Polysaccharide storage myopathies – or PSSMs – are rare but serious equine metabolic disorders. They are caused by an abnormal accumulation of glycogen, a complex sugar that serves as an energy source for muscle cells.
This build-up can have serious consequences for horses’ health, including as muscular weakening, discomfort, cramps, rapid weariness, and even damage to muscles.
But what are the different types of polysaccharide storage myopathies in horses? What are the symptoms, risk factors and consequences?
Different types of polysaccharide storage myopathies
In horses, there are various types of polysaccharide storage myopathies, each with its unique set of features and symptoms. Types I and II polysaccharide storage myopathy are the most frequent.
Type I polysaccharide storage myopathies
Polysaccharide Storage Myopathy Type I, also known as PSSM (Polysaccharide Storage Syndrome), is an inherited disease that mainly occurs in Quarter and Paint Horses. It is caused by a genetic mutation that results in an excessive accumulation of glycogen in muscle cells. Symptoms of the disease include muscle weakness, stiffness, pain, excessive sweating and reluctance to move.
Type II polysaccharide storage myopathies
Polysaccharide storage myopathy type II is a rare inherited disease characterized by a lack of alpha-glucosidase, an enzyme which helps in glycogen breakdown in muscle cells. Symptoms include generalized muscle weakness, weight loss, difficulty breathing and fatigue.
Consequences and risk factors
Polysaccharide storage myopathies in horses can have serious consequences for their health and well-being, with a significant influence on their life quality: irreversible muscle injury, decreased athletic performance, chronic pain, musculoskeletal diseases, and, in severe cases, early death. This is why it is important to diagnose the disease early, in order to set up an adapted care protocol to limit the harmful consequences.
Several risk factors are now known:
Some breeds of horses have been identified as being more likely to develop PSSM, such as Quarter Horses, Appaloosas, Paint Horses and Thoroughbreds.
Studies have shown that horses eating high-starch, high-sugar feeds – such as cereals, pellets and competition feeds – have a higher risk of developing polysaccharide storage myopathies. Horses with a history of the disease should be fed a low-starch, low-sugar diet to reduce the risk of developing the disease.
Polysaccharide storage myopathies are hereditary diseases that are transmitted from parents to foals. Horses having a family history of PSSM are more likely to develop it.
Genetic abnormalities that alter the body’s capacity to correctly metabolize glycogen cause polysaccharide storage myopathies. Horses with genetic mutations are more likely to suffer from this illness.
Diagnosis and management
Polysaccharide storage myopathies in horses can be difficult to diagnose since the symptoms are similar to those of other muscle illnesses. As a result, early detection is critical to avoiding future difficulties and emergencies.
– Clinical evaluation: Diagnosis begins with a thorough clinical assessment. As previously stated, common symptoms include muscle weakness, stiffness, excessive fatigue during exercise, excessive sweating, muscle tremors and cramps. These symptoms might develop as early as a few months of age or they can take years to appear.
– Laboratory tests: Blood tests can help confirm the diagnosis of PSSM. They may show higher levels of muscle enzymes including creatine kinase (CK) and aspartate aminotransferase (AST). Finally, urine testing may reveal the presence of acetylcholinesterase (AchE), an enzyme that is frequently increased in diseased horses.
– Muscle biopsy: Muscle biopsy is a procedure in which a small sample of muscle tissue is taken for microscopic examination. It can help to identify specific alterations in muscle cells linked with the disease. However, muscle biopsy is an invasive procedure that can cause pain and possible complications.
– Genetic testing: Polysaccharide storage myopathies can also be diagnosed by genetic testing. They can identify the genetic mutations associated with the disease and help determine whether a horse is a carrier of the mutation or not. They are also often recommended for horses with a family history of the disease.
Unfortunately, there is currently no curative treatment for polysaccharide storage myopathies in horses. Instead, management is aimed at relieving symptoms and preventing complications.
– Feeding: Proper nutrition is essential for horses with PSSM. They should be fed a high-fibre, high-fat diet with reduced carbohydrates and sugar content. This helps to reduce the workload on the digestive system and reduces the risk of disease-related attacks.
– Food supplements: Some supplements and medications can be prescribed, such as vitamin E and selenium, which help to support muscle function and protect muscle cells from oxidative damage.
– Training and daily work: Regular physical activity is important to maintain muscle health in these horses. However, exercise should be tailored to the condition of the horse. Low-intensity, short-duration exercise with frequent rest periods may be recommended to avoid excessive muscle glycogen accumulation.
– Regular veterinary follow-up: Frequent veterinary follow-up is important to monitor the progress of the disease and adjust treatment accordingly.
Polysaccharide storage myopathies are complex and heterogeneous diseases, and much research is underway to understand the underlying mechanisms and develop new treatments.
The following are some of the most intriguing studies directions:
Genetics and early diagnosis
Understanding the genetic alterations associated in different kinds of polysaccharide storage myopathies may aid in early identification and categorization of the illness. It would also make it possible to identify prospective therapeutic possibilities for the development of new medicines.
Gene therapy approaches have been explored. The objective is to introduce a functioning gene into muscle cells to fix the genetic mutation that causes the condition. Although this approach is still in the early stages of research, it has great potential for treating genetic diseases.
Many medicines, such as glycogen synthesis inhibitors and AMPK pathway modulators, have been studied for their efficacy. These medications can help minimize glycogen buildup in muscles and enhance muscular function. However, more research is needed to determine their long-term effectiveness and safety of use.
Cell therapy approaches have also been explored to treat polysaccharide storage myopathies in horses. The objective is to replace damaged muscle cells with healthy muscle cells sent into the horse’s muscles. This approach is still in the developmental phase, but it has great potential to treat muscle diseases.
Recent advances in understanding the pathogenesis of polysaccharide storage myopathies have led to significant progress in the diagnosis and management of affected horses. However, challenges remain in the management of these diseases, including the diversity of forms of the disease and the lack of a curative treatment.
To overcome these challenges, research continues intensively to understand the underlying mechanisms, identify new diagnostic tools, develop new treatments and improve the quality of life of affected horses.
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Naylor, R.J. (2015). Polysaccharide storage myopathy – the story so far. Equine Veterinary Education, 27(8), pp.414–419. doi:https://doi.org/10.1111/eve.12329.
Barrey, E., Mucher, E., Jeansoule, N., Larcher, T., Guigand, L., Herszberg, B., Chaffaux, S., Guérin, G., Mata, X., Benech, P., Canale, M., Alibert, O., Maltere, P. and Gidrol, X. (2009). Gene expression profiling in equine polysaccharide storage myopathy revealed inflammation, glycogenesis inhibition, hypoxia and mitochondrial dysfunctions. BMC Veterinary Research, 5(1), p.29. doi:https://doi.org/10.1186/1746-6148-5-29.
Keywords: polysaccharide storage myopathies, PSSM, equine muscle pathologies, muscle system, quantification tool, equine physiology, equine locomotion, EnvA, CIRALE