Thomsen Disease, also known as myotonia congenita, is a rare genetic disorder that affects the muscles' ability to relax after contracting. This condition is characterized by muscle stiffness and delayed relaxation after voluntary contraction and is caused by a mutation in the CLCN1 gene. The disorder is inherited in an autosomal dominant pattern, affecting males and females equally. There are no synonyms for Thomsen Disease, but it is sometimes referred to as congenital myotonia or Becker's myotonia. The diagnosis is often made based on a clinical evaluation and confirmed by genetic testing. While there is no cure for Thomsen Disease currently, various treatment options can help manage the symptoms.