This clinical trial is focused on studying a group of muscle disorders known as Non-Dystrophic Myotonias. These conditions cause muscle stiffness and difficulty relaxing muscles after use. The study will compare two forms of a medication called Mexiletine, which is used to treat these symptoms. The two forms being compared are Mexiletine PR (prolonged-release) and Mexiletine IR (immediate-release). The goal is to understand how safe and effective these medications are for treating muscle stiffness in adults with non-dystrophic myotonias, which include specific types like myotonia congenita, paramyotonia congenita, and sodium channel myotonia.

Participants in the study will receive either Mexiletine PR or Mexiletine IR over a period of 24 weeks. The study is designed as an open-label, randomized, cross-over trial, meaning that participants will switch between the two forms of the medication during the study. This approach helps researchers compare the effects of each medication form directly. The study will monitor the occurrence of any side effects and how well participants tolerate the medications. Additionally, the study will assess improvements in muscle stiffness and overall quality of life using various tests and questionnaires.

The primary focus is on the safety of the medications, looking at any adverse events that may occur during the treatment period. Secondary measures include video recordings of hand movements, quality of life assessments, and other tests to evaluate the impact of the medications on daily activities. The study aims to provide valuable information on the best treatment approach for managing symptoms in individuals with non-dystrophic myotonias.