External ear

The external ear consists of the auricle and the external auditory canal (EAC). In the EAC there is a lateral cartilaginous part and a medial bony part. The anterior, inferior and lower posterior part of the bony EAC is formed by the tympanic bone, while the roof and posterosuperior part is formed by the squamous part of the temporal bone. Also, see ear.

2. Pathology

Congenital malformation

One of the most frequent congenital anomalies of the temporal bone is atresia of the EAC, caused by aplasia of the tympanic bone. In the case of complete atresia, a bony plate is seen where the tympanic membrane is usually located. This deformity is associated with fusion of the neck of the malleus to the atresia plate (Fig.1). This anomaly can occur in isolation, with a normally developed tympanic cavity. Sometimes, there is a 'membranous' atresia of the EAC, with a 'plug' of soft tissue located at the site of the tympanic membrane.

CT is performed to rule out the presence of concomitant dysplasia of the middle ear or the presence of a (congenital or acquired) cholesteatoma. The position of the temporomandibular joint is frequently more posterior than normal, due to lack of posterior support by the tympanic bone. The mastoid portion of the facial nerve may run more anteriorly than normal, which is an important surgical consideration.

Inflammatory conditions

- external otitis: a common condition, also known as swimmer's ear. It does not require imaging in most cases.

- malignant otitis externa: also called necrotizing external otitis. This is an aggressive infection by Pseudomonas, which spreads beyond the confines of the EAC, both intra- and extracranially. This disease is seen in diabetic and immunocompromised individuals. The condition has a fatal outcome if untreated.

Trauma

Fractures of the tympanic bone are quite frequent; they are usually caused by the impact of a posteriorly displaced mandibular condyle. Longitudinal fracture temporal bone often extend into the EAC.

Benign neoplasia

- cholesteatoma is rarely seen to arise from the EAC; see cholesteatoma (VI:2), Fig. 4. It may be acquired (post-traumatic or secondary to an EAC stenosis), but often the pathogenesis is not clear  it is presumed to arise from ectopic epithelium.

- keratosis obturans: as in cholesteatoma, accumulations of exfoliated keratin, but often bilateral in patients with a history of sinusitis and bronchiectasis.

- exostoses and osteomata: exostoses are multinodular bony masses developing due to prolonged irritation of the EAC, most commonly secondary to excessive contact with cold sea water ('surfer's ear') (Fig.2). An osteoma is a typically solitary, unilateral and pedunculated bony growth; osteomata are less common than exostoses (Fig.3).

Malignant neoplasia

Several types of malignant neoplasms can arise in the external ear, or secondarily involve it. CT is normally the first study, as assessment of bone detail is critical to treatment planning. Surrounding structures are likely to become involved with further tumour growth, such as the parotid gland and temporomandibular joint. Perineural tumour spread head an neck along along the facial or auriculotemporal nerve, and intracranial spread is possible. MRI is frequently used as an adjunct in such cases. Spread to neck lymph nodes is possible (the parotid and higher parajugular lymph nodes are at risk) and a CT study of the neck is therefore desirable.

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