Semicircular Canal Dehiscence: Definition and Clinical Presentation
Semicircular canal dehiscence is an abnormal opening in the bone overlying a semicircular canal (most commonly the superior canal) that creates a "third mobile window" in the inner ear, disrupting normal inner ear mechanics and causing a characteristic syndrome of sound- or pressure-induced vertigo, autophony, conductive hearing loss, and pulsatile tinnitus. 1, 2
Anatomic Basis and Pathophysiology
The semicircular canals are specialized end organs within the inner ear responsible for balance and equilibrium, forming part of the vestibular system 3
Canal dehiscence creates an abnormal communication between the inner ear and surrounding structures (typically the middle cranial fossa for superior canal dehiscence), establishing a third mobile window that disrupts the normal closed hydraulic system of the inner ear 4, 2
Superior semicircular canal dehiscence (SSCD) is the most common and well-established form, first described by Minor and colleagues in 1998 2
Lateral and posterior semicircular canal dehiscences can also occur but are much rarer, with lateral canal dehiscence usually associated with chronic otitis media and cholesteatoma 4
Characteristic Symptoms
Vestibular Symptoms
Sound-induced vertigo and oscillopsia (Tullio phenomenon): episodic vertigo triggered by loud sounds 1, 5
Pressure-induced vertigo (Hennebert sign): vertigo provoked by changes in external auditory canal or middle ear pressure 1, 5
Auditory Symptoms
Autophony: hearing one's own voice or bodily sounds (breathing, eye movements, footsteps) abnormally loudly in the affected ear 1, 5, 6
Conductive hearing loss: paradoxical bone conduction hyperacusis where bone conduction thresholds are better than air conduction 1, 2
Pulsatile tinnitus: hearing one's own heartbeat or pulse 1, 6
Hyperacusis: increased sensitivity to sound 5
Aural fullness 6
Diagnostic Evaluation
High-resolution computed tomography (CT) of the temporal bone is the primary imaging modality, performed with fine-cut (0.5-0.6 mm) collimation reformatted parallel and orthogonal to the plane of the superior canal 3, 1
Vestibular evoked myogenic potentials (VEMPs) show characteristic abnormalities with lowered thresholds 1
Physical examination may demonstrate eye movements in the plane of the superior semicircular canal when ear canal pressure or loud tones are applied 2
Valsalva maneuvers and specific auditory testing can aid in detection 1
Potential Etiologies
Chronic otitis media with cholesteatoma (particularly for lateral canal dehiscence) 1, 4
Fibrous dysplasia 1
Treatment Approach
Conservative Management
- Mild symptoms can be managed conservatively with observation and symptom avoidance strategies 5
Surgical Intervention
Surgical repair is indicated for patients with debilitating symptoms and should only be performed in patients who exhibit clear clinical manifestations 1, 5
Middle fossa craniotomy approach provides better visualization of the dehiscence but carries higher morbidity than transmastoid approaches 5, 6
Surgical techniques include plugging, capping, or resurfacing the dehiscence, with plugging and capping associated with higher success rates than resurfacing without added risk of hearing loss 5
Surgical repair demonstrates high rates of symptom resolution, with 100% of patients reporting resolution in at least one symptom, and significant improvement in autophony, tinnitus, and sound/pressure-induced dizziness 6
Endoscopic assistance during middle cranial fossa approach may improve visualization 5
Clinical Pitfalls
SSCD remains an underdiagnosed and undertreated condition despite being well-established in the medical literature since 1998 6, 2
New symptoms can develop postoperatively, including imbalance (29%), sound/pressure-induced dizziness (20%), and aural fullness (18%) 6
All three semicircular canals (superior, lateral, and posterior) can be affected with similar clinical presentations, requiring careful diagnostic evaluation 4