What is PSSM in horses?

Polysaccharide Storage Myopathy PSSM is a disease in horses that causes glycogen the stored form of glucose to build up in the muscles. Usually glycogen is stored in the body and converted to energy as and when it is required.
However in a horse with PSSM the glycogen doesnt get used properly during exercise and therefore starts to build up. As well as storing too much glycogen affected horses often make too much of it as well. 
Why does it happen?

There are two types of PSSM Type 1 and Type 2. Type 1 is caused by a genetic mutation and is most commonly seen in Quarter horses and some of the draft breeds. The cause of Type 2 is still unknown and can affect Quarter Horses Arabs and Thoroughbreds.
What are the signs of PSSM?
Often the classic signs of tyingup are seen such as hard and painful muscles stiffness sweating and reluctance to move. The signs usually occur with exercise and dark coloured urine may also be seen. However some cases can be more subtle with a mild and intermittent hindlimb lameness being noticed.
How is PSSM diagnosed?
To confirm that a horse is tyingup a blood sample will be taken to check the levels of the muscle enzymes. These are usually raised during an episode and can stay raised for several hours afterwards. If the horse is one of the common breeds affected by Type 1 PSSM then it is likely that a genetic test will be recommended this can be carried out on a hair or blood sample.
If the horse is not one of the commonly affected breeds or genetic testing does not reveal a genetic mutation then a muscle biopsy will be advised. The horse will be sedated and then a small piece of muscle from the hindquarters will be removed