Résumé :
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Fatigue frequently occurs in various neuromuscular disorders specially in myotonic dystrophy type 1 (DM1). However its pathophysiology remains unclear. This study aimed to investigate the effects of exercise-related fatigability on central and peripheral properties in DM1. Ten DM1 patients and 10 healthy subjects were enrolled. Muscle, peripheral nerve and cortical excitability were studied using high spatial resolution surface EMG, stimulations of the ulnar nerve, and transcranial magnetic stimulation to obtain motor evoked potentials (MEP). Tests were performed before and after an isometric contraction of adductor digiti minimi sustained 45s at 60% of maximal voluntary contraction (MVC). Force and myoelectric characteristics [root mean square (RMS), mean power frequency (MPF) and muscle fibre conduction velocity (MFCV)] were analysed. At baseline DM1 patients displayed less intracortical facilitation of MEP (234.8?130.2% vs. 100.3?94.7%, p=0.0164), reduced supernormality and rheobase (107.7?22.3% vs. 124.4?7.5%, p=0.0065; 4.1?0.7mA vs. 5.6?1.8mA, p=0.0545 respectively). Mwave amplitude to ulnar stimulation and MFCV were smaller. Furthermore force, RMS, MPF and MFCV values were lower at MVC. During the exercise the force remained unchanged, MPF and MFCV decreased in both groups. Only controls presented RMS decline. After the task, the percentage of refractoriness decreased in DM1 patients (-14.66?12.29%, p=0.0078). Conversely MEP amplitude and intracortical inhibition were reduced, threshold and Mwave duration increased and RMS at MVC decreased in controls. Besides muscular impairments, DM1 patients displayed at baseline subtle impairments of motor cortex suggesting an alteration of pyramidal cells excitability and a deregulation of glutamaergic circuitry. Likewise changes in supernormality and rheobase may support an alteration of membrane potential. The exercise induced barely slight modifications of neuromuscular properties. Unlike DM1, the task implied important modifications of pyramidal cell excitability associated with functional changes in gabaergic circuitry and neuromuscular excitability adjustments in controls. This study provides new insights into pathophysiological mechanisms of fatigability in DM1.
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