PARAESTHESIAE AND HYPAESTHESIA FOLLOWING PROLONGED HIGH-FREQUENCY STIMULATION OF CUTANEOUS AFFERENTS

Abstract

The activity of cutaneous afferents was recorded in human subjects using microelectrodes inserted into individual fascicles of the median nerve at the wrist before and after a 10 min train of electrical stimuli at 200 Hz delivered to the appropriate digital nerve (via ring electrodes) or to individual afferent axons (via the microelectrode). Changes in neural activity produced by the stimulation were correlated with the time course of paraesthesiae and with changes in the ability to detect cutaneous stimuli. From approximately 20 s after the end of the stimulus train, there was a progressive increase in neural activity, and individual afferents became spontaneously active and discharged in high-frequency bursts. At this time the subjects began to experience paraesthesiae. Repetitive stimulation proximal to a complete digital nerve block induced paraesthesiae that were felt distal to the block in the insensate digit, indicating that they did not arise from the unmyelinated terminal segment of the axon or from a stimulus-induced disorder of receptor function. Recordings of the compound action potential evoked by submaximal test stimuli were made after the 10 min stimulus train and revealed evidence of an early transient increase in excitability superimposed on a long-lasting decrease in excitability, reaching a nadir approximately 30–40 min after the end of the repetitive stimulation. In parallel recordings, there was no detectable change in the cutaneous afferent volley evoked by mechanical stimulation of the skin. These findings are consistent with the views that the poststimulation positive symptom, paraesthesiae, can be attributed directly to a disturbance in peripheral afferent fibres, while the poststimulation negative symptoms such as hypaesthesia arise from stimulation-induced refractoriness at central synaptic relays.