PSSM in Horses: Causes and Management

In this week’s blog, we will look at PSSM (Polysaccharide Storage Myopathy) in horses, other names given to this syndrome include exertional rhabdomyolysis (ER),or tying-up.

PSSM is classified as a metabolic disease that results in the accumulation of high muscle glycogen and abnormal polysaccharide in skeletal muscles.

We will discuss some of the specific causes and the methods that can be used to prevent repeat episodes.

The commonality clinical signs of PSSM in horses vary, depending on the severity of the episode. Sometimes in mild cases horses can present as stiff after training whereas in severe cases the intense pain could immobilize a horse.

During training PSSM can be recognised when the horse presents with a short, stiff stride that worsens as the horse works. Sometimes they can adopt an unusual stance when standing. It is typically the large muscles of the hindquarters, back, and shoulders that are the most affected.

The affected muscles will be firm to the touch and painful for the horse. The muscles will be palpated when cramping is evident. The pain from tying-up can last for several hours with the horse sweating profusely due to an increased heart and respiratory rate.

In severe cases, horses can pass a dark, red-brown coloured urine (myoglobinuria) due to the presence of a muscle protein called myoglobin.

This happens when there is damage to the muscle fibres and a large amount of myoglobin, along with other muscle proteins, leak into the horse’s blood stream before being excreted through the kidneys.

Additional clinical signs could include depression, dehydration, hyperthermia, and “thumps” (synchronous diaphragmatic flutter), a condition characterized by hiccup-like contractions of the horse’s diaphragm. 

PSSM in horses should be distinguished from muscle soreness and strain which is very common in performance horses who overexert themselves.

Exercise in excess of the horses training level, inadequate warm-up, pre-existing lameness, electrolyte depletion, or dietary imbalances will predispose horses to muscle soreness and strain.

These horses usually respond well to rest, and a gradual return to their training regime, as well as balancing the diet.

Diagnosis of PSSM in horses includes:

• Blood work confirming the detection of elevated serum activities of the enzymes creatine kinase (CK), aspartate animotransferase (AST), and lactate dehydrogenase (LDH). Along with myoglobin, these proteins are contained within the muscle cells and are released when muscle cells become damaged during an episode of tying-up.
• A urinalysis is also commonly performed to detect electrolyte imbalances, as well as the presence of myoglobin, which would also indicate tying up.
• Muscle biopsy were samples are taken from the semimembranosis or semitendonosus muscle (hamstring muscles) and examined under a microscope.

Call your vet immediately if you suspect your horse is tying up!

As you wait do the following:

• Keep your horse still as forcing him to move may cause further damage to his muscles.
• Try and keep him calm as stress can make this condition worse.
• Offer him a drink of water with electrolytes.
• Warm up or cool down the affected muscles, according to the season. In cold weather, place a blanket over your horse to warm him up. If it is a hot day sponge your horse down to keep him cool.
• Monitor your horse if your horse urinates while you are waiting for the vet take note of the and if possible, collect a sample.
• Look out for additional clinical signs such as dehydration, hyperthermia, and thumps.

Dark mucous membranes and skin that does not snap back into place after being pinched are signs of serious dehydration.

Hyperthermia results when a horse can no longer cool himself by sweating. Signs of hyperthermia could be agitation, panting, excessive sweating, and skin that is noticeably hot to the touch. A horse who is exhausting his fluid reserves may develop anhidrosis, the loss of the ability to sweat.

Thumps will present itself as a distinctive jerking, in your horse’s flank that matches the rhythm of his heartbeat.

If you notice any of these other signs starting to develop. Call your vet with an update; so they can decide if your case is becoming critical.

There are two types of tying-up sporadic exertional rhabdomyolysis refers to horses that experience an episode of tying-up on rare occasions and chronic exertional rhabdomyolysis refers to a horse that experiences repeated episodes of tying up.

Management of the disease

Research has shown that management for all types of tying-up, regardless of the cause, can significantly improve the welfare for these horses. If the horse is overweight aiming to reduce and maintain a body condition score (BCS) of 5 on a 1-9 scale will be beneficial. 

You should customise a diet where the muscle will burn the excess glycogen instead of storing it. Do not restrict forage intake and if additional energy is required research the ingredients and choose feeds with a combined starch & sugar content of less than 10%.

Electrolyte imbalances within the individual horse rather than the diet may also be a concern in which case your vet may recommend a Fractional Electrolyte Excretion test.