Myotonia congenita can be either an autosomal dominant or autosomal recessive disease. With the autosomal dominant form, a person only needs to inherit defective genes from one parent, while inheriting the recessive form requires defective genes from both parents. The gene that causes this condition resides on chromosome 7.

The cause of myotonia congenita is believed to be an abnormality in the muscle cells' chloride channels. (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.

• There is often a family history of myotonia congenita.
• An EMG demonstrates myotonic potentials.
• A muscle biopsy may show an absence of type 2B fibers.

• Frequent choking, gagging, or difficulty swallowing in an infant
• Aspiration pneumonia, related to swallowing difficulties