CT
High-resolution, thin-section computed tomography (CT) is the most important test in patients with aural dysplasia. Since the diagnosis is clinical, the role of CT is preoperative evaluation. EAC dysplasia can be classifi ed as incomplete (stenosis) or complete (atresia), and membranous, bony, or mixed. In the membranous type, soft tissue is seen in the region of the tympanic membrane. In the bony type, the bony plate is variable in thickness. The external ear is also deformed, often small, and dysplastic. There is often poor pneumatization of the tympanic cavity and mastoid, the severity of which is a factor that affects access during surgery.
The ossicular abnormalities seen include fusion of the malleus and incus to the attic wall, fusion of the malleus to the atresia plate, fusion of the malleoincudal articulation, and a dysmorphic shape of
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Figure 6–1 External auditory canal atresia. (A) Axial and (B) coronal computed tomographic images illustrate lack of the resorption of the embryonic meatal plate ( long arrow ), result- ing in a complete bony external auditory canal atresia. The os-
sicles are malformed with fusion of the head of the malleus and short process of the incus ( short arrow ). The long process of the incus is also malformed ( curved arrow ). Note the micro- tia of the maldeveloped pinna ( double arrow ).
28 II EXTERNAL AUDITORY CANAL
the malleus and incus. There can be abnormalities of the oval and round window. Identifi cation of the stapes is very important to the surgeon. Similarly, evaluation of the otic capsule is important since a concurrent inner ear abnormality could preclude surgical treatment.
The course of the facial nerve is almost always abnormal. With the poor development of the tym- panic bone, the tympanic segment is often displaced caudally, and the posterior genu and the descend- ing mastoid segment of the facial nerve are typically located more anteriorly or anterolaterally ( Fig.
6–1 , Fig. 6–2 , and Fig. 6–3 ).
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Figure 6–2 Severe external auditory canal stenosis. (A) Ax- ial and (B) coronal images CT demonstrate severe stenosis of the external auditory canal. The dysplastic canal ( arrow ) is
fi lled with soft tissue. Abnormal soft tissue is present in the peri stapedial region an oval window ( curved arrow ), possibly representing an early cholesteatoma.
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Figure 6–3 Abnormal course of the facial nerve. Coro- nal CT obtained in a patient with external auditory canal atresia shows an abnormal course of the tympanic and de- scending segments of the facial nerve ( arrows ).
6 EXTERNAL AUDITORY CANAL ATRESIA AND STENOSIS 29
PE ARL S
•
External auditory canal dysplasia is classifi ed as bony, membranous, or mixed; and incomplete (stenosis) or complete (atresia).•
Associated middle ear deformities include reduced middle ear and mastoid pneumatization, os- sicular abnormalities involving the malleus and incus, and a higher incidence of cholesteatomas.•
Identifi cation of a normal inner ear and the presence of stapes are important for surgical planning.•
The course of the tympanic and mastoid segments of the facial nerve is abnormal. The facial nerve usually descends more anteriorly when compared with its normal course.
Suggested Readings
Hudgins PA. EAC atresia. In: Harnsberger HR, ed. Diagnostic Imaging: Head and Neck. Salt Lake City:
Amirsys, 2004:I-2–6
Remley KB, Swartz JD, Harnsberger HR. The external auditory canal. In: Swartz JD, Harnsberger HR, eds.
Imaging of the Temporal Bone, 3rd ed. New York: Thieme, 1997:16–46
Romo LV, Casselman JW, Robson CD. Temporal bone: congenital anomalies. In: Som PM, Curtin HD, eds.
Head and Neck Imaging, 4th ed. St. Louis: Mosby, 2003:1109–1172
Schuknecht HF. Congenital aural atresia and congenital middle ear cholesteatoma. In: Nadol JB, Schuknecht HF, eds. Surgery of the Ear and Temporal Bone. New York: Raven Press, 1993:263–274
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Epidemiology
Otitis externa is a common disease affecting all age groups and is usually infective in etiology. Though bacterial infections of the skin of the external auditory canal are the commonest cause, fungal infection can also occur (otomycosis). Infections are more prevalent in hot and humid climate conditions, and are often associated with trauma to the skin of the external auditory canal, such as during mechanical removal of cerumen. Swimming is considered a risk factor, hence the term swimmer’s ear . Malignant (necrotizing) otitis externa is a particularly aggressive form of infection occurring in a select patient population and is responsible for considerable morbidity and mortality.
Clinical Features
Otitis externa can cause a variable amount of pain (otalgia) and discharge (otorrhea), but it seldom causes signifi cant morbidity. Bacterial infections are usually more symptomatic. Depending on the degree of swelling and debris in the external auditory canal, there may be conductive hearing loss.
Systemic signs such as fever and (pretragal) lymphadenopathy may be present.
Malignant (necrotizing) otitis externa is a particularly aggressive life-threatening form of infec- tion caused by Pseudomonas aeruginosa infection typically in elderly diabetics and individuals with other immunosuppressed states, such as HIV patients or those who have undergone chemotherapy.
Clinically, a high index of suspicion in the susceptible patient population is required for diagnosis.
Nonspecifi c granulation tissue may be seen along the inferior wall of the external auditory canal (at the bony–cartilaginous junction) with exquisitely painful otorrhea. Cranial nerve palsies may result in advanced cases.