"Abductor Sparing": A New Selective Involvement in ALS

Various signs of selective muscle involvement have been reported in amyotrophic lateral sclerosis (ALS) but such studies for the lower limbs are scarce. We formed a preliminary impression that hip abductors (Ab) are often preserved in ALS. We named this phenomenon "abductor sparing", and this study aimed to verify our findings.

Patients with a confirmed diagnosis of ALS (ALS group) and patients with pyramidal weakness other than ALS (pyramidal group) were retrospectively identified. Medical Research Council (MRC) scores of 10 muscle groups in the lower limbs were evaluated. The proportion of patients with weakness (MRC score 4 or less) was compared between different groups.

We enrolled 61 patients in the ALS group and 27 patients in the pyramidal group. The most frequently weak muscle groups in both groups were big toe extensors and hip flexors. Ab was the third (70%) in the pyramidal group, whereas it was weak only in 30% of patients with ALS. This held true also for patients with ALS with shorter duration or less severity. "The lower limb flexor pattern", i.e., flexor muscles being weaker than extensor muscles, was observed both in ALS and pyramidal groups.

Patients with ALS generally showed similar muscle weakness patterns to those with pyramidal syndrome, except for abductor sparing. The reason for the latter phenomenon is unclear. Abductor sparing may be useful for early diagnosis of ALS, although larger studies with blinded evaluators are needed to confirm these findings.