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Fasciculations are a major clinical feature, present very early in the natural history of amyotrophic lateral sclerosis (ALS).1 They represent spontaneous discharges of motor units or portions of motor units, that can sometimes also be activated volitionally.2 There is no objective evidence that fasciculation potential (FP) discharges can be modulated by the patient or by the examiner. In this study we have addressed this issue in ALS, spinal muscular atrophy (SMA) and benign fasciculation syndrome (BFS).
We evaluated fasciculation firing frequency, using surface electromyographic (EMG) recordings from the first dorsal interosseous (DI) muscle in the hand at rest, and randomly after a period of maximal contraction, thus involving upper and lower motor neuron excitation, after radial sensory nerve stimulation and after supramaximal electrical stimulation of the ulnar nerve at the wrist, representing lower motor neuron excitation. We studied 75 patients with definite or probable ALS, excluding patients with polyneuropathy, diabetes or ulnar neuropathy (median age 61 years (range 27–75 years), median disease duration 12 months (range 3–24 months), 48 were men). One hand, in which first DI strength was 4 on the MRC (Medical Research Council) score 4 or 5, was studied in each patient. In each participant, FPs were recorded at rest in the first DI muscle. Six patients with SMA type 3 were studied (median age 45 years; range 28–60 years). …