The effects of medetomidine hydrochloride on the electroretinogram of normal dogs

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Abstract
To determine the effects of a standardized intravenous dose of an alpha-2 agonist (Domitor, Orion Pharma, distributed by Pfizer Animal Health, Exton, PA) on the electroretinogram (ERG) response in normal dogs. Twenty-five normal dogs were used to collect ERG responses including a- and b-wave implicit times (IT) and amplitudes (AMP) before and after administration of medetomidine. Dogs were dark adapted for 20 min and ERGs were obtained using the HMsERG (RetVetCorp Inc., Columbia, MO). The QuickRetCheck protocol (Narfström) was employed to provide the following flash intensities: 10 mcd s/m(2), 3 cd s/m(2), and 10 cd s/m(2). ERGs were repeated after 375 microg/m(2) of medetomidine intravenously. Statistical analysis of the difference between the responses before and after medetomidine at all flash intensities was performed using a mixed effects model for anova. The P value for the effect of medetomidine on each of the ERG responses was < 0.01. The estimates of the effect of medetomidine were (+)1.35 ms, (-)23 microV, (+)3.16 ms, and (-)47 microV for the a-wave IT, a-wave AMP, b-wave IT, and the b-wave AMP, respectively. Medetomidine significantly prolongs the implicit time and lowers the amplitude response of both the a- and b-waves in normal dogs at all flash intensities examined. Clinically, however, medetomidine only minimally affects the retinal responses and is a viable choice for use in dog ERGs.

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