Background:Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease involving both upper and lower motor neurons with no effective cure.Electrophysiological studies have found decremental responses during low-frequency repetitive nerve stimulation (RNS) except for diffused neurogenic activities.However,the difference between ALS and generalized myasthenia gravis (GMG) in terms of waveform features is unclear.In the current study,we explored the variation trend of the amplitudes curve between ALS and GMG with low-frequency,positive RNS,and the possible mechanism is discussed preliminarily.Methods:A total of 85 ALS patients and 41 GMG patients were recruited.All patients were from Peking Union Medical College Hospital (PUMCH) between July 1,2012 and February 28,2015.RNS study included ulnar nerve,accessory nerve and facial nerve at 3 Hz and 5 Hz stimulation.The percentage reduction in the amplitude of the fourth or fifth wave from the first wave was calculated and compared with the normal values of our hospital.A 15％ decrease in amplitude is defined as a decrease in amplitude.Results:The decremental response at low-frequency RNS showed the abnormal rate of RNS decline was 54.1％ (46/85) in the ALS group,and the results of different nerves were 54.1％ (46/85) of the accessory nerve,8.2％ (7/85) of the ulnar nerve and 0％ (0/85) of the facial nerve stimulation,respectively.In the GMG group,the abnormal rate of RNS decline was 100％ (41/41) at low-frequency RNS of accessory nerves.However,there was a significant difference between the 2 groups in the amplitude after the sixth wave.Conclusions:Both groups of patients are able to show a decreasing amplitude of low-frequency stimulation RNS,but the recovery trend after the sixth wave has significant variation.It implies the different pathogenesis of NMJ dysfunction of these 2 diseases.