Motor unit recruitment and amplitude of motor nerve evoked muscle action potential for the assessment of axon loss in cervical radiculopathy

The main purpose of this study is to assess the correlation between the motor unit recruitment (MUR) of C8 and T1-innervated muscles and amplitude of compound muscle action potential (A-CMAP) in motor nerve conduction study (NCS) of median or ulnar nerves, based on a retrospective study of electromyographic (EMG) reports of 24 patients with clinical diagnosis of cervical radiculopathy at C8+T1 levels. MUR was measured as the ratio of fastest motor unit firing rate to the number of firing motor units in 4 scales (from 0 to 3). A-CMAP was measured from the baseline to the peak of the evoked muscle action potentials recorded at abductor pollicis brevis or abductor digiti minimi muscles for median or ulanr nerve respectively. It was found that the mean MURs of abductor pollicis brevis and abductor digiti minimi muscles innervated by C8+T1 roots were significantly lower (p < 0.05) in patients with EMG evidence of C8+T1 radiculopathy compared to that with no definite EMG evidence of axon loss. Mean A-CMAP of median nerve or ulnar nerve was not significant lower (p > 0.5) in patients with EMG evidence of C8+T1 radiculopathy compared to that with no definite EMG evidence of axon loss. Based on analysis of linear regression, the correlation between MUR and A-CMAP was low (r < 0.6). When the patients with no definite EMG evidence of axon loss were excluded, there was still weak correlation (r < 0.6) between MUR of muscles innervated by C8+T1 and A-CMAP of median nerve or ulnar nerve. The results suggested that there is a poor correlation between MUR in EMG test and A-CMAP in patients with cervical radiculopathy, although both measurements are useful in the assessment of motor units loss. It was also suggested that MUR in EMG test was probably more sensitive to assess the motor unit loss in cervical radiculopathy than A-CMAP in motor NCS.