How do caloric test results help differentiate unilateral vestibular loss from bilateral hypofunction?

• A. Unilateral loss shows canal paresis with asymmetrical nystagmus
• B. Bilateral hypofunction shows symmetric nystagmus
• C. Unilateral loss shows decreased responses on one side
• D. Bilateral loss shows no response on either side

Answer: (A)

Caloric testing compares the input from each ear to reveal differences in vestibular function, focusing on canal paresis and the resulting eye movements. When one ear is weaker, the irrigation responses from that ear are diminished compared with the other ear, creating a canal paresis. This asymmetry drives nystagmus that is clearly biased toward the healthy side, producing an asymmetrical fast phase. That combination—reduced response on one side plus asymmetrical nystagmus—is characteristic of unilateral vestibular loss and is what the test uses to distinguish it from bilateral problems.

In bilateral hypofunction, both ears are underperforming, so the responses tend to be reduced on both sides and the resulting nystagmus, if present, is typically less directional or symmetric, not the pronounced asymmetry seen with a unilateral lesion. Therefore, the strongest distinguishing feature is the presence of canal paresis with asymmetrical nystagmus, which points to a unilateral deficit rather than bilateral hypofunction.