Hypoalgesia following epidural morphine: a controlled quantitative experimental study

The efficacy, duration, and spread of epidural morphine hypoalgesia were assessed by an experimentally induced pricking pain evoked by laser stimulation. Four mg of plain morphine was injected epidurally in 7 volunteers at the L2-L3 interspace. Thresholds to warmth and pain perception, and pain-evoked potentials were measured. In the first experiment, hypoalgesia was monitored each hour for 7 h at various dermatomes. Hypoalgesia was detected at S1 dermatome after 2 h, but 3 h elapsed before hypoalgesia could be detected at the L1, T12, T10, T8, and T6 dermatomes. No effect was found at C7. No conduction delay was found along the pain pathway during hypoalgesia. Hypoalgesia lasted more than 7 h at S1, whereas hypoalgesia could not be detected after 5 h at other dermatomes. In the second experiment, naloxone (0.8 mg i.v.) was injected 230 min after injection of epidural morphine, and the subsequent recording 10 min later showed that hypoalgesia had been partly reversed. The onset and duration of hypoalgesia are different for experimentally induced pain and clinical pain. Experimentally laser-induced pain has the advantage of being quantitative, and is, as such, useful to assess hypoalgesia, and to test the potency of narcotics.