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

Multifocal motor neuropathy (MMN) and amyotrophic lateral sclerosis (ALS) can be difficult to differentiate, particularly at early disease stages for 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 differentiation between MMN and ALS.

We retrospectively analyzed the clinical, laboratory, and electrophysiological characteristics of patients diagnosed with MMN and ALS who underwent an identical nerve conduction study protocol comprising extended motor stimulation.

A total of 125 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. The 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). The patients with ALS had greater muscle weakness, more frequent muscle atrophy and fasciculation, UMN signs, and body weight loss. Compared with both the overall ALS and hand-onset ALS groups, the MMN group had significantly lower serum creatine kinase (CK) levels 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 levels. Conduction blocks (CB) on nerve conduction studies were more common in the MMN group (87.5%) than in the overall ALS (19.7%, p < 0.001) and hand-onset ALS groups (31.0%, p = 0.005). MMN patients more frequently exhibited definite CBs involving multiple nerves (85.7%) compared with the overall ALS (17.4%, p = 0.002) and hand-onset ALS groups (7.7%, p = 0.001).

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.