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Congenital Myotonia (CM)


→ Impaired movement due to muscle rigidity.

Instructions for ordering:

  1. Enter the information of the first horse for which you want to order the test
  2. Add to cart
  3. Go to cart to check out, or
  4. Enter the information for a second horse
  5. For sample collection follow the instructions here.
  6. You can use this template to send in hair samples.
  7. Please also send the completed order form together with the samples.

For a breeder or veterinarian discount please contact us before ordering.

Any information you include here is optional, but will be included on the certificate of results (except “Aditional Info”).

Enter the name of your animal
Enter your animal's date/year of birth
Enter the breed name
Enter the registration number if known
Enter the microchip number
Is there anything else you would like to tell us about your horse? Reason for testing, symptoms etc.?

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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