Vestibular evoked myogenic potentials and saccular plasticity in divers

Introduction: Otolith function, which is dependent on linear velocity and acceleration, may be expected to change in underwater divers, who are submerged in a medium that is denser than air. The purpose of the present study was to examine possible changes in the sacculocollic reflex of professional divers and to investigate whether there might be diving-induced adaptation of the saccular response. Methods: We used the vestibular evoked myogenic potential (VEMP) response to evaluate saccular function in 12 professional divers shortly after a dive and after an interval of at least 24 h. The control group consisted of 12 matched non-divers. Wave latencies and amplitudes, asymmetry ratio, and the response threshold were compared between the groups. Results: Statistically significant shortening of N23-wave latency was found in the divers compared with the control group. The mean ± SE were 22 ± 0.1 and 22.1 ± 0.7 ms early and late after a dive in the divers group vs. 24.5 ± 0.5 ms in the control group. No significant differences were found in any of the VEMP parameters between the early and late post-dive recordings. Discussion: We suggest that the reduction in N23 latency reflects longterm adaptation of the sacculocollic reflex to underwater conditions. Increased sensitivity of the reflex is required to compensate for the decrease in linear velocity and acceleration, resulting in reduced stimulation of the otolith organ.