Intrathecal fentanyl depresses nociceptive flexion reflexes in patients with chronic pain

To investigate the selective role of intraspinal opioids on the perception and modulation of pain, seven subjects with chronic hip or back pain and one subject with C-6 quadriplegia received 25 micrograms of intrathecal fentanyl. The effect of lumbar intrathecal fentanyl on reported pain, nociceptive flexor withdrawal reflexes, a monosynaptic motor arc (H-reflex), and supraspinal effects such as miosis, nausea, respiratory depression was evaluated. In five of eight subjects the flexor withdrawal reflex was completely abolished within 15 min. In the others the reflex was significantly depressed from control values. Decreases in reported pain paralleled the decrease in the flexor reflex, H-reflexes remained unchanged, and no supraspinal side effects were observed. It is likely that these selective changes observed were from the isolated effect of fentanyl modulating nociception at the spinal cord level.