Old wine in new bottles…
Recently, the name PSSM2 (Polysaccharide Storage Myopathy 2) was changed to MIM (Muscle Integrity Myopathy) in German-speaking regions – a move made by the company that offers the associated genetic tests.
The genetic test is highly controversial from a scientific perspective. Studies conducted in 2020/21 have already shown that the genes being tested unfortunately have no connection to the histopathological condition known as PSSM2.
The term MIM does not represent a modified genetic test that detects different variants or a new disease; it is simply a rebranding. The intention behind the name change is to reflect that, in these myopathies, glycogen (sugar) accumulation in the muscle is not always present, as the term PSSM2 previously implied. Instead, there may be damaged muscle fibres or myofibrils, combined with delayed regeneration after exercise, similar to MFM (Myofibrillar Myopathy). Alternatively, the symptoms could point to a disorder of calcium regulation in the muscle, as seen in RER (Recurrent Exertional Rhabdomyolysis).
However, the connections between the genetic tests and the various myopathies have not been validated – or, if they have, the data has not yet been published. Accordingly, there is still no evidence that the detected gene variants or mutations identified by these tests are responsible for coding MFM, PSSM2, or RER.
The genetic tests on offer are still the same. There is still no published scientific validation showing that these tests are linked to any of the mentioned myopathies. Changing the name does not alter this fact.
The gold standard for diagnosing these myopathies remains a biopsy – a tissue sample taken from the muscle and examined in a laboratory. Of all equine myopathies, PSSM1 is still the only one that can be reliably detected through a genetic test.
For those who prefer not to perform a biopsy, it is also possible to assess the horse’s condition by adjusting the feeding and management regime to see whether the symptoms improve with appropriate support.
