Superior Canal Dehiscence Syndrome

The inner ear is normally a sealed bony chamber with two small windows for sound and pressure to enter and leave — the oval window (where the smallest bone in the body, the stapes, sits) and the round window (a flexible membrane lower down). In superior canal dehiscence syndrome, a thin spot or hole in the bone above one of the semicircular canals — the canal that arches up under the floor of the brain — creates an unintended third window.

The consequences are striking. Loud sounds can cause vertigo (the Tullio phenomenon). Holding the breath, lifting heavy objects, or even sneezing can do the same (the Hennebert phenomenon). Patients may hear their own heartbeat, eye movements, or footsteps inside the ear (autophony). Some lose hearing in the affected ear at low pitches — but paradoxically hear better than normal through bone conduction.

The condition is uncommon but well-defined. A CT scan showing the missing bone, plus the symptoms, plus an abnormal VEMP test, confirms the diagnosis. Mild cases are managed with reassurance and avoidance of triggers; severe cases benefit from a small surgical repair.

Superior canal dehiscence syndrome was first described in 1998 by Lloyd Minor and colleagues at Johns Hopkins, in a series of patients with sound- and pressure-induced vertigo whose CT scans showed an absent or thinned bony roof over the superior semicircular canal.1The mechanism is now conceptualised as a "third mobile window" in the bony labyrinth: in addition to the normal oval and round windows, the dehiscence creates a third compliant interface through which pressure and sound energy can be diverted away from the cochlea and into the dura or directly onto the membranous canal.

Anatomic dehiscence of the superior semicircular canal is present in approximately 0.5–1% of temporal bones at postmortem, with thinning (<0.1 mm) considerably more common.5 Symptomatic SCDS is rarer — most anatomic dehiscences are clinically silent, probably protected by the overlying dura. The syndrome typically presents in adulthood (4th to 6th decades), with up to half of patients showing bilateral anatomic dehiscence on imaging even when symptoms are unilateral.2

The diagnostic question is rarely "is there a dehiscence?" — CT will answer that — but rather "is the dehiscence the cause of these symptoms?". The Bárány Society 2021 criteria address this by requiring three elements together: characteristic symptoms, a physiologic test demonstrating third-window behaviour, and radiological confirmation of the anatomic defect.2

The biophysics of SCDS hinge on impedance mismatch. Normally, acoustic energy entering at the oval window travels through the cochlear partition (where it is transduced) and exits at the round window — a closed two-window system in which all input pressure must be balanced by an equal-volume displacement at the round window. With a third window present, a fraction of the input energy short-circuits through the dehiscence, producing two effects simultaneously: a relative reduction in air-conducted sound reaching the cochlea (manifest as a low-frequency air-bone gap) and an enhanced response to bone-conducted sound and to inertial stimuli (because the dehiscence becomes a low-impedance pathway for these stimuli to drive cochlear-fluid motion).6,4

The clinical fingerprint of SCDS therefore has two parts — vestibular and audiological — that often present together. Vestibular: sound- or pressure-induced vertigo (Tullio, Hennebert) with vertical-torsional nystagmus in the plane of the affected superior canal. Audiological: pseudo-conductive hearing loss with supranormal bone conduction (negative dB HL thresholds at low frequencies), autophony, pulsatile tinnitus, and audible eye movements or footfalls. The first dehiscent canal patient described by Minor reportedly heard her own pulse so loudly that she could not sleep on the affected side.3

The simulator above shows two ears side by side — one normal, one with the bone gap. Choose "Loud sound" or "Valsalva" to see the difference. In the normal ear, the pressure wave moves harmlessly through the cochlea. In the dehiscent ear, some of the energy escapes upward through the hole in the bone and pushes on the balance organ — which is why a loud noise can make a patient with SCDS feel like the room is spinning.

In the dehiscent labyrinth the bony roof of the superior canal is partly or wholly absent, leaving the membranous canal in contact with the overlying dura (and through it, CSF). This creates a fluid pathway between the labyrinth and the intracranial compartment that is normally impossible. Pressure that should remain bounded within the labyrinth — sound-driven stapes motion, ICP fluctuations, Valsalva-induced venous pressure — can now displace endolymph through the canal opening, deflecting the cupula of the superior canal and generating an aberrant excitatory signal to the central vestibular system.

The direction of the resulting eye movement is stereotyped: vertical-torsional, in the plane of the superior canal — slow phase up and torsional with the upper pole rotating away from the affected ear, fast phase the reverse. This pattern is pathognomonic when seen synchronously with sound or pressure stimulus.3

On the cochlear side, the third window changes the impedance of the inner ear in a frequency-dependent way. At low frequencies, where the stapes drives slow, sustained pressure changes, the dehiscence acts as a pressure-relief valve — energy escapes upward instead of driving the basilar membrane. The result is a low-frequency air-bone gap that superficially mimics otosclerosis. Crucially, bone conduction at low frequencies is enhanced rather than normal: vibrations applied to the skull are preferentially channelled through the low-impedance third window to the cochlear fluids, producing thresholds better than 0 dB HL.6

The supranormal bone-conduction threshold is the audiometric feature that makes SCDS impossible to confuse with otosclerosis on a careful audiogram. In otosclerosis, fixation of the stapes raises air-conduction thresholds but bone conduction is normal (or shows the Carhart notch at 2 kHz — a 5–15 dB depression that disappears after successful stapedectomy). In SCDS, bone conduction is supranormal — the low-frequency points sit above 0 dB HL on the audiogram. Miss the supranormal threshold and the patient gets a stapedectomy that fails to improve symptoms (and, worse, may produce a perilymph gusher).

Vestibular-evoked myogenic potentials reflect the same impedance shift. Both cVEMP (saccular) and oVEMP (utricular) show enhanced responses in SCDS — lower thresholds for cVEMP (often <65 dB nHL, vs the usual 75–95 dB nHL norm) and higher amplitudes for oVEMP. The Bárány criteria accept either an enhanced cVEMP, an enhanced oVEMP, or a low-frequency negative bone-conduction threshold as the physiologic test required for diagnosis.2

In a hearing test, a patient with SCDS shows two unusual findings on the affected side. The first is a hearing loss at low pitches that looks like a problem with the middle ear (the kind seen in glue ear or otosclerosis). The second is the give-away: when sound is delivered through the skull bone instead of through the air, the patient hears better than normal. Skipping this bone-conduction check is how SCDS gets misdiagnosed as otosclerosis and operated on inappropriately.

The Bárány Society accepts "low-frequency negative bone conduction thresholds on pure tone audiometry" as one of three acceptable physiologic-test findings for the diagnosis of SCDS.2"Negative" here means below 0 dB HL — better than the audiometric zero defined for a young, otologically normal population. In SCDS, this supranormal threshold typically appears at 250, 500, and sometimes 1000 Hz on the affected side, with normal bone conduction at higher frequencies and on the unaffected side.

Practical audiometric tips. Use insert phones for bone masking to avoid over-masking and missing the supranormal threshold. Calibrate bone-conduction at -10 or -15 dB HL — many clinical audiometers default to a 0 dB floor and will not register the diagnostic finding. Confirm with a Weber test (which should lateralise to the dehiscent ear despite the apparent "conductive" loss — the opposite of what one might expect for true middle-ear pathology).

Three audiological discriminators separate SCDS from its principal mimic, otosclerosis. First, supranormal bone conduction (SCDS) vs. normal or notched bone conduction (otosclerosis). Second, normal acoustic reflexes (SCDS) vs. absent reflexes (otosclerosis — the stapedial reflex requires a mobile stapes). Third, enhanced VEMP responses (SCDS) vs. normal or absent VEMPs (otosclerosis). A patient with apparent conductive hearing loss whose stapedial reflexes are present is, until proven otherwise, a third-window patient — image with high-resolution CT in the Pöschl plane before considering any stapes surgery.6,7

Doctors need three things to make the diagnosis: the characteristic symptoms (sound or pressure causing vertigo, hearing one's own pulse or eye movements), an abnormal balance or hearing test showing the third-window effect, and a CT scan that confirms the missing bone. Without all three, the diagnosis is not made.

The Bárány Society's 2021 diagnostic criteria require the presence of all three of:2

1. At least one symptom consistent with third-window pathophysiology:
   • Hyperacusis to bone-conducted sound (autophony, audible eye movements, audible footfalls)
   • Sound-induced vertigo and/or oscillopsia time-locked to the stimulus
   • Pressure-induced vertigo and/or oscillopsia time-locked to the stimulus
   • Pulsatile tinnitus
2. At least one physiologic test or signindicating a third mobile window:
   • Eye movements in the plane of the affected superior canal time-locked to sound or pressure
   • Low-frequency negative bone-conduction thresholds on pure-tone audiometry
   • Enhanced VEMP responses (low cVEMP threshold or elevated oVEMP amplitude)
3. High-resolution CT with multiplanar reconstruction in the plane of the superior canal (Pöschl view) and orthogonal to it (Stenvers view) consistent with a dehiscence.

And — explicitly — "not better accounted for by another vestibular disease or disorder."

Two technical points about the imaging deserve attention. Slice thickness must be <1 mm (ideally 0.625 mm or less); thicker slices over-call dehiscence because of volume averaging.7 The Pöschl reformat — parallel to the plane of the superior canal — is the best view for identifying the defect; the Stenvers reformat (perpendicular) is used to confirm. Avoid making the diagnosis on axial slices alone, where bone thickness can be ambiguous.

Other third-window pathologies share the audiometric signature: posterior canal dehiscence, lateral canal dehiscence (associated with chronic otitis media or cholesteatoma), large vestibular aqueduct syndrome (LVAS), and dehiscence of the cochlea into the carotid canal or internal auditory canal. The differential matters because the surgical approach differs by location. In ambiguous cases, electrocochleography may show an elevated SP:AP ratio (>0.4 — non-specific but supportive); the Tullio sign on video-oculography in the plane of the affected canal under sound stimulus is highly specific and worth seeking actively.

Many patients with mild SCDS manage with reassurance and by avoiding the triggers — wearing earplugs in loud environments, avoiding heavy lifting, learning to keep bowel movements gentle (to avoid Valsalva-type strain). For patients whose symptoms are severe enough to interfere with work or sleep, surgery to plug or resurface the affected canal resolves symptoms in about 90% of patients, although a small risk of hearing loss exists.

Management is stratified by symptom severity. Mild symptoms are managed conservatively: counselling and identification of triggers, hearing protection in loud environments, stool softeners and posture advice for Valsalva-sensitive patients. Anti-tinnitus strategies (masking, cognitive behavioural therapy) help with the pulsatile-tinnitus component. Most patients with mild SCDS live well with the condition.

Surgical repair is offered when symptoms are disabling. Two anatomical targets, two surgical approaches:

• Middle fossa craniotomy: the original Minor approach. Provides direct visualisation of the dehiscence and allows precise plugging, capping (cartilage or fascia overlay), or resurfacing. Requires temporal lobe retraction and a 2–3 day hospital stay.1
• Transmastoid approach: the dehiscence is accessed through the mastoid and plugged blind. No craniotomy; shorter hospital stay; lower recurrence rate in the Schwartz multi-institutional series compared with middle fossa.8

Across both approaches, vertigo symptoms resolve in approximately 90% of patients; the systematic review by Gioacchini and colleagues reported a pooled success rate of 94%.9 Hearing outcomes are mostly stable with a small risk (≈5–10%) of high-frequency sensorineural loss. Surgery should be reserved for patients with disabling symptoms because the risk-benefit calculation favours observation in mild cases.

The conversation about surgery is best framed around three axes: which symptom is dominant, how disabling it is, and what hearing-preservation tolerance the patient has. Patients whose primary complaint is autophony alone are generally less benefited by surgery than those with disabling Tullio or chronic disequilibrium — autophony resolves in ≈75% post-operatively, whereas vertigo resolves in >90%. Patients with bilateral anatomic dehiscence on imaging warrant particular caution: operate on the worst side first and reassess at 6 months; bilateral surgery is occasionally necessary but doubles the risk of hearing loss.4

A failed stapedectomy in a patient with an apparent otosclerotic audiogram is the classic SCDS late presentation. If the patient still has third-window symptoms after middle-ear surgery, image the temporal bones and check VEMPs before considering anything further on the stapes — there is a real risk of a perilymph fistula being created on a dehiscent canal.6