Abstract

BACKGROUND: The efficiency of inhibitory pain descending pathways (evaluated using conditioned pain modulation [CPM]) has not been studied in postherpetic neuralgia (PHN).OBJECTIVE: To compare CPM in PHN patients with healthy controls.METHODS: Nine PHN patients and nine control individuals were matched according to age and sex. Amplitudes of cortical thermal-evoked potentials were recorded on the surface of the scalp; clinical pain and thermal pain were evaluated on a 0 to 10 numerical rating scale, at baseline and at intervals during the 6 min after CPM (elicited by a cold pressor test, 8°C). A battery of cognitive tests was performed. Amplitude differences, percentages and related areas under the curve (AUC_CPM) were calculated and all data were compared between both groups; P<0.05 was considered to be statistically significant.RESULTS: AUC_{CPM0–6 min} was significantly lower in PHN patients compared with controls (−39±51 μV/min versus −144±66 μV/min; P=0.0012) and correlated (P=0.04) with clinical pain intensity. Pain ratings before CPM were similar in both groups but were significantly lower in the control group 3 min after the cold pressor test. Cognitive test results were not significantly different.CONCLUSION: Psychophysical and electrophysiological approaches have shown that patients with PHN exhibit a deficiency of pain inhibition modulation, which could signal a predisposing factor to developing chronic pain. This deficiency was not linked to the cognitive performance but rather to subtle in situ cognitivoemotional adaptations, which remain to be investigated.