Thomsen disease, also known as myotonia congenita, is a rare genetic disorder that affects muscle function. It is caused by mutations in the CLCN1 gene, which is responsible for encoding a chloride channel in muscle cells. This condition leads to muscle stiffness and difficulty relaxing muscles after contraction, particularly after periods of rest. Individuals with Thomsen disease often experience muscle tightness, especially in the arms and legs, but symptoms can vary in severity. The condition is usually inherited in an autosomal dominant pattern, meaning only one copy of the mutated gene is needed for a person to be affected. Treatment focuses on managing symptoms and improving muscle function.