Effect of surgery on sensory threshold and somatosensory evoked potentials after skin stimulation

We have studied the effect of surgical injury on cutaneous sensitivity and somatosensory evoked potentials (SSEP) to dermatomal electrical stimulation in 10 patients undergoing hysterectomy. Forty-eight hours after surgery, sensory threshold increased from 2.2 (SEM 0.3) mA to 4.4 (1.1) mA (P less than 0.01) and SSEP amplitudes in P1, N1, P2 and the peak-to-peak amplitudes decreased (P less than 0.05). Latency was prolonged only in the N3 component (P less than 0.05). No changes were seen in threshold and SSEP in a control group studied over the same interval. The results suggest that surgical injury activates antinociceptive modulatory systems.