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

Background: 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; three 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 < .001) compared with controls. HMFSE scores strongly correlated with ADM MUNIX (r 0.63).

Conclusions: 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.