Inhibition of hyperalgesia by conditioning electrical stimulation in a human pain model

Sensory gain (ie, hyperalgesia) and sensory loss (ie, hypoalgesia) are key features of neuropathic pain syndromes. Previously, we showed that conditioning electrical stimuli may provoke either sensory gain or decline in healthy subjects, depending on the stimulation frequencies applied. In the present study we sought to determine whether sensory decline induced by 20-Hz electrical stimulation preferentially of peptidergic C-nociceptors induces antihyperalgesic effects in a transdermal electrical pain model. Twelve healthy volunteers underwent 0.5-Hz noxious electrical stimulation of the right volar forearm for 35 minutes, leading to secondary mechanical hyperalgesia. In 5 sessions the 0.5-Hz stimulus was applied either alone (Stim1) or with concurrent noxious 20-Hz stimulation at different sites (Stim2: ipsilateral 5 cm distance; Stim3: ipsilateral 10 cm distance; Stim4: contralateral arm; Stim5: contralateral dorsal foot). Close concurrent 20-Hz stimulation (Stim2) inhibited the development of hyperalgesia, as measured using the mechanical pain threshold, while remote and contralateral 20-Hz stimulation had no impact on mechanical pain threshold. However, after ipsilateral (stim2, stim3) and contralateral (stim4) forearm stimulation the area of hyperalgesia around the 0.5-Hz stimulation site was significantly reduced. Thus, antihyperalgesia was induced in a homotopic and in a heterotopic ipsisegmental manner. Underlying mechanisms may include neuroplastic changes of pro- and antinociceptive systems at the spinal or supraspinal level. We conclude that 20-Hz noxious electrical stimulation may represent a neurostimulatory paradigm with antihyperalgesic properties. These findings may thus be of relevance for the future therapy of neuropathic pain syndromes as well.