Polysaccharide storage myopathy type 1

PSSM1 causes excess energy reserves to build up in muscle

Muscle uses glucose and stores some of it as glycogen. The GYS1 gene provides instructions for an enzyme, a protein that helps muscle build that reserve. The PSSM1 variant makes the enzyme more active or less effectively controlled, allowing excessive glycogen and abnormal polysaccharide material to accumulate in muscle cells.

Research identifying the variant linked the DNA change to increased enzyme activity. This is a gain of function, rather than the simple loss of a protein. The disturbed storage process is associated with muscle injury, particularly around exercise. PSSM stands for polysaccharide storage myopathy: a muscle disease involving the accumulation of long chains of sugars. Type 1 identifies the form associated with this variant in the GYS1 gene.

References [3] · [1]

PSSM1 muscle pain and stiffness vary between horses

Possible signs include stiffness, muscle pain, weakness and reluctance to move. An episode of rhabdomyolysis involves damage to muscle fibres and is often called tying-up. Sweating or muscle twitching may occur, although none of these observations identifies PSSM1 on its own. A horse with an episode needs assessment of the actual problem.

Some horses have striking episodes, others show subtler difficulties, and some remain without visible signs. Feeding, activity and other genetic factors can influence expression. An inherited variant is already present at conception, but the age when signs become apparent varies. Neither a normal-looking foal nor an uneventful period of work reveals the genotype.

References [2] · [1]

The PSSM1 variant has been found in several horse breeds

PSSM1 has been documented in Quarter Horses and related breeds, as well as several draught and sport-horse populations. Research across breeds indicates an old mutation, predating the formation of many modern breeds. It should not be attributed to a single recent stallion or treated as confined to one modern pedigree.

Documentation within a breed does not mean every horse carries the variant. The type of study also matters: a group of horses sent for muscle biopsy, the examination of a small piece of muscle, is not a random sample of the whole breed. Pedigree information can help frame the question, but a correctly identified genetic test provides evidence about the individual horse.

References [4] · [3] · [1]

Veterinary examination and DNA testing work together to diagnose PSSM1

A veterinarian considers the examination and the history of episodes, including work, feeding and recent changes. Blood tests may provide evidence of muscle damage. The DNA test establishes whether the specified GYS1 variant is present. Together these findings help distinguish genetic status from the cause and severity of a current clinical episode.

Other muscle disorders and other health problems can produce pain or abnormal movement. Persistent signs therefore deserve investigation even after a negative PSSM1 result. Depending on the case, muscle biopsy or further tests may be appropriate. Matching a horse to an online description cannot reliably distinguish all these possibilities.

References [2] · [1] · [4]

One copy of the PSSM1 variant can cause muscle problems

The targeted test distinguishes no detected copy, one copy and two copies of the variant. One copy can be sufficient for PSSM1 to produce effects: this is autosomal dominant inheritance, affecting males and females. Horses with two copies may be more severely affected, but copy number is not a personal forecast of symptoms.

The names in older reports can be confusing. PSSM1 refers to the identified GYS1 variant. PSSM2 has been used for muscle abnormalities without that variant; a negative PSSM1 result does not establish PSSM2. A study in Quarter Horses found no association between the commercially tested P2, P3 and P4 variants and a muscle-histology diagnosis of PSSM2. Those findings do not support using these variants as diagnostic tests for PSSM2 in that population.

References [1] · [4] · [5]

Adjusting feeding and activity to help reduce PSSM1 episodes

Long-term care aims to reduce episodes and maintain comfort and muscle function. The veterinarian helps review what the horse eats, its regular activity and changes in its daily routine. The plan needs to account for how much energy the horse needs, its body condition and the muscle problems found during assessment. A feeding programme developed for another horse is not automatically suitable for this one.

Some horses improve substantially with appropriate management, while others have more limiting disease. Prognosis depends on clinical expression and response over time. Management does not remove the variant from the DNA. Marked muscle pain or a horse unable to move normally calls for prompt veterinary assessment rather than an exercise or feeding experiment.

References [2] · [1]

A horse can pass the PSSM1 variant to its foals without showing symptoms

A parent with one copy has a 50% chance of passing it on at each pregnancy. With a parent carrying no copy of the variant, each foal therefore has a 50% chance of inheriting one. That is a probability of inheriting DNA, not a guarantee of matching the parent’s symptoms, athletic ability or response to management.

A parent with two copies of the PSSM1 variant passes one copy to every foal. If the mare and stallion each have one copy of this variant, every pregnancy has a 25% chance of a foal without it, a 50% chance of a foal with one copy and a 25% chance of a foal with two copies. These are chances for each pregnancy, not a guaranteed pattern across four births. Breeding decisions should consider both parents’ results, future foal welfare and the population context, rather than a parent’s appearance or current performance alone.

References [1]

Questions about the condition: Polysaccharide storage myopathy type 1

Scientific sources about the condition: Polysaccharide storage myopathy type 1

This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.

1. 1.
   Polysaccharide Storage Myopathy (PSSM1)

   UC Davis Veterinary Genetics Laboratory · Test documentation

2. 2.
   Polysaccharide storage myopathy in horses

   University of Minnesota Extension · University resource

3. 3.
   McCue et al. (2008) — GYS1 mutation and skeletal muscle glycogenosis

   Genomics · Research

4. 4.
   McCue et al. (2008) — GYS1 mutation in diverse horse breeds

   Journal of Veterinary Internal Medicine · Research

5. 5.
   Valberg et al. (2023) — P2/P3/P4 tests and muscle histopathology in Quarter Horses

   Equine Veterinary Journal · Research