What is PSSM in horses?

and sent to the laboratory for examination. Performing this biopsy is necessary to diagnose PSSM and to distinguish the signs from other causes of tyingup. 
How is PSSM treated?
If you think your horse is showing signs of tyingup you should call the vet immediately as it can be very painful and stressful for the horse. Exercise should be stopped straight away and the horse should be kept calm until the vet gets there.
The vet will give pain relief and may even sedate the horse if dehydration is present then intravenous fluids may be required and therefore the horse may need to be hospitalised. The horse should be boxrested until the condition improves after this turnout in a small paddock can be allowed which will encourage the breakdown of the glycogen in the muscle.
No cure but it can be managed
Unfortunately PSSM can not be cured it can however be managed through careful control of diet and exercise. 
Changing diet
The aim of diet control is to prevent sudden increases in blood glucose and reduce the amount of glycogen that is stored in the muscle. Hard feed should be avoided with good quality forage making up the majority of the diet. Where possible this should have a low watersoluble carbohydrate level.
Any extra feed should be low in calories and sugar aiming for feeds that release energy slowly. Controlled access to grazing and the use of a grazing muzzle may be required to reduce the amount of sugar that the horse consumes from the grass especially if they are gooddoers. Vitamins and minerals should be provided in the form of a balancer. Oils can be used as an alternative energy source. 
Increased exercise and preparation
Exercise helps to break up the glycogen that is stored in the muscle and therefore a horse with PSSM should
have regular light exercise. Ideally a small amount of exercise should be carried out every day and consistency is key. Before undertaking hard or fast work the horse should be allowed to stretch and warmup properly frequent breaks from collected work are also recommended.
If careful attention is paid to an appropriate exercise regime and diet the frequency and severity of the episodes can be reduced and many horses with PSSM can live long and happy lives.