A 60-year-old man presented with muscle stiffness. He had tended to fall since childhood. His parents were first cousins, and his relatives had no symptoms. Examination showed myotonia in ocular, hand, and limb muscles, followed by improvement with repeated activity (warm-up phenomenon; Video 1). Cold exposure did not aggravate myotonia. His lower legs showed muscle hypertrophy. Genetic tests showed a homozygous p.M560T mutation in the CLCN1 gene, which encodes skeletal muscle chloride channel 1, consistent with autosomal recessive myotonia congenita (Becker disease).1 Although treatment with mexiletine or phenytoin did not improve his symptoms, carbamazepine (350 mg/d) did, especially the walking disturbance (Video 2). Although the warm-up phenomenon is sometimes also observed in sodium channel myotonia,2 it is characteristic of myotonia congenita and an important clue for the diagnosis.
Video 1
Carbamazepine(−): The patient has myotonia during gait initiation, with the warm-up phenomenon and also grip and eyelid myotonia.Download Supplementary Video 1 via http://dx.doi.org/10.1212/013010_Video_1
Video 2
Carbamazepine(+): The myotonia during gait initiation improves dramatically with carbamazepine, and it suggests that myotonia may improve with medications that block voltage-gated sodium channels. Muscle hypertrophy is observed in his lower legs (latter part).Download Supplementary Video 2 via http://dx.doi.org/10.1212/013010_Video_2
Study Funding
The authors report no targeted funding.
Disclosure
The authors report no disclosures relevant to the manuscript. Go to Neurology.org/N for full disclosures.
Appendix Authors
Footnotes
Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article.
Teaching slides links.lww.com/WNL/B639
- © 2021 American Academy of Neurology
Disputes & Debates: Rapid online correspondence
REQUIREMENTS
If you are uploading a letter concerning an article:
You must have updated your disclosures within six months: http://submit.neurology.org
Your co-authors must send a completed Publishing Agreement Form to Neurology Staff (not necessary for the lead/corresponding author as the form below will suffice) before you upload your comment.
If you are responding to a comment that was written about an article you originally authored:
You (and co-authors) do not need to fill out forms or check disclosures as author forms are still valid
and apply to letter.
Submission specifications:
- Submissions must be < 200 words with < 5 references. Reference 1 must be the article on which you are commenting.
- Submissions should not have more than 5 authors. (Exception: original author replies can include all original authors of the article)
- Submit only on articles published within 6 months of issue date.
- Do not be redundant. Read any comments already posted on the article prior to submission.
- Submitted comments are subject to editing and editor review prior to posting.