In this clinical study, the aim is to investigate whether there is a difference in treatment of myotonia using two drugs. A difference there can justify the significantly higher cost when treated by Namuscla versus Lamotrigine.

According to the current corona pandemic, the investigators designed an app to use for data collection in the study. The app also ensures that patients who live far from the clinic more easily can participate.

On 18 May 2020, the National Board of Health in Denmark approved Namuscla for the treatment of myotonia. Myotonia is a congenital muscle disease that delays muscle relaxation after a muscle contraction. Myotonia can be disabling, but with treatment, most are able to work. Namuscla is the first registered drug to treat myotonia, however, the active part mexiletine is not new. It has been used off-label to treat myotonia for decades. The price for the new preparation is high with a daily cost (approx. 70 EU/dollar). In 2017, the investigators found evidence that treatment with lamotrigine is efficacious against myotonia. Treatment with lamotrigine can reduce the price to 150 EU/dollar per year.

Therefore, the aim of the study is to investigate whether there is a difference in the effects om myotonia and side effects of the two drugs, which can justify the significantly higher cost when treated with Namuscla in a double blinded, inferiority, cross-over study lasting 4 months.

According to the current corona pandemic, the study is designed without need of physical attendance so that inclusion and collection of experimental data can take place, by phone conversations, video-guided testing, and diary registration via a study-App. It also ensures that patients who live far away more easily can participate. Blood tests and ECGs can be performed locally.

Inclusion Criteria:

1. Capable adult women and men (age ≥ 18 years).
2. Diagnosed with Myotonia Congenita (Becker and Thomsen type), Paramyotonia Congenita, or Hyperkalemic periodic paralysis.
3. Myotonia under treatment or which significantly limits the daily activities (MBS> 2).

Exclusion Criteria:

1. Allergy to lamotrigine, mexiletine, or the inactive ingredients in trial medication.
2. Disease, which is affected by trial medication such as heart disease (ischemia and arrhythmia), epilepsy, and significant renal or hepatic failure.
3. Treatment that, in the opinion of the project manager, can affect the study result - medication with significant interactions with trial medication.
4. In case of smoking, start or cessation during the study.
5. Pregnant or breastfeeding during the study period. Fertile women with a positive pregnancy test at the time of entry into the trial, or who do not use safe contraception during the project period.