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Diffuse noxious inhibitory controls in humans: A neurophysiological investigation of a patient with a form of Brown‐Séquard syndrome

In normal subjects, the application of heterotopic painful stimuli induces simultaneous and parallel decrease in the sensation of pain and of the spinal nociceptive flexion (RIII) reflex evoked by electrical stimulation of the sural nerve. This inhibition of the RIII reflex is absent in tetraplegic patients with clinically complete spinal cord transections and can be triggered only from the analgesic hand in patients with Wallenberg's syndrome. These findings suggest that the inhibitory phenomena observed in normal subjects are likely to be examples of diffuse noxious inhibitory controls (DNICs), being sustained by a loop involving supraspinal structures, the ascending part of which is localized in the spinoreticular tract. We now report an exceptional case of a patient with Brown‐Séquard syndrome due to a 4‐year‐old spinal cord lesion (left side, T‐6 level) produced by a knife‐wound in the back. Nociceptive flexion (RIII) reflexes elicited by stimulation of cutaneous afferents in the ulnar and sural nerves were studied in the upper and lower limbs by recording from the biceps brachialis and biceps femoris muscles, respectively. For each limb, the RIII reflex threshold was determined. The reflex was then elicited regularly by stimuli of 1.2 times threshold before, during, and after periods of nociceptive electrical conditioning stimulation (15 mA; 4Hz; 1 min) applied successively to the other three limbs. Inhibitions of around 90% followed by after effects (2–3 min) were observed in all situations except that (1) no inhibition could be obtained when the conditioning stimuli were applied to the lower right limb (contralateral to the spinal lesion) and (2) the RIII reflex in the lower left limb was completely insensitive to any of the conditioning stimuli. These results suggest that in humans (1) the ascending part of the loop subserving DNICs is completely crossed at the spinal level and (2) the descending part is confined to the white matter ipsilateral to the limb being tested.