about inheritance of myotonia congenita

Myotonia congenita is a genetic, neuromuscular disorder that affects muscles used for movement ( skeletal muscles). It is congenital, meaning that it is present from birth. It amongst other problems causes delayed relaxation of the muscles (myotonia & rigidity). The disorder is caused by mutations in a gene responsible for shutting off electrical excitation in the muscles, muscle fiber membranes have an unusually exaggerated response to stimulation (hyperexcitability).

The two major types of myotonia congenita are known as Thomsen disease and Becker disease. More recently, as the individual mutations that gave rise to this condition have started to be identified, these limited classifications are being less widely used. These conditions are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness, and pain. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not seen in people with Thomsen disease. In many cases, muscles of the eyelids, hands, and legs may be most affected.

The two forms of myotonia congenita have different patterns of inheritance. Thomsen disease is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one parent with the condition.

Becker disease is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder each carry one copy of the altered gene, but do not show signs and symptoms of the disorder.

Because several CLCN1 mutations can cause either Becker disease or Thomsen disease, doctors usually rely on characteristic signs and symptoms to distinguish the two forms of myotonia congenita.