Subclinical involvement of small hand muscles in early amyotrophic lateral sclerosis: Selective susceptibility leads to 'split hand' phenomenon.

Objective: Split-hand phenomenon is common in patients with amyotrophic lateral sclerosis (ALS), but it is unknown if first dorsal interosseous (FDI) and abductor pollicis brevis (ABP) are affected earlier than abductor digiti minimi (ADM). We aimed to address this issue.

Methods: One clinically normal hand from ALS patients was investigated, including needle EMG of the FDI, motor amplitude, distal latency, F-waves, neurophysiological index (NI) and split-hand index (SHI). Hands were categorised as G1 (normal FDI) and G2 (FDI with neurogenic changes). In patients who agreed EMG of the 3 muscles was done. A subset of G1 patients underwent a second evaluation 4-5 months later.

Results: We studied 133 patients; EMG of the 3 muscles was done in 77 patients. There was no evidence for an earlier loss of motor units in FDI/ABP. In G2 patients, CMAP amplitude and NI were significantly lower (p < 0.001), but ADM changes were minor. Reassessment of G1 patients confirmed significant SHI, and amplitude and NI decrease in all muscles, but F-waves frequency remained stable in ADM.

Conclusions: Loss of motor units in the 3 hand muscles began in parallel, but ADM spinal motoneurons showed stronger resistance to degeneration. Conclusions: Dysfunction of intrinsic spinal circuits can influence split-hand phenomenon.