Superior Canal Dehiscence Syndrome
Superior canal dehiscence syndrome (SCDS) was first described by Lloyd Minor and colleagues in 1998 as a novel vestibular disorder caused by an absence of bone overlying the superior (anterior) semicircular canal, creating an abnormal “third window” into the inner ear. This dehiscence allows sound energy and pressure changes to displace endolymph through the superior canal, stimulating vestibular receptors and producing a characteristic combination of vestibular and auditory symptoms. Though anatomic thinning of the tegmen over the superior canal is found in approximately 5% of temporal bone specimens, symptomatic SCDS is much rarer. The condition is frequently misdiagnosed — particularly the conductive hearing loss component, which can be mistaken for otosclerosis — making clinical awareness essential for neurologists and otologists.
Bottom Line
- Third window physiology: Bony dehiscence over the superior canal creates an abnormal pathway for sound and pressure to stimulate the vestibular system
- Pathognomonic symptoms: Tullio phenomenon (vertigo from loud sounds) and autophony (hearing own voice, heartbeat, or eye movements abnormally loudly) are highly specific for SCDS
- Commonly misdiagnosed as otosclerosis: Low-frequency conductive hearing loss with normal tympanometry and intact acoustic reflexes should raise suspicion (otosclerosis has absent reflexes)
- Best screening test: VEMP testing — cervical VEMP with decreased thresholds and ocular VEMP with enhanced amplitudes; highly sensitive
- Confirmatory imaging: High-resolution CT of temporal bones with 0.5 mm cuts and Poschl/Stenvers reformats showing the bony defect
- Definitive treatment: Surgical canal plugging or resurfacing via middle fossa craniotomy for disabling symptoms; conservative management with symptom avoidance is appropriate for mild cases
Anatomy and Pathophysiology
Normal Inner Ear Windows
The normal inner ear has two mobile windows: the oval window (footplate of the stapes) and the round window. Sound energy enters through the oval window, travels through the cochlear fluids, and exits through the round window. This two-window system ensures that acoustic energy is efficiently transmitted to the cochlear hair cells.
Third Window Effect
In SCDS, the dehiscence creates a third mobile window at the site of the bony defect, fundamentally altering inner ear mechanics:
Third Window Pathophysiology
- Sound-induced vestibular stimulation (Tullio phenomenon): Airborne sound transmitted through the middle ear creates pressure waves that, instead of being confined to the cochlea, shunt through the dehiscent superior canal, displacing endolymph and stimulating the superior canal ampulla
- Pressure-induced vestibular stimulation (Hennebert sign): Changes in middle ear or intracranial pressure (Valsalva, coughing, straining, tragal pressure) similarly displace endolymph through the abnormal window
- Conductive hearing loss: The third window diverts acoustic energy away from the cochlea, reducing air-conducted sound transmission at low frequencies, creating an air-bone gap — this is a “pseudo-conductive” loss because the middle ear mechanism is normal
- Bone conduction hyperacusis: Bone-conducted sound is enhanced because the third window provides a lower-impedance pathway for bone-conducted vibrations to reach the cochlea; this causes autophony and the perception of internal body sounds
- Pulsatile tinnitus: Transmission of vascular pulsations through the dehiscent bone to the inner ear
Etiology of the Dehiscence
- Developmental: The bone overlying the superior canal is among the last areas of the otic capsule to ossify; failure of complete ossification may leave a congenitally thin tegmen
- Acquired: Thinning may progress with age, chronic intracranial pressure elevation, or trauma; symptoms often develop in the 3rd–5th decade, suggesting a two-hit model (congenital thinning + later trigger)
- Prevalence: CT studies show thinning or dehiscence in ~5% of temporal bones; clinically symptomatic SCDS is estimated at approximately 1–2 per 100,000
- Bilateral: Anatomic dehiscence is bilateral in ~30–40% of cases on CT, though bilateral symptomatic disease is less common
Clinical Features
The clinical presentation of SCDS encompasses two categories of symptoms — vestibular and auditory — reflecting the dual impact of the third window on both the semicircular canal and cochlear systems.
Vestibular Symptoms
| Symptom | Description | Mechanism |
|---|---|---|
| Tullio phenomenon | Vertigo and/or nystagmus triggered by loud sounds; patients may report dizziness in noisy environments | Airborne sound energy shunts through the dehiscence, displacing endolymph in the superior canal |
| Hennebert sign | Vertigo induced by pressure changes — Valsalva maneuver (straining, nose-blowing), tragal pressure, coughing, sneezing | Middle ear or intracranial pressure changes transmitted through the dehiscence |
| Chronic disequilibrium | Persistent sense of imbalance or spatial disorientation; may be the predominant symptom in some patients | Tonic asymmetry in vestibular signaling from the affected superior canal |
| Oscillopsia | Visual blurring with certain sounds or pressure changes | Sound/pressure-induced eye movements (nystagmus in the plane of the superior canal) |
Auditory Symptoms
| Symptom | Description | Mechanism |
|---|---|---|
| Autophony | Hearing own voice abnormally loudly; hearing own heartbeat, eye movements, footsteps, or joint sounds internally | Enhanced bone conduction through the third window pathway |
| Conductive hearing loss | Low-frequency air-bone gap (typically 10–30 dB at 250–1000 Hz); often misdiagnosed as otosclerosis | Acoustic energy shunted away from the cochlea through the third window |
| Pulsatile tinnitus | Rhythmic tinnitus synchronous with heartbeat | Vascular pulsations conducted through dehiscent bone to inner ear |
| Hyperacusis to bone-conducted sound | Increased sensitivity to body sounds; may hear eyes moving in their sockets | Lowered impedance for bone-conducted vibrations to reach the cochlea |
Clinical Pearl: Symptoms Patients Describe
- “I can hear my eyeballs move” — classic autophony from bone conduction enhancement
- “Loud restaurants make me dizzy” — Tullio phenomenon from ambient noise
- “When I strain or lift something heavy, the room spins” — Hennebert sign from Valsalva
- “My own voice echoes in my ear” — autophony
- “I hear my heartbeat in my ear” — pulsatile tinnitus
- Many patients have seen multiple physicians over years before diagnosis; a high index of suspicion is key
Examination Findings
Nystagmus Patterns
When sound or pressure stimulation provokes nystagmus in SCDS, it characteristically aligns with the plane of the superior semicircular canal: primarily vertical (upward) and torsional (top pole of the eye rotating toward the affected ear). Examination should include:
- Sound-evoked nystagmus: Apply a loud tone (e.g., 500–2000 Hz via audiometer or tuning fork) to the affected ear and observe for nystagmus with Frenzel goggles
- Pressure-evoked nystagmus: Positive tragal pressure (pressing on the tragus to compress the ear canal), Valsalva against pinched nostrils (increases middle ear pressure), or Valsalva against a closed glottis (increases intracranial pressure)
- Fistula test: Pneumatic otoscopy with observation for induced nystagmus
Tuning Fork Examination
The Weber test lateralizes to the affected ear, which may initially suggest a conductive hearing loss from middle ear pathology. However, in SCDS, the lateralization is due to enhanced bone conduction from the third window rather than middle ear disease.
Diagnostic Workup
VEMP Testing
Vestibular evoked myogenic potential (VEMP) testing is the most sensitive screening test for SCDS and is essential in the diagnostic evaluation.
| VEMP Type | Normal | SCDS Finding | Significance |
|---|---|---|---|
| Cervical VEMP (cVEMP) | Threshold ~90–100 dB nHL; moderate amplitude | Decreased threshold (≤75 dB nHL); increased amplitude | Saccular pathway — the third window lowers the threshold for sound to stimulate the saccule |
| Ocular VEMP (oVEMP) | Low amplitude (~5–10 μV) | Enhanced amplitude (often >15–20 μV) | Utricular pathway — most sensitive and specific single test; third window enhances bone-conducted vibration to the utricle |
High-Resolution CT Temporal Bone
CT Imaging Protocol and Interpretation
- Technique: High-resolution CT with ≤0.5 mm slice thickness; multiplanar reformats are essential
- Poschl reformat: Reformatted parallel to the plane of the superior canal — shows the arch of the canal and whether bone is present or absent over its apex
- Stenvers reformat: Reformatted perpendicular to the superior canal — shows the canal in cross-section and the thickness of overlying bone
- Findings: Absence of bone overlying the arcuate eminence of the superior canal; the canal lumen communicates directly with the middle cranial fossa
- Caution: Partial volume averaging can create false-positive dehiscence on axial images alone; multiplanar reformats are mandatory to avoid overdiagnosis
- Bilateral assessment: Always evaluate both sides; bilateral anatomic dehiscence is common (~30–40%)
Audiometry
The audiometric pattern in SCDS is distinctive and, when recognized, is an important diagnostic clue:
| Audiometric Feature | SCDS | Otosclerosis |
|---|---|---|
| Air-bone gap | Low-frequency (250–1000 Hz) | Low-frequency (250–2000 Hz); Carhart notch at 2000 Hz |
| Bone conduction | Supranormal (may be <0 dB, i.e., better than normal) | Normal or mildly reduced |
| Tympanometry | Normal (Type A) | Normal or shallow (Type As) |
| Acoustic reflexes | Present (intact) | Absent |
| Mechanism of CHL | Third window shunting (pseudo-conductive) | Stapes fixation (true conductive) |
Pitfall: Misdiagnosis as Otosclerosis
- SCDS is commonly misdiagnosed as otosclerosis due to the low-frequency conductive hearing loss
- Patients have undergone unnecessary stapedectomy for presumed otosclerosis, which fails to improve hearing and may worsen symptoms
- Key distinguishing feature: In SCDS, acoustic reflexes are present (the middle ear mechanism is normal); in otosclerosis, acoustic reflexes are absent (stapes is fixed)
- Supranormal bone conduction thresholds (better than 0 dB) are virtually pathognomonic for SCDS
- Any patient with conductive hearing loss, present acoustic reflexes, and vestibular symptoms should be evaluated for SCDS
Differential Diagnosis
| Condition | Key Distinguishing Features |
|---|---|
| Otosclerosis | Absent acoustic reflexes; no vestibular symptoms typically; no autophony; Carhart notch on audiometry; CT may show lucency around oval window (otospongiosis) |
| Perilymphatic fistula | History of trauma, surgery, or barotrauma; SNHL (not conductive); vertigo with pressure changes but no Tullio phenomenon; CT normal |
| Enlarged vestibular aqueduct | SNHL (often mixed), fluctuating; associated with Pendred syndrome; CT/MRI shows dilated vestibular aqueduct |
| Patulous Eustachian tube | Autophony present (voice, breathing) but no vertigo with sound; symptoms improve when lying down or with head-dependent position; tympanometry shows respiratory fluctuations |
| Meniere disease | Episodic vertigo with SNHL (not conductive), tinnitus, aural fullness; no Tullio phenomenon; normal CT |
| Vestibular migraine | Episodic vertigo with migraine features; no hearing loss; no sound/pressure-evoked nystagmus; normal VEMP and CT |
| Other third-window lesions | Posterior canal dehiscence (very rare), large vestibular aqueduct, otic capsule dehiscence near cochlear aqueduct — similar third-window physiology |
Treatment
Conservative Management
For patients with mild or tolerable symptoms, conservative management is appropriate and may be the preferred approach for those who are not surgical candidates or prefer to avoid surgery:
- Symptom avoidance: Minimize exposure to triggering loud sounds; avoid heavy lifting, straining, or other activities that provoke Valsalva-related symptoms
- Hearing protection: Ear plugs or noise-canceling devices in environments with predictably loud sounds
- Vestibular rehabilitation: May improve chronic disequilibrium through central adaptation
- Patient education: Understanding the diagnosis is often therapeutic in itself, particularly for patients who have been undiagnosed for years
- Monitoring: Periodic audiometry and symptom assessment
Surgical Treatment
Surgery is indicated for patients with disabling symptoms that significantly impair quality of life and have not responded to conservative measures.
| Approach | Technique | Advantages | Disadvantages |
|---|---|---|---|
| Middle fossa craniotomy — canal plugging | Craniotomy above the ear; direct visualization of dehiscence; canal lumen occluded with bone wax, fascia, or bone pate | Highest success rates (>90% vestibular symptom improvement); directly addresses the defect; prevents endolymph flow through the canal | Requires craniotomy; risks include SNHL (5–10%), CSF leak, epidural hematoma, temporal lobe injury; canal function permanently eliminated |
| Middle fossa craniotomy — canal resurfacing | Craniotomy; fascia and bone placed over the dehiscence without entering the canal lumen | Preserves superior canal function; theoretically lower risk of SNHL | Higher recurrence rate than plugging (graft may resorb); may have lower long-term success for vestibular symptoms |
| Transmastoid approach | Mastoidectomy; canal accessed from behind; plugging or capping via this route | Avoids craniotomy; lower morbidity; may be preferred in certain anatomic situations | Limited visualization of the dehiscence; technically more challenging for some surgeons; outcomes comparable but less extensively studied |
| Endoscopic approaches | Emerging endoscopic middle fossa or transmastoid techniques | Minimally invasive; reduced morbidity | Limited availability; long-term outcomes data still accumulating |
Surgical Outcomes
Expected Results After Canal Plugging
- Vestibular symptoms: >90% of patients report significant improvement or resolution of vertigo, Tullio phenomenon, and Hennebert sign
- Autophony: Improves in ~80–90% of cases
- Conductive hearing loss: Air-bone gap often closes partially or completely
- Pulsatile tinnitus: Usually improves
- SNHL risk: 5–10% may experience new or worsened sensorineural hearing loss
- Vestibular recovery: Transient vertigo and imbalance expected post-operatively (days to weeks) as the brain compensates for the plugged canal; vestibular rehabilitation accelerates recovery
- Recurrence: Rare after successful plugging; more common after resurfacing alone
Special Considerations
Bilateral SCDS
Bilateral anatomic dehiscence is found in approximately 30–40% of CT scans showing unilateral SCDS, though symptomatic bilateral disease is less common. When bilateral surgery is needed, it is typically staged (one side at a time, separated by several months) to allow vestibular compensation between procedures.
SCDS and Raised Intracranial Pressure
Conditions that chronically elevate intracranial pressure (idiopathic intracranial hypertension, obesity) may contribute to progressive thinning of the tegmen and potentially precipitate or worsen SCDS symptoms. Patients with SCDS should be evaluated for signs of raised intracranial pressure, and concurrent IIH should be treated.
SCDS in the Neurology Clinic
Neurologists may encounter SCDS in several contexts: as a cause of unexplained dizziness, as a mimic of vestibular migraine, or as a cause of “conductive hearing loss” referred from audiology. Asking about autophony, Tullio phenomenon, and pressure-evoked vertigo should be part of the vestibular review of systems.
References
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- Minor LB. Superior canal dehiscence syndrome. Am J Otol. 2000;21(1):9–19.
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