Myotonia congenita, also known as congenital myotonia, is a muscle condition that causes stiffness, making it hard for muscles to relax normally.
Myotonia can affect any part of the body but is most common in the legs. Some people experience mild stiffness, while others find it harder to do activities like walking, running, or climbing stairs. Other symptoms may include muscle cramps, larger muscles, difficulty getting up, trouble letting go after shaking hands, and muscle pain. Symptoms often worsen in cold weather.
There are two main types of myotonia congenita: Becker disease and Thomsen disease. Becker disease is more common, with symptoms usually starting between ages 4 and 12. Becker disease tends to cause more severe stiffness and can lead to mild muscle weakness over time, especially in those assigned male at birth. Thomsen disease is less common and often starts earlier, from a few months old to around age 3. There’s no cure for myotonia congenita, but treatments can help manage symptoms. These may include light exercise after rest to reduce stiffness, avoiding cold weather, and changing the diet to prevent choking. Medications are not typically used, but they might help with severe muscle cramps and stiffness.
Both conditions are caused by pathogenic (disease-causing) variants in the CLCN1 gene. Becker diserase exhibits autosomal recessive inheritance. This means that both parents must be carriers to have a 25% chance of having a child with the condition. The risk of being a carrier is based on a person’s ancestry or ethnic background. Thomsen disease exhibits autosomal dominant inheritance. This means that only one pathogenic variant is needed to develop the condition and individuals have a 50% chance of having a child with the condition.
Resources:
Muscular Dystrophy Association
Written August 2024
