Teaching Case

Enlarged vestibular aqueduct syndrome (also known as large endolymphatic duct and sac syndrome) is one of the commonest radiological abnormalities of the inner ear. We present a case of paediatric enlarged vestibular aqueduct syndrome and demonstrate how high resolution CT of temporal bones and MR imaging aid the diagnosis.

A 14-year-old boy was referred to the ENT outpatient clinic with a history of progressive and fluctating hearing loss. A preliminary audiological test demonstrated bilateral sensorineural hearing loss.

Enlarged vestibular aqueduct syndrome (EVAS - also known as large endolymphatic duct and sac) is the commonest cause of congenital deafness, in one study being present in 12% of children with congenital sensorineural hearing loss. It tends to progress from infancy in a stepwise fashion.
The vestibular aqueduct extends from the posteroinferior surface of the temporal bone to the medial wall of the vestibule. It contains the endolymphatic sac, which is connected to the utricle and saccule by the endolymphatic duct. Its exact physiological role is uncertain.
The vestibular aqueduct is considered enlarged if it is morethan 1.5mm at the point halfway between the crus communis and the intracranial aperture of the aqueduct, making it bigger than the posterior semicircular canal which runs parallel, and immediately anterior, to it.
EVAS is bilateral in 50-60% of patients, with a female to male ratio of 3:2. Most patients who have an enlarged vestibular aqueduct are born with normal hearing. The simple presence of an enlarged vestibular aqueduct has not been reliably correlated with either clinical symptoms or prognosis.
The mechanism of hearing loss is unknown, but several theories have been proposed. Possibilities include the transmission of hyperosmolar proteins through the enlarged vestibular duct causing damage to the neuroepithelium, or elevation in pressure transmitted to the cochlea causing injury to the hair cells. These theories have been supported by studies that have shown a positive correlation of the size of abnormality with the degree of hearing loss, but other studies have not supported this correlation.
EVAS may be associated with other abnormalities of the inner ear. The vestibular aqueduct and the portioning of the cochlea are the last stages of development of the inner ear. A teratogenic insult at this late stage may affect both structures. Early studies suggested that associated abnormalities occur in approximately 60% of patients, but subsequent studies with higher resolution CT indicate that this association may be stronger. Cochlear anomalies that have been demonstrated in association with EVAS include modiolar deficiency (Mondini malformations). The semicircular canals may also be abnormal. The two genetic syndromes associated with EVAS are Pendreds syndrome and branchial otorenal syndrome.
High resolution CT of the temporal bones clearly demonstrates the bony outline of the vestibular aqueduct, and any associated enlargement.
T2W MRI demonstrates the fluid filled structures within the aqueduct i.e. the endolymphatic duct and sac, which may be distinguished from CSF either by visualisation of a thin dural sac or by the slightly differing fluid signals. This is best seen on high resolution, heavily T2 weighted 3D sequences.