Absence of split hand in the flail arm variant of ALS

Flail arm syndrome (FAS), a variant of amyotrophic lateral sclerosis (ALS), has many similarities with upper limb onset ALS (UL-ALS). This study analyzed the compound muscle action potentials (CMAPs) recorded from abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles in patients presenting these two types of ALS variants, compared to normal controls. The APB/ADM CMAP amplitude ratio was lower in the UL-ALS group (mean±SEM: 0.56±0.37), consistent with a split hand, compared to the FAS (1.05±0.29) or control (1.11±0.30) groups. An abnormally reduced APB/ADM CMAP amplitude ratio (<0.6) was found in 40% of UL-ALS patients and 11% of FAS patients. However, absent CMAP in the APB or extremely low APB/ADM CMAP amplitude ratio (<0.25) were observed only in UL-ALS patients (27%) and could be used as diagnostic criteria to differentiate UL-ALS from FAS variant.