most commonly occurs between the age of 30 and 50 y (2:1 male predilection) with gradual onset of ophthalmoplegia and orbitofrontal headache. Large chordoma may affect optic nerve, chiasm, and optic tracts, CN VII or VIII, and CN IX to XII with consequent cranial nerve abnormalities. Chordomas rarely metasta-size (lymph nodes, lung, skeleton).


Fig. 5.6

Tends to arise offmidline in the petroclival fissure, petro-occipital synchondrosis, parasellar region, at the sphenoethmoid junction, and at the junction of the sphenoethmoid sinuses and vomer. Characteristically involves the clivus and prepontine region, the cerebellopontine angle, or the parasellar region. Bone destruction may be extensive, and sequestrations may form. Most tumors show prominent enhancement of the soft tissue portions. Chondroid calcification (45%–60%) may appear as arcs, rings, snowflakes, or coarse amorphous calcifications.

Chondrosarcomas of the skull base are rare and occur from childhood through old age (mean age: 43 y) without sex predilection. Commonly present with cranial nerve findings (abducens palsy; other cranial nerve palsies CN 2–5; CN 9, 11, 12). Late symptoms of tumor progression include increased intracranial pressure accompanied by headache or nausea and vomiting. Osteosarcoma and Ewing sarcoma originate infrequently in the skull base.


Osteolytic bone destruction of the central skull base, often with a large, bulky intra- and extracranial soft tissue mass.

Rhabdomyosarcoma occurs almost exclusively in the pediatric age group, with the origin of the tumor in the nasopharynx, orbit, paranasal sinuses, and middle ear. A cranial neuropathy is possible.

Metabolic/dysplastic lesions

Fibrous dysplasia

Fig. 5.7

Expanded thickened bone with heterogeneous decreased (“ground glass”) bone density is typical, with abrupt transition zone between lesion and normal bone. Involvement by fibrous dysplasia is usually unilateral, which leads to asymmetry. May have cystic regions in the early, active phase of the disease, with centrally lucent lesions and thinned but sclerotic borders. The pagetoid (mixed) pattern of fibrous dysplasia shows mixed radiopacity and radiolucency. The base of the skull is preferentially involved by diffuse areas of sclerosis. Obstructs osseous canals, foramina, pneumatic system, and sinuses. Contrast enhancement is often difficult to appreciate except in areas of lucent bone.

Benign, developmental skeletal disorder, most common in the axial skeleton. The skull is frequently affected (craniofacial region, clivus, posterior skull base). The medullary cavity of the affected bone fills and expands with fibrous tissue. The fibrous tissue then variably ossifies. Can be monostotic (75%) or polyostotic. Typically seen in adolescents and young adults.

McCune-Albright syndrome is a subtype of polyostotic fibrous dysplasia (usually unilateral) with endocrine dysfunction (precocious puberty) and cutaneous hyper-pigmentation (café-au-lait spots).

Other skeletal dysplasias affecting the skull base are neurofibromatosis type 1, achondroplasia, osteogenesis imperfecta, and craniotubular dysplasia. Diffuse skull base involvement also occurs with mucopolysaccharidosis and severe anemias. Other fibro-osseous lesions, such as ossifying fibroma, intraosseous hemangioma, osteoma, osteoblastoma, osteoclastoma, chondroblastoma, chondromyxoid tumors, hemangiopericytoma, chondroma, and aneurysmal bone cyst, are unusual entities and very rarely found in the skull base.

Paget disease

Fig. 5.8

Demineralization, “cotton wool” appearance, or marked sclerosis and bony enlargement with diploic widening and inner and outer table thickening of the osseous skull base are apparent. Often more diffuse and symmetric than fibrous dysplasia. Involvement of the skull base may lead to bone softening and result in basilar impression with the tip of the odontoid and anterior atlas arch above Chamberlain’s line. Sarcomatous degeneration, an uncommon sequela, presents with cortical destruction, masslike marrow replacement, and soft tissue masses.

Involves temporal bone and calvarium more frequently than the craniofacial area. Usually an incidental finding in individuals older than 40 y with male predominance. Headache, ataxic gait, cranial nerve palsies, myelopathy, and hydrocephalus may be clinical manifestations. Sclerosis and thickening of the base of the skull, including the petrous bone bilaterally with stenosis or obliteration of the internal auditory canal, facial nerve canal, and otic capsule, are signs associated with a variety of congenital disorders, including osteopetrosis, pyknodysostosis, craniometaphyseal dysplasia, cleidocranial dysplasia, Camurati-Engelmann disease, and osteopathia striata. Other conditions that can cause bony sclerosis are myelofibrosis, fluorosis, mastocytosis, sickle cell disease, and tuberous sclerosis.


Skull base fracture

Linear lucency with irregular borders representing fracture line, traversing the skull base, with or without separation and displacement of the fragments. It is generally associated with a dural tear. Indirect fracture signs, such as incorrectly located air (pneumocephalus, air in the labyrinth) or soft tissue densities (air–fluid levels in the tympanic cavity, mastoid cells, or sinus; meningoencephalocele), can provide additional diagnostic assistance. In fractures of the base of the skull, the temporal bone is usually involved. Fractures in the anterior fossa are of a cribriform, frontoethmoidal, lateral frontal, or complex type. Fractures resulting from temporal blows tend to propagate medially along a course parallel to the long axis of the petrous pyramid. They usually terminate in the floor of the middle fossa or in the sphenoid base. Some extend across the midline to be continuous with a contralateral temporal bone fracture. A minority extend anteriorly to exit the cranium through the anterior fossa floor laterally or through the midline and the cribriform plate. Strong occipital blows characteristically produce a fracture that disrupts the foramen magnum ring and then propagates anteriorly across the petrous pyramid at right angles to its long axis, passing to the floor of the middle fossa. The fracture may pass lateral to, through, or medial to the otic capsule. Occipital condylar fracture results from a high-energy blunt trauma with axial compression, lateral bending, or rotational injury to the alar ligament. Fractures of the clivus are subdivided into longitudinal, transverse, and oblique types. The site of the leak in CSF otorrhea and rhinorrhea can be detected accurately using CT water-soluble iodinated contrast cisternography.

Skull base fractures represent 19% to 21% of all skull fractures. The skull base is prone to fracture following severe head trauma at the thin squamous temporal and parietal bones over the temples and the foramen magnum, the petrous temporal ridge, the sphenoid sinus, and the inner parts of the sphenoid wings at the skull base. The middle cranial fossa is the weakest, with thin bones and multiple foramina. Other places prone to fracture are the cribriform plate and the roof of the orbits in the anterior cranial fossa and the areas between the mastoid and dural sinuses in the posterior cranial fossa. Skull base fractures are usually the result of extension of a vault fracture. The most important complications of these fractures are CSF leakage (otorrhea, rhinorrhea), related infection, and pneumocephalus with fistula, extracerebral and intracerebral hemorrhages, and cranial nerve and intracranial major vessel injury. Anterior and middle cranial base fractures generally cause upper cranial nerve injuries (CN I, II, III, IV, V, and VI) and vascular injuries to the carotid artery and middle cerebral artery. Posterior cranial base fractures are associated with injury to the lower cranial nerves (CN IX, X, XI, and XII) and major venous sinuses. Laterobasal fractures, including those of the petrous bone, are usually associated with deficits of facial and vestibulocochlear nerves (CN VII and VIII). Other clinical signs of a skull base fracture occurring after craniocerebral trauma are bleeding from the nose and ears, perilymphatic fistula, periorbital ecchymosis (raccoon eyes), and ecchymosis of the mastoid process of the temporal bone (Battle sign).

Miscellaneous lesions

Langerhans cell histiocytosis

Single (eosinophilic granuloma) or multiple areas of pure osteolysis in the skull base, temporal bone, or calvarium. Well-marginated lytic bone destruction may be extensive. A heterogeneously enhancing soft tissue mass may be associated. Fragments of bone within the soft tissue component are common.

Poorly understood, nonneoplastic disease. Children and young adults primarily are affected (F:M = 1:2). Most common signs with temporal bone lesion are otalgia, otorrhea, conductive or sensorineural hearing loss, facial nerve palsy, vertigo, and postauricular swelling.

Fig. 5.1 Meningioma (en plaque). Axial bone computed tomography (CT) image shows marked hyperostotic changes of the right wings and body of the sphenoid bone, narrowing of the right optic canal and inferior orbital fissure, and proptosis of the right eye.
Fig. 5.2 Meningioma (globose). Axial contrast-enhanced CT image reveals a moderate enhancing soft tissue mass (M) arising from the left middle cranial fossa and expanding into the orbit and masticator space. Also seen is osteolytic destruction of the left greater wing of the sphenoid.
Fig. 5.3 Osteoblastic metastases from prostatic carcinoma. Axial bone CT demonstrates a diffuse sclerotic involvement of the middle cranial fossa and posterior skull base.
Fig. 5.4 Osteolytic metastasis from adenoid cystic carcinoma. Axial bone CT shows a large destructive soft tissue mass in the central skull base.
Fig. 5.5 Clival chordoma. Axial contrast-enhanced CT image reveals a heterogeneously enhancing clival mass, which invades the right cavernous sinus and displaces the brainstem posteriorly. (Courtesy of Dr. A. von Hessling, Zurich.)
Fig. 5.6 Chondrosarcoma. Axial bone CT image demonstrates a skull base expansile mass centered on the right petro-occipital fissure. Also seen is associated bone destruction of the right petrous apex and petroclival junction with a sharp, narrow, nonsclerotic transition zone adjacent to normal bone. Notice the small bony fragments/calcifications in the tumor matrix. (Courtesy of Dr. A. von Hessling, Zurich.)
Fig. 5.7 Fibrous dysplasia. Axial bone CT image reveals diffuse “ground glass” expansion of the skull base, facial bones, and calvarium, as well as an abrupt transition zone between the lesion and normal bone. Note the obliteration of the frontal, ethmoid, and sphenoid sinuses.
Fig. 5.8 Paget disease. Axial bone CT image reveals asymmetric, polyostotic, marked irregular thickening with both sclerotic and lytic changes of the skull base, facial bones, and both tables and diploic space of the cranial vault.

The paired temporal bones each form part of the middle and posterior cranial fossae and contribute to the skull base. They are composed of five parts: the squamous bone, the mastoid bone, the petrous bone, the tympanic bone, and the styloid process.

The squamous part of the temporal bone is broad and flat and serves as the lateral wall of the middle cranial fossa and as the bony floor of the suprazygomatic masticator space. A portion contributes to the fossa of the temporomandibular joint and the roof of the external auditory canal. The zygomatic process projects from its lower surface.

Both the petrous and the squamous portions of the temporal bone form the mastoid segment. The mastoid antrum is the large central mastoid air cell. The aditus ad antrum connects the epitympanum to the mastoid antrum. Körner septum is part of the petrosquamosal suture that runs posterolaterally through the mastoid air cells and serves as a barrier to the extension of infection from the lateral mastoid air cells to the medial mastoid air cells.

The pyramidal-shaped petrous bone has three surfaces: the anterior, which is close to the temporal lobe; the posterior, which is close to the brainstem and cerebellum; and the inferior surface, an area that helps to form the carotid canal and jugular foramen. The posterior surface of the petrous bone contains the porus acusticus, the vestibular aqueduct (which transmits the endolymphatic duct), and the cochlear aqueduct (which transmits the perilymphatic duct). The petrous apex is defined as the portion of the temporal bone lying anteromedial to the inner ear, between the sphenoid bone anteriorly and the occipital bone posteriorly. It is separated from the clivus by the petro-occipital fissure and the foramen lacerum. Meckel cave and the cavernous sinus are in close proximity to the petrous apex.

The tympanic bone has anterior, inferior, and posterior walls that form the majority of the adult bony external auditory canal.

The styloid portion of the temporal bone forms the styloid process.

The temporal bone contains three cavities: the external, middle, and inner ear (Figs. 5.9 and 5.10). The external auditory canal is composed of fibrocartilage laterally and bone medially (the tympanic bone and the vertical retromeatal portion of the squamous bone). The medial border of the external auditory canal is formed by the tympanic membrane. It attaches to the scutum superiorly and to the tympanic annulus inferiorly.

Fig. 5.9 Axial temporal bone anatomy.




external auditory canal


facial nerve canal (mastoid segment)


incus (long process)


malleus (neck)




posterior semicircular canal


sinus tympani



Fig. 5.10 Coronal temporal bone anatomy.


basal turn of the cochlea


external auditory canal






internal auditory canal


lateral semicircular canal


stapes (footplate in oval window)


superior semicircular canal


tympanic membrane



The tympanic cavity is a small cleftlike, air-containing space (~20 × 10 × 2 mm) within the petrous portion of the temporal bone bound by the anterior wall (carotid wall with the ostium tympanicum of the musculotubal canal in the hypotympanum), the posterior wall (with the aditus ad antrum in the upper part, the pyramidal eminence, the sinus tympani, and the facial nerve recess in the lower part), the tympanic membrane laterally, the labyrinthine wall with the cochlear promontory and the oval and round windows medially, the tegmen tympani superiorly, and the floor (jugular wall) inferiorly. The epitympanum (attic) is the tympanic cavity above the line drawn between the inferior tip of the scutum and the tympanic portion of the facial nerve. The aditus ad antrum connects the epitympanum to the mastoid antrum. Within the epitympanum are the malleus head and the body and short process of the incus. Prussak space, the most common site of pars flaccida cholesteatoma, is the area between the incus and the lateral sidewall of the epitympanum. The mesotympanum extends from the inferior tip of the scutum above to the line drawn parallel to the inferior aspect of the bony external auditory canal. The posterior inferior wall is comprised of the pyramidal eminence, the sinus tympani, and the facial nerve recess (contains the descending facial nerve). The mesotympanum contains the manubrium of the malleus, the long process of the incus, and the stapes, whose vibrations are modulated by the tensor tympani muscle (inserts on the malleus) and the stapedius muscle (attaches on the head of the stapes). The hypotympanum is a shallow trough in the floor of the middle ear and contains no vital structures. A vascular mass in the middle ear upon otoscopic inspection may represent a high-riding or dehiscent jugular bulb, aberrant carotid artery, persistent stapedial artery, vascular granulation tissue, cholesterol granuloma, paraganglioma, or other tumor (e.g., hemangioma and meningioma).

The inner ear contains the membranous labyrinth set within the bony labyrinth (otic capsule), which forms the cochlea, vestibule, semicircular canals, and vestibular and cochlear aqueducts. The cochlea has approximately two and one half turns encircling a central bony axis, the modiolus. The basal first turn opens posteriorly into the round window niche. The vestibule, containing the utricle and saccule, is the central part of the labyrinth and is separated laterally from the middle ear by the oval window niche. The semicircular canals project off the superior, posterior, and lateral aspects of the vestibule. The upper bony margin of the superior semi-circular canal forms a convexity on the petrous pyramid roof, called the arcuate eminence. The posterior semicircular canal points posteriorly along the line of the petrous ridge. The lateral semicircular canal juts into the epitympanum. The cochlear aqueduct, which contains the perilymphatic duct, is 6 to 10 mm in length and extends from the scala tympani of the basal turn of the cochlea (just anterior to the round window orifice) posteromedially to the lateral border of the jugular foramen and posteroinferiorly to the internal auditory canal. The vestibular aqueduct encompasses the endolymphatic duct. It extends from the vestibule, coursing posteroinferiorly to the posterior wall of the petrous pyramid, where it joins the endolymphatic sac.

The internal auditory canal enters the petrous pyramid from the posteromedial surface and functions as a conduit for the facial nerve, intramediate nerve, and vestibulocochlear nerves as they course from the brainstem to the inner ear. The medial opening of the internal auditory canal is known as the porus acusticus. The porus acusticus internus is shaped much like the beveled tip of a needle, with the maximum diameter in the same axis as the petrous pyramid. The posterior, superior, and inferior lips of the porus are prominent and made up of dense bone. The anterior lip is usually poorly demarcated because it blends smoothly with the posteromedial surface of the petrous bone. Normally, the two internal auditory canals of the same patient are symmetric, but their shape varies considerably from one individual to the next. In 50% of cases, the canals are cylindrical, in 25% they have an oval shape, and in the remaining 25% the canals taper either medially or laterally. The canal’s vertical diameter varies from 2 to 12 mm (mean 5 mm) and its length from 4 to 15 mm (mean 8 mm). In 95% of normal individuals, the difference between the two internal auditory canals does not exceed 1 mm in diameter and 2 mm in length. The lateral end of the internal auditory canal, known as the fundus, is separated from the inner ear by a vertical plate of bone that is perforated to allow passage of the nerves. The fundus is subdivided by a horizontal plate (falciform crest) and a vertical plate (Bill bar) into four compartments. The facial nerve lies anterosuperi-orly with its nervus intermedius, the superior vestibular nerve posterosuperiorly, the cochlear nerve anteroinferiorly, and the inferior vestibular nerve posteroinferiorly.

The facial nerve canal originates at the foramen faciale of the fundus meatus acustici interni and terminates in the foramen stylomastoideum. The facial nerve canal has three segments and two genus. The labyrinthine segment courses with a gentle curve anterolaterally in the vestibulocochlear groove between the cochlea and vestibular labyrinth. At the genicu-late fossa, the canal forms an acute angle (first genu), then courses posteriorly and laterally to become the tympanic segment. It runs along the superior portion of the internal wall of the tympanic cavity, above and medial to the cochleariform process, and beneath the plane of the horizontal semicircular canal above the oval window, faces the promontory that separates the round and oval windows, and extends to the posterior wall of the tympanum. The posterior extremity of the short process of the incus marks the point where the facial canal begins its second turn (posterior genu) into the styloid complex to become the mastoid segment. The stylomastoid foramen, the caudal opening of the mastoid portion of the intratemporal facial nerve canal located in the exocranial skull base surface anteromedial to the mastoid tip and posterome-dial to the styloid process, transmits the facial nerve (CN VII) and extends directly into the parotid space. There may be congenital bony dehiscences in any portion of the facial canal (into the anterior epitympanic air cell, in the tympanic segment, or into the jugular fossa).

The carotid canal, located in the anterior part of the petrous pyramid, transmits the internal carotid artery and sympathetic plexus. Entering the skull base, the canal ascends vertically for ~1 cm, then proceeds horizontally in an anteromedial direction before ending above the foramen lacerum. The vertical portion is located inferior to the cochlea, anterior to the jugular fossa, and medial to the tympanic cavity. The horizontal portion is located anteromedial to the protympanum and posteromedial and parallel to both the eustachian tube and the semicanal for the tensor tympani muscle.

The jugular foramen is a bony channel that extends ante-riorly, laterally, and inferiorly from the endocranium to the exocranium between the anterolaterally temporal and posteromedially occipital bones and transmits vessels and cranial nerves through the skull base into the carotid space. It can be divided into three compartments: a larger posterolateral venous compartment (sigmoid part), containing the sigmoid sinus and small meningeal arterial branches; a smaller anteromedial venous compartment (petrosal part), containing the inferior petrosal sinus; and a neural or intrajugular compartment (pars nervosa), situated between the sigmoid and petrosal compartments, containing CN IX to XI. The sigmoid and the petrosal parts are separated by the intrajugular processes, which originate from the opposing surfaces of the temporal and occipital bones, as well as by a dural septum, which connects these two bony structures. The jugular foramen is separated from the hypotympanum by the bony lateral jugular plate, which may be normally dehiscent (dehiscent jugular bulb), and is medial to the descending facial canal and inferomedial to the posterior semicircular canal. The jugular foramen is separated from the anteromedial carotid canal by the caroticojugular spine and from the inferomedial hypoglossal canal by the jugular tubercle. Lesions can arise within the fossa or grow into the fossa from neighboring structures. The differential diagnosis of an erosive or destructive lesion in or about the jugular foramen includes paraganglioma, schwannoma (CN IX, X, XI, and XII), metastasis, meningioma, chordoma, chondrosarcoma, invasive squamous neoplasms (nasopharynx or external auditory canal), arteriovenous malformations (enlarging bulb), and cholesteatoma (primary or acquired).

The musculotubal canal proceeds from the inferior surface of the petrous bone, near the sphenopetrosal fissure, cranially in a dorsolateral direction to its orifice in the anterior wall of the tympanic cavity; it is divided by the cochleariform process into two semicanals: the lower for the bony part of the pharyngotympanic (auditor) tube, the upper for the tensor tympani muscle.

Imaging of the ear is requested for three main reasons: hearing loss (conductive or sensorineural), tinnitus, and dizziness. In other cases the main sign is external auditory meatus flow, facial palsy, or auricular malformation.

Conductive hearing loss may be congenital (congenital ossicular anomalies, which are isolated or associated with external auditory canal dysplasia, and congenital middle ear anomalies) or associated with cerumen, foreign body, exostosis, otitis, or tumor of the external ear; acute otitis media, serous otitis media, tympanic membrane perforation, tympanosclerosis, post-inflammatory ossicular fixation, traumatic ossicular disruption, cholesteatoma, glomus tympanicum tumor, fenestral otosclerosis, and superior semicircular canal dehiscence syndrome; fibrous dysplasia; and Paget disease.

Neurosensory hearing loss may be caused by peripheral lesions (75%–80% of all cases of neurosensory hearing loss), such as congenital malformations of the labyrinth, transverse fractures of the petrous pyramid, labyrinthitis (serous, toxic, viral, or bacterial), ototoxicity (drugs such as streptomycin, gentamicin, and quinine), tumor destruction of the labyrinth, and otodystrophies (otosclerosis and Paget disease) or by retrocochlear processes (20%–25% of all cases of pure sensorineural hearing loss), such as cerebellopontine angle lesions (acoustic schwannoma, meningioma, and vascular loop), petrous apex lesions (congenital cholesteatoma, cholesterol granuloma, and glomus tumor), or central pathology involving the brainstem, cerebellum, and central auditory pathways (multiple sclerosis, tumors, ischemia, aneurysm, and intra-axial hemorrhage).

Tinnitus may be from intrinsic (vestibulocochlear) or extrinsic (muscular or vascular) causes. Intrinsic tinnitus is a common complaint, subjective and audible only to the patient (with Ménière disease, viropathies, drugs, allergy, noise, or systemic diseases). Often the cause is unclear, and treatment is lacking. Extrinsic tinnitus is far rarer, often objective, and potentially audible also to the examiner. Muscular tinnitus (with myoclonus of the palatal muscles or the tensor tympani) can be pulsatile but is not usually pulse synchronous. Vascular tinnitus is always pulse synchronous. Causes for vascular tinnitus may be arterial, arteriovenous, or venous. The arterial causes include the aberrant arteries (aberrant carotid artery, persistent stapedial artery, and laterally displaced artery), the stenotic arteries (fibromuscular dysplasia, atherosclerosis of the internal and external carotid, and styloid carotid compression), and petrous carotid aneurysm. Arteriovenous causes include paragangliomas, other vascular tumors, Paget disease of the bone, cerebral arteriovenous malformations, dural arteriovenous fistulas, and vertebral fistulas. Venous tinnitus may be caused by chronic anemia, pregnancy, thyrotoxicosis, intracranial hypertension, or a large or exposed jugular bulb, or it may be idiopathic.

Peripheral facial nerve paralysis may occur with intracranial intra-axial lesions (cavernoma, brainstem glioma, metastasis, multiple sclerosis, cerebrovascular accident, or hemorrhage), intracranial extra-axial lesions (cerebellopontine angle tumor: acoustic schwannoma, meningioma, or epidermoid; cerebellopontine angle inflammation: sarcoidosis or meningitis; and vascular: vertebrobasilar dolichoectasia, arteriovenous malformation, or aneurysm), intratemporal processes (fracture through the facial nerve canal, Bell palsy, otitis media, cholesteatoma, paraganglioma, hemangioma, facial nerve schwannoma, or metastasis), extracranial lesions (forceps delivery, penetrating facial trauma, malignant otitis externa, parotid surgery, or parotid malignancy) or miscellaneous processes (Möbius syndrome, diabetes mellitus, myasthenia gravis, or hyperparathyroidism).

Sixty percent of congenital anomalies of the temporal bone occur in the external auditory canal (range from mild stenosis to complete agenesis; pinna deformity [microtia] is often associated), middle ear (range from minor hypoplasia to agenesis; ossicular changes, e.g., rotation, fusion, or absence), or both. Inner ear abnormalities account for 30% of congenital defects and, because of a different embryogenesis of the inner ear, are not associated with external and middle ear deformities. Combined anomalies involving all three compartments make up to 10% and are limited to craniofacial dysplasias and trisomies (13, 18, and 21).

Computed tomography (CT) is ideal for outlining the bony architecture, whereas magnetic resonance imaging (MRI) provides soft tissue details. The causes of conductive hearing loss are best depicted by CT. MRI offers the best chance to find pathology in a clinical picture of vertigo, sensorineural hearing loss, or tinnitus. In cases such as congenital malformations, petrous apex lesions, and mixed hearing loss, CT and MRI are complementary and are often used together to demonstrate the full extent of disease.

The differential diagnosis list of diseased temporal bone is discussed in Tables 5.3 , 5.4 , 5.5 , 5.6 , and 5.7 .

Table 5.3 Temporal bone: diseases of the external auditory canal


CT Findings


Congenital/developmental lesions


Fig. 5.11

In cases of stenosis, CT shows unilateral or bilateral narrowing of the external auditory canal from external opening of the canal to the tympanic membrane.

Membranous atresia is characterized by cartilaginous plug in a bony external auditory canal.

Complete atresia results in absence of the bony external auditory canal. The associated bony overgrowth about a deformed tympanic bone (atresia plate) may be thick, thin, complete, or incomplete.

Middle ear findings depend on severity of atresia (hypoplastic middle ear and mastoid complex, ossicular chain deformities [rotation, fusion, or absence], hypoplasia or aplasia of the oval or round window, anterior displacement of the facial canal, and associated congenital cholesteatoma).

Inner ear and internal auditory canal are usually normal.

Congenital bony, soft tissue, or mixed dysplasia of entire external auditory canal, including membranous and bony portions, usually unilateral by a 6:1 ratio, more commonly in right ear and in male patients.

Patients usually present with a small deformity of the auricle, no visibly patent canal, or conductive hearing deficit. May be associated with inherited syndromes, including mandibulofacial dysostosis (Treacher Collins), acrofacial dysostosis (Nager), craniofacial dysostosis (Crouzon), oculoauriculovertebral dysplasia (Goldenhar), and Pierre Robin syndrome.

Low-lying dura

The low-lying dura may cover the roof of the external auditory canal, and when the canal is not developed, the middle cranial fossa deepens to form a groove lateral to the attic and labyrinth.

A depression of the tegmental plate is not unusual, particularly in patients with congenital atresia of the external auditory canal.

Inflammatory/infectious conditions

Postinflammatory medial canal fibrosis

Homogeneous crescent soft tissue formation in the medial external auditory canal, unilateral or bilateral, abutting the tympanic membrane without underlying bone erosion or middle ear/mastoid involvement. May show slight enhancement of inflamed/edematous thickened external auditory canal walls in early stage.

Very rare. Distinct entity characterized by the formation of fibrous tissue in the medial bone external auditory meatus. Most cases occur in older patients (mean age 50 y; M:F = 1:2), with conductive hearing loss, tinnitus, otorrhea after chronic otitis externa and/or media, or as a complication of ear surgery.

Necrotizing (malignant) external otitis

Fig. 5.12

Enhancing external auditory canal soft tissue mass with aggressive underlying bony changes (cortical bone erosions and associated osteomyelitis especially affect the inferior portion of the external auditory canal and mastoid), thickened auricle, extension to the temporomandibular joint, adjacent cellulitis or abscesses of the parotid, masticator and parapharyngeal spaces, and opacification of the middle ear and mastoid air cells. May progress to skull base osteomyelitis. Intracranial extension can lead to sigmoid sinus thrombosis, meningitis, intracranial empyema, and abscess.

Severe invasive infection of the external ear, usually caused by Pseudomonas aeruginosa. Occurs in elderly diabetic or otherwise immunocompromised patients (M:F = 2:1) with persistent otorrhea, otalgia, and cranial neuropathy (CN VII, IX to XII).

Keratosis obturans

Fig. 5.13

Homogeneous soft tissue plug in the external auditory canal, often bilateral, without focal bone erosion of the canal. If the canal is diffusely widened, the bone walls appear smooth, without bone fragments. No enhancement of the tissue.

Rare abnormal accumulation and obstruction of the external auditory canal from desquamated keratin. Patients (< 40 y of age) present with acute, severe bilateral otalgia and a conductive hearing loss. Associated abnormalities are chronic sinusitis and bronchiectasis.


Soft tissue mass within the external auditory canal with erosive osseous changes, usually seen as focal scalloping or irregular erosion of the inferior and/or posterior external auditory canal wall underneath the cholesteatoma mass. Foci of bony fragments (sequestrations of necrotic bone) may be present within the cholesteatoma matrix. The cholesteatoma may extend into the middle ear cavity or mastoid, or it may involve the facial nerve canal or tegmen tympani. May demonstrate rim enhancement.

Cholesteatoma of the external auditory canal is a rare entity. Most cases are spontaneous or occur after surgery, trauma, or with ear canal stenosis or obstruction. Occurs in older patients (age 40–75 y), usually as a unilateral process without other associated disease.

Common symptoms are persistent otorrhea, chronic dull otalgia, and, less commonly, conductive hearing loss.

Benign neoplasms


Fig. 5.14

Broad-based bony expansion of the osseous external auditory canal wall with secondary stenosis. Bilaterality is the rule. The circumferential encroachment is usually located medial to the isthmus. Complete occlusion of external auditory canal is rare.

Benign bony overgrowth (not a true tumor) of bony external auditory canal in young adult male with chronic history of prolonged cold seawater exposure (surfer’s ear, cold water ear).

Conductive hearing loss may develop if the lesions are large. Other symptoms may be ear infection, pain, and tinnitus.


Solitary, unilateral, pedunculated, narrow-based, well-defined, osseous density mass, most commonly located lateral to the isthmus, near the osseous-cartilaginous junction of the external auditory canal. There are two varieties: the compact “ivory” type appears as a homogeneous dense bony mass, the “cancellous” type as a partially ossified mass.

Usually an incidental finding composed of mature bone. Much less common than exostoses. Occasionally identified at extracanalicular sites within the temporal bone, particularly in the mastoid.

Secondary bacterial external otitis can occur medial to an obstructing osteoma. Cholesteatoma of the external auditory canal and/or middle ear may be an associated abnormality.

Benign ceruminous gland tumor

Polypoid soft tissue mass in the external auditory canal without bony destructive changes.

Benign tumors of ceruminous gland origin in the external auditory canal include ceruminous adenoma, chondroid syringoma, syringocystadenoma papilliferum, and ceruminous pleomorphic adenoma.

They are very rare and are seen in adult patients (mean age 54 y; equal M:F distribution), which present with a painless mass of the outer half of the external auditory canal or with hearing changes. Occasionally symptoms of this tumor (pain, otorrhea) can result from an otitis externa secondary to meatus obstruction. The lesions have a tendency to recur after surgery and may degenerate into carcinoma.

Malignant neoplasms


Malignant tumors of the external auditory canal are relatively rare (squamous cell carcinoma > basal cell carcinoma > adenoid cystic carcinoma), mucoepidermoid carcinoma, adenocarcinoma; melanomas, and sarcomas are much less common.

Squamous cell carcinoma

Fig. 5.15

Homogeneously or heterogeneously enhancing soft tissue mass filling the external auditory canal. Bone destruction may be quite extensive without bony flecks. Extension into the pinna, parotid gland, and soft tissues below temporal bone and mastoid tip is common. Involvement of the middle ear cavity and superior extension into middle cranial fossa are rare. Nodal drainage is to pre- and postauricular and parotid nodes.

Squamous cell carcinoma is the most common primary malignancy of the external auditory canal. Tumors may begin within the canal or in the middle ear.

Patients (median age 65 y, more common in women) present with ulcerating external auditory canal mucosal lesion (may mimic otitis externa or external auditory canal cholesteatoma), otorrhea, otalgia, and conductive hearing loss. Late symptoms are facial nerve paresis, extensive bulky tumor, and nodal disease.

T staging:

T1: Tumor limited to external auditory canal without bony erosion or soft tissue involvement.

T2: Tumor with limited osseous erosion or soft tissue involvement.

T3: Tumor eroding osseous external auditory canal with limited soft tissue/middle ear/mastoid involvement.

T4: Tumor eroding deeper inner ear structures/temporomandibular joint/extensive soft tissue extension, or facial nerve paresis.

Parotid malignancy (local invasion)

Enhancing inhomogeneous ill-defined soft tissue mass of the parotid gland with extraparenchymal extension and invasion of adjacent structures (skull base, deep spaces of the suprahyoid neck) filling the external auditory canal. There may be marked bony erosion.

When invasive parotid malignancy occurs in the cephalad aspect of the parotid gland, it may invade the external auditory canal and present as a mass in this area.


Osteolytic and/or osteoblastic lesions in the bony external auditory canal, usually not associated with prominent soft tissue masses.

Originate most often from carcinomas of the lung, breast, prostate, and kidney or may present multiple myeloma and malignant lymphoma manifestations.

Fig. 5.11 Agenesis, external auditory canal. Coronal right T-bone CT image shows the absence of the bony external auditory canal. The mastoid and middle ear cavity are well developed and clear.
Fig. 5.12 Malignant necrotizing external otitis. Axial left T-bone CT image shows inflammatory changes in the region of the external auditory canal and auricle. The anterior bony external auditory canal is partially eroded (arrows).
Fig. 5.13 Keratosis obturans. Coronal bone CT reconstruction reveals a homogeneous soft tissue filling (arrow) of the right external auditory canal without osseous changes. The middle ear is unaffected, and the overall size of the canal is slightly enlarged.
Fig. 5.14 External auditory canal exostosis. Axial right T-bone CT image demonstrates broad-based osseous external auditory canal encroachment on both anterior and posterior walls (x) medial to the isthmus with severe narrowing of the lumen.
Fig. 5.15 External auditory canal squamous cell carcinoma. Axial left T-bone CT image shows a soft tissue density filling the external auditory canal with dorsal osseous destruction and invasion of the mastoid. The middle ear is unaffected.

Table 5.4 Temporal bone: diseases of the middle ear and mastoid


CT Findings


Congenital/developmental lesions

Middle ear dysplasia

Malformations of the middle ear vary from minor hypoplasia with coarcted small and underdeveloped but aerated tympanic cavity to almost complete agenesis. Middle ear findings depend on severity of external auditory canal atresia and auricular dysplasia. Anomalies of the ossicular chain include rotation, fusion, or absence of the ossicles and abnormalities of the suspensory ligaments. Oval window atresia may be associated.

An anomalous course of the intratemporal facial nerve is usual in middle ear dysplasia. The tympanic segment may be dehiscent, overlying oval, or round windows. The mastoid segment migrates anteriorly away from its normal position.

A congenital cholesteatoma behind the atresia plate is identifiable in fewer than 10% of cases.

Middle and external ear dysplasia represents a spectrum of congenital anomalies resulting from abnormal embryogenesis of the first and second branchial arches and tympanic ring. The middle and external ear develop in concert from these structures, which explains the usual association of these anomalies. Abnormalities of the inner ear are less commonly associated, as these structures develop earlier in gestation. Dysplastic auricle, absent or stenotic external auditory canal, and conductive hearing loss are the most common symptoms. Occurs more commonly in male patients.

Mastoid dysplasia

CT reveals the development of the mastoid and the degree of pneumatization. Pneumatization can be completely absent, with absence of the corresponding mastoid process or with a solid block of bone.

In case of a hypoplastic dense, sclerotic mastoid the pneumatization may be limited to a small mastoid antral cell. Possible associated findings are external auditory canal, middle ear and, less frequently, inner ear malformations.

Hyperpneumatization of the temporal bone with extension into the occipital bone and even the parietal bone is a rare condition (associated complication: pneumatocele, pneumocephalus).

The development of the mastoid air cell system begins in utero. Unilateral or bilateral absence of the mastoid antrum is a rare clinical finding and it is usually systemic. Underdevelopment of mastoids may be isolated or in association with children with microtia, Treacher-Collins syndrome, Cornelia de Lange syndrome, trisomy 13, Down syndrome, and bran-chiooculofacial syndrome.

Primary cholesteatoma

Globular, well-marginated nonenhancing, hypodense, expansile mass, located in the posterior epitympanum (at tympanic isthmi) or near the stapes or the antero-superior middle ear (adjacent to eustachian tube and anterior tympanic ring, medial to ossicles).

Larger mass (late in disease) may erode ossicles, middle ear wall, and lateral semicircular canal or tegmen tympani and extend throughout cavity and mastoid complex.

Most common cause of mass in the middle ear cavity behind an intact tympanic membrane (other T-bone locations: petrous apex, mastoid, external auditory canal, squamous portion of the temporal bone). Histologically identical to epidermoid; congenital cholesteatomas consist of exfoliated keratin and solid cholesterin within a sac of stratified squamous epithelium, originating from ectodermal rests. Rarely associated with first branchial cleft remnant or middle and external ear dysplasia.

Patients are usually between 4 and 20 y (M:F = 3:1) and present with unilateral progressive conductive hearing loss and no history of inflammatory ear disease. Late in disease large lesions can obstruct eustachian tube with resultant middle ear effusion and infection. Otoscopically, a white mass behind intact tympanic membrane may be seen.


Fig. 5.16

Well-defined middle ear mass with heterogeneous, mixed density (due to variable amounts of CSF and brain tissue) localized at the upper part of the attic with an apparent focal bone defect usually of the tegmen tympani. No contrast enhancement. Both ossicular chain and scutum are preserved.

MRI is the best modality to clarify the temporal lobe herniation.

Protrusion of cranial contents into middle ear. Spontaneous in origin with congenital defect in the tegmen tympani or mastoideum, or as a result of previous temporal bone surgery or trauma. Can be associated with CSF leaks. Herniated brain is usually nonfunctional.

Aberrant internal carotid artery

Fig. 5.17

Rounded tubular soft tissue density that enters the middle ear cavity posterolateral to the cochlea, crosses the mesotympanum along the cochlear promontory, and exits anteromedially to become the horizontal portion of the carotid canal. Carotid foramen and vertical segment of petrous internal carotid artery are absent. The inferior tympanic canaliculus, just anterolateral to the jugular bulb and posterior and lateral to where normal carotid foramen would be, is enlarged. Enhancement is equivalent to other arteries. CTA shows the aberrant nature of the enlarged collateral vessel traversing middle ear when internal carotid artery fails to develop.

Congenital vascular pseudomass: when the cervical portion and the first petrous portion of the internal carotid artery fail to develop, inferior tympanic artery, a branch of ascending pharyngeal artery, supplies petrous internal carotid artery via middle ear and caroticotympanic artery. Very rare disorder, with female and right side preference.

May be an incidental finding or present with pulsatile tinnitus or conductive hearing loss. Otoscopically, a red retrotym-panic pulsatile mass is seen.

Lateralized internal carotid artery

Dehiscent lateral wall of the petrous internal carotid artery as it borders the anterior middle ear cavity with protrusion of the enhancing internal carotid artery into middle ear.

At the level of the cochlear promontory, CTA confirms the course of the laterally displaced petrous internal carotid artery (normal in size and contour) projecting into middle ear.

Vary rare vascular variant with failure of formation or ossification of the lateral wall of the petrous internal carotid artery. May be an incidental finding or present with pulsatile tinnitus. When prominent, a vascular retrotympanic mass may be seen otoscopically.

Persistent stapedial artery

CT scans will show absent ipsilateral foramen spinosum posterolateral to the normal foramen ovale (nonspecific; may be seen also when the middle meningeal artery takes its origin from the ophthalmic artery) and Y-shaped enlargement of the geniculate fossa with encroachment upon the adjacent anterior epitympanic recess.

CTA may show the persistent stapedial artery arising from the genu of vertical and horizontal infracochlear petrous internal carotid artery and in the absence of a normal middle meningeal artery.

It may occur with or without aberrant internal carotid artery.

The persistent stapedial artery is a very rare vascular congenital anomaly. If the embryological stapedial artery fails to involute (in the third fetal month), the artery courses from the infracochlear carotid through the stapedial obturator foramen, passes through the stapes footplate, crosses the medial wall of the middle ear cavity over the cochlear promontory, and passes superiorly into an enlarged anterior facial nerve tympanic segment. Intracranially, it becomes the middle meningeal artery.

Most commonly asymptomatic finding on CT or during surgery.

Dehiscent jugular bulb

Fig. 5.18

Soft tissue mass low in the middle ear, contiguous with the internal jugular vein through a focal jugular (sigmoid) plate defect. The other margins of the adjacent jugular foramen are smooth and intact. Commonly seen with high-riding jugular bulb. The superolateral outpouching demonstrates similar enhancement characteristics to jugular bulb, sigmoid sinus, and internal jugular vein.

Congenital vascular pseudomass. Otoscopically, a retrotym-panic vascular mass may be seen in the lower part of the middle ear behind the intact tympanic membrane. Does not grow with time.

This is usually asymptomatic, but may cause pulsatile tinnitus or conductive hearing loss.

Prolapsing facial nerve

Soft tissue “mass” in the oval window niche along the undersurface of the lateral semicircular canal, best seen on coronal images.

Concomitant anomalies of the stapes may be present.

Relatively rare congenital lesion. Facial nerve protrudes through a bony dehiscence as it courses along undersurface of the lateral semicircular canal. Usually it is an incidental finding.

Inflammatory/infectious conditions


Fig. 5.19

In acute uncomplicated otomastoiditis , nonspecific fluid and enhancing inflammatory debris are apparent in the middle ear, and some or all of the mastoid and petrous apex air spaces are opacified by fluid or mucosal edema. The presence of air–fluid levels is pathognomonic for effusion.

Acute coalescent otomastoiditis is characterized by erosion of bony walls, rarefaction of septa, and dehiscence into the inner ear and intracranial compartment. When there is associated abscess, a rim-enhancing fluid collection adjacent to eroded middle ear/mastoid cortex is present extracranially, either in the post- or preauricular soft tissues (subperiosteal abscess) or in and around the sternocleidomastoid muscle (Bezold abscess), or intracranially in the temporal lobe. The presence of pneumolabyrinth in the cochlea and vestibule indicates a fistula.

Long-standing history of chronic otitis media may initiate ossicular erosions with absence of segment of ossicular chain. Tympanosclerosis appears as multifocal calcifications or ossifications within soft tissue debris in the middle ear cavity. Oval window involvement with a focus of calcification within occurs. An underdeveloped, poorly aerated sclerotic mastoid, granulation tissue in the middle ear, and retraction of thickened tympanic membrane also may be visible.

Acute otomastoiditis is a common infectious process that stems from upper respiratory infection and usually affects children. Bacterial etiology is the rule, with Streptococcus pneumoniae and Haemophilus influenzae accounting for the majority of cases. Common clinical symptoms are otalgia, fever, and postauricular swelling. Most aggressive cases are in young children.

Tuberculous otitis and fungal disease are uncommon but occur more often in immunocompromised patients. Diagnostic imaging plays a key role in diagnosing complications, which may include subperiosteal abscess, labyrinthitis, meningitis, brain abscess, dural sinus thrombosis, and Bezold abscess.

Manifestations of chronic otomastoiditis include cholesteatoma, granulation tissue, cholesterol granuloma, tympano-sclerosis, postinflammatory ossicular erosions, and bony or fibrous tissue fixation.

Acquired cholesteatoma

Fig. 5.20

Fig. 5.21

Fig. 5.22

Fig. 5.23

Nondependent, rounded, homogeneous hypodense mass (density less than that of the brain), associated with bone erosion or ossicular destruction. Cholesteatomas do not enhance with contrast. Tympanic membrane retraction may arise in the pars flaccida or pars tensa.

Pars flaccida cholesteatoma , the most common form of acquired cholesteatoma (82%), develops primarily within Prussak space along the lateral attic wall, lateral to the ossicles, and may extend posteriorly into the posterolateral attic, then through the aditus ad antrum into the mastoid or inferiorly to the posterior middle ear recess. Erosion of the scutum is a classical finding in attic cholesteatoma and remodeling of the lateral attic wall.

Pars tensa cholesteatoma , the far less common form of acquired cholesteatoma (18%), begins in the posterior mesotympanum medial to the ossicles and commonly involves the sinus tympani. Extension is toward the mastoid antrum medial to the incus.

The acquired mural cholesteatoma , a particular type of lesion, does not present as a bulky mass, but appears to be quite invasive and has a propensity for developing numerous manifestations of bony erosion, including automastoidectomy, ossicular involvement, and fistula formation (at surgery, a fine non-CT- detectable membrane of cholesteatoma may be identified).

Secondary findings relate to progressive bone erosion and may include destruction of the ossicles, facial nerve canal, labyrinth (particularly the lateral semi-circular canal), mastoid (including Körner septum), tegmen tympani, or floor of the middle cranial fossa. Intracranial infections, sigmoid sinus thrombosis, and CSF leaks are rare complications.

Acquired cholesteatomas are usually a complication of chronic otomastoiditis, more common in men, and of any age. The accumulation of exfoliated keratin within a sac of stratified squamous epithelium (keratoma) within the middle ear produces a mass effect that erodes the bony walls and ossicles. It is believed to result from ingrowth of squamous epithelium through marginal tympanic membrane perforations, from retraction pockets, or from ingrowth into the middle ear of the basal layer of the tympanic membrane. Common symptoms are smelly aural discharge, conductive hearing loss, and otalgia. Late complications are vertigo, venous sinus thrombosis, and intracranial infections.

Cholesterol granuloma

Fig. 5.24

Expansive, hypodense middle ear mass with smooth bony margins that does not enhance with contrast administration. When there is associated hemorrhage, a fluid level may be present. Although the mass fills the middle ear cavity, the ossicles are intact in most patients.

Cholesterol granuloma is a complication of chronic otomastoid inflammation that is characterized pathologically by specialized granulation tissue, blood breakdown products, and cholesterol crystals. Eustachian tube dysfunction is considered the most likely etiology, with secondary decreased intratympanic pressure, mucosal edema, and blood vessel rupture. In the temporal bone, cholesterol granuloma is seen within the middle ear cavity, mastoid, and petrous apex. When the lesion is in the middle ear, otoscopic examination reveals a “blue” tympanic membrane in the absence of pulsatile tinnitus.

Granulation tissue

On CT, granulation tissue causes contrast-enhancing, nondependent opacification of the middle ear without erosive bone changes; however, when there is associated hemorrhage, a fluid level may be present. Stranding soft tissue in middle ear is associated with inflammatory debris. (When the debris is globular, however, it cannot be differentiated from early cholesteatoma, where scalloping has not yet taken place.)

The development of granulation tissue in the middle ear is common both as an isolated phenomenon and in conjunction with other middle ear maladies, such as effusion and cholesteatoma. Granulation tissue is often vascular and has a distinct tendency to bleed, causing hemotympanum.

Benign neoplasms

Glomus tympanicum paraganglioma

Fig. 5.25

Enhancing focal mass with flat base on the cochlear promontory, without connection to the jugular fossa. A large glomus tumor fills the middle ear cavity, creating attic block and resulting in fluid collection in mastoid, or extends into mastoid air cells or through tympanic membrane into the external auditory canal. The floor of the middle ear cavity is intact. The ossicles typically are spared.

Glasscock-Jackson classification

Glomus tympanicum paraganglioma:

Type I: small mass limited to the promontory.

Type II: mass completely fills the middle ear space.

Type III: mass completely fills the middle ear space and extends into the mastoid.

Type IV: mass fills the middle ear space and extends into the mastoid or through tympanic membrane to fill the external auricular canal; may extend anterior to carotid; may have intracranial extension.

Glomus tympanicum paraganglioma is a benign, neural crest tumor localized to the cochlear promontory of the middle ear cavity (along the course of the inferior tympanic nerve). The glomus tumor is the second most common cause of mass in the middle ear cavity behind an intact tympanic membrane. It is rarely associated with multicentric paragangliomas. Patients are usually female and between 40 and 60 y of age with pulsatile tinnitus and conductive hearing loss, rarely with facial nerve paralysis. Otoscopically, a red vascular mass is seen behind the anteroinferior quadrant of the eardrum.

Glomus jugulotympanicum paraganglioma

Fig. 5.26

Homogeneous, intensely enhancing mass extending superolaterally from the jugular foramen into the middle ear cavity. Permeative-destructive bone changes along the superolateral margin of the jugular foramen mark extent of tumor. Bony floor of the middle ear cavity is invaded, and the jugular spine of the jugular foramen is eroded. The vertical segment of the petrous internal carotid artery is often dehiscent. Larger lesions may extend into the petrous bone, external auditory canal, and mastoid.

Glomus jugulare paraganglioma (GJP) is a benign tumor arising from paraganglia in and around the jugular foramen (jugular bulb, Jacobson nerve, and Arnold nerve). When middle ear extension occurs, such a tumor is called a glomus jugulotympanicum paraganglioma. GJP is the most common jugular foramen tumor and the second most common temporal bone tumor. In sporadic GJP, it is multicentric in 5% to 10% of patients. When familial (inherited as an autosomal dominant disease), multicentricity amounts to 25% to 50%. Patients are usually female and between 40 and 60 y of age with pulsatile tinnitus, conductive hearing loss, and cranial nerve involvement (CN IX, X, XI, and XII; VII and VIII less often). Otoscopically, a red vascular retrotympanic mass is seen.

Middle ear adenoma

Well-marginated, rounded enhancing soft tissue mass in the middle ear with minimal erosion. The ossicles may be encased. The mastoid is well pneumatized.

Very rare benign, indolent epithelial middle ear tumor, arising from modified respiratory mucosa.

Mean age at presentation with conductive hearing loss is 45 y. No history of chronic otitis media. Otoscopically, a pink soft tissue mass is seen behind an intact tympanic membrane.

Middle ear schwannoma

Fig. 5.27

Well-marginated, lobular, homogeneous, uniformly enhancing soft tissue mass in the middle ear cavity. Facial nerve schwannomas are most commonly centered on geniculate ganglion and emanating from facial canal.

Jacobson nerve schwannoma reveals erosion of the cochlear promontory, possible enlargement of the inferior tympanic canaliculus, and absence of facial nerve involvement.

Schwannoma is one of the common benign middle ear space tumors. They may originate from the nerves of the tympanic cavity (facial nerve, Jacobson nerve, chorda tympani nerve, or Arnold nerve) or by expansion from outside the middle ear space (CN VIII, IX, X, and XI).

Patients are adult and present with neural palsy and conductive hearing loss. Otoscopically, a fleshy white mass is seen behind an intact tympanic membrane.


Poorly marginated, intensely enhancing mass with “honeycomb” bony matrix and/or spiculated appearance with intratumoral bone flecks. Most common sites of involvement are the labyrinthine segment of the facial nerve and the geniculate fossa area, but any segment of the facial nerve can be affected. Multiseg-mental involvement is also possible.

Intratemporal hemangiomas are rare extraneural vascular lesions arising from capillaries around the facial nerve, which can secondarily grow into the nerve.

Often occurs with chronic, progressive, peripheral facial nerve paralysis. Pulsatile tinnitus may be present.

Middle ear meningioma

Intensely uniform or heterogeneous enhancing, globular (most common) or en plaque soft tissue mass within the middle ear cavity with permeative-sclerotic bony changes in surrounding bones. Tegmen tympani and mastoid bones are affected in tegmen tympani meningioma; sigmoid plate and middle ear floor are affected in jugular foramen meningioma. Middle ear component may represent “tip of the iceberg” for larger jugular foramen or tegmen tympani meningioma.

Benign tumor arising from arachnoid cap cells. May extend up from jugular foramen into middle ear, extend down from dura overlying tegmen tympani, or arise within middle ear. Patients are usually middle-aged and female with conductive hearing loss (M:F = 1:3, average age at presentation 45 y). Otoscopically, a vascular (blue) retrotympanic mass may be seen.

Malignant neoplasms

Squamous cell carcinoma

Fig. 5.28

Holotympanic enhancing soft tissue mass. Bony destruction should be rather extensive. Destruction of the tegmen tympani or sinodural plate leads to intracranial involvement.

Primary malignant middle ear tumors in adults are rare and usually associated with a history of chronic otitis media. Squamous cell carcinoma can occur, as can various types of adenocarcinoma, particularly adenoid cystic carcinoma. Squamous cell carcinomas have a definite male predominance. The peak incidence is in the fifth to seventh decades. They are believed to originate in most patients from the tympanic mucosa.

Adenocarcinomas of the middle ear have an equal gender incidence.

Middle ear rhabdomyosarcoma

Large, irregular, middle ear/mastoid mass with osteolytic, destructive bone and ossicle changes. The tumor enhances homogeneously. Possible areas of extension are the external auditory canal, internal auditory canal, middle and posterior cranial fossae, nasopharyngeal carotid space, masticator space, and parotid space.

Although rare, rhabdomyosarcoma is the most common primary middle ear tumor in the pediatric age group (bimodal: in children < 5 y and teens 15–19 y) with male predominance. Clinical symptomatology usually includes bloody otorrhea and ear pain; 30% of patients have neurologic deficits and nodal metastases at the time of diagnosis.


Metastases involving the middle ear and mastoid usually extend from elsewhere, particularly the petrous apex, or disseminate hematogenously, and may be osteoblastic or osteolytic.

Primary sites include breast, lung, kidney, prostate, head and neck squamous neoplasms, and stomach.




Damage to the temporal bone typically requires the application of great force and may cause fracture, hemorrhage, nerve trauma, vascular damage, or disruption of the middle or inner ear structures. Associated intracranial injuries, such as extra-axial hemorrhage, shear (or diffuse axonal injury), and brain contusion, are common. Twenty percent of patients with skull fracture have temporal bone fractures. Majority of fractures are actually oblique, transverse, longitudinal, or mixed. Potential complications of temporal bone fracture include infection (meningitis), hearing loss, facial (and other cranial) nerve injury, and CSF leak with otorrhea and/or rhinorrhea, and perilymphatic fistula.

Longitudinal fractures of the temporal bone

Fig. 5.29a, b

Longitudinal fractures of the temporal bone course parallel to the long axis of the petrous pyramid, typically extralabyrinthine. It starts in the pars squamosa, mastoid, or external auditory canal, extends through the posterosuperior bony external auditory canal, continues across the roof of the middle ear space anterior to the labyrinth, and ends anteromedially in the middle cranial fossa in close proximity to the foramen lacerum and ovale. The most common course of the fracture is anterior and extralabyrinthine; however, although rare, intralabyrinthine extension is possible. Facial canal involvement is less common. Bilateral temporal bone fractures are present in 8% to 29% of all fractures.

The oblique fracture crosses the external auditory canal in a horizontal plane and then extends upward obliquely toward the middle fossa. The fracture misses the otic capsule and may extend toward the petrous apex, where the fracture line may extend to the foramen lacerum. Conversely, the longitudinal fracture line is oriented in a more vertical plane.

Longitudinal fractures of the temporal bone are most common (70%–90% of all temporal fractures). They are usually secondary to blunt temporoparietal head trauma. Longitudinal fractures usually present with classic findings of laceration of the ear canal, tympanic membrane perforation, hematotympanum, ossicular injury (most commonly the incus), facial paralysis, and hearing loss. The hearing loss is predominantly conductive but may have a sensorineural component as well. Facial paralysis, often delayed and incomplete, occurs in ~10% to 20% of longitudinal fractures. Patients with longitudinal fractures usually develop CSF leaks from the tegmen, where the fracture line perforates the dura.

Transverse fractures of the temporal bone

Fig. 5.29c

Transverse fractures of the temporal bone course perpendicular to the long axis of the petrous pyramid, usually medial or lateral to the arcuate eminence. Medially situated fractures involve the vestibule, cochlea, fundus of the internal auditory canal, and crus commune. A more unusual type of transverse fracture occurs medial to the vestibule and bisects the inner auditory canal. Laterally placed fractures involve the promontory, vestibule, and horizontal and posterior semicircular canals. Facial nerve canal involvement (geniculate fossa) is common.

Transverse fractures of the temporal bone are less common (10%–30% of all temporal fractures). They are usually secondary to blunt fronto-occipital head trauma. The fracture commonly begins in the vicinity of the jugular foramen or foramen magnum and extends to the middle cranial fossa. The tympanic membrane is usually spared. The ossicles are often spared.

Clinical findings include persistent vertigo (due to transection of the vestibule, vestibular nerves, or vestibular aqueduct, perilymph fistula, labyrinthine concussion, or cupulolithiasis), often with spontaneous nystagmus, and permanent sensorineural hearing loss (due to damage to the cochlea or transection of the cochlear nerve). Facial paralysis is common (50%), often immediate and complete, due to edema, intraneuronal hematoma, impingement by fracture fragments, and complete transection. Patients with transverse fractures are more likely to develop CSF leaks from the vestibule defect. Also, because the tympanic membrane may be intact, they are more likely to present with CSF otorhinorrhea.

Ossicular injuries

Fig. 5.30

Incudomalleolar joint separation appears as displacement of the head of the malleus (the “scoop of the ice cream” is usually displaced laterally) from the body and short process of the incus (the “cone” is usually displaced medially and anteriorly).

Incudostapedial joint separation appears as abnormal enlargement of the dark cleft between the head of the stapes and the long process of the incus, as a fracture of the lenticular process of the incus, or as a fracture through the stapes superstructure.

Dislocation of the incus: When incudomalleolar joint separation is associated with incudostapedial joint separation or a fracture of the stapes, the incus may remain in the epitympanic recess with rotation and superiorly, posteriorly, and laterally displace, prolapse into the lower part of the tympanic cavity or external auditory canal, or even disappear.

Dislocation of the malleoincudal complex may be associated with an incudostapedial joint separation. The direction can be outward, inward, or downward.

In cases of stapediovestibular dislocation , CT shows absence of the footplate from the oval window without labyrinthine fracture. The stapes is dislocated in the tympanic cavity or depressed into the vestibule. Pneumolabyrinth or perilymphatic fistula may be associated.

Fracture of the malleus occurs at the neck or manubrium and is usually associated with other severe derangements.

Fractures of the incus affect the long or lenticular process or the body of the incus.

Fractures of the stapes may involve one crus or the arch and the footplate with or without displacement of fragments.

Trauma to the ossicular chain is a frequent complication of temporal bone injury after blows to the temporal, parietal, or occipital region, blasts, barotraumas, and lightning. Ossicular disruption can also occur following direct trauma to the ear by penetrating injury through the external auditory canal. Ossicular injury usually occurs as a dislocation. Incudostape-dial and incudomalleolar disarticulation and dislocation of the incus and malleoincudal complex are common injuries, whereas stapediovestibular dislocation is rare. Fracture of the malleus, incus, or stapes is uncommon. Both axial and coronal images are needed for evaluation. Reformatted images may also be useful.

There is a high incidence of conductive hearing loss secondary to ossicular injury.

Miscellaneous lesions

Langerhans cell histiocytosis

CT can demonstrate a sharp-marginated, lytic temporal bone lesion with destruction of the middle and inner ear, with potential fistula formation toward the cochlea and all the semicircular canals. The soft tissue component enhances strongly. Fragments of bone within the soft tissue component are common.

Aggressive nonneoplastic lesion in the temporal bone, often bilateral or with other associated osseous lesions, causing extensive bone destruction with associated soft tissue mass; extracalvarial and/or intracranial extradural in young children with conductive hearing loss and otorrhea.

Mastoid pneumocele

Affect the mastoid air cells with surrounding extensive pneumatization of the skull base. All areas of pneumatization intercommunicate. There is focal or diffuse thinning of the surrounding bony structures and loss of the bony trabeculae. There may be dehiscence of the outer cortex of the temporal bone. Air may also be present within the atlanto-occipital joints and within adjacent extracranial soft tissues.

Uncommon symptomatic acquired lesion with abnormal pneumatization of the skull base extending from the temporal bone. Persistently increased intraluminal pressure has been proposed as a mechanism of pneumocele formation that causes the mastoid cells to expand throughout the skull base.

Fig. 5.16 Postoperative cephalocele, right middle ear. Right coronal T-bone CT image reveals a postmastoidectomy right ear with broad bone defect of the tegmen tympani (arrow) and soft tissue in the epitympanum.
Fig. 5.17 Aberrant internal carotid artery. Left axial T-bone CT image shows the horizontal portion of the carotid canal extending too far posterolaterally into the middle ear (arrow). Note the absence of posterolateral margination. (Courtesy of Dr. B. Stinn, Zurich.)
Fig. 5.18 Dehiscent jugular bulb. Axial bone CT image shows the sigmoid plate is dehiscent on the right side (arrow) with a soft tissue mass in the posteroinferior middle ear cavity contiguous with an enlarged jugular bulb.
Fig. 5.19 Acute coalescent otomastoiditis. Left axial T-bone CT image demonstrates mastoid debris and confluence of mastoid air cells with trabecular and cortical erosions (arrow). Also seen is postauricular soft tissue swelling.
Fig. 5.20 Acquired pars flaccida cholesteatoma. Left coronal T-bone CT image shows an atticoantral nondependent, homogeneous soft tissue mass filling Prussak space. The scutum, lateral attic wall, and tegmen tympani are eroded. Also seen is the medially displaced malleus.
Fig. 5.21 Cholesteatoma with labyrinthine fistula. Right coronal T-bone CT image reveals opacification of the epitympanum with associated dehiscence of the lateral semicircular canal and tegmen tympani.
Fig. 5.22 Automastoidectomy (acquired mural cholesteatoma). Right coronal T-bone CT image demonstrates erosion of the scutum, lateral attic wall, tegmen tympani, and ossicles, as well as a ventilated common cavity connecting the enlarged middle ear and antrum.
Fig. 5.23 Recurrent cholesteatoma. Coronal T-bone CT image of the left postmastoidectomy ear reveals an abnormal soft tissue mass beneath a tegmen tympani defect (arrow).
Fig. 5.24 Cholesterol granuloma, middle ear. Left axial T-bone CT image shows a smoothly expansile hypodense mass (M) of the middle ear, expanding into the external ear, without bone erosion or remodeling.
Fig. 5.25 Glomus tympanicum paraganglioma. Right coronal T-bone CT image shows complete opacification of the middle ear and mastoid air cells. The floor of the middle ear cavity is intact.
Fig. 5.26 Glomus jugulotympanicum paraganglioma. Right axial T-bone CT image demonstrates a large mass in the jugular foramen with adjacent permeative-destructive bone changes. The lesion extends through the bony floor into the middle ear. There is erosion of the posterior wall of the petrous carotid canal. (Courtesy of Dr. B. Stinn, Zurich.)
Fig. 5.27 Facial nerve schwannoma. Left sagittal T-bone CT image reveals a tubular soft tissue mass involving the mastoid and tympanic segments of the facial nerve. The tumor pedunculates into the middle ear cavity (arrow) and produces an expansion of the bony canal (arrowheads).
Fig. 5.28 Squamous cell carcinoma, middle ear. Left coronal T-bone CT image shows a large soft tissue mass filling the middle ear. There is marked bony erosion of the cavity walls and ossicular destruction.
Fig. 5.29a–c Temporal bone fractures. Left axial T-bone CT image (a) demonstrates an extralabyrinthine longitudinal fracture extending from the mastoid (arrow), crossing the middle ear cavity with associated ossicular derangement and hemotympanum, the carotid canal, and the greater wing of the sphenoid, reaching the clivus (arrow). Right axial T-bone CT image (b) reveals a mixed fracture with both longitudinal and oblique components. Also seen are associated ossicular derangement and hemotympanum, pneumolabyrinth, and air within the internal auditory canal. Left axial T-bone CT image (c) shows a transverse fracture extending through the bony labyrinth. Middle ear fluid is present, as well as an abnormal density filling of the mastoid cells.
Fig. 5.30 Ossicular injuries. Right coronal T-bone CT image shows a comminuted mastoid fracture with ossicular disruption. The incus is rotated and displaced inferolaterally (arrow). Also present are a hemotympanum and blood in the external auditory canal.

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Jul 5, 2020 | Posted by in GENERAL RADIOLOGY | Comments Off on 5 Skull Base and Temporal Bone(Table 5.1 – Table 5.4)
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