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. Twenty-five people undergoing clinically indicated cervical spine surgery were consented to map motor responses. During surgery, stimulation was performed in midline and lateral positions at multiple exposed segments; six 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 despite similar latency and shape of responses. 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 response sizes 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.
New and noteworthy A map of muscle responses to cervical epidural stimulation during clinically indicated surgery revealed strongest activation when stimulating laterally compared to midline, and differences to be weaker than expected across different segments. In contrast, waveform shapes and latencies were most similar when stimulating midline and laterally indicating activation of overlapping circuitry. Thus, a map of the cervical spinal cord reveals organization and may help guide stimulation to activate arm and hand muscles strongly and selectively.
Graphical abstract
Competing Interest Statement
Jason B. Carmel is a Founder and stock holder in BackStop Neural and a scientific advisor for SharperSense. Michael S. Virk has been a consultant and has received honorarium from Depuy Synthes, Globus Medical and BrainLab. Noam Y. Harel is a consultant for RubiconMD. K. Daniel Riew: Consulting: Happe Spine (Nonfinancial), Nuvasive; Royalties: Biomet; Speaking and/or Teaching Arrangements: Biomet, Medtronic (Travel Expense Reimbursement); Stock Ownership: Amedica, Axiomed, Benvenue, Expanding Orthopedics, Osprey, Paradigm Spine, Spinal Kinetics, Spineology, Vertiflex. The other authors have nothing to disclose.
Footnotes
↵* J. B. Carmel and M. S. Virk should be considered joint senior authors.
Abbreviations
- mJOA
- Modified Japanese Orthopaedic Association;
- MEP
- Motor Evoked Potential;
- TA
- Tibialis Anterior;
- EDB
- Extensor Digitorum Brevis;
- AH
- Abductor Hallucis;
- AUC
- Area Under the Curve;
- SCI
- Spinal Cord Injury;
- SEM
- Standard Error of the Mean.
