Recurrent otitis externa (Fig. 67-2) may result in a fibrotic band of soft tissue occluding the medial EAC. It is important for the radiologist to answer the following questions: Is there any erosion of the adjacent bony walls to suggest a more aggressive process? What is the depth of the occluding soft tissue? Does the fibrotic tissue extend to involve the tympanic membrane (TM)? Is there a normally pneumatised middle ear cleft deep to the occluding soft tissue? Always review the contralateral side as this pathology is commonly bilateral.

The EAC osteoma usually arises spontaneously, but exostoses are associated with repeated exposure to cold water and are also known as ‘surfer’s ear’. The two pathologies (Fig. 67-3) cannot be distinguished histopathologically; however, exostoses are usually broad-based and bilateral, whereas osteoma are pedunculated, unilateral and lateral to the bony isthmus of the canal. They may cause sufficient narrowing of the EAC to require surgery. It is important when assessing EAC exostoses and osteoma to answer the following questions: What is the maximum depth and transverse diameter of the exostosis? What is the exact site of origin of the osteoma? What is the distance between the medial aspect of the exostosis and the TM and the deep aspect of the exostosis and the descending facial nerve canal? Are there any associated obstructed secretions in the medial EAC and is the middle ear cleft normally pneumatised?

Keratosis obturans (Fig. 67-4) is usually a bilateral accumulation of keratin within the EAC. The CT appearances are of soft tissue filling the EAC that may be associated with expansion of the canal and remodelling but not erosion of the bony canal walls. Keratosis obturans is associated with otalgia and conductive hearing loss and most frequently occurs in young men.

An EAC cholesteatoma (Fig. 67-5) can be differentiated from keratosis obturans as it is associated with erosion of the floor/posterior wall of the bony EAC and is usually seen in an older population (>40 years).

Also known as malignant otitis externa, a misleading term used because of what used to be the high mortality associated with this condition. The pathology is a necrosis usually of the walls and in particular floor of the EAC at the bony cartilaginous junction typically in the elderly diabetic patient who presents with severe otalgia. Pseudomonas is the usual organism. The infection commonly extends inferiorly via the fissures of Santorini (see the section ‘Anatomy’, above), causing a skull base osteomyelitis, and involves the soft tissues inferior to the skull base where the lower cranial nerves (VII to XII) may be affected (Fig. 67-6). Anterior extension into the temporomandibular joint may cause destruction of the mandibular condyle.

These are rare tumours presenting in the elderly (F > M) with a painful, discharging ulcerative lesion of the EAC that most frequently extends inferiorly. CT is the usual imaging technique (Fig. 67-7), but MRI is recommended if there is suspected superior extension into the middle cranial fossa or medial extension into the middle ear cleft (both are rare).

The EAC may be stenosed or completely atretic (absent). The atresia may be membranous, bony or a mixture of the two. The atresia is usually unilateral in non-syndromic and bilateral in syndromic patients (Fig. 67-8). The rule of thumb is the more severe the EAC abnormality, the greater the auricular deformity (microtia/anotia). The inner ear structures are usually not affected unless there is severe EAC atresia with an absent auricle, small middle ear cleft and absent ossicles. If surgery is being considered it is important to assess the following on CT:

The CT appearance is of foci of calcification, punctate or web-like in the middle ear cleft (Fig. 67-13) and tympanic membrane. Usually this is associated with a long history of otitis media (OM). The suspensory ligaments and muscles may also calcify. There is varying conductive hearing loss depending on the degree of ossicular fixation. If the tympanic membrane only is involved (calcified), the condition is known as myringosclerosis.

Nearly all ossicular disruption is associated with a temporal bone fracture. Incudostapedial joint (ISJ) disruption is the commonest derangement. Malleoincudal disruption results in loss of the normal ‘ice cream cone’ appearance on axial images of the head of malleus articulating with the body of the incus (Fig. 67-15). Complete dislocation of the incus is the next most common finding. Much more rarely found are fractures of the individual ossicles. The incus is the ossicle most frequently disrupted as the malleus is supported by three ligaments and the tensor tympani muscle and the stapes by the annular ligament and the stapedius muscle.

When assessing ossicular trauma, look first for the normal ‘ice cream cone’ appearance of the malleoincudal joint and then closely assess the alignment of the distal long process of the incus with the head of stapes at the ISJ and compare with the normal contralateral side.

Venous Sinus Thrombosis

The sigmoid sinus is an immediate posterior relation to the mastoid air cells and mastoiditis may result in sinus thrombosis with occasionally severe intracranial complications (Fig. 67-16). The transverse sinus and jugular bulb may also be involved, although the latter is usually thrombosed due to retrograde extension from the neck.