14 - Enlarged Vestibular Aqueduct

Abstract

Enlargement of the vestibular aqueduct (EVA) is the most commonly detected inner ear anomaly in children with sensorineural hearing loss. EVA may be the sole radiologic abnormality or associated with additional inner ear anomalies. The hearing loss is variable in severity and can fluctuate or progress, often in stepwise increments that can be precipitated by minor head trauma or barotrauma. It can be unilateral and is typically asymmetric when bilateral. EVA is considered nonsyndromic when there are no accompanying features other than hearing loss. EVA can be associated with syndromes including distal renal tubular acidosis with deafness, branchio-oto-renal or branchio-oto syndrome, Waardenburg syndrome, or CHARGE syndrome. The most common syndrome associated with EVA is Pendred syndrome, an autosomal recessive disorder characterized by prelingual-onset bilateral hearing loss and EVA, and thyroid goiter associated with an iodine organification defect. Pendred syndrome often presents as nonsyndromic hearing loss in children caused by delayed onset and incomplete penetrance of the goiter. The causal gene for Pendred syndrome is SLC26A4, encoding the anion exchanger pendrin. Results of genetic testing for SLC26A4 mutations have been correlated with auditory and thyroid phenotypes, and the probability of EVA affecting a sibling of the patient. Mouse studies indicate that the developing endolymphatic sac requires the protein products of EVA genes such as pendrin to absorb Na+, Cl−, and water from the sac lumen. Disruption of this process leads to an abnormal increase in volume and altered composition of endolymph in the sac and throughout the inner ear, followed by hearing loss fluctuation and progression associated with functional and structural degeneration of the stria vascularis.