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.

Fig. 4.178a, b A large vestibular aqueduct is visible (arrow in a) in a 5-year-old boy with a perceptive (sensorineural) hearing deficit. A large vestibular aqueduct is visible (arrow in a). The cochlea is malformed with an absent cochlear modioli (arrow in b).
Fig. 4.179 A 16-month-old boy with a left-sided deafness of unknown cause. On the right side, a normal cochlear nerve is visible (arrow) on the T2-weighted image. On the left side, the cochlear nerve is absent. The facial nerve and vestibular nerves are present.

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Jul 12, 2020 | Posted by in PEDIATRIC IMAGING | Comments Off on The Temporal Bone: Congenital Anomalies
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