Effect of epidural morphine on the Hoffman-reflex in man

Four healthy, young subjects undergoing knee surgery with epidural anaesthesia were given epidural morphine for the relief of post-surgical pain. The Hoffman (H)-reflex was tested before and after the injection of morphine in the epidural space. The reflex was chosen as a monitor of the function of monosynaptic pathways in the spinal cord. Analogue scales for the measurement of pain revealed a satisfactory decrease of nociception after morphine injection, while the H-reflex was not significantly affected. This study may confirm that monosynaptic reflexes are not affected by epidural administration of morphine and that they are probably not involved in pain transmission.