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Original research
Split hand and motor axonal hyperexcitability in spinal and bulbar muscular atrophy
  1. Kazumoto Shibuya,
  2. Sonoko Misawa,
  3. Akiyuki Uzawa,
  4. Setsu Sawai,
  5. Atsuko Tsuneyama,
  6. Yo-Ichi Suzuki,
  7. Tomoki Suichi,
  8. Yuta Kojima,
  9. Keigo Nakamura,
  10. Hiroki Kano,
  11. Mario Prado,
  12. Satoshi Kuwabara
  1. Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan
  1. Correspondence to Dr Satoshi Kuwabara, Neurology, Chiba University, Chiba 263-8522, Japan; kuwabara-s{at}


Objective The ‘split hand’ sign refers to preferential wasting of the thenar and first dorsal interosseous muscles with relatively sparing of the hypothenar muscles in amyotrophic lateral sclerosis (ALS) and both cortical and spinal/peripheral excitotoxic mechanisms have been proposed. We aimed to study split hand and axonal excitability in spinal and bulbar muscular atrophy (SBMA) in which cortical motor neurons are intact.

Methods In 35 patients with genetically confirmed SBMA, 55 with ALS, 158 with other neuromuscular diseases and 90 normal controls; split hand was strictly determined by amplitudes of compound muscle action potentials. Nerve excitability testing of median motor axons was performed in 35 SBMA and 55 patients with ALS and 45 normal controls.

Results Split hand was as frequently found for patients with SBMA (57%) and ALS (62%), compared with disease (20%) and normal (0%) controls. Excitability testing showed that in both SBMA and ALS, strength-duration time constant was longer, and threshold changes in depolarising threshold electrotonus and superexcitability in the recovery cycle were greater than in normal controls (p<0.01).

Conclusions Split hand is not specific to ALS and can be caused by the peripheral mechanism alone in SBMA, whereas the effect of upper motor neuron lesion cannot be excluded in ALS. Our results also suggest that SBMA and ALS share common axonal excitability changes; increased nodal persistent sodium and reduced potassium currents that may accelerate motor neuronal death and differently affect axons-innervating different muscles. Ion channel modulators could be a therapeutic option for both SBMA and ALS.

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  • Contributors KS and SK designed the study. KS, SM, AU, SS, AT, Y-IS, TS, YK, KN, HK and MP collected clinical, neurophysiological and genetic data. KS analysed the data. KS and SK wrote the manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval This study was approved by the Ethics Committee of Chiba University School of Medicine. Data acquisition for nerve conduction studies was approved by the local ethics committee.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available upon reasonable request. Data are available upon reasonable request, within ethical approval.

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