Myotonia Congenita (It isn’t just a ‘goat thing’)

This brief article appeared as a sidebar to a piece about Myotonic goats that first appeared in the November-December 2007 issue of Hobby Farms magazine.

Goats, dogs, cats, horses, mice, water buffalo and humans; all can be affected by Myotonia congenita, the cell disorder that makes fainting goats ‘faint’.

Myotonia congenita is an inherited neuromuscular disorder characterized by the inability of muscles to relax after contraction. Mutations in the CLCN1 gene cause Myotonia congenita. The CLCN1 gene provides the body with instructions for manufacturing a protein that is critical for the normal function of skeletal muscle cells. The flow of charged atoms (ions) into and out of muscle cells control muscle contraction and relaxation. Normally, protein produced according to instructions from the CLCN1 gene forms a channel that controls the flow of negatively charged chlorine ions into muscle cells. This channel stabilizes the cells’ electrical charge and this in turn prevents muscles from contracting abnormally. Mutations in the CLCN1 cells alter the structure of chlorine channels so that they can’t properly regulate ion flow; this disruption in chloride ion flow causes prolonged skeletal muscle contractions, the hallmark of Myotonia congenita.

Myotonia congenita affects an estimated 1 in 100,000 people worldwide, although it’s more common in northern Scandinavia, where it occurs in approximately 1 in 10,000 people. Two forms affect humans: Thomsen and Becker diseases. The most common form, Becker disease (named for Dr. Julius Becker, the Danish physician who described the disease, citing episodes experienced by his own family members), causes more pronounced muscle stiffness than Thomsen disease, particularly in males. Many people with Becker disease also experience temporary attacks of muscle weakness, particularly in the arms and hands, that are often triggered by movement after periods of rest; this muscle weakness isn’t seen in people with Thomsen disease.

People with Thomsen disease inherit the condition in an autosomal dominant pattern; typically, affected persons have one parent with Myotonia congenita. Becker disease is inherited in an autosomal recessive pattern; in most cases the parents of a person with Becker disease each carry a copy of the mutated gene although they don’t experience symptoms of the disorder.

Although Myotonia can affect any of the body’s skeletal muscles, it occurs most often in the legs. The good news is that while myotonic muscle stiffness can interfere with movement, the condition isn’t painful.

Researchers at the University of Minnesota are currently studying foals affected by Equine Myotonia congenita. The affliction manifests somewhat differently in horses, causing affected foals to develop a cramp within a muscle group that produces a noticeable bump with a dimple beneath it. Very young foals affected by this type of Myotonia have well-developed musculature but in time their muscles waste and the foals develop a pot-bellied appearance.

Chow Chows, West Highland Whites, Great Danes, Staffordshire Bull Terriers and Labrador Retrievers all experience canine Myotonia congenita but to date DNA tests have been developed to pinpoint carriers in only two breeds: Miniature Schnauzers and Australian Cattle Dogs.

The condition is being studied in cats, as well.

Did You Know?

A congenital neuromuscular disorder closely related to Myotonia congenita occurs in American Quarter Horses and related breeds like Appaloosas and Paints. Called Hyperkalemic Periodic Paralysis (HYPP), it’s characterized by intermittent attacks of muscle tremors and triggered by a similar mutation in the sodium (rather than the chlorine) gene channel.