During a caloric test, ice-cold water is instilled into the external auditory canal of an alert patient with an intact brainstem. The expected response is:
- A Slow phase toward the irrigated ear, fast phase away from it
- B Purely tonic eye deviation with no nystagmus
- C Fast phase toward the irrigated ear, slow phase away from it ✓
- D Vertical nystagmus regardless of head position
Correct answer: C. Fast phase toward the irrigated ear, slow phase away from it
Explanation
By the mnemonic COWS (Cold Opposite, Warm Same), cold water in the left ear reduces the firing rate of the left horizontal canal afferents, mimicking a rightward head turn. The slow component drifts away from the irrigated side while the fast corrective phase beats toward the right, that is, toward the stimulated side. In coma with a destroyed brainstem, the fast phase disappears and only tonic deviation remains, so option A describes warm water, not cold.
Reference: Ganong Review of Medical Physiology, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.