Introduction: Space motion sickness (SMS), caused by a canal-otolith conflict, is currently treated with intramuscular promethazine. However, the drug has an inconsistent efficacy against SMS. We hypothesize that pharmacological depression of the semicircular canals (SCC) might relieve SMS. The aim of the present study was to identify the effects of meclizine (25 mg), dimenhydrinate (40 mg) combined with cinnarizine (25 mg), and promethazine (25 mg) combined with d-amphetamine (10 mg) on the SCCs and the otoliths.
Methods: This double-blind, placebo-controlled study was performed on 20 healthy men. Function of the SCC was evaluated by means of an electronystagmography, whereas utricular function was assessed by a unilateral centrifugation test. A cervical vestibular evoked myogenic potentials test evaluated saccular function.
Results: Meclizine (0.54 +/- 0.05 vs. 0.38 +/- 0.06) and dimenhydrinate with cinnarizine (0.54 +/- 0.05 vs. 0.45 +/- 0.05) decreased the vestibulo-ocular reflex gain. Promethazine with d-amphetamine decreased the latency of the saccadic eye response (right eye: 185 +/- 3.8 ms vs. 165 +/- 4.5 ms; left eye: 181 +/- 4.9 ms vs. 165 +/- 4.8 ms) and also increased the phase of ocular counterrolling measured during unilateral centrifugation (0.32 +/- 0.35 degrees vs. 1.5 +/- 0.45 degrees).
Discussion: It is hypothesized that meclizine and dimenhydrinate with cinnarizine affect the medial vestibular nucleus. Promethazine is a vestibular suppressor, but study results show that d-amphetamine counterbalances this depression and abolishes the effect of fatigue on the saccadic reaction time The hypothesis that a SCC-suppression alleviates SMS should be further evaluated.