Science News: Nerve Transfer After Cervical Spinal Cord Injury: Who Has a "Time Sensitive" Injury Based on Electrodiagnostic Findings?

Summary: Injuries sustained during a traumatic spinal cord injury (tSCI) are pure (central nervous system) CNS lesions and many have evidence of lower motor neuron lesion (LMNL). During the care of these individuals, LMNL may go undetected during early to subacute management. Individuals that sustain significant denervation of intrinsic hand muscle may miss the ideal opportunity for early nerve transfer and thereby potentially reducing their chances for improved outcome. While the literature has demonstrated good outcomes after 1 year of injury, clinically 6 months is often used as the ideal time in which to complete a nerve transfer. This study investigated the use of nerve conduction studies (NCSs) to investigate the degree of injury and change overtime to help identify the subpopulation of tSCI that would best be served by early consideration of nerve transfer. 

 

In a retrospective review of records using the European Multicenter Study About SCI database, 79 individuals with tSCI underwent formal NCS evaluation at 3 and 6 month injury and completed standardized muscle testing using the Medical Research Council score. An independent limb analysis was conducted to account for individual heterogeneity, resulting in a total of 145 limbs analyzed. Only individuals with a score of four or higher at the C5-6 level and partial innervation of intrinsic hand muscles with a grade below three were included in the review. The ulnar motor compound motor action potential (CMAP) amplitude was utilized as a surrogate for C8-T1 spinal levels and results were categorized into normal (>6.0 mV), sub-normal (1.0-5.9 mV), and very abnormal (<1.0 mV).

 

At baseline, 74.7% of subjects had American Spinal Injury Association (ASIA) Impairment Scale scores of A or B. At 3 months, 87.2% had CMAP that were either subnormal or very abnormal, which decreased to 80% at 6 months. Over the 3 month period, only four limbs (9.1%) improved at all, and none reached normal range. Twelve of the limbs  (16%) improved to normal level. Comparing complete versus incomplete tSCI, there was no difference in CMAP amplitude (p=0.31). However, incomplete tSCI showed a median CMAP that was 2.4 mV greater than incomplete injuries (p=.001). Utilization of a CMAP amplitude cut off of <1.0 mV resulted in a positive predictive value of 0.73 and 0.78 for muscle strength of zero versus 1-2 for C8 and T1 segments respectively.

Comments: Following tSCI there exists a subpopulation of individuals who are unlikely to regain functional hand use and are also amenable to nerve transfers. Individuals with damage resulting significantly in the denervation of intrinsic hand muscle, in essence, have a finite time before atrophy and fibrosis makes successful nerve transfer unlikely. While ulnar CMAP amplitudes are often tolerable of axonal loss up to 50%, the result of this study provides an objective cut off for clinicians to identify at-risk individuals in a timely manner. It also provides a 3-month time period for clinicians to provide stabilizing care and for confirmation of LMN findings, but also a significant amount of time in which individuals can be evaluated as surgical candidates. Traumatic SCIs are rarely just injuries of the SCI and there often exist concurrent peripheral nerve injuries. Providers rely on the expertise and clinical knowledge of EDX physicians in evaluating and fully characterizing these injuries. The insights gained from this study can assist clinicians in managing and improving outcomes for patients with SCI, while also drawing attention to potentially overlooked peripheral nerve injuries.