Vestibular modulation of skin sympathetic nerve activity in sopite syndrome induced by low-frequency sinusoidal motion

Sopite syndrome, centred around the drowsiness, lethargy and irritability associated with motion sickness, can be induced by exposure to low-frequency motion. It is known that the vestibular apparatus plays an important role in the pathogenesis of motion sickness, which features several autonomic responses, and we have previously documented increased vestibular modulation of skin sympathetic nerve activity (SSNA) and an increase in skin blood flow associated with nausea. Here we assessed whether imperceptibly slow sinusoidal motion, sufficient to induce sopite syndrome but not nausea, also modulates SSNA and skin blood flow. Participants were seated upright and exposed to a randomised set of sinusoidal linear accelerations, ranging from 0.03 Hz at 0.5 mG to 0.2 Hz at 5 mG, via a motorised platform. At all frequencies vestibular modulation was greater than the cardiac modulation of SSNA, but cardiac modulation and skin blood flow were both significantly lower during the motion than at baseline. We conclude that sopite syndrome is associated with a marked modulation of sympathetic outflow to the skin and cutaneous vasoconstriction.