MUNIX of abductor digiti minimi correlates with upper limb function in adult patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is characterized by remarkable phenotypic variability. In adults, the disease is milder, and progression is slower, which makes the objective perception of worsening using motor scales challenging. With the advent of new disease-modifying therapies, the identification of potential biomarkers is essential, especially for this group of patients. In clinical practice, tracking upper limb function in adults with SMA becomes relevant. To assess whether motor unit number index (MUNIX) of abductor digiti minimi (ADM) correlates with upper limb function in adults with SMA. We evaluated 30 adult patients with SMA (2 type 4 SMA, 26 type 3 SMA and 2 type 2B SMA). The same board-certified neurophysiologist recorded 3 CMAPs and MUNIX sequentially in the right ADM for all patients. Average MUNIX values were then used for further analyses. Studies were performed with a commercially available electromyography device (Neurosoft, Neuro-MEP-Micro). The Revised Upper Limb Module (RULM) scale was performed to assess arm motor function in each individual. To assess correlations, we used Spearman's nonparametric correlation coefficient. Nineteen male and eleven female adult patients were evaluated, with a mean age of 33.06 years. We found a strong correlation between MUNIX of ADM and RULM scores (rho=0.748, p<0.001). When patients with null MUNIX values were excluded, the correlation remained significant (rho= 0.626 and p<0.001). MUNIX of ADM is an easy-to-perform and reliable neurophysiological parameter that correlates with upper limb function in adults with SMA. It emerges as a promising biomarker for this population, but longitudinal studies are needed to fully validate its usefulness. Spinal muscular atrophy (SMA) is characterized by remarkable phenotypic variability. In adults, the disease is milder, and progression is slower, which makes the objective perception of worsening using motor scales challenging. With the advent of new disease-modifying therapies, the identification of potential biomarkers is essential, especially for this group of patients. In clinical practice, tracking upper limb function in adults with SMA becomes relevant. To assess whether motor unit number index (MUNIX) of abductor digiti minimi (ADM) correlates with upper limb function in adults with SMA. We evaluated 30 adult patients with SMA (2 type 4 SMA, 26 type 3 SMA and 2 type 2B SMA). The same board-certified neurophysiologist recorded 3 CMAPs and MUNIX sequentially in the right ADM for all patients. Average MUNIX values were then used for further analyses. Studies were performed with a commercially available electromyography device (Neurosoft, Neuro-MEP-Micro). The Revised Upper Limb Module (RULM) scale was performed to assess arm motor function in each individual. To assess correlations, we used Spearman's nonparametric correlation coefficient. Nineteen male and eleven female adult patients were evaluated, with a mean age of 33.06 years. We found a strong correlation between MUNIX of ADM and RULM scores (rho=0.748, p<0.001). When patients with null MUNIX values were excluded, the correlation remained significant (rho= 0.626 and p<0.001). MUNIX of ADM is an easy-to-perform and reliable neurophysiological parameter that correlates with upper limb function in adults with SMA. It emerges as a promising biomarker for this population, but longitudinal studies are needed to fully validate its usefulness.