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

Area of Science:

• Neuroscience
• Neurology
• Clinical Electrophysiology

Background:

• The split hand (SH) phenomenon, characterized by thenar muscle atrophy exceeding hypothenar atrophy, has been historically associated with amyotrophic lateral sclerosis (ALS).
• This association has supported the hypothesis of primary cortical degeneration as the underlying cause of ALS.
• However, the precise localization of the pathology in SH remains debated.

Purpose of the Study:

• To investigate the origin of the split hand phenomenon.
• To determine if SH exclusively indicates cortical involvement or can arise from spinal cord pathology.
• To differentiate between first and second motor neuron involvement in SH.

Main Methods:

• Clinical examination of muscle atrophy patterns in the thenar and hypothenar complexes.
• Electrophysiological studies to assess nerve conduction and muscle activity.
• Comparative analysis of SH phenomenon across different neurodegenerative diseases.

Main Results:

• The split hand phenomenon was observed clinically and electrophysiologically in conditions affecting the second motor neuron.
• These conditions included autosomal dominant spinal muscular atrophy, spinocerebellar ataxia type 3, and juvenile muscular atrophy.
• These diseases primarily involve spinal motor neurons, not cortical neurons.

Conclusions:

• The split hand phenomenon is not exclusive to diseases with first motor neuron (cortical) degeneration like ALS.
• Neurogenic loss presenting as a split hand distribution can originate from spinal cord pathology.
• This finding challenges the theory of primary cortical degeneration based solely on the SH phenomenon in ALS.