What is myotonia congenita?
Myotonia congenita is a genetic, neuromuscular disorder characterized by the slow relaxation of the muscles. Myotonia congenita can be either an autosomal dominant or autosomal recessive disease. With the autosomal dominant form, a person only needs to inherit the mutation from one parent to be affected. Inheriting the recessive form means a person must
inherit the mutation from both parents to be affected. The gene that causes this condition resides on chromosome 7.
The cause of myotonia congenita is believed to be an abnormality in the chloride channels of muscle cells (chloride ions are required for a muscle to relax). The abnormal chloride channels also cause an accumulation of potassium outside the cells and an activation of sodium channels in the muscle cells (sodium ions trigger muscle contraction). When the cells have more than enough sodium but not enough chloride, abnormal repetitive electrical discharges cause a stiffness called myotonia.
The hallmark of this condition is the myotonia -- the inability of the muscle to quickly relax after voluntary contraction. For example, after a handshake, the affected individual is only very slowly able to open and disengage his hand. Early symptoms may include gagging and difficulty in swallowing, because oropharyngeal muscles are slow to relax. Initial movements may be stiff but improve with immediate repetition. Children with myotonia congenita often appear to be muscular and well-developed. There may not be symptoms of myotonia congenita until the child is 2 or 3 years old.
Most cases of myotonia congenita do not require treatment. Sometimes, however, symptoms of the disorder may be relieved with quinine, phenytoin and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may be used to help muscle function. Genetic counseling is available. |
