Science News: Preservation of Functional Descending Input to Paralyzed Upper Extremity Muscles in Motor Complete Cervical Spinal Cord Injury

Published August 31, 2023

Science News

Submitted by:  Eman Tawfik, MD
Edited by: Pritikanta Paul, MD

Sharma P, Naglah A, Aslan S, et al. Preservation of functional descending input to paralyzed upper extremity muscles in motor complete cervical spinal cord injury. Clin Neurophysiol. 2023;150:56-68. doi:10.1016/j.clinph.2023.03.003

Summary: According to the widely used American Spinal Injury Association (ASIA) impairment scale (AIS), complete spinal cord injury (SCI) is characterized by complete absence of sensory and motor function below the injury level. However, voluntary activation of paralyzed lower limbs muscles in patients with complete SCI has been previously observed. Whether similar activation of paralyzed upper limb muscles in complete cervical SCI is possible or not has not been proven.

In this study, the authors investigated neurophysiological evidence of motor involvement in clinically paralyzed muscles in patients with complete cervical SCI using surface EMG.

Eighteen individuals with motor complete cervical SCI and five age-matched non- injured individuals were recruited. EMG activity was recorded from biceps brachii (BB), triceps brachii (TB), extensor carpi radialis (ECR), flexor digitorum (FD), and abductor digiti minimi (ADM) using surface EMG. The participants were instructed to perform a controlled and isolated 3- second movement trials with full force, termed ‘events’ for each muscle against manual resistance resulting in no visible movement of the target muscle.

Surface EMG activity was recorded from the clinically paralyzed muscles (FD, ADM). Moreover, greater activation of the paralyzed muscles was observed when the patients were instructed to perform events involving relatively proximally innervated non-paralyzed muscles (BB, ECR, TB). These findings indicate presence of residual pathways across the injury establishing supra-lesional control over the sub-lesional neural circuitry. The greater activation of the paralyzed muscles during events involving the non-paralyzed muscles can be considered as reinforcement techniques which are known to facilitate the spinal cord excitability below the injury level.

Comments: A profound and disabling injury, complete SCI significantly affects patients' lives, often leading to diminished hope for recovery. This study revealed presence of voluntary subclinical muscle activation through surface EMG recordings in paralyzed muscles. These findings shed light on neurophysiological changes in upper limb muscles after clinically complete motor loss and offer insights for creating neuromodulation rehabilitation strategies to enhance upper limb recovery after cervical SCIs.

The limitation includes small number of the recruited patients. Therefore, larger scale studies are needed. Further investigation of the effect of neuromodulation techniques and spinal cord stimulation on the EMG activity of the paralyzed muscles is also warranted.