The motor unit number index (MUNIX) profile of patients with adult Spinal Muscular Atrophy

Publication date: Available online 13 September 2018

Source: Clinical Neurophysiology

Author(s): Giorgia Querin, Timothée Lenglet, Rabab Debs, Tanya Stojkovic, Anthony Behin, François Salachas, Nadine Le Forestier, Maria del Mar Amador, Lucette Lacomblez, Vincent Meininger, Gaelle Bruneteau, Pascal Laforêt, Sophie Blancho, Véronique Marchand-Pauvert, Peter Bede, Jean-Yves Hogrel, Pierre-François Pradat

Abstract

Objective

Objective of this study is the comprehensive characterisation of motor unit (MU) loss in type III and IV Spinal Muscular Atrophy (SMA) using motor unit number index (MUNIX), and evaluation of compensatory mechanisms based on MU size indices (MUSIX).

Methods

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. Compound motor action potential (CMAP), MUNIX and MUSIX were studied in the abductor pollicis brevis (APB), abductor digiti minimi (ADM), deltoid, tibialis anterior and trapezius muscles. A composite MUNIX score was also calculated.

Results

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. Interestingly, in SMA patients MUNIX was much lower in the ADM than in the ABP, a pattern which is distinctly different from that observed in Amyotrophic Lateral Sclerosis.

Conclusions

MUNIX is a sensitive measure of MU loss in adult forms of SMA and correlates with disability.

Significance

MUNIX evaluation is a promising candidate biomarker for longitudinal studies and pharmacological trials in adult SMA patients.

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