Few things unsettle an owner more than a horse that suddenly refuses to move, sweats up and stands with hard, painful muscles. Often the cause is polysaccharide storage myopathy, better known as PSSM. It is one of the most common inherited muscle disorders in horses, and while there is no cure, most affected horses can be kept comfortable and working with the right feeding and exercise routine.
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What Is PSSM in Horses
PSSM is a disorder of how muscle cells store and use sugar. Glycogen is the form of sugar stored in muscle, and horses with PSSM build up abnormally high amounts of it, sometimes along with an abnormal polysaccharide that the muscle cannot use as fuel at all. Affected horses can carry one and a half to four times the normal amount of glycogen in their skeletal muscle.
The result is a strange contradiction. The muscle is full of stored sugar, yet the cell runs short of usable energy during work. That energy shortfall shows up as cramping, stiffness and pain, commonly called tying up or exertional rhabdomyolysis.
PSSM is not the only cause of tying up. Malignant hyperthermia, glycogen branching enzyme deficiency and myosin heavy chain myopathy can all look similar from the outside, which is why a proper diagnosis matters before you rebuild a horse's whole feeding programme.
The Two Types of PSSM
Researchers recognise two forms of the disease. They look much the same in the stable but differ in cause and in how they are confirmed.
Type 1 PSSM
PSSM1 is caused by a known mutation in the glycogen synthase 1 gene, usually written as GYS1. The mutation leaves the glycogen synthase enzyme permanently switched on, so muscle cells keep producing glycogen when they should stop.
It is an autosomal dominant condition, which means a single copy inherited from either parent is enough to affect the horse. A foal from one affected parent has at least a 50 percent chance of carrying the variant.
Diet and exercise still decide how badly the horse is affected. Two horses with identical test results can have very different careers depending on how they are managed.
Type 2 PSSM
PSSM2 describes horses with abnormal glycogen storage in muscle who do not carry the GYS1 mutation. The underlying cause has not been identified, and there may well be several causes grouped under one label. Some of these horses turn out to have myofibrillar myopathy, a separate condition involving structural damage to the muscle fibres.
Clinical signs overlap heavily with PSSM1, though poor performance tends to be more typical than dramatic tying-up episodes. Warmbloods and Arabians are the breeds most often diagnosed.
Which Breeds Are Most at Risk
The GYS1 mutation has been found in more than twenty breeds, and it is thought to have emerged well over a thousand years ago in the heavy horses bred to carry armoured knights.
Halter-bred Quarter Horses show the highest prevalence at roughly 28 percent, while the general Quarter Horse population sits closer to 6 to 10 percent. Paints and Appaloosas fall around 6 to 8 percent. Draft breeds of continental European origin are heavily affected, with estimates for Belgian Drafts as high as 36 percent, whereas British-derived breeds such as Shires and Clydesdales show low numbers.
Light breeds like Thoroughbreds and Arabians rarely carry the type 1 mutation, but that does not put them in the clear. They are simply more likely to fall into the type 2 group.
Signs of PSSM in Horses
Signs range from barely noticeable to life-threatening, and some horses never show anything at all.
Everyday Signs Under Saddle
The subtle picture is often the more common one. A dressage horse that suddenly resists collection, a trail horse that runs out of energy ten minutes into a ride, or a gelding who has become grumpy about being saddled and will not stand for the farrier are all worth investigating.
Owners frequently report tail swishing, high muscle tone at rest, a shortened stride, shifting lameness and a sour attitude towards work. Some horses stop and stretch out as if to urinate. In draft horses the leading complaint is often weakness and muscle loss rather than pain, sometimes to the point of struggling to rise.
Signs of a Severe Episode
An acute episode looks alarming. The horse sweats profusely, trembles, refuses to move and has hard, painful muscles over the hindquarters and back. Pawing and rolling can make it look like colic, which is one reason a vet's opinion is needed rather than a guess.
The most serious warning sign is dark brown or red urine. That indicates muscle proteins are being released into the bloodstream, and it puts the kidneys at risk. Severe cases can be fatal, so treat this as an emergency.
What to Do During a Tying-Up Episode
Stop work immediately. Continuing to move the horse causes further muscle damage. Get the horse to a stable or small pen and phone your vet straight away.
Keep the horse comfortable while you wait. Rug a cold horse, or hose down one that is hot and sweating. Offer fresh water in small amounts, with electrolytes if dehydration is a concern. Withhold all grain and offer only small amounts of hay until the signs settle. Do not give anti-inflammatories on your own initiative, as some can worsen kidney damage in a dehydrated horse.
How Vets Diagnose PSSM in Horses
Blood Testing and Exercise Tolerance
Bloodwork usually comes first, both to confirm muscle involvement and to rule out colic. Vets look at creatine kinase and aspartate aminotransferase, two enzymes that rise when muscle is damaged. An exercise tolerance test involves a blood sample taken four to six hours after no more than fifteen minutes of light work. Affected horses show a marked rise, often of 1,000 U/L or more.
Genetic Testing
Type 1 PSSM is confirmed with a simple genetic test for the GYS1 variant, run on a hair or blood sample. Results come back as two normal copies, one variant copy, or two variant copies. Both of the last two mean the horse is affected.
Muscle Biopsy
Type 2 PSSM can only be confirmed by muscle biopsy, usually taken from the hamstring muscles. The sample is stained for glycogen and examined for abnormal deposits and damaged fibres. A biopsy is worth considering for any horse showing classic signs that has tested negative for the GYS1 variant.
A Word on PSSM2 Genetic Tests
Several laboratories sell genetic tests claiming to identify PSSM2 variants, marketed as P2, P3 and P4. Research has found these variants occur just as often in healthy horses as in horses diagnosed by biopsy, so a positive result tells you very little. There is currently no validated genetic test for PSSM2.
Feeding a Horse with PSSM
Diet does most of the heavy lifting in managing this condition.
Cutting Sugar and Starch
Sugar and starch trigger insulin release, and insulin drives glucose into muscle cells where the faulty enzyme turns it into yet more glycogen. Horses with PSSM are unusually insulin sensitive, so they clear sugar from the blood into muscle quickly.
The standard target is to keep nonstructural carbohydrates under 12 percent of the diet, and some nutritionists work to a stricter figure of under 10 percent of energy from starch and simple sugars. Practically, that means cutting out grain-based concentrates, choosing hay tested at 12 percent or less, soaking higher-sugar hay for thirty minutes in warm water, and limiting grazing on lush pasture with a muzzle or a dry lot.
Never cut forage to lower sugar intake. Aim for 1.5 to 2 percent of ideal body weight in hay daily, which is roughly 8 to 10 kilograms for a 500 kg horse, and use a slow-feeder net to spread intake across the day.
Safe Calorie Sources
Many PSSM horses are easy keepers who need no extra calories at all. When a horse does need more condition, fat is the usual choice because it does not trigger insulin and is not stored as muscle glycogen. Up to 20 percent of calories can come from fat, with ground flaxseed, rice bran or oils rich in omega-3 being sensible options. Introduce fat slowly over two to three weeks to avoid digestive upset.
High-energy soluble fibre is a useful alternative, particularly for horses that already carry too much weight. Beet pulp, lucerne and linseed meal cake provide calories along with protein for muscle repair.
Vitamin E, Selenium and Protein
Vitamin E and selenium work together to support muscle function and recovery. Horses prone to tying up are often given 1,500 to 2,500 IU of vitamin E and around 3 mg of selenium daily in the total diet. Selenium has a narrow safety margin, so never stack supplements without adding up the total.
Because cutting out concentrates also cuts out the vitamins and minerals they carried, a low-calorie ration balancer or comprehensive mineral supplement is usually needed. Good-quality protein matters too, especially for horses showing muscle wastage.
Exercise and Turnout
Daily movement is not optional. Exercise helps the muscle burn stored glycogen instead of hoarding it, and even ten minutes a day makes a measurable difference. Turnout should be as generous as the horse's sugar intake allows, with a grazing muzzle where the pasture is rich.
Long spells in a stable are one of the most reliable ways to trigger an episode. If more than three or four days pass without work, drop back to a very light session rather than picking up where you left off.
Coming Back After an Episode
Strict box rest after tying up is counterproductive. Once the horse is willing to move freely again, reintroduce turnout or gentle hand-walking.
Build work back gradually. Start with three to five minutes a day of uncollected walk and trot, adding about two minutes per day. Duration matters more than intensity, so keep sessions short rather than slow. Once the horse is working comfortably for fifteen minutes, add a five-minute walk break between trot intervals and hold that pattern for at least three weeks before asking for canter. Horses with type 2 PSSM particularly benefit from a long warm-up and regular stretching breaks.
Prognosis and Breeding
The numbers are encouraging. Roughly 50 percent of horses with PSSM1 improve on dietary changes alone, and around 90 percent of those kept on both a controlled diet and a consistent exercise routine have few or no episodes. Signs tend to return quickly if the routine slips, which is the main reason management fails.
For breeders, genetic testing is the clearest tool available. Because PSSM1 is autosomal dominant, an affected horse passes the variant to at least half its foals. Horses diagnosed with PSSM2 by biopsy are also best left out of a breeding programme, since a genetic basis is suspected even though it has not been pinned down.
Living with PSSM
A PSSM diagnosis is not the end of a horse's working life. Plenty of affected horses compete, hack out and hold down normal careers on a low-sugar diet and a routine that keeps them moving every day. The horses that struggle are usually the ones whose management is inconsistent rather than the ones with the worst test results. Get the forage right, keep the grain out, move the horse daily, and work with your vet on a plan built around that individual animal.
Frequently Asked Questions About PSSM in Horses
Can you still ride a horse with PSSM?
Yes, and in most cases you should. Regular work is part of the treatment, not something the horse needs protecting from. Horses managed on a low-sugar diet with consistent daily exercise compete and hack out normally. The exception is during and immediately after an episode, when your vet will guide the return to work.
Can PSSM in horses be cured?
No. Both types are lifelong conditions with no cure. What they do have is a well-tested management plan. Around 90 percent of horses that stick to both the dietary and exercise guidelines have few or no tying-up episodes, but signs usually return within days or weeks if the routine lapses.
At what age does PSSM show up?
Most horses with the type 1 mutation start showing signs by six years of age, though the range is wide. Some show problems as yearlings, others only in their mid-teens. Muscle damage can be present on biopsy from about a month old, long before an owner would notice anything. Draft horses are typically diagnosed around eight years.
Is PSSM the same as tying up?
No. Tying up is a symptom, not a diagnosis, and PSSM is only one of its causes. Malignant hyperthermia, glycogen branching enzyme deficiency and myosin heavy chain myopathy all produce similar signs, and a horse can also tie up simply from a heavy grain feed followed by hard work after a rest day. That is why testing matters before you commit to a management plan.
Should I buy a horse that has tested positive for PSSM?
That depends on what you want the horse for and how consistent your yard routine is. An affected horse in a well-run programme with controlled forage and daily turnout can be a reliable partner. A horse that will be stabled most of the week, worked irregularly and fed whatever the yard provides is a much riskier proposition. Ask what the current diet and exercise routine looks like, and have the horse's history of episodes disclosed in writing.
