This is a non-interventional, prospective, observational, multicentre study to evaluate the long-term safety and effectiveness of Namuscla in adult patients with NDM.

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.

Inclusion Criteria:

1. Adult, male or female patients with non-dystrophic myotonic disorders planned to be started on Namuscla according to the approved SmPC
2. Patients already receiving Namuscla/mexiletine for the treatment of NDM; (for patients on mexiletine other than Namuscla, only those who switch to Namuscla will be enrolled).
3. Patients who understand and are willing to provide informed consent.

Exclusion Criteria:

1. Patients who are enrolled or participating in any other clinical trial for an investigational product. -
2. Hypersensitivity to mexiletine, or to any of the excipients of Namuscla, or hypersensitivity to any local anaesthetic
3. Ventricular tachyarrhythmia
4. Atrial tachyarrhythmia, fibrillation or flutter
5. Complete heart block (ie, third-degree atrioventricular block) or any heart block susceptible to evolve to complete heart block (first-degree atrioventricular block with markedly prolonged PR interval (≥ 240 ms) and/or wide QRS complex (≥ 120 ms), second-degree atrioventricular block, bundle branch block, bifascicular and trifascicular block),
6. Myocardial infarction (acute or past), or abnormal Q-waves
7. Symptomatic coronary artery disease
8. Heart failure with reduced ejection fraction <50%
9. Sinus node dysfunction (including sinus rate < 50 bpm)
10. Patients receiving drugs that can induce torsades de pointes
11. Patients receiving medicinal products with narrow therapeutic index (ie, theophylline, tizanidine, digoxin, lithium, phenytoin or warfarin)
12. Patients who are pregnant or lactating.

The study will target the enrolment of 50 patients who are treated with Namuscla.