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Congenital Myotonia (CM)


→ Impaired movement due to muscle rigidity.

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Product Information

Affected breeds: New Forest Pony and related breeds
Test #: H109
Description: Congenital Myotonia Genetic Testing

This test detects the single basepair change in exon 15 in the CLCN1 gene.

Samples: Hair, blood
Time: 10 – 14 days

General Information

  • Foal may seem healthy when born but symptoms may evolve and worsen over time.
  • Symptoms mentioned in the “Symptoms” tap are caused by the general muscle stiffness in the affected horse.


  • Difficulties rising
  • Gait abnormalities
  • Muscular rigidity, especially in hindquarters
  • Vertigo and 0ccasional falling
  • Impaired flexibility of fetlock joints
  • Temporary protruding third eyelid
  • Inward protuberant eyeballs
  • Excessively muscled

Inheritance and Genotypes

→ CM is an autosomal recessive hereditary disease, meaning that horses with only one copy of the genetic variant (N/cm) are clinically normal carriers while horses with two copies of the genetic variant (cm/cm) are affected.

Genotype: The horse is: Effects:
N/N normal. The horse does not have any copies of the genetic variant causative for CM and therefore cannot pass it on to any offspring.
N/cm a carrier. The horse is clinically healthy. It has one copy of the genetic variant causitive for CM, which will be passed on to its offspring with a probability of 50%.
cm/cm affected. The genetic variant causative for CM will be passed on to all offspring. All offspring will be carriers (N/cm).


  • Carriers may be bred to normal animals (N/cm x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
  • Breeding two carriers (N/cm x N/cm) is not recommended due to the possibility of 25% of the offspring being affected.
  • Affected animals (cm/cm) should not be used for breeding.


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. Neuromuscul Disord 22:361-7, 2012. Pubmed reference: 22197188. Doi: 10.1016/j.nmd.2011.10.001.

Further information is available at Online Mendelian Inheritance in Animals.

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