Intraoperative electrical stimulation of the human dorsal spinal cord reveals a map of arm and hand muscle responses

Abstract

While epidural stimulation of the lumbar spinal cord has emerged as a powerful modality for recovery of movement, how it should be targeted to the cervical spinal cord to activate arm and hand muscles is not well-understood, particularly in humans. We sought to map muscle responses to posterior epidural cervical spinal cord stimulation in humans. We hypothesized that lateral stimulation over the dorsal root entry zone would be most effective, and responses would be strongest in the muscles innervated by the stimulated segment. Eighteen people undergoing clinically indicated cervical spine surgery were enrolled. During surgery, stimulation was performed in midline and lateral positions at multiple exposed segments; eight arm and three leg muscles were recorded on each side of the body. Across all segments and muscles tested, lateral stimulation produced stronger muscle responses than midline. These stronger responses were due to a combination of a decreased response threshold and steeper recruitment slope. Muscles innervated at a cervical segment had the largest responses from stimulation at that segment, but responses were also observed in muscles innervated at other cervical segments and in leg muscles. The cervical responses were clustered in rostral (C4-C6) and caudal (C7-T1) cervical segments. Strong responses to lateral stimulation are likely due to the proximity of stimulation to afferent axons. Small changes in responses to stimulation of adjacent cervical segments argues for local circuit integration, and distant muscle responses suggest activation of long propriospinal connections. This map can help guide cervical stimulation to improve arm and hand function.