5 Skull Base and Temporal Bone(Table 5.1 – Table 5.4)
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.
external auditory canal
facial nerve canal (mastoid segment)
incus (long process)
posterior semicircular canal
basal turn of the cochlea
external auditory canal
internal auditory canal
lateral semicircular canal
stapes (footplate in oval window)
superior semicircular canal
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.