Monday, May 23, 2011

Your Audiology Tutorial: Enlarged Vestibular Aqueduct

The inner ear contains two organs: the cochlea (for hearing) and the semicicular canals, or vestibular labyrinth (for balance). Vestibular aqueducts are the bony canals leading from the inner ear in the temporal bone into the skull. The fluid known as endolymph fills a tube which runs through the aqueduct. How the endolymph moves influences how the brain interprets motion (grossly simplified explanation).

If the aqueduct is enlarged (determined by imaging studies such as MRIs or CT scans of the internal auditory canals), the sac in which endolymph resides may swell and cause imbalances of ions which drive signals up the vestibulococochlear nerve (cranial nerve VIII). These electrical signals are how the brain recognizes stimuli and accordingly regulates hearing and balance. If the aqueduct is enlarged, there may concurrently be a significant loss of hearing sensitivity. Much lit. is devoted to studies of children with enlarged vestibular aqueducts (EVA) and inner ear hearing loss. The etiology of EVA is usually attributed to a mutation of the PDS gene, or the SLC26A4 gene on chromosome 7. It is important to note that not every case of EVA is caused by such mutations. Other causes (including environmental) are being traced, with varying success.

The genetic disorder Pendred Syndrome, a cause of hearing loss during childhood, often presents with a symptom of EVA. The hearing loss often degrades at varying rates over time, sometimes leading to deafness. Balance function may also be affected. However, the brain is often able to achieve compensation : an ability to adapt to an impaired vestibular system in one or both ears.

Treatment for EVA is not clear cut. Surgical options are risky and can actually damage the inner ear. If sudden sensorineural hearing loss (SSHL), a condition that can literally overnight cause hearing to degrade significantly (sometimes due to barotrauma, autoimmune disease, etc.)is present, the ear, nose, and throat doctor may prescribe an oral or transtympanically injected course of steroids. According to the National Institute on Deafness and Other Communication Disorders (NIDCD), no scientific studies support steroidal intervention as being effective for EVA.

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