Horizontal canal benign paroxysmal positioning vertigo: reversible ipsilateral caloric hypoexcitability caused by canalolithiasis?

Bithermal caloric stimulation in a patient with a benign paroxysmal positioning vertigo of the horizontal (lateral) semicircular canal (HC-BPPV) revealed a significant hypoexcitability of the affected ear that was reversible when treated by liberatory maneuvers. Both the positional vertigo and caloric hypoexcitability in HC-BPPV are caused by canalolithiasis, a concept strongly supported by the intensity of the positioning nystagmus being maximal when the patient turned his head around the longitudinal z-axis from the left lateral to right lateral position while recumbent (maximum slow-phase velocity [SPV] 176 deg/sec) and being much lower (maximum SPV 55 deg/sec) when he turned his head to the right lateral position while in a supine position (nose up). The dependence of this difference in nystagmus intensity on the initial head position and direction of rotation indirectly proves that the clot moves freely (to and fro) within the segment of the HC diametrically opposite to the ampulla. The reversibility of ipsilateral caloric hypoexcitability is attributed to the clot's (functional) plugging of the HC, which agrees with studies in squirrel monkeys. These data support the view that physiologic caloric responses consist of a major convective and minor nonconvective component.