Electrodiagnosis of upper limb weakness in acute quadriplegia

Clinical and neuropathologic observations after cervical spinal cord injury suggest varying involvement of gray and white matter. The resulting upper limb weakness may reflect varying degrees of upper motoneuron (UMN) and/or lower motoneuron (LMN) involvement. This study uses electrophysiologic measures, including compound muscle action potential (M response) amplitude, root mean square (RMS) of the surface electromyographic activity during voluntary muscle contractions and the firing rate of motor units, to distinguish UMN and LMN weakness in upper extremities after acute quadriplegia. M response amplitude did not correlate with strength; many muscles had large M responses given their strength. These muscles manifest: (1) high M/RMS ratios (ratio of electrically elicited to voluntarily recruited electromyographic activity) and (2) slow firing rates of single motor units during maximal isometric contractions. For muscles with normal M amplitudes, M/RMS ratio correlates inversely with strength. For muscles with normal M/RMS ratios, M amplitude correlates positively with strength. Cluster analysis was used to distinguish UMN, LMN or Mixed types of weakness. Distinguishing these different types of weakness in acute quadriplegia may allow individualized rehabilitation for the type of weakness present.