Rinsho Shinkeigaku (Clinical Neurology)

Case Report

An unusual case of sodium channel myotonia with transient weakness upon initiating movements which is characteristic in Becker disease

Junpei Yamamoto, M.D.1), Keiichi Hokkoku, M.D.1), Yuki Hatanaka, M.D.1), Shunichi Sakoda, M.D.2), Jun-Hui Yuan M.D.2) and Masahiro Sonoo, M.D.1)

1)Department of Neurology, Teikyo University School of Medicine
2)Department of Neurology and Geriatrics, Kagoshima University Graduate School of Medical and Dental Sciences

We reported a 32-year-old man who was a sporadic case of myotonic syndrome with muscle stiffness or transient weakness of limbs upon initiating movements after rest. On examination, he showed painless myotonia with warm-up phenomenon, Hercules-like hypertrophic musculature and myotonic discharges in EMG. The clinical findings resembled to those of Becker disease rather than Thomsen disease. But electrodiagnosis suggested sodium channel myotonia instead of chloride channelopathy. Genetic testing detected a novel missense mutation (p.V1166A) in the SCN4A gene but not in the CLCN1 gene. Transient weakness upon initiating movements is usually observed in Becker disease but rare in Thomsen disease, which is not reported in sodium channel myotonia so far. He was probably the first case of sodium channel myotonia with transient weakness upon initiating movements, which was confirmed by 10 Hz repetitive nerve stimulation test as depolarization block.
Movie legends
Movie 1. Eyelid myotonia.
The patient cannot open his eyes immediately due to involuntary continuous contraction of the orbicularis oculi muscle after strong eye closure. The contraction persists more than 10 seconds.
Movie 2. Grip myotonia.
It takes more than 10 seconds to fully extend the fingers after a sustained grip.
Movie 3. Myotonic discharges on the right biceps brachii.
Needle EMG shows typical myotonic discharges. The waxing and waning of the amplitude and frequency are observed.
Full Text of this Article in Japanese PDF (931K)

(CLINICA NEUROL, 57: 287|292, 2017)
key words: myotonia, transient weakness, Becker disease, sodium channel myotonia, SCN4A

(Received: 12-Nov-16)