Electrophysiological characteristics of the split hand syndrome in amyotrophic lateral sclerosis and the variant

Objective To study the electrophysiological characteristics of hands muscle of upper limb onset amyotrophic lateral sclerosis (UL-ALS), and the variant-flail arm syndrome (FAS) for diagnosis and differential diagnosis. Methods We chose 55 UL-ALS and 12 FAS patients as the cases, 20 cervical spondylotic amyotrophy (CSA) patients as the case controls, and 20 healthy volunteers as the normal controls from January 2013 to March 2018 in the Third Central Hospital of Tianjin. Conventional nerve conduction studies of the median nerve and ulnar nerve were performed in all the patients. The main analysis was done in the compound muscle action potential (CMAP) recorded on the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) and the ratio of the two. Results The ratio of CMAPAPB/CMAPADM of ALS was 0.59 (0.25, 0.79), which was depressed obviously compared with FAS (1.02 (0.92, 1.18), Z=-4.440, P=0.000), CSA (1.88 (1.42, 3.19), Z=-5.902, P=0.000) and the normal controls (0.96 (0.88, 1.15), Z=-5.416, P=0.000). The low ratio of CMAPAPB/CMAPADM (<0.6) was encountered in 40% (23/55) ALS patients, 0 CSA patient and 1/12 FAS patients. An absent APB CMAP and an abnormally low APB/ADM CMAP amplitude ratio (<0.25) were observed only in 25.4% (14/55) ALS patients. The area under receiver operating characteristic curve in patients of UL-ALS was 0.911 (P=0.000), and in FAS was 0.518 (P=0.559). Using a cut-off value of CMAPAPB/CMAPADM=0.7 for diagnosing ALS yielded 85.5% sensitivity and 95.0% specificity. Conclusion The split hand syndrome is not specific for ALS; however, the low APB/ADM CMAP amplitude ratio may help predict prognosis and can be the diagnostic marker for ALS. Key words: Amyotrophic lateral sclerosis; Flail arm syndrome; Split hand syndrome; Action potential