The effects of analgesics on pain-related somatosensory evoked potentials

Human cerebral evoked potentials can be employed to test the effectiveness of analgesics. In the present study, pain-related cortical responses were elicited by chemical stimulation of the nasal mucosa in combination with electrical stimulation of dental pulp. In alternating sequence, 16 series of 6 chemical and 6 electrical stimuli each were presented. Interstimulus and interseries intervals were 2 s and 50-60 s, respectively. Stimulus intensities were 54% carbon dioxide (v/v) and 4 dB over the electrical current threshold of 21-45 microA for noxious chemical and electrical stimulation, respectively. Four volunteers participated in the study. The EEG was recorded at 8 sites of the international 10-20 system. After each series of stimuli, the test subjects, using visual analogue scales, rated pain intensity relative to a single standard stimulus they had received initially. After running through the procedure once, without their noticing it, the subjects intravenously received 5 ml (2.5 g) metamizol or 5 ml 0.9% NaCl, which were slowly (over a period of 12 min) added to an infusion. The entire stimulation procedure was then repeated 10 min later. Whereas the evoked potentials clearly exhibited habituation effects due to stimulus repetition within a series, the latter were absent in the subjects' pain ratings. Despite the comparatively high i.v. dose of 2.5 g metamizol, pain ratings markedly different from those given under placebo were not observed. In contrast, potential amplitudes after metamizol administration exhibited a clear tendency towards smaller amplitudes in both types of stimulation. In addition, metamizol effects on pain rating could be demonstrated successfully by applying continual chemical stimulation.