Superior Semicircular Canal Dehiscence

Absence of bone over the superior semicircular canal, creating a third mobile window into the labyrinth in addition to the oval and round windows.

Third window physiology, which explains every symptom

Normally acoustic energy enters at the oval window, travels through the cochlea, and exits at the round window. A third window provides an alternative low impedance path for energy to dissipate.

• Air conducted sound is shunted away from the cochlea, raising air conduction thresholds, particularly at low frequencies. This produces an apparent conductive loss with a completely normal middle ear.
• Bone conducted sound is transmitted more efficiently because the third window enhances the differential motion across the cochlear partition, lowering bone conduction thresholds, sometimes to negative values below 0 dB. This is a pathognomonic finding.
• The combination produces a substantial air bone gap, typically 20 to 50 dB and maximal at 250 and 500 Hz, with entirely normal middle ear mechanics and therefore preserved acoustic reflexes.
• Sound and pressure become able to move endolymph in the superior canal, producing vertigo and eye movements in the plane of that canal, which is the Tullio phenomenon and the Hennebert sign.
• Internal body sounds that would normally dissipate are now amplified and perceived, producing autophony of the voice, audible eye movements, audible footsteps, and hearing of one's own heartbeat.

Aetiology

A two hit hypothesis is generally accepted: a congenitally thin or absent bone over the canal, present in around 1 to 2 percent of temporal bones, followed by a second event such as head trauma, barotrauma, straining or gradual thinning with age, which completes the dehiscence and produces symptoms. This explains why symptoms often begin in the fourth or fifth decade after an apparently trivial event.

Auditory symptoms, which are often the most distressing

• Autophony: the patient's own voice sounds abnormally loud and distorted, described as booming or hearing oneself in a barrel. This is the single most characteristic symptom.
• Hearing one's own eye movements, which is close to pathognomonic when reported spontaneously.
• Hearing one's footsteps, chewing, neck movement and joint sounds.
• Pulsatile tinnitus, hearing the heartbeat in the ear.
• Aural fullness.
• Apparent hearing loss with an air bone gap on testing.
• Hyperacusis to bone conducted sound, so patients may be able to hear a tuning fork placed on the ankle or elbow, which is a useful bedside demonstration.

Vestibular symptoms

• Tullio phenomenon: vertigo and oscillopsia induced by loud sounds.
• Hennebert sign: vertigo induced by pressure change, including nose blowing, straining, coughing, lifting and tragal compression.
• Chronic disequilibrium and unsteadiness.
• Oscillopsia during the provoked episodes, with the visual world appearing to move in the plane of the superior canal, that is with a vertical torsional pattern.

Examination

• Nystagmus provoked by loud sound or by pressure, beating in the plane of the superior semicircular canal, that is upward with a torsional component away from the affected ear on excitation.
• Weber lateralising to the affected ear despite a sensorineural type of symptom picture, which is because of the enhanced bone conduction.
• Rinne may be negative in the affected ear.
• A 512 Hz tuning fork placed on the medial malleolus or elbow may be heard in the affected ear, which is a striking and specific bedside sign.
• Valsalva against a pinched nose or against a closed glottis provoking nystagmus.

Diagnosis requires the combination of characteristic symptoms, physiological evidence of a third window, and imaging confirmation. Imaging alone overdiagnoses the condition, since radiological dehiscence is more common than symptomatic disease.

• Pure tone audiometry showing a low frequency air bone gap with normal tympanometry, and bone conduction thresholds that may be better than 0 dB.
• Acoustic reflexes present, which is the key discriminator from otosclerosis and other middle ear causes of an air bone gap. Present reflexes with an air bone gap should always prompt consideration of a third window lesion.
• Vestibular evoked myogenic potentials, which are the key physiological test. Cervical potentials show markedly reduced thresholds, typically below 70 dB compared with a normal 80 to 95 dB, with increased amplitudes. Ocular potentials show increased amplitudes, typically above 17 microvolts. These reflect the reduced impedance of the labyrinth.
• High resolution computed tomography of the temporal bone with sub millimetre collimation, reconstructed in the plane of the superior semicircular canal, that is the Pöschl and Stenver planes. Standard axial and coronal images overestimate dehiscence because of partial volume averaging, and reformatting is essential. Report the dehiscence length, since larger defects correlate with more severe symptoms.
• Magnetic resonance imaging is not the primary modality but may be used to exclude other pathology.
• Electrocochleography showing a raised summating potential to action potential ratio, which is supportive but non specific.

Differential diagnosis and the discriminating feature

• Otosclerosis: air bone gap with absent acoustic reflexes and normal vestibular evoked myogenic potentials. Performing a stapedotomy on a patient with superior canal dehiscence is a well documented error that produces no hearing improvement and may worsen vertigo. Present acoustic reflexes are the safeguard.
• Perilymphatic fistula: overlapping pressure and sound sensitivity, but normal vestibular evoked myogenic potential thresholds and no dehiscence on imaging.
• Ménière disease: episodic vertigo with progressive low frequency sensorineural loss, no autophony of internal sounds.
• Patulous Eustachian tube: autophony of the voice and of breathing, with breath synchronous drum movement and relief on lying down. Autophony of breathing points to patulous tube; autophony of eye movements points to dehiscence.
• Other third window lesions: enlarged vestibular aqueduct, posterior canal dehiscence, cochlear dehiscence adjacent to the carotid canal, and X linked stapes gusher syndrome.

Management is graded according to symptom severity, since many patients with radiological dehiscence have mild or no symptoms.

Conservative management

• Appropriate for mild symptoms and for patients who can adapt their behaviour.
• Explanation of the mechanism, which many patients find enormously reassuring after years of being told nothing is wrong.
• Avoidance of triggers: loud noise, straining, heavy lifting, nose blowing, and pressure changes.
• Earplugs in noisy environments.
• Treatment of any coexisting condition that increases straining, including constipation, chronic cough and nasal obstruction.
• Vestibular rehabilitation for chronic disequilibrium, though it does not address the sound and pressure induced symptoms.
• Avoid diving and consider occupational restrictions.

Surgical management

Indicated for disabling symptoms that fail conservative management, particularly severe autophony, frequent sound or pressure induced vertigo, and functional impairment. Surgery is not indicated for an incidental radiological finding.

Middle fossa craniotomy approach

• Provides direct access to the dehiscence for plugging, resurfacing or capping.
• Plugging with fascia, bone wax, bone pate and bone chips occludes the canal lumen and is the most reliable method for abolishing symptoms, at the cost of loss of superior canal function.
• Resurfacing lays material over the defect without occluding the lumen, preserving canal function but with a higher failure and recurrence rate.
• Requires temporal lobe retraction, with risks of seizure, dysphasia, epidural haematoma and cerebrospinal fluid leak.

Transmastoid approach

• Avoids craniotomy and temporal lobe retraction, with lower morbidity and shorter recovery.
• The canal is plugged on both sides of the dehiscence without directly visualising the defect, which is an advantage where the dehiscence is adjacent to the superior petrosal sinus.
• May be technically difficult in a sclerotic mastoid or with a low lying tegmen.

Round window reinforcement

• Transcanal reinforcement of the round window with soft tissue or cartilage, which is far less invasive and may reduce the third window effect by dampening the round window.
• Outcomes are inconsistent and less durable than canal plugging, but it is a reasonable option for patients unfit for or unwilling to accept craniotomy.

Outcomes and complications

• Autophony and sound induced vertigo resolve in around 80 to 95 percent after canal plugging.
• The air bone gap closes in most patients.
• Complications: sensorineural hearing loss in around 5 to 25 percent depending on approach and technique, which is the principal risk and must be discussed explicitly; benign paroxysmal positional vertigo postoperatively, which is common and treatable; prolonged postoperative disequilibrium for weeks to months as the patient compensates for loss of superior canal function; cerebrospinal fluid leak; and seizure or temporal lobe injury with the middle fossa approach.
• Recurrence after resurfacing is higher than after plugging.
• Bilateral dehiscence is present in up to a third. Operate on the more symptomatic side only, and reassess before considering the second side, since bilateral canal plugging produces greater vestibular deficit.

Postoperative care and follow up

• Expect disequilibrium for several weeks. Begin vestibular rehabilitation early.
• Audiometry at 6 weeks and 3 months.
• Repeat vestibular evoked myogenic potentials at 3 months to confirm normalisation of thresholds, which correlates with symptom resolution.
• Test for and treat postoperative benign paroxysmal positional vertigo, which is frequently the cause of ongoing symptoms mistaken for surgical failure.