Clinical and electrophysiological features for differentiating MMN from hand-onset ALS

BACKGROUND: Multifocal motor neuropathy (MMN) and amyotrophic lateral sclerosis (ALS) can be difficult to differentiate, particularly at early disease stages in patients with hand-onset weakness and without upper motor neuron (UMN) signs. This study aimed to identify clinical and electrophysiological features that may facilitate early distinction between MMN and ALS. METHODS: We retrospectively analyzed the clinical, laboratory, and electrophysiological characteristics of patients diagnosed with MMN and ALS and receiving identical nerve conduction study protocol comprising extended motor stimulation. RESULTS: One hundred and twenty-five patients (74 men and 51 women) were included, consisting of eight patients with MMN and 117 patients with ALS, including 42 with hand-onset ALS. Patients with MMN had a significantly younger mean age at symptom onset than those with ALS (43.1 vs. 58.7 years, p = 0.004). ALS patients had more severe muscle weakness, more frequent muscle atrophy and fasciculation, UMN signs, and body weight loss. Compared with both the overall ALS and the hand-onset ALS groups, MMN patients showed significantly lower serum creatine kinase (CK) level and higher serum IgM levels. Elevated CK levels were observed in approximately one-third of patients with hand-onset ALS, whereas none of the MMN patients had elevated CK. Conduction block (CB) on nerve conduction studies was more common in MMN (87.5%) than in all ALS cases (19.7%, p < 0.001) or hand-onset ALS (31.0%, p = 0.005). MMN patients more frequently exhibited definite CBs involving multiple nerves (85.7%) compared with all ALS (17.4%, p = 0.002) and hand-onset ALS (7.7%, p = 0.001) patients. CONCLUSION: Our findings suggest that a combination of clinical features, serum CK and IgM levels, and electrophysiological evidence of CB provides valuable clues for distinguishing MMN from ALS.