Study of cortical motor representation and cortical excitability in refractory neuropathic pain

Objective: Repetitive transcranial magnetic stimulation over the primary motor cortex has been shown to provide an analgesic effect on refractory neuropathic pain. It is thought that the primary motor cortex may be involved in pain–related cognitive processing. In this study, navigation–guided transcranial magnetic stimulation (TMS) was applied to investigate cortical motor representation and cortical excitability related to pain. Methods: Subjects were seven patients with refractory neuropathic pain (60.4 ± 13.5 years; stroke, n=5 ; peripheral nerve injury, n=1 ; brachial plexus avulsion, n=1). Pain intensity was measured using a visual analog scale, a numeric rating scale and the short–form McGill Pain Questionnaire 2 (SF–MPQ–2). We measured motor representation and cortical excitability assessed by motor evoked potentials with navigation–guided TMS around the primary motor cortex. A resting motor threshold (RMT), and motor map area and extent were measured in the both hemispheres. The relations between pain assessment items and each measurement (the RMT ratio of affected hemisphere (AH) to unaffected hemisphere (UH), AH ⁄ UH area ratio, and AH ⁄ UH extent ratio) were examined. Results : The RMT of AH trended to be higher than that of UH (p=0.07). The AH ⁄ UH area ratio significantly correlated to SF–MPQ2 (rs=−0.85, p=0.02). The other analyses showed no significant correlations between pain assessment items and each measurement with TMS. Conclusions: This study suggested that refractory neuropathic pain might lead to changes of cortical motor representation and cortical excitability.