Motor unit number index in children with later‐onset spinal muscular atrophy

Abstract

Introduction Motor unit number index (MUNIX) is a validated electrophysiological biomarker in amyotrophic lateral sclerosis. MUNIX studies in spinal muscular atrophy (SMA) are limited. Methods Later‐onset SMA children (n=13, 3 SMN2 copy number) were evaluated for Hammersmith Motor Function Scale Expanded (HMFSE) and MUNIX of right abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. Age‐matched healthy controls (n=8) were enrolled to obtain normative APB and ADM MUNIX values. Results Mean APB and ADM MUNIX values in SMA subjects were significantly reduced (p<0.001) compared to controls. HMFSE scores strongly correlated with ADM MUNIX (r 0.63). Discussion APB and ADM muscle MUNIX studies are feasible in SMA type 2 children. ADM MUNIX correlated with disease severity on motor function testing. MUNIX studies in later‐onset SMA could be a potential biomarker of motor neuron loss.