Split-hand phenomenon quantified by the motor unit number index for distinguishing cervical spondylotic amyotrophy from amyotrophic lateral sclerosis.

Objective: To investigate and compare split-hand phenomenon quantified by motor unit number index (MUNIX) between patients with cervical spondylotic amyotrophy (CSA) and those with amyotrophic lateral sclerosis (ALS).

Methods: MUNIX was performed on abductor pollicis brevis (APB), abductor digiti minimi (ADM) and first dorsal interosseous (FDI) in 46 CSA patients, 39 ALS patients and 41 healthy subjects. Split-hand measurements including split-hand index (SHI=ABP×FDI/ADM), ratio of APB to ADM (AA), ratio of FDI to ADM (FA) were measured by compound muscle action potential (CMAP) and MUNIX.

Results: There was a significant difference in both AA and SHI measured by two different methods between ALS and CSA patients (P<0.05). Receiver operating characteristic (ROC) curve and logistic regression analysis demonstrated good differential diagnostic accuracy for AA, SHI and their combination between ALS and CSA. A larger area under the curve (AUC) was observed in these measurements calculated by MUNIX than those measured by CMAP (AA: 0.885 vs. 0.700, SHI: 0.865 vs. 0.703, Combination: 0.925 vs. 0.750; P<0.05). Sub-group analysis of ROC curves revealed an AUC of 0.893 for AAMUNIX, 0.801 for SHIMUNIX and 0.896 for their combination in differentiating "clinically possible" ALS (Awaji-Shima criteria) from CSA (P<0.05).

Conclusions: Both AA and SHI measured by two different methods are useful in distinguishing ALS from CSA, and those quantified by MUNIX may be a better differential diagnostic marker to provide an accurate and noninvasive additional test for distinguishing CSA from ALS, even in their early stages.