In an alert patient undergoing bithermal caloric testing, warm water is irrigated into the left external auditory canal. The expected direction of the fast phase of induced nystagmus is:
- A Toward the right ear
- B Toward the left ear ✓
- C Vertical, upward beating
- D Rotatory toward the irrigated side
Correct answer: B. Toward the left ear
Explanation
Caloric responses follow the mnemonic COWS: Cold Opposite, Warm Same. Warm water in the left ear causes endolymphatic convection currents that excite the left lateral semicircular canal, producing nystagmus with its fast phase toward the left. Cold water produces the opposite, with fast phase away from the irrigated ear. Vertical nystagmus is not produced by unilateral lateral canal stimulation in this setting, and the response is horizontal rather than rotatory when tested with fixation removed.
Reference: Dhingra's Diseases of Ear, Nose and Throat, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.