The "split hand" phenomenon: evidence of a spinal origin.

The clinical phenomenon of a split hand, dominant muscle atrophy in the thenar as compared to the hypothenar complex, has been used to support the theory of primary cortical degeneration in amyotrophic lateral sclerosis. However, the same phenomenon, both clinically and electrophysiologically, was observed in three diseases with a second but not first motor neuron affection: autosomal dominant spinal muscular atrophy, spinocerebellar ataxia type 3, and juvenile muscular atrophy. Neurogenic loss in a split hand distribution points to a spinal instead of cortical origin.