Responses to electrical stimulation of the median nerve in the human newborn

Abstract

Four normal full-term newborns and 4 adults were stimulated by constant-current square-wave pulses to the median nerve at the wrist. Thumb and finger movement thresholds were determined by visual observation and by recording movement potentials from the thenar eminence. Afferent nerve action potential thresholds were determined by recording from the median nerve above the elbow. Subjective sensory thresholds were also determined by verbal report in the adults. Somesthetic evoked Iesponses (SEP) were recorded from the contralateral scalp at movement threshold and 2 times the movement threshold in both infants and adults. The mean afferent nerve action potential threshold of the newborns was 3 times higher than that of the adults. The mean movement thresholds of newborns and adults were not significantly different. The mean action potential threshold was not different from the mean subjective sensory threshold in the adults. The mean action potential threshold was not different from the mean movement threshold in the newborns. Shocks of movement threshold intensity are not always adequate for elicitation of afferent action potentials and SEP's in newborns. In stimulating peripheral nerves to elicit SEP's, the positions of the stimulating electrodes are critical.The present study, like a previous one (Ellingson & Ellis, 1970), was undertaken to provide a firmer basis for choosing stimuli to elicit somesthetic evoked potentials (SEP's) in newborns. In the previous study we found that (1) thresholds for thumb or finger movement responses to shocks to the median nerve at the wrist were 1.75 times higher in newborns than in adults, (2) thumb and finger movement thresholds were 3 4 times higher than subjective sensory thresholds in adults, and (3) movement thresholds were variable in some newborns, but no relationship could be established with the wakefulness-sleep cycle. In practice, however, stimuli at movement threshold sometimes fail to elicit SEP's in newborns, despite the fact that similar stimuli considerably exceed sensory thresholds in adults. Further investigation of the relationships among afferent and efferent nerve fiber thresholds and SEP thresholds was therefore needed.