Low dose epidural morphine does not affect non-nociceptive spinal reflexes in patients with postoperative pain

In order to determine the selectivity of the antinociceptive effect of epidural morphine, the electrophysiological features of a monosynaptic reflex were investigated before and after epidural morphine (2-3 mg) required for pain relief, in 4 patients suffering from postoperative pain. The reflex tested was the H reflex from the soleus muscle. Not only the classical Hmax/Mmax ratio, but also the threshold and the slope of the rising phase of the recruitment curve of H and M responses were analyzed. It was found that epidural morphine did not change significantly any of these parameters, at least during the 60 min post-injection period. By contrast, as usually observed clinically, epidural morphine produced an onset of pain relief by the 25th-30th min following injection. These data support the hypothesis that epidural morphine produced a 'selective spinal analgesia in man.'