Enlarged Vestibular Aqueduct (EVA)

Definition and epidemiology

The vestibular aqueduct is a narrow bony canal that runs from the vestibule to the posterior surface of the temporal bone and contains the endolymphatic duct. When it measures wider than normal on computed tomography (CT), it is called an enlarged vestibular aqueduct (EVA). Valvassori and Clemis described the condition in 1978 and proposed the term “large vestibular aqueduct syndrome” for a midpoint width greater than 1.5 mm (Valvassori and Clemis, 1978). EVA is the most common radiological inner ear malformation associated with sensorineural hearing loss (Smith et al., 2025) and is often bilateral (Ho et al., 2017).

Diagnostic criteria have changed over time. Measurements in 146 ears of 73 children with normal hearing gave a 95th percentile of 0.9 mm at the midpoint and 1.9 mm at the operculum (Vijayasekaran et al., 2007). Under the definition known as the Cincinnati criteria, an aqueduct is enlarged if it measures 1.0 mm or more at the midpoint or 2.0 mm or more at the operculum; using these criteria, EVA proved more common in children with sensorineural loss than previously thought (Boston et al., 2007). EVA may occur on its own or together with Pendred syndrome, incomplete partition type II and other inner ear malformations (Smith et al., 2025).

Symptoms and signs

Hearing loss is usually noticed in childhood; it may be congenital, prelingual or postlingual, and many children pass newborn hearing screening (Smith et al., 2025). The loss ranges from normal hearing to profound, fluctuates and often progresses in steps: after a drop, hearing may partly recover or settle at a new level. The risk of progressive loss has been shown to rise linearly with aqueduct width (Boston et al., 2007).

Sudden drops sometimes follow a minor head injury, a pressure change or loud noise. In a meta-analysis covering 179 patients, about a third of patients (61/179) and 30% of ears had a sudden drop after such a trigger (Noordman et al., 2015). Vestibular symptoms are also common: in a series of 106 patients, 45% had vestibular symptoms or signs (Zalewski et al., 2015). In young children this may show as clumsiness, late walking or head tilting (Smith et al., 2025).

Causes and risk factors

The best-known genetic cause of EVA is variants in SLC26A4, which are associated both with Pendred syndrome and with non-syndromic EVA without thyroid involvement (DFNB4). The proportion of patients with EVA who carry two variants is 67-90% in those of East Asian origin and about 25% in northern Europeans, about half of whom have no variant identified (Smith et al., 2025). EVA and cochlear hypoplasia are not specific to SLC26A4; congenital cytomegalovirus infection and other genetic conditions can produce a similar picture (Smith et al., 2025).

How the hearing loss arises is not fully understood. In the third-window model, part of the sound energy that would normally pass between the oval and round windows is dissipated through the enlarged aqueduct; this worsens air-conduction thresholds while improving bone-conduction thresholds, producing an air-bone gap mainly below 1 kHz (Ho et al., 2017). Changes in endolymph composition and pressure waves transmitted to the inner ear fluids during trauma are thought to contribute to the sensorineural loss, but the evidence here is largely observational.

Audiological and clinical assessment

EVA is diagnosed on imaging, but audiological findings often raise the suspicion. EVA should be considered in a child with fluctuating or stepwise sensorineural loss and a low-frequency air-bone gap despite a normal tympanogram. Expected findings are:

• Pure-tone audiometry: a bilateral or asymmetric sensorineural loss, or one that looks mixed, sloping towards the high frequencies, with an inner ear air-bone gap at low frequencies. Bone-conduction thresholds may be better than 0 dB HL (negative values) (Ho et al., 2017; Smith et al., 2025).
• Tympanometry and acoustic reflexes: the tympanogram is normal. Tympanometry and acoustic reflex testing should be interpreted together to show that the air-bone gap does not come from the middle ear (Ho et al., 2017), so that unnecessary middle ear surgery for presumed otosclerosis or effusion is avoided.
• Serial audiograms and speech tests: the key tools for documenting fluctuation and stepwise drops; each new complaint is compared with earlier results.
• VEMP: low cervical VEMP thresholds and increased amplitudes support a third-window effect; low thresholds have been reported in 92% of children with EVA (Zhou and Gopen, 2011), although not every series finds this.
• Other vestibular tests: caloric testing, video head impulse test (vHIT) and rotational testing may show vestibular hypofunction (Zalewski et al., 2015).
• OAE and ABR: help estimate thresholds and the type of loss in young children.
• Imaging and genetics: temporal bone CT or high-resolution MRI, with the aqueduct measured at the midpoint and operculum. SLC26A4 testing and thyroid assessment are arranged to identify Pendred syndrome (Smith et al., 2025).

The differential diagnosis includes other third-window lesions that produce an inner ear air-bone gap, particularly superior semicircular canal dehiscence, and conductive losses of middle ear origin (Ho et al., 2017).

Treatment and audiological rehabilitation

No treatment corrects EVA itself. Hearing aids are fitted for mild to severe loss, and settings need frequent updating because the loss fluctuates. Evidence for medical treatment of sudden drops is limited, and the decision rests with the physician. For unilateral loss, approaches that reduce the head shadow effect and CROS systems may be considered; see unilateral hearing loss.

A cochlear implant (CI) is used for severe to profound loss when hearing aids give insufficient benefit. A systematic review including adults and children with EVA found improvements in pure-tone average, speech perception and language measures after CI in all studies; perilymph gusher, cerebrospinal fluid leak and partial electrode insertion occurred during surgery (Alahmadi et al., 2022). Transient vertigo is common after surgery, so vestibular assessment is advised before CI (Smith et al., 2025).

Head injury advice and level of evidence: patients with EVA have traditionally been told to avoid contact sports and to wear helmets. This advice, however, rests largely on small retrospective case series. In the meta-analysis, the only factor significantly associated with a sudden drop after minor trauma was pre-existing fluctuating hearing loss (odds ratio 8.6); Pendred syndrome, aqueduct width and degree of loss were not associated. The authors concluded that blanket restrictions for all patients with EVA appear unnecessary and that advice could be limited to those with fluctuating loss or a previous drop after trauma (Noordman et al., 2015). There is no evidence that avoidance prevents progression, and families should be encouraged to make their own decisions (Smith et al., 2025). For hearing and balance problems after head injury, see the related entry.

Impact on quality of life and advice

For children with EVA and their families, the hardest part is that hearing can change unpredictably. After a drop, a child may no longer follow the teacher in class, hearing aid settings may become inadequate and parental anxiety rises. With a fluctuating course, regular checks make it possible to spot changes early and keep rehabilitation going without interruption. Balance problems can limit sport and play.

Practical advice:

• Note the days when you notice a change in hearing and any events before them (a fall, a knock, a flight, a cold), and share them with your audiologist.
• Discuss sport and helmet use with your doctor, bearing in mind that the evidence is limited; more caution may be reasonable in children with previous fluctuations.
• For pressure changes (flying, diving), review the precautions in ear barotrauma.
• At school, a remote microphone system and sitting near the front reduce listening effort; let the teacher know that hearing may fluctuate.
• Talk to your doctor about genetic assessment and thyroid checks.