What is Superior Canal Dehiscence?

Superior canal dehiscence (SCD) is a condition in which a thin or missing segment of bone overlies the superior semicircular canal, one of the balance organs of the inner ear. This opening acts as a third window into the inner ear, altering pressure dynamics and causing sounds or changes in intracranial pressure to abnormally stimulate the balance system.

Recognizing the Symptoms

The symptoms of SCD are often highly specific and can be provoked by everyday activities. Key clues include:

• Sound-induced vertigo (Tullio phenomenon): Dizziness or a sense of motion triggered by loud noises, such as a telephone ring, a dog bark, or music.
• Pressure-induced vertigo (Hennebert sign): Dizziness arising from pressure changes in the ear canal, for example when straining, coughing, sneezing, lifting, or bending over.
• Pulsatile tinnitus: A rhythmic whooshing or thumping sound in the ear that may align with the heartbeat.
• Autophony: Unusual hearing of one’s own voice, breathing, eye movements, or even the sound of footsteps inside the affected ear.
• Low-frequency conductive hearing loss: Reduced ability to hear deep tones, despite a normally functioning middle ear, because the dehiscence creates an alternative path for sound energy.
• Chronic disequilibrium: A persistent feeling of unsteadiness or swaying, especially while walking or during head movements.
• Nystagmus: Involuntary, rhythmic eye movements that may be triggered by sound or pressure in the affected ear.

Diagnostic Steps

Diagnosing SCD begins with a careful history and physical examination. A clinician will look for the characteristic symptom pattern described above. Several provocative tests help reproduce symptoms and eye movements in the clinic:

• Fistula testing with a pneumatic otoscope: Applying pressure to the ear canal while observing the eyes for nystagmus or asking the patient to report dizziness.
• Tympanometry: This is often normal in SCD, which helps distinguish it from middle ear disorders.
• Vestibular evoked myogenic potentials (VEMPs): Cervical VEMPs and ocular VEMPs often show abnormally low thresholds and increased amplitudes in affected ears, reflecting the hypersensitive response of the saccule or utricle due to the third window.
• Pure-tone audiometry: Hearing tests typically reveal a low-frequency air-bone gap, even when the middle ear is healthy.

While these tests provide strong supporting evidence, the definitive confirmation of SCD is obtained through high-resolution computed tomography (CT) of the temporal bone. Thin-slice CT scans, ideally with images reconstructed in the plane of the superior semicircular canal, can directly demonstrate the missing or thinned bone over the canal.

Differential Considerations

Several other inner ear and middle ear conditions can mimic SCD symptoms. These include otosclerosis, Meniere disease, perilymphatic fistula, and superior semicircular canal apraxia. Because the treatment for each of these differs substantially, a precise diagnosis based on the combination of symptoms, physiologic testing, and imaging is essential.

When to Seek Evaluation

Anyone who notices recurrent dizziness triggered by loud sounds, pressure changes, or straining—especially when accompanied by autophony or a low-frequency hearing loss—should consider an evaluation by an ear, nose, and throat specialist or a neuro-otologist. Early diagnosis can help avoid unnecessary treatments and guide appropriate counseling about activity modifications and potential surgical repair if symptoms are severe.