Congenital insensitivity to pain and the “morphine-like” analgesic system

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Abstract
Congenital insensitivity to pain remains without a satisfactory physiopathological explantation. In an electrophysiological study on a nociceptive flexion reflex of the lower limb, the effects of naloxone and of placebo were compared in 8 normal subjects and in a patient with congenital insensitivity to pain. In normal subjects, no significant change in the reflex threshold was observed with naloxone or with placebo. Two electrophysiological abnormalities characterized the patient: spontaneous elevation in the nociceptive reflex threshold of 350% as compared to control, and a large (67%) and rapid (2-3 min) fall of this threshold for about 10 min following naloxone administraiton. These results raise the problem of the relationship between congenital insensitivity to pain and hyperactivity of a naturally occurring morphine-like pain-inhibitory system.