Development of the Clinical Myotonia Rating Scale and a Mexiletine Prescribing and Monitoring Algorithm for Patients with Non-Dystrophic Myotonia

The two most common forms of non-dystrophic myotonia (NDM) are myotonia congenita (Thomsen disease or Becker-type) and paramyotonia congenita. Symptoms, including muscle stiffness, cramps, and transient weakness, can affect a person’s quality of life. An unmet need for a validated tool to assess myotonia symptom severity and frequency, as well as disability caused by myotonia, led to the development of the Clinical Myotonia Rating Scale (CMRS). At the 9th Congress of the European Academy of Neurology (EAN), Budapest, Hungary, 1st–4th July 2023, a poster was presented regarding validation and reliability testing of the CMRS, the results of which are discussed here. Such a tool is needed when first assessing myotonia symptoms in a patient with NDM, as well as when assessing their response to myotonia-targeting medication. One such drug is mexiletine, a Class 1B antiarrhythmic agent that is approved for the treatment of myotonia symptoms in adults with NDM. Although a number of studies, including clinical and real-world trials in people with NDM, have not found mexiletine to be associated with impaired cardiac function, but as an antiarrhythmic drug, cardiac assessment is required with mexiletine prescription. Also presented at the 2023 EAN meeting is an algorithm to aid prescribers in understanding patients in whom mexiletine may be contraindicated, tests needed prior to mexiletine prescribing, and cardiac monitoring under treatment in patients with NDM. This algorithm was developed utilising expert opinion, the mexiletine summary of product characteristics, and a literature review of mexiletine safety data in NDM.