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LESSON 146

Enlarged vestibular aqueduct

Year 2
Spring semester
Disorders of the Auditory and Vestibular Systems
Inner ear and retrocochlear disorders
Duration: 10:16

We explain the diagnostic criteria for enlarged vestibular aqueduct, the fluctuating and stepwise progressive hearing loss of childhood, and sudden drops after minor head trauma. We also cover the SLC26A4 gene, Pendred syndrome, the low-frequency air-bone gap and audiological monitoring.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn about the enlarged vestibular aqueduct, an inner ear difference that is common in children.
We will cover the diagnostic criteria, the fluctuating and progressive hearing loss, the related genes and Pendred syndrome.
The vestibular aqueduct is a thin bony canal that runs from the inner ear into the cranial cavity.
It contains the endolymphatic duct, which leads to the endolymphatic sac on the back surface of the temporal bone.
A normal aqueduct is very narrow; its width at the midpoint is usually less than one millimeter.
An enlarged vestibular aqueduct, or EVA, is one that is wider than normal.
It is one of the most common inner ear differences seen with sensorineural loss in children.
The enlargement is usually bilateral, but it can also be unilateral.
EVA is diagnosed by measuring the aqueduct on computed tomography or magnetic resonance imaging.
By the classic Valvassori criterion, the aqueduct is enlarged if its midpoint width exceeds 1.5 millimeters.
The newer Cincinnati criteria count a midpoint width of 1 millimeter or more as enlarged.
Under the same criteria, the aqueduct is also enlarged if its operculum, the outer opening, is 2 millimeters or wider.
The midpoint lies halfway between the common crus and the outer opening of the aqueduct.
The choice of criterion changes the diagnosis rate; the images are interpreted by the radiologist and physician.
EVA can occur alone or together with other structural differences of the cochlea.
The most common associated difference is incomplete partition type two, in which the apical septa of the cochlea are underdeveloped.
The classic name for this combination is the Mondini deformity.
The endolymphatic sac is often enlarged too; magnetic resonance imaging shows this well.
These structural differences are important information for the surgical team when planning a cochlear implant.
In EVA, the loss is often found at birth or in early childhood; sometimes it appears later in children who passed screening.
This example graph shows one child's average threshold over the years.
The loss does not progress in a straight line; it worsens in steps, with sudden drops.
In this example, the two drops followed minor head injuries.
After a drop, hearing can partly recover, but it often does not fully return to its earlier level.
The degree of loss varies widely, from mild to profound.
Among the reported triggers of sudden drops, minor head trauma is the one most often named.
Pressure changes such as diving or flying can also be triggers.
Situations that raise intracranial pressure, such as heavy lifting and straining, have also been proposed.
This is why physicians give some families advice about head knocks and contact sports; this advice comes from the physician.
Some drops come with dizziness; some children with EVA have vestibular symptoms.
Why hearing declines in EVA is not fully known.
The width of the aqueduct is not thought to explain the loss by itself; it is more a marker of the underlying problem.
One view is that protein-rich fluid from the sac flows back into the cochlea and damages hair cells.
Another proposed mechanism is a disturbed ionic composition and volume of endolymph when pendrin is lacking.
Easy transmission of pressure changes to the cochlea through the wide canal is another idea that tries to explain sudden drops.
In this example, the right ear shows a loss that increases toward the high frequencies.
At the low frequencies, masked bone conduction is better than air conduction, so there is an air-bone gap.
With a normal tympanogram and middle ear, this gap is explained by the third window effect of the wide aqueduct.
This finding rests on the same mechanism as the gap we saw in superior canal dehiscence in the previous lesson.
The left ear also has a similar sensorineural loss; this example fits bilateral EVA.
A low-frequency gap can be mistaken for middle ear fluid; the tympanogram and acoustic reflexes help tell them apart.
In an ear with EVA, the tympanogram is usually normal, which shows that the gap does not come from the middle ear.
The vestibular evoked myogenic potential, or VEMP, may appear at a lower threshold than normal because of the third window effect.
Serial audiograms document the drops and recoveries, which helps clarify the course.
For children who complain of dizziness, vestibular tests are also planned.
All findings are passed in full to the physician, who decides on imaging.
The best-known genetic cause of EVA is changes in the SLC26A4 gene.
This gene codes for pendrin, a protein that transports ions in the inner ear and the thyroid gland.
In the inner ear, pendrin helps keep the ionic balance of endolymph; in the thyroid, it moves iodide for hormone production.
Inheritance is autosomal recessive; an affected child has changes in both copies of the gene.
Some people with EVA have changes in both copies; this proportion varies markedly between populations.
There are also people with EVA who have a change in only one copy, or none at all; other genes are being studied.
SLC26A4 changes also cause the condition known as Pendred syndrome.
In this syndrome, sensorineural hearing loss is accompanied by an enlarged thyroid gland, or goiter.
The goiter is usually absent at birth; it most often appears in late childhood or adolescence.
Most patients have normal thyroid hormone levels; some, however, may develop hypothyroidism.
Pendred syndrome is considered one of the most common causes of syndromic hearing loss.
A clinical tip: the family of a child with EVA should know that hearing needs prompt testing after a drop.
If a sudden drop in performance is reported in a child who uses a device, check the device first, but also rule out a hearing drop.
Testing after a drop is an important starting point for the physician's follow-up and any treatment decision.
Comparing each new audiogram with the previous one reveals stepwise progression early.
Evaluating a child with EVA needs several specialties working together.
The audiologist carries out a detailed hearing evaluation and passes the findings to the physician.
The physician orders imaging; EVA is diagnosed by imaging.
Genetic testing and genetic counseling help clarify the cause and the recurrence risk in the family.
Thyroid evaluation for Pendred syndrome is also planned by the physician.
In children with EVA, regular audiological monitoring is the basis for tracking the course of the loss.
Hearing aids are used according to the degree of loss; the fitting is updated with each new audiogram.
When the loss becomes profound, a cochlear implant is a good option; results in children with EVA are usually good.
If there are vestibular complaints, vestibular evaluation and, if needed, rehabilitation are added to the plan.
The family is told clearly that the loss may fluctuate and progress; this keeps expectations realistic.
EVA is diagnosed with imaging; the cutoff is 1.5 millimeters by Valvassori and 1 millimeter by the Cincinnati criteria.
The loss fluctuates and progresses in steps; sudden drops can follow minor head trauma.
A low-frequency air-bone gap despite a normal middle ear is explained by the third window effect.
The best-known genetic cause of EVA is SLC26A4; the same gene also causes Pendred syndrome.
Test yourself: In enlarged vestibular aqueduct, what most often triggers sudden drops in hearing?
The correct answer is B: minor head trauma.
Pressure changes can also be triggers; hearing should be tested promptly after a drop.
In the next lesson, we will examine genetic hearing loss and the distinction between syndromic and nonsyndromic forms.
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Prerequisites

LESSON 145 The third window: superior canal
LESSON 34 Genetics Basics: Genes, Alleles, Inheritance Patterns Coming soon

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