Lumbosacral spinal cord somatosensory evoked potentials for quantification of nociception in horses

REASONS FOR PERFORMING STUDY: There is a need for objective evaluation and quantification of the efficacy of analgesic drugs and analgesic techniques in horses.

OBJECTIVES: To determine whether lumbosacral spinal cord somatosensory evoked potentials (SSEP) can be a useful and reliable tool to assess nociception in equines.

METHODS: SSEPs and electromyograms (EMG) from the epaxial muscles were recorded simultaneously, following electrical stimulation applied to the distal hindlimb in lightly anaesthetised Shetland ponies (n=7). In order to validate the model, the effect of increasing stimulus intensity was documented and the conduction velocities (CV) of the stimulated nerves were calculated. The effect of epidurally applied methadone (0.4 mg/kg bwt) in a randomised, crossover design was investigated.

RESULTS: Two distinct complexes (N1P1 and N2P2) were identified in the SSEP waveform. Based on their latency and conduction velocity and the depressant effect of epidurally applied methadone, the SSEP N2P2 was ascribed to nociceptive Adelta-afferent stimulation. The SSEP N1P1 originated from non-nociceptive Abeta-afferent stimulation and was not influenced by epidurally applied methadone.

CONCLUSIONS AND POTENTIAL RELEVANCE: The nociceptive Adelta component of the SSEP, the N2P2 complex, is presented as a valid and quantitative parameter of spinal nociceptive processing in the horse. Validation of the equine SSEP model enables the analgesic effects of new analgesics/analgesic techniques to be quantified and analgesia protocols for caudal epidural analgesia in equidae improved.