• Claudia Spadavecchia, Ole K Andersen, Lars Arendt-Nielsen, Luciano Spadavecchia, Marcus Doherr, and Urs Schatzmann.
• Anesthesiology Section, Department of Clinical Veterinary Sciences, University of Berne, Langassstrasse 124, 3012 Berne, Switzerland.
• Am. J. Vet. Res. 2004 Jul 1; 65 (7): 901-8.

ObjectiveTo investigate whether facilitation of the nociceptive withdrawal reflex (NWR) can be evoked and quantified as a measure of temporal summation from the distal aspect of the left forelimb and hind limb in standing nonsedated horses via repeated stimulations of various subthreshold intensities and frequencies.Animals10 adult horses.ProcedureSurface electromyographic activity evoked by stimulation of the digital palmar and plantar nerves was recorded from the common digital extensor and cranial tibial muscles. For each horse, the NWR threshold intensity to a single stimulus was determined for the forelimb and hind limb. Repeated stimulations were performed at subthreshold intensities and at frequencies of 2, 5, and 10 Hz. The reflex amplitude was quantified, and the behavioral responses accompanying the stimulations were scored.ResultsRepeated stimulations at subthreshold intensities were able to summate and facilitate the NWR in conscious horses. The reflex facilitation was significantly related to the intensity of the repeated stimuli, whereas no effect of stimulation frequency was found. Reaction scores increased significantly for increasing stimulation intensities.Conclusions And Clinical RelevanceTemporal summation obtained by repeated stimulations of subthreshold intensity appears to represent a new tool for investigating nociceptive pathophysiologic processes in horses; this experimental model may be useful to examine the mode of action and efficacy of analgesic and anesthetic interventions and possibly to assess sensory dysfunction in clinical settings.

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