Possible startle response contamination of the spinal nociceptive withdrawal reflex.

The objective of this study was to examine the possibility that the spinal nociceptive withdrawal reflex, otherwise known as the RIII reflex, is contaminated by the startle response, which is a non-pain-related supraspinal response. Startle response contamination of the RIII reflex would seriously compromise the RIIIs ability to measure spinal nociceptive processes in man, since a change in the startle response affecting EMG amplitude in the RIII latency range would be erroneously interpreted as a change in a spinal nociceptive process. EMG responses evoked by electrical stimulation of the sural nerve were recorded from the orbicularis oculi, neck, biceps, and biceps femoris muscles in 31 healthy human volunteers. The startle response was elicited under conditions often used to record the RIII reflex. Procedures are described that will completely eliminate the startle response. Comparisons between subjects that did and did not elicit a startle response revealed that the startle does not appear to significantly contaminate the biceps femoris RIII reflex, at least when performing group comparisons. There are, however, situations not dealt with in this study in which the startle might significantly contaminate the RIII reflex, such as patients with pre-existing negative emotional states, experimental procedures that induce fear and/or anxiety, and single case studies. It is important, therefore, that investigators using the RIII reflex be cognizant of the startle response and take appropriate precautions to monitor and if necessary eliminate the startle before attributing a change in the RIII reflex to a spinal nociceptive process.