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

Abstract

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.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers – an updated review

“…Here 4 mg morphine was administered in the epidural space. This dose is at upper end of the therapeutic range [37]. In the study by Staahl et al analgesia was seen for several pain parameters in various tissues and this group also applied a high dose, compared with the normal dose used in the clinic, although in the therapeutic range [5].…”

“…It should be emphasized that pain models which evoke higher pain intensities recruit more C-fibres than pain intensities close to or under the pain detection threshold. Accordingly suprathreshold pain measures are traditionally thought to be more sensitive in drug research than the pain detection threshold [37]. Part of the pain system can also be evaluated objectively with neurophysiologic techniques.…”

“…pain [5,37,53,81]. For heat pain to be sensitive to morphine it has been argued that it needs to be applied with slow temperature rises (<1°C s -1 ) [82,83].…”

“…For heat pain to be sensitive to morphine it has been argued that it needs to be applied with slow temperature rises (<1°C s -1 ) [82,83]. However, morphine has also shown effect on pain from rapid increases in temperature [37,81,[83][84][85].Three studies found sensitivity of electrical pain for morphine, whereas another did not [5,37,53]. Pain evoked by pinching of the skin was sensitive to morphine [5].…”

“…Pain evoked by pinching of the skin was sensitive to morphine [5]. Two studies showed an effect of morphine on the warmth detection threshold and pain detection threshold to heat, pressure and electrical stimulation [37,84,85]. Electrically-induced pain in the teeth and skin has been assessed by electroencephalogram (EEG) recordings.…”