The Temporal Bone: Congenital Anomalies

10.1055/b-0034-87909

The Temporal Bone: Congenital Anomalies

During embryonic development, the external and middle ear are derived from the first and second branchial groove and pouch. The inner ear is derived from surface ectoderm. Its development starts in the third week and is completed around the twelfth week. In the majority of children with congenital sensorineural hearing loss, no anatomic anomaly is demonstrated. Combined anomalies of external, middle, and inner ear occur in approximately 10% of patients.

In external ear atresia, it is important to note whether the atretic plate is composed of soft tissue or bone. The extent of ossicular chain malformation can differ from a fusion of the mallear head and the incudal body to a small clump of malformed chain. Bilateral external ear canal atresia is often associated with a syndrome, while unilateral atresia is usually nonsyndromal.

The lateral semicircular canal develops last. In malformations of the semicircular canals, the lateral canal is most commonly affected.

The vestibular aqueduct is widened when its diameter is larger than the diameter of the posterior semicircular canal, which corresponds with a diameter at its midpoint of 1.5 mm. It is almost always associated with cochlear modiolar deficiencies.

Absence of the cochlear nerve is rare. The internal auditory canal can be narrowed, but it can also occur with an internal auditory canal of normal diameter. On heavily T2-weighted images perpendicular to the internal auditory canal, the anteroinferiorly located cochlear nerve is absent.

**Table 4.55 Congenital anomalies of the external and middle ear**
Diagnosis	Findings	Comments
External ear atresia	Membranous atresia.	Often with external auditory canal stenosis.
	Bony atresia.	Usually also middle ear anomalies.
	Ossicular chain anomalies, contracted middle ear.	Spectrum from isolated mallear fusion to lateral wall to absence of chain.
	Anterior position of mastoidal part of facial nerve canal.	Important if surgery is contemplated.

**Table 4.56 Congenital inner ear anomalies**
Diagnosis	Findings	Comments
Large vestibular aqueduct Fig. 4.178a, b	Diameter is larger than diameter of the posterior semicircular canal.	Search for cochlear malformation, especially absence of bony modiolus.
Mondini malformation	Incomplete partition of cochlea.
Michel malformation	No cochlea/single cavity.	Rare.
Deformed lateral semicircular canal	Can be fused with vestibule.	Common among inner ear anomalies.
Absence of cochlear nerve Fig. 4.179, p. 404	No nerve seen on MRI.	Inferior and superior vestibular nerve split at the fundus.