Enlarged Vestibular Aqueduct Syndrome
What is it?
Enlarged vestibular aqueduct syndrome (EVA) is a congenital disorder characterized by a vestibular aqueduct larger than 1.5 mm in diameter (Bostic, et al. 2018).
Symptoms:
EVA often presents as a mixed or sensorineural hearing loss that ranges in severity and configuration (Ralli, et al. 2017). EVA can also cause vestibular issues ranging from feeling slightly imbalanced to episodic vertigo (Ralli, et al. 2017). Symptoms may not present until as late as early adulthood, although most likely will present in childhood (Ralli, et al. 2017). Onset of symptoms is sudden and progressive and often follows head trauma, upper respiratory infections, or drastic changes in air pressure (Bostic, et al. 2018; Zalewski, et al. 2015).
Prevalence:
Prevalence of EVA is underestimated because imaging is required for a full diagnosis, and many without symptoms may never realize they have EVA at all. Estimates of prevalence are between 5-15% of pediatric patients, with females being slightly more likely to have EVA than males (Hamid, 2018).
Causes:
There is no singular known cause of EVA, but it is thought to have a genetic component. Genetic testing often, but not always, reveals that EVA is associated with mutation of the SLC26A4 gene. (Zalewski, et al. 2015).
How it affects the auditory system:
The vestibular aqueduct is a small bony canal that connects the endolymphatic sac to the fluid-filled spaces of the cochlea/semicircular canals. The purpose of the endolymphatic sac is not completely understood, but it is thought to be crucial to maintaining endolymph volume and ion composition (Hamid, 2018). Because the vestibular aqueduct is so large, the endolymphatic sac/duct are not able to maintain inner ear ion homeostasis. If the onset of symptoms follows head trauma, sudden fluctuations in CSF (hyperosmolar reflux) are thought to force concentrated proteins into the cochlear duct, damaging inner ear structures like the organ of Corti (Hamid, 2018; Noordman, et al. 2015). A conductive hearing loss is thought to be a result of increased endolymphatic pressure, which keeps the stapes from moving in the oval window (Merchant & Rosowski, 2008).
Diagnosis:
While an audiologist may notice the mixed/SN hearing loss on an audiogram, or vestibular-related complaints, the diagnosis of EVA must be confirmed with imaging. An MRI is considered a superior diagnostic imaging tool to CT scan by some researchers (Ralli, et al. 2017). CT scans only show the bony labyrinth, and T2-weighted MRIs also show you the extraosseous portion of the endolymphatic sac. (Ralli, et al. 2017). Both can be used to measure the diameter of the vestibular aqueduct.
Effects on Audiology Test Battery and Treatment
The presentation of EVA is variable, and therefore your test battery must support a differential diagnosis. For example, because EVA often is characterized by a conductive or mixed hearing loss, a middle ear pathology must be ruled out to support a EVA diagnosis. Also, there are many predictive factors for EVA that you can ask about in your case history. Finally, the progressive nature of the hearing loss associated with EVA affects how we prescribe treatment for these clients. A thorough test battery has been included below, as well as treatment/counseling considerations.
Test Battery
• Case History
o Predictive factors to ask about:
♣ Thyroid issues or direct family history- This could be indicative of Pendred syndrome, a syndrome characterized by EVA.
♣ Kidney or neck issues- This could be a sign of branchio-oto-renal syndrome, a syndrome characterized by EVA.
♣ Any instance of head trauma or airplane traveling before the onset of symptoms.
♣ Any upper respiratory infections recently.
♣ Any disequilibrium or vertigo.
• If the client is a child, ask the parent if the child’s developmental milestones have been delayed. When did they start walking, sitting, standing, etc.?
• Otoscopy and Immittance
Used to rule out: Otitis Media, middle ear fluid, Eustachian tube dysfunction, middle ear fluid, ossicular concerns, etc.
o If EVA:
♣ Otoscopy should be unremarkable.
♣ Tympanometry results should normal (Wolf, 2015).
♣ Stapedial reflexes could be present at normal, reduced, or elevated sensation levels, or be absent, depending on severity and type of hearing loss.
• Pure Tones (Air and Bone) and Word Recognition
o If EVA:
♣ Audiogram should indicate a mixed or sensorineural hearing loss of ranging severities or configurations.
♣ Conductive and mixed components are most likely to occur in the low frequencies (Wolf, 2015).
♣ As hearing declines, loss often shifts from conductive/mixed to sensorineural (Wolf, 2015).
♣ Word recognition deficits are often directly correlated to the severity of the hearing loss (Wolf, 2015).
• Vestibular Evoked Myogenic Potentials (VEMPs)
o Used to rule out: middle ear pathology. In cases of middle ear pathology VEMP responses are typically absent (Wolf, 2015).
o If EVA: VEMP responses are present (Wolf, 2015).
♣ cVEMP responses typically have low thresholds (Wolf, 2015).
♣ oVEMP responses will have high amplitudes (Wolf, 2015).
Management
• Counseling:
o Clients (and parents of clients) should be counseled on the importance of avoiding further head injury, as head trauma may exacerbate their symptoms and make them worse (Hamid, 2018).
o Clients should be counseled to:
♣ Take protective measures (i.e. wear a helmet) when participating in activities wear head trauma could be possible.
♣ Avoid contact/high-impact sports.
• Referrals:
o If hearing loss is identified, and case history reveals potential red flags for EVA, refer client to an ENT for imaging to confirm diagnosis.
o You can also refer a family to a geneticist for genetic testing.
• Treatment:
o No cure and no way to reverse the effects of EVA or, reduce the size of the vestibular aqueduct.
o Steroids are sometimes administered in cases of sudden hearing loss, but has typically not been effective (Hamid, 2018).
o Surgical removal of the endolymphatic sac is considered unnecessarily harmful and is not a treatment option (Hamid, 2018).
o Hearing aids can be beneficial, but recommended hearing aids must have flexible programming capabilities due to progressive nature of HL (Bostic, et al. 2018).
o AAs must be conducted regularly to monitor progression. (Hamid, 2018).
Cochlear implants can also be a viable option, as long the patient does not have any other contraindications for cochlear implants (Bostic, et al. 2018).