Congenital Myotonia in the New Forest Pony

Congenital myotonia (myotonia congenita) is an inherited muscle disorder characterized by delayed relaxation of the muscles after contraction or mechanical stimulation. In horses, this form of myotonia has been described in the New Forest pony. It is caused by impaired function of chloride channels in muscle cells, leading to increased excitability of muscle fibers and their persistent contraction. The main clinical manifestation of the disease is marked muscle stiffness and delayed muscle relaxation after contraction. Problems are particularly noticeable when standing up and lying down. Temporary protrusion of the third eyelid may also occur due to retraction of the eyeball.

Congenital myotonia is associated with the c.1775A>C mutation in the CLCN1 gene, which encodes the chloride channel of skeletal muscle. This channel is important for maintaining the electrical stability of the muscle cell membrane and for proper muscle relaxation after contraction.

The mutation is inherited in an autosomal recessive manner. This means that the disease develops only in individuals that inherit the mutated gene from both parents. Carriers of the mutated gene are clinically healthy but transmit the mutation to their offspring. When two heterozygous individuals are bred, theoretically 25% of the offspring will be completely healthy, 50% will be carriers, and 25% will inherit the mutated gene from both parents and will therefore be affected by the disease.

Genetic testing enables reliable identification of mutation carriers and helps prevent the birth of affected foals through appropriate breeding planning.

.

References:

Wijnberg, I.D., Owczarek-Lipska, M., Sacchetto, R., Mascarello, F., Pascoli, F., Grünberg, W., van der Kolk, J.H., Drögemüller, C.: A missense mutation in the skeletal muscle chloride channel 1 (CLCN1) as candidate causal mutation for congenital myotonia in a New Forest pony. Neuromuscular Disorders 22:361–7, 2012. PubMed reference: 22197188