Split phenomenon of antagonistic muscle groups in amyotrophic lateral sclerosis: relative preservation of flexor muscles

Objective: In addition to the split hand sign, other split phenomena of different muscles also exist in amyotrophic lateral sclerosis (ALS). We analyzed the incidence of split phenomena in multiple antagonistic muscle groups in ALS patients and explored whether clinical factors affected their occurrence.Methods: 618 ALS patients were included from a single ALS center. Muscle strength in upper and lower limbs was evaluated using the modified Medical Research Council (MRC) scoring system (range from 1 to 13). Split phenomena between different antagonistic muscle groups were summarized, and the correlations with clinical factors were analyzed.Results: Split phenomena were detected in 22.3% antagonistic muscles for flexion and extension of the elbow, 11.9% for the wrist, 23.9% for fingers, 18.2% for the ankle, and 14.7% for toes. These manifestations were characterized by preferential wasting of the elbow, wrist, and finger extensor muscles compared with the flexor muscles, and the ankle and toe dorsiflexor muscles compared with the plantar flexor muscles. The presence of muscle wasting was more common when the muscle strength was stronger than a modified MRC grade 6. No definite correlation was found between split phenomena and clinical factors, including age-at-onset, gender, disease duration, the region of onset, and pyramidal tract damage.Discussion: Split phenomena of antagonistic muscle groups widely exist in ALS patients. No definitive and consistent clinical factors were observed that affected the occurrence of these phenomena.