How does a superior canal dehiscence alter VEMP responses, and what physiological mechanism explains this finding?

• A. Lower thresholds and higher amplitudes in VEMP due to third-window effect increasing vestibular system sensitivity to sound and pressure
• B. Higher thresholds and lower amplitudes due to reduced sensitivity
• C. No change in VEMP responses
• D. VEMP responses become inverted (negative) due to canal plugging

Answer: (A)

The main idea is that a superior canal dehiscence creates a third window in the inner ear, which changes how sound and pressure drive the vestibular organs and, in turn, how VEMP responses appear.

VEMP responses come from vestibular receptors (the saccule for the cervical VEMP and the utricle for the ocular VEMP) being activated by sound or vibration. Normally, the inner ear has two main windows, and energy from air-conducted sound mostly drives the cochlea with limited spillover to vestibular parts. When a dehiscence forms, this additional opening lowers the impedance of the labyrinth and provides an easier pathway for acoustic and pressure energy to move the inner-ear fluids, especially exciting the vestibular end organs.

Because more energy reaches the saccule and utricle for the same stimulus, the threshold for eliciting a VEMP becomes lower and the resulting reflex is larger in amplitude. This exaggerated response is a hallmark of the third-window effect seen with superior canal dehiscence. The mechanism isn’t about inverting the response; it’s about amplifying vestibular stimulation from sound and pressure due to the extra window.