Serial Compound Muscle Action Potential Recordings in Duchenne Muscular Dystrophy
Mary Vollmer1, Sumit Verma2
1Nova Southeastern University Kiran C. Patel College of Osteopathic Medicine, 2Pediatrics and Neurology, Emory University School of Medicine
Objective:
To assess serial compound muscle action potential (CMAP) amplitudes as an objective, sensitive biomarker in Duchenne muscular dystrophy (DMD).
Background:
Ambulatory DMD boys are monitored for their functional status using a standardized six-minute walk test (6MWT) and North Star ambulatory assessment (NSAA). However, lack of motivation and associated comorbidities like attention deficit hyperactivity and autism can lead to underperformance on these functional tests, highlighting the need for sensitive objective measures. This study evaluates CMAP amplitudes, an electrophysiologic measure of the number and size of motor units, as a sensitive indicator of disease severity and interval change in DMD patients at baseline and at six-months follow-up.
Design/Methods:
Peak negative CMAP amplitudes were recorded from the bilateral deltoid, biceps brachii, and tibialis anterior (TA) muscles. Supramaximal motor nerve stimulation was applied to the bilateral Erb’s point and fibular heads. Active electrodes were positioned on the motor endpoints of the muscles using a trial-and-error approach to achieve maximal negative CMAP amplitude.
Results:
Twenty-three genetically confirmed DMD boys with mean age 8.4 + 2.6 years were enrolled with age-matched healthy controls. Six-month follow-up 6MWT distance, NSAA score, and CMAP measures were obtained in 65% of patients (15/23). The baseline CMAP amplitudes in DMD patients were significantly reduced (p<0.001) compared to controls in all muscle groups tested with no significant changes in NSAA scores (p =0.70) or 6MWT distance (p = 0.25) from baseline to six months. A sub-group analysis of patients receiving chronic steroid therapy (n = 9) and in those initiating steroids at the time of enrollment (n = 4) showed significant increase only in the left (p = 0.02) and right fibular TA muscle CMAP amplitudes (p = 0.002).
Conclusions:
Peak negative serial CMAP amplitudes are a sensitive biomarker of disease severity and treatment response in DMD.
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