-- as reported in the WBFSH State of the Industry Report 2023
Type 2 Polysaccharide Storage Myopathy (PSSM2) was discovered when not all the horses diagnosed with PSSM by muscle biopsy had the glycogen synthase 1 genetic mutation. Like PSSM1, PSSM2 has a similar type of tying up and muscle damage. PSSM2 does not have excessive glycogen in their muscles and horses do not have muscle damage as their creatine kinase activity is normal in the bloodstream.
There is a subset of PSSM2 which is referred to PSSM2-ER which has high creative kinase in the bloodstream during tying-up episodes. PSSM2-ER responds to the same diet and exercise recommendations as PSSM1.
PSSM2 affected horses show a reluctance to go forward under saddle, reluctance to engage the hindquarters and exercise intolerance. Symptoms improve only 50% if the horse if given diet and exercise recommendations for PSSM1 (indicating a difference from PSS2-ER). In warmblood horses, this group of PSSM2 horses have myofibrillar myopathy (MRM) and their management is different from PSSM2-ER. Measurement and CK activity on the muscle biopsy is required to distinguish between PSSM2-ER and PSSM2. In Warmbloods most cases of exercise intolerance and normal CK levels are likely related to myofibrillar myopathy.
Affected breeds include Dutch Warmbloods, Swedish Warmbloods, Hanoverians, Selle Francais, Westphalian, Canadian Warmblood, Irish Sport Horse, Gelderlander, Husien. In warmbloods there can be an undiagnosed gait abnormality, sore muscles and drop in energy level and unwillingness to perform after 5-10 minutes of exercise. They have a painful, firm back and hindquarter muscles, reluctance to collect and engage hindquarters, poor rounding over fences, and slow onset of atrophy especially when out of work. The mean age of onset is between 8-11 years of age.
The specific cause of PSSM2 has not been identified. Research has identified two types of PSSM2. Firstly, PSSM2-ER as a glycogen storage disease that has high creatine kinase (CK) activity. Secondly, PSSM2-MFM which is characterised by exercise intolerance and normal CK activity. There is a possible link between PSSM2 and Myofibrillar Myopathy as some horses showed abnormal glycogen muscle accumulation and were diagnosed with PSSM2. Myofibrillar Myopathy is associated with abnormal clumping of a protein called desmin in muscles, which causes tying up symptoms as well. The link between these two diseases is not clear.
At this time researchers are not recommending a genetic test for PSSM2 or MFM as no scientific validated evidence that the variants for which genetic testing is available is linked to PSSM2 or MFM. Current there is no company is providing commercial tests for PSSM2 nor MFM.
Horses with PSSM2-ER can be managed by a diet and exercise regime. Horses with PSSM2- MFM have responded positively to increased amino acid intake and fat for calories and exercise regimes but the research not conclusive.