An Observational Study in Adult Patients With Non-dystrophic Myotonic Disorders

This is a non-interventional, prospective, observational, multicentre study to evaluate the long-term safety and effectiveness of Namuscla in adult patients with NDM. Namuscla should be prescribed as per the approved Summary of Product Characteristics (SmPC).

Adult patients with non-dystrophic myotonic disorders who have been prescribed Namuscla by the treating physician, and who meet the eligibility criteria will be enrolled in this study.

This includes:

* Patients newly initiated on Namuscla for the treatment of NDM (newly exposed) * Patients already on Namuscla/ mexiletine at enrolment - For patients receiving mexiletine other than Namuscla, only those who switch to Namuscla will be included in the study.

Patients already being treated with Namuscla/ mexiletine at the time of enrolment will be considered for enrolment provided they meet the eligibility criteria.

The study will be initiated at specialized centres for the treatment of myotonic disorders ("reference centres") in the United Kingdom (UK), France, and Germany, depending on availability of Namuscla in the specific country. Depending on the enrolment and marketing status (availability) of Namuscla in other countries in the EU, inclusion of additional sites in other countries will be considered.

The study population will comprise patients who are diagnosed with non-dystrophic myotonic disorders and considered suitable candidates for the treatment by Namuscla by the investigators according to the approved SmPC. Patients will be enrolled over an approximate 2-year enrolment period and will be followed-up on-treatment for up to 3 years. Each enrolled patient will be observed for 3 years or until discontinuation (if discontinued early).

For all enrolled patients, the baseline would be the latest data available at the enrolment visit.

For the patients already on Namuscla, cumulative data (data related previous exposure as well as current data) will be collected for adverse events (AEs) on Namuscla treatment.

No drug will be supplied for this study; patients will receive medicines through local standard practices. All evaluations and investigations during the study will be performed according to the routine clinical practices and discretion of the treating physician.