What Is MIM (PSSM2)?

MIM also known as PSSM2 stands for muscle integrity myopathy, which is a form of exertional myopathy.

What Is Exertional Myopathy?

Exertional Myopathy is a myopathy that leads to an abnormal response when the muscles are subjected to either normal strain (e.g walking around the field), or increased strain (e.g exercise).

Under the current MIM test there is 6 genes responsible for causing exertional myopathy, some of the genes discovered so far are associated with MFM (Myofibrillar myopathy), & RER (Recurrent Exertional Rhabdomyolysis).

Muscle biopsy showing a MIM affected horse, notice the breaks in the muscle

What Types Of MIM (PSSM2) Are There?

There are currently 6 known variants of MIM (PSSM2) these include:

P2– A likely variant of MFM affecting the Myotilin protein in muscles
P3– A variant of MFM affecting the Filamin C protein in muscles
P4– A variant of MFM affecting the Myozenin 4 protein in muscles
P8– A variant of MFM affecting the Pyridine nucleotide-disulphide oxidoreductase domain 1 protein in muscles
K1– A muscle disease affecting the COL6A3 protein in muscles
PX– A likely variant of RER affecting the CACNA2D3 gene in muscles

With up to 32 genes in horses, research is still very much ongoing into more variants of MIM, P5 & P6 have recently been discovered but research is still in early stages. As more variants of MIM are discovered and information is released we will add them to this list.

Is MIM (PSSM2) Hereditary?

Yes MIM (PSSM2) is hereditary a horse inherits the mutation for the disease from its parents, a horse only needs one copy of the gene mutation to pass it on to its offspring.

Can A Horse Be Affected With Both PSSM1 & MIM (PSSM2)?

Yes- In fact one of our Admin’s Christine has a horse affected with both PSSM1 & MIM (PSSM2)

Can A Horse Be Affected With More Than One Variant Of MIM (PSSM2)?

Yes, a horse can have up to 1, 2, 3, or even all variants of MIM (PSSM2)

What Are The Symptoms Of MIM (PSSM2)?

Symptoms of MIM are very similar to those of PSSM1 and can occur before or after work these usually include:

What Breeds Are Affected By MIM (PSSM2)?

ANY BREED can be affected with MIM some breeds it has been detected in so far are:

  • Warmbloods
  • American Quarter horses
  • American Paint horses
  • Appaloosas
  • Thoroughbreds
  • All types of Arabians
  • Iberian horses
  • Native Ponies
  • Draught Horses
What Is PSSM2?
Karlo my n/p4 homebred warmblood, after becoming mysteriously lame as a 4 year old he ended up at Rossdales where he had scans, x rays, and ultrasounds which were inconclusive. The MIM test came back n/p4

How Do I Test For MIM (PSSM2)?

Hair strand test or a muscle biopsy although a muscle biopsy will not tell you the specific variant they are affected with.

Interpreting The Results Of A Hair Strand Test

When you receive your MIM (PSSM2) results back it can often be confusing as to what all the terminology means, but we are here to explain that for you.

The main confusion with MIM (PSSM2) results stands with the terms Heterozygous & Homozygous as this is the terminology that is usually used to determine whether a coloured horse will throw a coloured foal.

Because of this people and also some vets automatically assume that if a horse is Heterozygous for MIM (PSSM2) it only means it has a 50% chance of having the condition.

This is FALSE regardless of if your horse is Heterozygous or Homozygous they 100% have MIM (PSSM2), the only time they would not have it in a test result is if their results come back N/N which means negative for that specific MIM gene.


Means your horse IS affected and carries 1 copy of the gene, if a horse is Heterozygous for MIM (PSSM2) it means it has a 50% chance of passing on the PSSM gene to it’s offspring, an example of a Heterozygous MIM result would be N/P2 or N/P4.


Means your horse IS affected and carries 2 copies of the gene, if a horse is Homozygous for MIM (PSSM2) it means it will have a 100% chance of passing on the MIM gene to it’s offspring, an example of a Homozygous MIM result would be P4/P4 or P2/P2.

Regardless of if a horse is Heterozygous or Homozygous for MIM they should not be bred from, even with a 50% chance of passing on the gene it is still to much of a higher risk to take.

Where To Test For MIM (PSSM2)?

Europe (MIM Alone)
Cag (Center for animal genetics)

Europe (PSSM1 & MIM combined test)
Cag (Center for animal genetics)

USA (MIM Alone)

Is MIM (PSSM2) Manageable?

MIM is manageable in some horses, however manageable is a broad context, and not all MIM horses go back to being the top competitive performance horses they once were.

MIM is much trickier to manage, and maintain, due to the problem being in that of the horse’s whole muscle structure.

There is also the issue of MIM being degenerative which means over time as the horse ages it will get worse and could cause secondary further issues down the line like arthritis, breathing issues, and hind end weakness due to the natural progression, and the extra strain the myopathy itself puts on the whole body.

The management for MIM (PSSM2) is a diet of high protein and/or tri aminos, some MIM horses also do better on lots of grass.

For more information on MIM check out our YouTube video by clicking this link