The split hand syndrome in amyotrophic lateral sclerosis
- 1University of British Columbia, Vancouver, British Columbia, Canada
- 2Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan
- Correspondence to Dr S Kuwabara, Department of Neurology, Graduate School of Medicine, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan;
Contributors EA and SK wrote the manuscript and contributed equally to this review.
- Received 18 September 2011
- Revised 12 October 2011
- Accepted 14 October 2011
- Published Online First 19 November 2011
In amyotrophic lateral sclerosis (ALS), hand muscle wasting preferentially affects the ‘thenar (lateral) hand’, including the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles, with relative sparing of the hypothenar muscles (the abductor digiti minimi (ADM)). This peculiar pattern of dissociated atrophy of the intrinsic hand muscles is termed the ‘split hand’ and is rarely seen in diseases other than ALS. The muscles involved in the split hand are innervated through the same spinal segments (C8 and T1), and FDI and ADM, which are differentially affected, are both ulnar nerve innervated. The physiological mechanisms underlying the split hand in ALS are incompletely understood but both cortical and spinal/peripheral mechanisms are probably involved. Motor potentials evoked by magnetic stimulation are significantly smaller when recorded from the thenar complex, compared with the hypothenar muscles, supporting a cortical mechanism. But peripheral axonal excitability studies have suggested that APB/FDI motor axons have more prominent persistent sodium currents than ADM axons, leading to higher axonal excitability and thereby more ready degeneration. Pincer or precision grip is vital to human hand function, and frequent use of thenar complex muscles may lead to greater oxidative stress and metabolic demands at both upper and lower motoneurons innervating the APB and FDI. The split hand is a useful diagnostic sign in early ALS, and recent objective studies indicate that the sign has a high degree of specificity.
See ALS and FTD Special Edition, p 355
Funding This work was supported in part by Grants-in-Aid from the Research Committee of CNS Degenerative Diseases, the Ministry of Health, Labour and Welfare of Japan (SK).
Competing interests None.
Provenance and peer review Commissioned; externally peer reviewed.