This study aimed to characterise motor unit (MU) loss in type III and IV Spinal Muscular Atrophy (SMA) using the motor unit number index (MUNIX), and to evaluate compensatory mechanisms based on MU size indices (MUSIX). Nineteen type III and IV SMA patients and 16 gender- and age-matched healthy controls were recruited. Neuromuscular performance was evaluated by muscle strength testing and functional scales. SMA patients exhibited significantly reduced MUNIX values (p < 0.05) in all muscles, while MUSIX was increased, suggesting active re-innervation. Significant correlations were identified between MUNIX/MUSIX and muscle strength. Similarly, composite MUNIX scores correlated with disability scores.The authors concluded that MUNIX is a sensitive measure of MU loss in adult forms of SMA and correlates with disability.
