Short Reports Significant Cmap Decrement by Repetitive Nerve Stimulation Is More Frequent in Median than Ulnar Nerves of Patients with Amyotrophic Lateral Sclerosis

Introduction:. Several studies have shown a significant amplitude decrement in compound muscle action potentials (CMAPs) on repetitive nerve stimulation (RNS) of muscles involved in amyotrophic lateral sclerosis (ALS). In ALS, muscle wasting preferentially affects the thenar muscles (APB) rather than the hypothenar muscles (ADM). Methods:. We performed RNS studies in the APB and ADM muscles of 32 ALS patients to determine whether the effect of RNS differs between the median and ulnar nerves. Results:. The decremental responses to RNS were greater in the APB than in the ADM. Reduced CMAP amplitude was negatively correlated with CMAP decrement in median but not in ulnar nerves. Conclusions:. The greater CMAP decrement in median nerve is attributable to preferential involvement of the APB in the pathophysiology of ALS or some underlying difference in the biology of the two muscles/nerves. Further investigations will better our understanding of the pathophysiology of ALS. Muscle Nerve 45: 426–428, 2012 Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disease of unknown etiology that can selectively affect both upper and lower motor neurons. It is diagnosed on the basis of clinical and electromyographic findings and by excluding other diseases with similar symptoms, because there are no specific tests or biological markers to confirm the diagnosis. A decremental motor response to repetitive nerve stimulation (RNS) in ALS patients was originally reported in 1959 by Mulder and colleagues. In previous studies, the decremental response was attributed to failure of conduction of repeated stimuli by degenerating motor axon branches or by regenerating nerves undergoing collateral sprouting. For a further understanding of the pathophysiology of ALS, we sought to determine whether the effect of RNS differs between the median and ulnar nerves in ALS patients because it is known that different nerves/muscles have different physiological properties. METHODS The procedures followed were in accordance with the standards of the Committee on Human Experimentation at Kumamoto University. Thirty-two consecutive patients diagnosed with ALS were examined between July 2010 and March 2011. The patients fulfilled the revised El Escorial criteria for definite, probable, or probable laboratory-supported ALS. The study group included 11 men and 21 women ranging in age from 17 to 83 years (mean 6 SD: 60.4 6 18.3 years). Disease duration ranged from 4 to 66 months (17.4 6 13.7 months). The scores for the revised ALS Functional Rating Scale (ALSFRS-R), which is a wellestablished and widely used scoring system for the functional status of patients with ALS, ranged from 28 to 47 (39.8 6 5.6). Surface electrodes were used to record the CMAP of the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles in response to 3-HZ stimulation of the median and ulnar nerves at the wrist. The median nerve was stimulated at the wrist, and recordings were made by placing the active electrode (G1) over the belly of the APB muscle and the reference electrode (G2) on the tendon. The ulnar nerve was stimulated at the wrist with recording from the belly of the ADM muscle (G1) and G2 on the tendon. The recordings were obtained using a Neuropack device (Nihon-Kohden, Tokyo, Japan). For each train of repetitive stimuli, the amplitudes of the first and fifth compound muscle action potentials (CMAPs) at supramaximal stimulus intensity were compared, and the resulting decrement of the latter was expressed as a percentage. A significant CMAP decrement was defined as a difference of >10%. All tests were performed in a warm room, and the skin temperatures over the APB and ADM muscles were maintained at >32 C. CMAP amplitudes were divided by each cut-off value (median 4.6 mV, ulnar 3.8 mV) and then compared. Data were analyzed using the two-tailed Student t-test. Regression analyses were performed by the Pearson correlation test.