Delivered by Ingenta to: Guest User IP: 146.185.206.35 On: Sun, 26 Jun 2016 04:03:27 Copyright: Aerospace Medical Association Aviation, Space, and Environmental Medicine x Vol. 81, No. 2 x February 2010 103 RESEARCH ARTICLE L AVON H, T AL D, K AMINSKI-G RAIF G, H ERSHKOVITZ D, S HUPAK A. Vestibular evoked myogenic potentials and saccular plasticity in div- ers. Aviat Space Environ Med 2010; 81:103–6. 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 re- flex 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: Statisti- cally significant shortening of N23-wave latency was found in the divers compared with the control group. The mean 6 SE were 22 6 0.1 and 22.1 6 0.7 ms early and late after a dive in the divers group vs. 24.5 6 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 long- term adaptation of the sacculocollic reflex to underwater conditions. Increased sensitivity of the reflex is required to compensate for the de- crease in linear velocity and acceleration, resulting in reduced stimulation of the otolith organ. Keywords: adaptation, otoliths, vestibular function tests. T HE VESTIBULAR SYSTEM can undergo plastic changes when exposed to exceptional stimuli in order to adjust its reaction to the changing environment. Previous studies have demonstrated changes in the vestibulo-ocular reflex in response to magnifying or direction-changing prisms (3), prolonged low-frequency yaw motion (4), microgravity (6), and rough sea conditions (14). Changes in the vestibulo-spinal reflex, intended to reduce conflicting vestibular input in the control of balance to a minimum, were reported in astronauts (5) and naval crew (11). Adaptation of the otolith end organs to weightlessness has been reported mainly in connection with spaceflight. This includes a higher firing rate of the otolith neurons (1) and augmentation of the static otolith reflex (10). Morphological changes in otolith hair-cell innervations may explain some of these observations (8). Underwater divers are submerged in a medium which is denser than air. In these conditions, when there is greater resistance to head movements, the angular ve- locities and accelerations to which the semicircular canals have to respond will be greatly diminished while it may be expected that there will be reduced stimulation of the otolith organ, which depends on linear velocities and accelerations. In a previous study, prolongation of the primary time constant of the vestibulo-ocular reflex was reported in professional divers. This adjusts the hori- zontal semicircular canal response to the attenuated ve- locity and acceleration stimuli during submersion (12). The purpose of the present study was to examine possible changes in saccular function in professional divers. We also investigated whether there might be diving-in- duced adaptation of the saccular response. METHODS Subjects The study group consisted of 12 male professional divers. The control group was composed of 12 matched non-diver volunteers. The age of the study group partici- pants was 25.3 6 6.1 yr and that of the controls 26.8 6 7.8 yr (mean 6 SD). The average height was 173.7 6 6.5 cm in the divers and 176.4 6 7.6 cm in the controls. A detailed history was taken from all study parti- cipants to exclude any previous cochlear or vestibular injury. Each subject underwent an otoneurological ex- amination that included otoscopy, bedside testing for static, dynamic, positional, and positioning nystagmus, and postural tests. Pure tone air and bone, speech, and impedance audiometry were carried out. Exclusion cri- teria included previous vestibular or cochlear injury, positive findings on otoneurological examination, hearing loss, or abnormal findings in impedance audiometry. To examine the short-term effects of diving on the sac- culocollic reflex, we made two recordings of vestibular evoked myogenic potentials (VEMP) for each subject in the study group using a randomized, crossover design. The first was made shortly after the subject completed a dive and the second after an interval of at least 24 h. To examine long-term effects of diving, VEMP results were From the Motion Sickness and Human Performance Laboratory, Is- rael Naval Medical Institute, Israel Defense Forces Medical Corps, Haifa, Israel. This manuscript was received for review in September 2009. It was accepted for publication in November 2009. Address correspondence and reprint requests to: Avi Shupak, M.D., Motion Sickness and Human Performance Laboratory, Israel Naval Medical Institute, P.O. Box 8040, Haifa 31080, Israel; shupak@internet- zahav.net. Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.2672.2010 Vestibular Evoked Myogenic Potentials and Saccular Plasticity in Divers Haim Lavon, Dror Tal, Gil Kaminski-Graif, Dov Hershkovitz, and Avi Shupak