Equine polysaccharide storage myopathy
[2] Additionally, some horses have been shown to have insulin sensitivity, which improves glucose uptake by muscle cells and contributes to excessive glycogen storage that is already elevated secondary to the GSY1 mutation.
This condition is caused by a mutation of the GBE1 gene responsible for producing the glycogen branching enzyme.
Type 2 PSSM is a category for disorders that lead to abnormal deposition of glycogen in the skeletal muscles of the horse that is not due to mutations in GSY1 or GBE1.
[3] It has been suggested that the GSY1 mutation provided some benefit to hard working animals with poor-quality diets, and is now damaging members of those "thrifty" breeds that are managed with moderate to low levels of work and diets high in non-structural carbohydrates.
[1] Horses with Type 1 PSSM usually appear normal at rest, but show signs of exertional rhabdomyolysis ("tying up") such as shortened stride, stiffness, firm musculature, sweating, pain or reluctance to exercise, when asked to perform light work.
[3][5] These clinical signs usually first become apparent when the horse is placed into training as a young animal; however, affected horses will show histological changes consistent with muscle damage at one month of age, and may also show elevations in creatine kinase (CK), an enzyme that elevates with muscle damage.
[6] Concurrent illness, such as respiratory or gastrointestinal infection, can lead to elevations in CK and potentially life-threatening rhabdomyolysis, even without exercise.
[1] However, environmental factors such as diet and exercise, whether the horse is heterozygous or homozygous for the mutated GSY1 allele, and the presence of modifying genes all play a role.
[8] Additionally, some affected horses may have PSSM Type 2, which will produce different cellular changes and subsequently different phenotypic effects.
[9] A combination of both of these genes can cause severe rhabdomyolysis should the horse become recumbent due to an HYPP attack.
Often a muscle biopsy is recommended for horses displaying clinical signs of PSSM but who have negative results for GSY1 mutation.
Horses with Type 1 PSSM will usually have between 1.5-2 times the normal levels of glycogen in their skeletal muscle.
[2] The exact mechanism of abnormal glucose metabolism has not yet been established, but it may have similarities to phosphofructokinase deficiency in humans.
PSSM horses fed diets high in nonstructural carbohydrates (NSC), which stimulate insulin secretion, have been shown to have increased severity of rhabdomyolysis with exercise.
Additionally, they are much more likely to develop muscle stiffness and rhabdomyolysis if they are exercised after prolonged stall rest.
[6] Horses generally have fewer clinical signs when asked to perform short bouts of work at maximal activity level (anaerobic exercise), although they have difficulty achieving maximal speed and tire faster than unaffected horses.
