MIM is Not a New Muscle Disease

I have been receiving an increasing number of questions regarding how to treat a horse with MIM. What is MIM?

MIM is being proposed as a new term for horses with PSSM2

MIM stands for muscle integrity myopathy. It’s unclear where it originated but apparently it was in Europe, possibly with the German genetics lab Generatio.

PSSM2 is currently the moniker for any biopsy showing accumulation of glycogen in abnormal locations that is not positive for the genetic test for PSSM1 – the GYS-1 mutation.

While the name polysaccharide storage myopathy does not tell you anything about the disease process, it is descriptive of the pathological changes. Muscle integrity myopathy isn’t an improvement. In fact, it is described as a group of exertional myopathies, which means exercise brings it on, but they go on to describe some with weakness and muscle atrophy, with no mention of exercise as a trigger.

The main problem though is this entire collection of myopathies is based entirely on the test results from the EquiSeq screen, which has never been validated as actually detecting any genetic myopathy, even after being in business for 8 years and promising to publish.

EquiSeq even has the nerve to state on their web site that the University of Minnesota is collaborating with them, when scrutinizing would be more accurate.

Reaction among the veterinary community in Europe has not been positive either, from the premier equine community at Utrecht University https://www.uu.nl/en/news/pssm-in-horses-a-summary-of-the-scientific-facts to the British Equine Veterinary Association https://www.veterinary-practice.com/2023/beva-warns-against-the-use-of-unvalidated-genetic-tests-for-pssm2-mfm-and-mim .

Pay particular attention to the two articles linked in the BEVA statement which specifically refute the validity of several of EquiSeq’s tests. The American College of Genetics and Genomics in conjunction with the Association for Molecular Pathology has published guidelines for describing genetic variants as pathogenic, likely pathogenic, uncertain, likely benign or benign and what needs to be done to ascertain the significance of any given variant https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4544753/ . There is no evidence that EquiSeq or it’s cohort Generatio are doing that.

There seems to be no shortage of unproven approaches to diagnosis and treatment of horses. That will probably continue as long as people like the founder of EquiSeq describe horse owners as “a niche market with money to burn” https://www.krqe.com/news/albuquerque-entrepreneurs-pitch-for-shark-tank-like-contest/

Don’t get taken in.

Eleanor Kellon, VMD