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Research paper
Split hand syndrome in amyotrophic lateral sclerosis: different excitability changes in the thenar and hypothenar motor axons
  1. Kazumoto Shibuya1,
  2. Sonoko Misawa1,
  3. Saiko Nasu1,
  4. Yukari Sekiguchi1,
  5. Satsuki Mitsuma1,
  6. Minako Beppu1,
  7. Shigeki Ohmori1,
  8. Yuta Iwai1,
  9. Shoichi Ito1,
  10. Kazuaki Kanai1,
  11. Yasunori Sato2,
  12. Satoshi Kuwabara1
  1. 1Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan
  2. 2Clinical Research Center, Chiba University Hospital, Chiba, Japan
  1. Correspondence to Dr Satoshi Kuwabara, Department of Neurology, Graduate School of Medicine, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan; kuwabara-s{at}faculty.chiba-u.jp

Abstract

Background In 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.

Objective To elucidate the peripheral axonal pathophysiology of split hand.

Methods In 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.

Results In 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.

Conclusions APB 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.

  • Motor Neuron Disease
  • Neurophysiology
  • Channels
  • Neurophysiology, Motor

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