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→ Impaired movement due to muscle rigidity


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

General Information:

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

Inheritance: autosomal recessive

→ Only animals with two copis of the genetic variant (cm/cm) are affected. Animals with only one copy (N/cm) are clinically healthy carriers.

Possible Genotypes:

Genotype The horse is:
N/N normal. The horse has no copies of the genetic variant causative for Congenital Myotonia and therefore cannot pass it on to its offspring.
N/cm a carrier The horse is clinically healthy. The genetic variant causative for Congenital Myotonia will be passed on to its offspring with a probability of 50%.
cm/cm affected. The genetic variant causative for Congential Myotonia 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.

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

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.

Breeds affected:

Test #: H109

Samples: Blood, Hair

Price: 35 € (net)

Time required: 10-14 days

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