Polysaccharide Storage Myopathy Type 1
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
Breeding advice appliesUse the screening guidance and veterinary or genetics advice before breeding an affected or at-risk animal.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Polysaccharide storage myopathy type 1, or PSSM1, is an inherited skeletal-muscle disorder caused by a gain-of-function mutation in the GYS1 glycogen-synthase gene. The autosomal dominant mutation causes muscle to store excessive glycogen and abnormal polysaccharide, disrupting energy use during exercise. One copy can cause disease; two-copy horses may be more severely affected.
PSSM1 occurs in Quarter Horse-related breeds, draught horses, Morgans and numerous others. Signs often begin with exercise after one or more rest days. The horse may become stiff, shorten its stride, sweat, tremble, tuck the abdomen, stretch out or refuse to move. Episodes can look like colic. Muscle cells rupture, releasing enzymes and sometimes dark myoglobin into urine; severe damage can threaten the kidneys. Some draught horses show progressive weakness and muscle loss rather than classic tying-up.
The mutation is not equivalent to every syndrome marketed as “PSSM2”. A negative GYS1 test excludes PSSM1 but does not diagnose another myopathy, and commercial genetic panels for unvalidated variants should not replace clinical investigation. Likewise, a positive test shows genetic status but does not prove that every episode of lameness or poor performance is caused by PSSM1.
Expression is strongly influenced by diet, fitness and regularity of exercise. Many affected horses remain functional when management is consistent, whereas sudden rest followed by work and high-starch meals trigger recurrence. Because the mutation is inherited, breeding choices affect future foals and should be discussed openly rather than judged from clinical appearance alone.
A gain-of-function GYS1 variant drives abnormal glycogen production in skeletal muscle. Some homozygous horses are more severely affected, yet genotype does not predict every episode because diet and activity modify expression. Clinical signs range from reluctance to engage the hindquarters to painful tying-up and dark urine; many carriers remain unnoticed until work or feeding changes.
Screening & Early Detection
A hair-root or blood DNA test from an accredited laboratory identifies the GYS1 mutation and distinguishes zero, one or two copies. Identity verification is important when results guide breeding or sale. Relatives of a confirmed horse can be tested, but there is no need to provoke an episode merely to establish genetic status.
During signs, serum creatine kinase rises within hours and AST remains elevated longer. Urinalysis assesses myoglobin, while electrolytes and kidney values identify severe complications. Baseline enzymes may be normal between episodes. A controlled exercise challenge with pre- and post-exercise CK is sometimes used after stabilisation to evaluate management response.
History should record rest days, exact concentrate and forage, exercise duration, excitement and previous dark urine. Lameness, gastric pain, RER and other myopathies can mimic reluctance to move. Muscle biopsy is unnecessary to diagnose genetically confirmed PSSM1 but may be useful when signs persist despite a negative test or when another disease is suspected. Screening results are interpreted with clinical findings rather than used as a universal explanation.
Treatment & Management
During an acute episode, exercise stops and the horse remains quietly in place rather than being forced to walk. Analgesia, sedation and fluids are selected according to pain, hydration and the extent of muscle damage. Severe myoglobinuria requires intravenous fluid therapy and monitoring of kidney function and electrolytes. Transport is delayed until the horse can move safely unless hospital care is essential.
Long-term control combines daily turnout or exercise with a low-starch, low-sugar ration. Forage is analysed, grain reduced or removed and additional energy supplied from fat only when body condition and workload require it. Adding oil to an obese horse is counterproductive. Changes are introduced gradually, and vitamin E is adequate for dietary fat and work.
Exercise resumes slowly after CK and comfort normalise. Consistency matters more than occasional hard work: prolonged stall rest is minimised and conditioning builds aerobic capacity over weeks. A rest day still includes safe turnout where possible, and the first session after unavoidable lay-off is light. Repeat CK can show whether the programme is tolerated. Breeding an affected horse risks transmitting the dominant mutation; genetic counselling and mate testing support informed decisions, but management cannot change inherited status.
Breeds or species affected
American Quarter Horse
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American Paint Horse
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Appaloosa
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Morgan
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Friesian
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Dutch Warmblood
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Hanoverian
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Holsteiner
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Oldenburg
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Westphalian
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Trakehner
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Selle Français
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Belgian Warmblood
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Danish Warmblood
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Swedish Warmblood
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Swiss Warmblood
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Australian Warmblood
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Irish Sport Horse
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Württemberger
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Zangersheide
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Quarter Pony
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Clydesdale
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Shire
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Percheron
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Belgian Draft
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Suffolk Punch
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American Cream Draft
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Ardennes
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Breton
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Boulonnais
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Comtois
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Dutch Draft
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North Swedish Horse
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Haflinger
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Gypsy Cob
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Irish Draught
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Finnhorse
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Black Forest Horse
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