Which vestibular pathology is commonly associated with a low-threshold, high-amplitude VEMP response, known as a third window phenomenon?

• A. Superior canal dehiscence syndrome (SCDS)
• B. Ménière disease
• C. Vestibular neuritis
• D. Benign paroxysmal positional vertigo

Answer: (A)

A third window in the inner ear creates an extra open doorway for energy to reach the vestibular organs, which lowers the amount of energy needed to trigger a reflex. In superior canal dehiscence syndrome, a defect in the bone over the superior semicircular canal acts as that third window. This makes the vestibular system unusually sensitive to sound and pressure, so even soft stimuli can provoke large vestibular responses. That’s why VEMP measurements show a low threshold and high amplitude when stimulated with sound or pressure in this condition.

Other vestibular disorders don’t produce this pattern. Ménière disease involves fluctuating hearing loss and vertigo with different VEMP changes that aren’t characterized by such a dramatic, low-threshold, high-amplitude response. Vestibular neuritis typically causes unilateral hypofunction, leading to reduced VEMP responses on the affected side rather than exaggerated, easily triggered responses. Benign paroxysmal positional vertigo mainly causes vertigo with specific head positions and usually shows normal VEMP results. So the combination of easily elicited, large VEMP responses due to an extra third window points to superior canal dehiscence syndrome.