Motor unit number index: A potential electrophysiological biomarker for pediatric spinal muscular atrophy

Abstract

Introduction/Aims: In adult spinal muscular atrophy (SMA), the motor unit number index (MUNIX) has been shown to be an useful electrophysiological biomarker. This study evaluated the feasibility and the clinical relevance of using the MUNIX technique for patients with pediatric SMA (Ped-SMA) and correlated MUNIX results with clinical scores. Methods: Fourteen patients with type II Ped-SMA (11 females; median age 11 y [interquartile range (IQR), 4.8-17 y]) and 14 controls (nine females; median age 10.75 y [IQR, 6.5-13.4 y]) were enrolled and matched by sex, age, height, weight, and body mass index. Clinical examination included manual muscle testing, dynamometry (grasp and pinch), and motor function measure (MFM). The MUNIX technique was evaluated in the abductor digiti minimi (ADM) and abductor pollicis brevis (APB) on two sides when possible.Results: In the patients with Ped-SMA, the MUNIX and compound muscle action potential (CMAP) amplitudes were significantly decreased and the motor size unit index (MUSIX) was significantly increased in the ADM and APB when compared to controls. The intraclass correlation coefficient was good for the intrarater variability of the CMAP amplitude, MUNIX, and MUSIX in the ADM (0.95, 0.83, and 0.89, respectively) and the APB (0.98, 0.96, and 0.94, respectively). The total CMAP amplitude correlated with the grasp and pinch scores (P < .05), and the MUNIX measurements correlated with the MFM scores.Discussion: The MUNIX technique, which accurately estimated lower motor neuron loss and the number of remaining functional motor units, was shown to be a useful electrophysiological biomarker for disease progression and a potential biomarker for treatment response.

Neurophysiological Characteristics in Type II and Type III 5q Spinal Muscular Atrophy Patients: Impact of Nusinersen Treatment

“… 25 , 26 In comparison to MUNE, CMAP is non-invasive, painless, requires less time, is more easily accepted by young patients, has good reproducibility, and is less influenced by changes in the expertise of the operator. 27 , 28 …”

Long‐term follow‐up of 64 children with classical infantile‐onset Pompe disease since 2004: A French real‐life observational study

“…The presence of progressive motor neuron dysfunction could therefore favor the worsening of disease evolution in some children. Recently, the MUNIX (motor unit number index) has been shown to be an useful electrophysiological biomarker for disease progression in pediatric spinal muscular atrophy and for treatment response [30]. Korlimarla et al [31] recently [9,13,16,32].…”

Biomarkers in 5q-associated spinal muscular atrophy—a narrative review

“…It has been well established in ALS [ 147 , 148 ] as a reliable and valid marker for motor unit loss. Recent studies in patients with SMA showed relevant correlations of MUNIX and motor function and disease progression in both adults and children [ 139 , 145 , 149 , 150 ]. MUNIX further detects disease-specific patterns of motor unit loss, allowing a precise distinction of SMA versus ALS patients [ 139 ].…”