Responses to median and tibial nerve stimulation in patients with chronic neuropathic pain

Somatosensory evoked magnetic fields and electrical potentials were measure d in eight patients with unilateral neuropathic pain. After median nerve st imulation on the painful side, the amplitudes of the evoked responses were enhanced 2 to 3 times at a latency of about 100 ms compared to the response s of the contralateral, unaffected side. After posterior tibial nerve stimu lation an enhancement was found at latencies around 110 ms and 150 ms. The scalp distribution of the magnetic field at the latencies of "abnormal" res ponses was dipolar and the responses could be ascribed to a current dipole. Three (of the eight) patients underwent spinal cord stimulation (SCS) for their pain. The enhancement of the evoked responses to stimulation of the p ainful side decreased after spinal cord stimulation. After a long period of spinal cord stimulation only (e.g., a year) during which the patient repor ted to be pain free, these "abnormal" responses were no longer observed.