Enlarged Vestibular Aqueduct Syndrome

The vestibular aqueduct is a tiny canal shielded by the temporal bone, one of the densest bones in the body. It houses the endolymphatic duct and sac (see Label 15 on VEDA’s Ear Diagram) along with a vein and artery.

If a vestibular aqueduct has a diameter greater than 1½ millimeters—approximately the size of the head of a pin—it is considered to be an enlarged vestibular aqueduct (EVA). An EVA is usually accompanied by an enlargement of the endolymphatic duct and sac. The function of the fluid-filled duct and sac is not totally understood, but it is believed that they help maintain the volume and ionic composition of endolymph necessary for transmitting hearing and nerve signals to the brain. 

An EVA can be associated with genetic conditions such as Pendred syndrome, which affects thyroid function and hearing, and branchio-otorenal syndrome, which affects kidney function and hearing. It can also be associated with anatomical problems such as Mondini’s deformity, when the cochlea develops incompletely. When EVA causes hearing loss or balance symptoms, it is referred to as enlarged vestibular aqueduct syndrome (EVAS).

Causes and prevalence
As yet incompletely understood genetic or environmental factors are thought to result in EVAS, but research has consistently shown it to be associated with a mutation of gene SLC26A4 (also called the PDS gene). EVAS is considered to be rare, but as with many inner ear disorders, its true prevalence is difficult to assess because it is not always recognized during a medical evaluation. Estimates fall between 1% to 12%, but improvements in medical technology and clinical training have pushed the number as high as 5% to 15% in pediatric patients. More women than men are affected; for every two males with EVAS, there are three females who have the disorder.

Symptoms
Hearing loss is what usually brings EVAS to the attention of a physician. Such loss can be sensorineural, conductive, or both. However, it is most commonly a sensorineural hearing loss, meaning deafness usually related to the cochlea, but sometimes to the vestibulocochlear nerve or the brain’s auditory system. Some people are born with hearing loss, but most hear normally in the first years of life and then notice losses later in childhood, or less commonly in adolescence or early adulthood. Generally, this occurs after a minor or major head impact, upper respiratory infection, or air pressure trauma, such as occurs during rapid depressurization of an airplane.

EVAS can also produce problems with balance, although vestibular symptoms can be difficult for a child to describe and thus may not always be reported. Symptoms may include episodic spinning vertigo, mild unsteadiness, trouble watching revolving objects, and decreased visual acuity, among others. A young child may also grab his or her head and walk in circles.

Clinical evaluation
In addition to a complete medical history and physical examination, the diagnostic process for uncovering EVAS usually involves hearing and balance testing and radiological assessment. Fast spin-echo magnetic resonance imaging (MRI) is generally considered to be the most appropriate test because it permits precise imaging and measurement of the endolymphatic duct and sac soft tissues. High resolution computerized tomography (CT) scans are also often used. Thyroid, renal, and cardiac function are also usually analyzed, and genetic screening is sometimes also performed.

Treatment
Currently, the cornerstone of EVAS management is prevention, primarily by protecting the head from trauma that will worsen the progression of symptoms. People with EVAS are advised to avoid head blows and contact sports. No treatments can reverse or stop the progression of hearing and/or vestibular losses from EVAS. Hearing aids or cochlear implantation may help. Vestibular rehabilitation therapy might also be helpful for managing imbalance and dizziness resulting from an enlarged vestibular aqueduct.

From VEDA publication F-28, Enlarged Vestibular Aqueduct Syndrome (EVAS).