Skin Sympathetic Nerve Activity is Modulated during Slow Sinusoidal Linear Displacements in Supine Humans

Utricular dysfunction in patients with orthostatic hypotension

PURPOSE

To delineate the association between otolithic dysfunction and orthostatic hypotension (OH).

METHODS

We retrospectively reviewed the medical records of 382 patients who presented with orthostatic dizziness at a tertiary dizziness center between July 2017 and December 2021. Patients were included for analyses when they had completed ocular (oVEMP) and/or cervical vestibular-evoked myogenic potentials (cVEMP), and head-up tilt table test with a Finometer (n = 155). We compared the results between the patients with OH (n = 38) and those with NOI (normal head-up tilt table test despite orthostatic intolerance, n = 117).

RESULTS

Thirty-eight patients with OH were further categorized as either classic (n = 30), delayed (n = 7), or initial (n = 1) types. Multivariable logistic regression showed that OH was associated with high baseline systolic BP (p = 0.046), presence of heart failure (p = 0.016), and unilateral oVEMP abnormalities (p = 0.016). n1 latency of oVEMP were negatively correlated with the maximal changes of systolic blood pressure (BP) in 15 s ([Formula: see text]SBP_15s, p = 0.013), 3 min ([Formula: see text]SBP_3min, p = 0.005) and 10 min ([Formula: see text]SBP_10min, p = 0.002). In contrast, the n1-p1 amplitude was positively correlated with [Formula: see text]SBP_15s (p = 0.029). Meanwhile, p13 latency of cVEMP was negatively correlated with [Formula: see text]SBP_10min (p = 0.018).

CONCLUSIONS

Our study provides evidence of utricular dysfunction related to OH.

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

Sopite syndrome, centered 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 randomized set of sinusoidal linear accelerations, ranging from 0.03 Hz at 0.5 mG to 0.2 Hz at 5 mG, via a motorized 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.NEW & NOTEWORTHY Little is known about the autonomic consequences of sopite syndrome-the drowsiness that can be induced by low-amplitude cyclic motion. We recorded skin sympathetic nerve activity (SSNA) in seated participants exposed to slow sinusoidal linear acceleration (0.03-0.2 Hz), which preferentially activates hair cells in the utricular part of the otolithic organs, at amplitudes that generated no sensations of motion. At all frequencies, there was a clear vestibular modulation of SSNA and cutaneous vasoconstriction.

Random-amplitude sinusoidal linear acceleration causes greater vestibular modulation of skin sympathetic nerve activity than constant-amplitude acceleration

We tested the hypothesis that random variations in the magnitude of sinusoidal linear acceleration cause greater modulation of skin sympathetic nerve activity (SSNA), but not muscle sympathetic nerve activity (MSNA), than sinusoidal stimuli of the same frequency but constant amplitude. Subjects (n = 22) were seated in a sealed room mounted on a linear motor that could deliver peak sinusoidal accelerations of 30 mG in the antero-posterior direction. Subjects sat on a padded chair with their neck and head supported vertically, thereby minimizing somatosensory cues, facing the direction of motion in the anterior direction. Each block of sinusoidal motion was delivered at 0.2 Hz, either with a constant-amplitude (root mean square 14 mG) or randomly fluctuating amplitudes of the same mean amplitude. MSNA (n = 12) and SSNA (n = 10) were recorded via tungsten microelectrodes inserted into muscle or cutaneous fascicles of the common peroneal nerve. Cross-correlation analysis was used to measure the magnitude of vestibular modulation. The modulation index for SSNA was significantly higher during delivery of random vs constant-amplitude acceleration (31.4 ± 1.9 vs 24.5 ± 2.5%), but there was no significant difference in the modulation indices for MSNA (28.8 ± 2.9 vs 33.4 ± 4.1%). We conclude that the pattern of vestibular stimulation affects the magnitude of modulation of sympathetic outflow to skin but not to muscle. Presumably, this is related to the subperceptual development of nausea, which is known to be associated with greater vestibular modulation of SSNA but not MSNA.