An animal model of ocular vestibular-evoked myogenic potential in guinea pigs

This study aimed to establish an animal model of ocular vestibular-evoked myogenic potential (oVEMP) in guinea pigs. Ten healthy and 10 gentamicin-treated guinea pigs underwent oVEMP test using a hand-held bone-conducted vibrator placed on the animal’s forehead. All 10 healthy animals exhibited bilateral oVEMPs at the stimulus intensity of 139 dB force level (FL), with a mean threshold and latencies of peak nI and pI of 130 ± 4 dBFL, 3.17 ± 0.37 ms and 4.72 ± 0.38 ms, respectively. Similar to response rate, the nI–pI amplitude decreased markedly in magnitude as stimulus intensity decreased. Another 10 animals administered with gentamicin (2 mg) on the left ear 1 week after surgery had 100% clear oVEMPs beneath the left eye (ipsilateral to the lesion side), whereas oVEMPs were absent and reduced beneath the right eye (opposite to the lesion side) in 7 and 3 animals, respectively. Morphological study of animals with absent oVEMPs identified substantial damage to the hair cells of the utricular macula. Quantitative analysis revealed that histological density of intact hair cells of the utricular macula from control and lesion ears were 194 ± 15 and 66 ± 9 per 130 × 130 μm² field, respectively, showing a 68% reduction in the latter. Further, the stereocilia of the residual hair cells were either fused or deformed, and pointed outward randomly. In conclusion, this study establishes the animal model of oVEMP in guinea pigs using bone-conducted vibration stimuli, which sets the stage for investigating the pathophysiology of the utricular disorders.