SPLIT-HAND PHENOMENON IN AMYOTROPHIC LATERAL SCLEROSIS: A MOTOR UNIT NUMBER INDEX STUDY

Introduction: The split-hand phenomenon refers to preferential wasting of the thenar muscles with relative sparing of the hypothenar muscles in amyotrophic lateral sclerosis (ALS). Methods: We compared the split-hand index (SI) calculated from the compound muscle action potential (CMAP; SICMAP) with that calculated from the motor unit number index (MUNIX; SIMUNIX). We performed MUNIX on the abductor policis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles of 39 ALS patients and 40 age-matched, healthy controls. SI is derived by multiplying the CMAP (or MUNIX) recorded over the APB and FDI and dividing by the CMAP (or MUNIX) recorded over the ADM. Results: Receiver-operating characteristic curve analysis revealed good diagnostic accuracy for both indices, but better performance of SIMUNIX than SICMAP. Conclusion: SIMUNIX and SICMAP were useful in differentiating ALS patients from healthy controls. SIMUNIX appears to be a better electrophysiological marker than SICMAP for the split-hand sign of ALS.