Quantitative sensory examination during epidural anaesthesia and analgesia in man: effects of morphine

In a double-blind placebo-controlled cross-over study the effects of epidural morphine (4 mg) on somatosensory functions were investigated in 10 healthy volunteers. Detection, pain detection and pain tolerance thresholds to thermal, mechanical and electrical stimuli as well as magnitude rating of short-lasting stimuli of the same modalities were monitored before and for 10 h after epidural administration of 4 mg of morphine or saline. Epidural morphine induced a naloxone-reversible (0.1 mg/kg, i.v.) increase in pain detection threshold to heat and mechanical stimuli and in pain tolerance threshold to heat, mechanical and electrical stimuli. Morphine induced a more pronounced increase in the pain tolerance than in the pain detection threshold. Magnitude rating of short-lasting radiant heat (argon laser) stimuli were reduced by epidural morphine in comparison to placebo while there was no significant difference between the effects of morphine and placebo on magnitude rating of short-lasting mechanical and electrical stimuli. The warm detection threshold was increased (naloxone reversible) by morphine. Segmental distribution of pruritus was reported by 7 subjects following epidural morphine which was replaced by a short-lasting burning sensation following naloxone administration. Naloxone (0.1 mg/kg) preceeded by placebo did not change somatosensory functions. These results indicate that the somatosensory effect of epidural morphine is dependent on the types of afferent fibres activated as well as on the duration and intensity of the stimulus.