O-MN003. Decremental response to repetitive nerve stimulation occurs in early stage of amyotrophic lateral sclerosis

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Introduction . This study aimed to clarify the features and clinical significance of decremental response during slow-rate repetitive nerve stimulation (RNS) in amyotrophic lateral sclerosis (ALS) patients of early stage. Methods . 3-Hz RNS tests were performed in 175 nerve/muscle pairs from 22 ALS patients, whose duration of illness was less than 1 year, and lower motor neuron signs were limited in upper limbs. Both proximal and distal muscles were included. All patients were given a follow- up evaluation by telephone every 3 months. 15 patients had an electromyography (EMG) reexamination in 6 months. Besides, 81 nerve/muscle pairs in 37 normal controls were collected. Results . In ALS patients, compound muscle action potentials (CMAP) decrement percentages to RNS in all tested nerve/muscle pairs were greater than in controls (p < 0.01). Muscles without EMG spontaneous activity (including fibrillation potentials and positive sharp waves) showed higher decrement percentage than normal controls (p < 0.05), but were significantly lower compared to muscles with spontaneous activity (p < 0.05). Additionally, in muscles without EMG spontaneous activity, 14/58 pairs had RNS decrements over 5%, mainly in the deltoid (38.5%) and trapezius (35.3%). Furthermore, nerve/muscle pairs with higher RNS decrement percentage had greater drops in CMAP and higher increases in EMG spontaneous activity 6 months later, but had no correlation with the changes of ALSFRS-R scores. Conclusion . RNS decrements changed in the early stage of ALS, even before the denervation of corresponding muscles. In ALS patients within 12 months, higher RNS decrement percentage indicated faster low motor neuron injury and denervation during the progression of the disease. RNS could be a possible electrophysiological biomarker to evaluate prognosis.