Polysaccharide Storage Myopathy.

Polysaccharide Storage Myopathy (PSSM) is a genetic muscle disorder in horses that leads to abnormal glycogen storage and can cause muscle damage, especially during exercise. There are two main types, PSSM1 and PSSM2, each with distinct causes and diagnostic methods, but both respond well to dietary management and exercise.

Overview of Polysaccharide Storage Myopathy (PSSM)

• PSSM is a glycogen storage disorder that affects skeletal muscles in horses, causing abnormal accumulation of polysaccharides.
• It primarily presents with exertional rhabdomyolysis (ER), which is muscle pain and damage linked to exercise.
• The disease is inherited and has been identified in over 20 horse breeds.

Type 1 Polysaccharide Storage Myopathy (PSSM1)

• PSSM1 is an autosomal dominant condition, meaning a single copy of the mutated gene can cause the disease.
• It is caused by a genetic mutation leading to excessive glycogen storage with abnormal polysaccharides that are resistant to amylase digestion.
• Diagnosis can be confirmed by genetic testing detecting this mutation, or through muscle biopsy identifying muscle fibers with abnormal amylase-resistant polysaccharides.
• PSSM1 occurs in many horse breeds, highlighting its broad relevance.

Type 2 Polysaccharide Storage Myopathy (PSSM2)

• PSSM2 has been recently subdivided into subtypes, one of which is PSSM2-ER.
• PSSM2-ER primarily affects Quarter Horses and presents similarly with exertional rhabdomyolysis.
• Unlike PSSM1, PSSM2-ER does not currently have a known genetic mutation associated with it.
• Diagnosis of PSSM2-ER relies mainly on muscle biopsy demonstrating abnormal glycogen storage features consistent with a glycogen storage disorder.

Diagnosis and Clinical Signs

• Both types of PSSM may cause muscle stiffness, pain, and weakness, typically triggered by exercise (exertional rhabdomyolysis).
• Diagnosis is based on genetic testing for PSSM1 and muscle biopsy for both PSSM1 and PSSM2.
• Muscle biopsy reveals muscle fibers containing abnormal polysaccharide that resists amylase digestion, a key diagnostic hallmark.

Treatment and Management

• Both PSSM1 and PSSM2-ER respond favorably to nutritional and management strategies.
• A diet low in nonstructural carbohydrates (such as starch and sugars) and high in fat is recommended.
• Regular, appropriate exercise is also critical to managing symptoms and preventing episodes of rhabdomyolysis.
• This combined management approach helps reduce muscle damage and improves quality of life for affected horses.

Significance of the Article

• The article summarizes current knowledge about two forms of PSSM in horses, highlighting their genetic/biopsy diagnosis and clinical management.
• It underscores the importance of differentiating PSSM1 and PSSM2 for appropriate diagnosis and treatment.
• The information is valuable for veterinarians and horse owners for understanding this complex muscle disorder and optimizing care for affected horses.