Changes in caloric nystagmus induced by vasodilating drugs.

Systemic blood flow, blood flow to the peripheral skin, middle ear, and temporal bone, and that to the inner ear of normal volunteers was manipulated selectively through the use of vasodilator drugs. The effect of selective vasodilation was monitored via changes in the temporal course of the slow-phase velocity of nystagmus evoked by bithermal caloric irrigation. Drug influenced eye velocities were compared with control values measured before drug administration. Eye velocities measured after niacin (a peripheral vasodilator) administration generally were greater than control values, whereas eye velocities measured after papaverine (a central vasodilator) and histamine (a systemic vasodilator) administration typically were less than control values. These findings, with the exception of the cool caloric/niacin condition, are consistent with a model of vestibular dynamics and measurements of cochlear blood flow in animal models.