• Kazumoto Shibuya, Sonoko Misawa, Saiko Nasu, Yukari Sekiguchi, Satsuki Mitsuma, Minako Beppu, Shigeki Ohmori, Yuta Iwai, Shoichi Ito, Kazuaki Kanai, Yasunori Sato, and Satoshi Kuwabara.
• Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan.
• J. Neurol. Neurosurg. Psychiatr.. 2013 Sep 1;84(9):969-72.

BackgroundIn amyotrophic lateral sclerosis (ALS), muscle wasting preferentially affects the abductor pollicis brevis (APB) and first dorsal interosseous over the abductor digit minimi (ADM), and this is termed 'split hand'. Previous axonal excitability studies have suggested increased nodal persistent sodium current and reduced potassium current in motor axons in ALS, but the extent of excitability changes in APB and ADM axons in ALS has never been compared.ObjectiveTo elucidate the peripheral axonal pathophysiology of split hand.MethodsIn both APB and ADM motor axons of 21 patients with ALS and 17 age-matched normal controls, threshold tracking was used to measure excitability indices such as strength-duration time constant (SDTC; a measure of persistent sodium current) and threshold electrotonus.ResultsIn normal controls, SDTC was significantly longer for APB than ADM axons, suggesting that axonal excitability is physiologically higher in APB axons. Compared with normal controls, patients with ALS had longer SDTC and greater threshold changes in depolarising threshold electrotonus in both APB and ADM axons. Furthermore, the difference in extent of SDTC prolongation between normal subjects and patients with ALS was greater in APB than ADM axons.ConclusionsAPB axons have physiologically higher excitability than ADM axons, and, in ALS, the hyperexcitability is more prominent in APB axons. Although cortical mechanisms would also be involved, more prominent hyperexcitability of APB axons may contribute to development of split hand, and the altered axonal properties are possibly associated with motor neuronal death in ALS.

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