PSSM1 (Polysaccharide Storage Myopathy)

PSSM1 – Polysaccharide Storage Myopathy

Polysaccharide Storage Myopathy (PSSM) is a group of muscle disorders found in various horse breeds, particularly in Quarter Horses, Draft breeds, and Warmbloods. This condition leads to abnormal storage of glycogen (sugar) in the muscles, causing muscle pain, stiffness, and, in severe cases, muscle atrophy. PSSM is also commonly referred to as “Tying-Up” or Equine Rhabdomyolysis.

Causes of PSSM: PSSM1 vs. PSSM2

PSSM is characterized by the excessive accumulation of glycogen in the muscles, which disrupts normal muscle function. There are two main forms of PSSM:

• PSSM1 is caused by a mutation in the GYS1 gene, which is crucial for glycogen synthesis. This mutation is inherited in an autosomal dominant manner, meaning that a horse only needs one copy of the mutated gene to develop the condition.
• PSSM2 is a group of muscle disorders that also leads to increased glycogen storage in the muscles. However, unlike PSSM1, it is not caused by the GYS1 gene mutation. While several genetic mutations appear more frequently in horses with PSSM2, it has not been conclusively proven that these mutations cause the condition. The exact cause of PSSM2 remains unknown.

Symptoms of PSSM

The symptoms of PSSM can range from mild to severe, and may include:

• Muscle stiffness
• Sweating after light exercise
• Difficulty or reluctance to move
• Muscle atrophy
• Dark, coffee-colored urine
• Episodes of muscle tremors or recurrent “tying-up” (exertional rhabdomyolysis)

Symptoms often worsen after exercise, particularly following periods of rest.

Diagnosis of PSSM

Veterinarians typically suspect PSSM based on a horse’s clinical signs, often combined with elevated muscle enzymes in the blood. A muscle biopsy can reveal an increased concentration of glycogen in the muscles, but it cannot distinguish between PSSM1 and PSSM2. A DNA test is required to confirm if PSSM1 is the cause of the symptoms.

DNA testing for PSSM

A DNA test can accurately identify whether a horse carries the GYS1 mutation responsible for PSSM1. This information helps breeders, owners, and veterinarians make informed decisions regarding breeding and management. Horses can be classified as:

• Normal: No copies of the PSSM1 mutation, will not develop PSSM1.
• Carrier: One copy of the PSSM1 mutation, likely to develop PSSM1.
• Affected: Two copies of the PSSM1 mutation, likely to develop PSSM1.

Be aware that a horse that is a carrier or affected by PSSM will only show symptoms depending on its exercise regimen. If a horse shows symptoms of PSSM and is a carrier or affected by the GYS1 mutation, it is highly likely that PSSM1 is the cause. If the horse does not carry the mutation, the condition is likely due to PSSM2 or another underlying cause.

Treatment of PSSM

There is currently no cure for PSSM. However, episodes of tying-up can be minimized through dietary adjustments and carefully managed exercise routines. If your horse is carrier of or affected by PSSM1, consult your veterinarian to discuss the best management plan.

Relevant tests

• P678
• P803