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LESSON 145

The third window: superior canal

Year 2
Spring semester
Disorders of the Auditory and Vestibular Systems
Inner ear and retrocochlear disorders
Duration: 11:20

We explain the sound- and pressure-induced vertigo, autophony and third window effect of superior semicircular canal dehiscence. We cover the low-frequency air-bone gap, bone-conduction thresholds below zero, preserved acoustic reflexes and VEMP findings, and compare them with otosclerosis.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn about third window syndromes and their best-known example, superior canal dehiscence.
We will look at vertigo triggered by sound and pressure, the air-bone gap, and vestibular evoked myogenic potential, or VEMP, findings.
We know that the inner ear has two windows opening into the middle ear: the oval window and the round window.
When the stapes pushes the oval window, a pressure wave passes through the cochlea and the round window membrane bulges outward.
In this arrangement, most of the energy is used to vibrate the basilar membrane.
If a third opening forms in the bony labyrinth, part of the energy escapes through this new path.
These conditions are known together as third window syndromes.
The best-known third window is superior semicircular canal dehiscence.
In this condition, the bone covering the top of the superior semicircular canal becomes thin and opens.
The gap lies between the canal and the middle cranial fossa; there, the canal is covered only by the dura.
The condition was first described in 1998 by Lloyd Minor and colleagues.
In cadaver studies, a gap was found in about five per thousand temporal bones, and very thin bone in more.
The bone is thought to be thin from birth and to open later with trauma or pressure changes.
The gap makes the superior canal sensitive to sound and pressure.
A loud sound or a pressure change is transmitted to the inner ear through the stapes.
Part of the energy flows not to the cochlea but through the superior canal, toward the low-resistance gap.
This flow deflects the cupula in the canal's ampulla, and a false rotation signal goes to the brain.
The result is vertigo and eye movements triggered by sound or pressure.
Vertigo or oscillopsia brought on by sound is called the Tullio phenomenon.
A symptom brought on by coughing, straining, nose-pinched blowing or ear canal pressure is called the Hennebert sign.
Hearing one's own voice as echoing and loud is called autophony; it is a very common complaint.
Some patients say they hear their own pulse, their footsteps, or even their eye movements in the ear.
Aural fullness is also common and can easily be mistaken for a middle ear problem.
Symptoms can be unilateral or bilateral; some people with a gap have no symptoms at all.
Symptoms usually begin in adulthood, sometimes after a pressure event such as head trauma or heavy lifting.
Some patients have a gap in one ear, others in both ears.
Complaints can go undiagnosed for years; patients have often visited several clinics.
Constant autophony and dizziness from sound can markedly affect daily life.
So asking specifically how symptoms relate to sound and pressure is the most important first step toward diagnosis.
In this example, air-conduction thresholds in the right ear are slightly raised at the low frequencies.
The striking finding, however, is bone conduction.
Masked bone-conduction thresholds fall below zero, to minus ten decibels, at two fifty and five hundred hertz.
The result is a clear low-frequency air-bone gap; this is called an inner-ear or pseudoconductive gap.
The third window lets part of the air-conducted energy escape, while it increases bone-conduction sensitivity.
The left ear is within normal limits in this example; tympanograms and acoustic reflexes are normal in both ears.
Let us look more closely at why bone conduction is heard better than normal.
Bone-conducted vibration moves the basilar membrane by creating a pressure difference between the two sides of the cochlea.
Normally, the vestibular side has high impedance; the third window lowers the impedance on this side.
So in bone conduction, the pressure difference between the two sides grows, and the cochlea is stimulated more strongly.
In air conduction, part of the energy escapes through the gap; together, the two effects create the air-bone gap.
The vestibular evoked myogenic potential, or VEMP, is one of the most valuable physiological tests for dehiscence.
The cervical VEMP, or cVEMP, is obtained at a clearly lower intensity than normal in the dehiscent ear.
The amplitude of the ocular VEMP, or oVEMP, grows markedly; in many studies, this is the most distinctive finding.
A clear oVEMP to a four thousand hertz stimulus, which normally gives a weak response, can also suggest dehiscence.
In otosclerosis, by contrast, an air-conducted stimulus usually gives no VEMP; this difference is very valuable in diagnosis.
Because cutoff values vary with device, stimulus and laboratory, interpretation uses local norms.
A low-frequency air-bone gap first brings otosclerosis to mind, so telling the two apart is very important.
In otosclerosis, the stapes footplate is fixed, so the acoustic reflex is usually absent.
In superior canal dehiscence, the middle ear is intact, and the acoustic reflex is usually present.
In otosclerosis, bone conduction does not fall below zero; a Carhart notch is often seen around two thousand hertz.
In dehiscence, symptoms from sound and pressure and enlarged VEMP responses complete the picture.
Stapes surgery done on a wrong diagnosis does not relieve the symptoms, so reflex testing should not be skipped.
On tuning fork tests, the Weber may lateralize toward the dehiscent ear.
Some patients even hear, in that ear, a tuning fork placed on the bony prominence of the ankle.
These findings point to oversensitivity to bone-conducted sound, called bone-conduction hyperacusis.
Electrocochleography, or ECochG, may show a raised ratio of summating potential to action potential in the dehiscent ear.
Because this rise is also seen in Meniere's disease, it is not distinctive on its own.
Symptoms can also be shown during the examination.
The eyes are watched carefully while pressure is applied to the ear canal or a loud sound is presented.
In dehiscence, a vertical and torsional eye movement can appear in the plane of the open superior canal.
When the direction of pressure changes, the direction of the eye movement also reverses.
If this examination is done with video goggles, even small movements can be recorded and reviewed later.
A clinical tip: do not stop searching once you find a bone-conduction threshold at zero decibels.
Also test whether the patient responds at minus five or minus ten decibels, and record the result.
When a normal tympanogram, a low-frequency air-bone gap and present acoustic reflexes occur together, think of a third window.
Stating this pattern clearly in the report helps the physician request the right imaging.
In 2021, the Barany Society published diagnostic criteria for superior canal dehiscence syndrome.
These criteria require a symptom consistent with a third window together with at least one physiological test finding.
In addition, high-resolution CT must show the gap on images reconstructed in the plane of the canal.
Because CT can make thin bone look like a gap, the imaging finding alone is not enough.
The physician weighs treatment options based on symptom severity; with severe symptoms, surgical repair may be considered.
The third window effect is not seen only in the superior canal.
An enlarged vestibular aqueduct can also give a similar low-frequency gap and large VEMP responses.
Posterior canal dehiscence and gaps where the canal lies next to a large blood vessel have also been described.
In an X-linked inner ear malformation, a wide connection between the internal auditory canal and the cochlea has a similar effect.
What these conditions share is an air-bone gap despite a normal middle ear.
The audiologist may be the first to notice findings that suggest a third window.
Measuring bone conduction down to negative values, testing reflexes and running VEMPs are key contributions here.
Asking in the history how symptoms relate to sound and pressure also guides the diagnosis.
In patients who have surgery, hearing and vestibular function are compared before and after.
Results are reported clearly to the ENT physician, in line with the diagnostic criteria.
Superior canal dehiscence creates a third window in the inner ear, and part of the energy escapes through it.
Vertigo from sound and pressure, autophony and hearing internal body sounds are typical symptoms.
The audiogram can show a low-frequency air-bone gap and bone-conduction thresholds below zero.
Present acoustic reflexes and enlarged VEMP responses help distinguish it from otosclerosis.
Test yourself: In a patient with a low-frequency gap, which finding suggests dehiscence rather than otosclerosis?
The correct answer is C: a low cervical VEMP threshold.
An absent reflex and a Carhart notch fit otosclerosis better.
In the next lesson, we will examine another third window example, the enlarged vestibular aqueduct.
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