Evaluation of the lower motor neuron integrity of upper extremity muscles in high level spinal cord injury

Abstract

Purpose: To evaluate the lower motor neuron (LMN) integrity of upper extremity muscles of persons with high tetraplegia (C1-C4) in order to determine muscles available for stimulation. Methods: Fourteen subjects (23 arms) were evaluated for LMN integrity. Muscles that elicited a functional response (grade 3 or better) to surface electrical stimulation were considered to have intact LMN and good candidates for FES. Strength-duration (S-D) curves were generated on muscles that showed weak (less than grade 3) or no response to surface stimulation. Muscles were considered denervated if S-D curves were discontinuous or depicted steep, increasing amplitude for pulse durations greater than 1 m. Results: Muscles for grasp and release had intact LMN in 19 of 23 (83%) arms. The wrist extensors and¯exors and pronator were excitable in 17 (74%), 20 (87%) and 19 (83%) arms, respectively. The supinator demonstrated LMN lesion in 80% of the arms. Over 90% of the biceps muscles were unresponsive to electrical stimulation and 85% and 87% of the deltoid and supraspinatus muscles, respectively, were not electrically excitable. The latissimus dorsi and triceps muscles were typically innervated (78% and 91%, respectively) and slightly more than half (52%) of the pectoralis major muscles were excitable. Conclusion: These data suggest that application of FES in high tetraplegia for hand and arm function would require augmentation because of the inability to stimulate the elbow¯exors, deltoid and rotator cu muscles. These data also show that several paralyzed proximal muscles with intact LMN that have been historically transferred to address shoulder paralysis in other patient populations are available for transfer and stimulation in the population with high level spinal injuries.

Motoneuron Death after Human Spinal Cord Injury

“…For example, various muscles innervated from the lesion epicenter have been unresponsive to electrical stimulation of their peripheral nerves, consistent with complete muscle denervation. [14][15][16][17][18] Other studies have shown signs of partial denervation (low motor unit counts, positive sharp waves, small amplitude compound muscle action potentials, muscle weakness that exceeds that expected from disuse atrophy, atrophied and angular muscle fibers), including indications of intramuscular motor axon sprouting, an important compensatory mechanism for recovery of muscle innervation after death of some motoneurons in a motor pool (large polyphasic motor unit potentials, stronger than usual motor unit forces, increased motor unit fiber density, increased jitter in re-innervated muscle fibers). 12,[19][20][21][22][23] In this study, we address an important and often overlooked aspect of SCI-the effect of injury on the lesioned spinal cord segment and denervation of skeletal muscle as a contributor to muscular weakness.…”

Patterns of lower extremity innervation in pediatric spinal cord injury

“…If a muscle responded weakly to surface stimulation, strength duration testing was performed to that muscle to determine if the muscle should be classified as innervated, denervated, or partially denervated. A full description of performing and interpreting strength duration curves is identified in Mulcahey et al 21 Data analysis Subjects were grouped according to levels of injury (Table 2). These groupings were based upon the authors' hypotheses about potential sources of denervation due to clinical experience and anatomy.…”

“…The literature presents mixed findings in regard to mechanism of injury and innervation. Doherty et al 1 found no relationship between mechanism of injury and innervation, while Mulcahey et al 21 anecdotally reported increased LMN damage in the upper extremities of children whose cervical SCI had viral, vascular, or iatrogenic causes, and Triolo et al 23 reported a relationship between SCI due to transverse myelitis and denervation. Despite anatomical differences reported in the spines of children less than 10 years of age, we found no relationship between age at injury and innervation status, indicating that the anatomical differences in children likely do not impact innervation status.…”

“…Studies have attempted to predict whether the neurological level will correspond to the type of lesion in the lower extremities, 1 upper extremities, 21 and the bladder. 22 Doherty et al 1 found a significant trend that the incidence of LMN lesions affecting the lower extremities decreased as the level of injury progressed in a cephalad manner in adults with SCI, using reflex testing to distinguish those with UMN and LMN lesions.…”

“…25,26 Viral, vascular, and iatrogenic causes of SCI have also been linked to LMN damage. 21,23 Differences may exist in patterns of UMN and LMN lesions between injuries sustained by children and by adults. Injuries to the spinal cord can differ in presentation between adults and children.…”