The split hand syndrome in amyotrophic lateral sclerosis

Area of Science:

• Neurology
• Neuroscience
• Muscle Physiology

Background:

• Amyotrophic lateral sclerosis (ALS) causes progressive muscle degeneration.
• A characteristic pattern of hand muscle wasting, termed the "split hand," is observed in ALS.
• This pattern involves preferential atrophy of thenar muscles (abductor pollicis brevis, first dorsal interosseous) with relative sparing of hypothenar muscles (abductor digiti minimi).

Purpose of the Study:

• To investigate the physiological mechanisms underlying the "split hand" phenomenon in ALS.
• To evaluate the diagnostic utility of the "split hand" sign in early ALS detection.

Main Methods:

• Analysis of motor potentials evoked by magnetic stimulation to assess cortical mechanisms.
• Axonal excitability studies to investigate peripheral nerve mechanisms.
• Review of objective studies on the specificity of the "split hand" sign.

Main Results:

• Motor potentials are smaller in thenar muscles compared to hypothenar muscles, suggesting a cortical involvement.
• Peripheral axonal excitability studies indicate greater persistent sodium currents in thenar motor axons (APB/FDI) than hypothenar axons (ADM), potentially leading to degeneration.
• The "split hand" sign demonstrates a high degree of specificity for ALS.

Conclusions:

• The "split hand" in ALS likely results from a combination of cortical and spinal/peripheral mechanisms.
• Preferential degeneration of thenar motor axons may be linked to increased metabolic demands or oxidative stress due to frequent use in precision grip.
• The "split hand" is a valuable and specific clinical sign for the early diagnosis of ALS.