Diseases affecting any muscle group can influence performance and intended function. Genetic muscle disorders, metabolic, nutritional, toxic, infectious, neurogenic, immune-mediated, traumatic, and even neoplastic etiologies have to be considered when diagnosing equine muscle disease.
There are two types of skeletal muscle fibers. Type 1, the red aerobic fiber, rich in myoglobin and oxidative enzymes, is slow contracting and resists fatigue. Type 2, the pale anaerobic fiber, rich in glycogen, is fast in contracting and rapidly fatigues. It follows a glycolytic pathway for energy supply. Type 2 muscle fibers can be subdivided into types 2A and 2B. Most muscles contain both type 1 and type 2 fibers.
A review article of equine muscle disorders proposed dividing muscle diseases into nonexertional and exertional entities. The presence or absence of rhabdomyolysis creates a subdivision and secondary classification. A third category includes myopathies resulting from altered membrane structures.
I. Genetic muscle disorders
Genetic disorders of equine skeletal muscle include single gene abnormality or gene pool abnormality and follow either an autosomal or dominant mode of inheritance. Examples for such muscle disorders in the horse are two types of glycogenoses: (1) glycogen branching enzyme deficiency (GBED), recessive; and (2) types 1 and 2 polysaccharide storage myopathy (PSSM), dominant. Other examples of genetic disorders are hyperkalemic periodic paralysis (HYPP), dominant; malignant hyperthermia (MH), dominant; and recurrent exertional rhabdomyolysis (RER), dominant.
1. Glycogen branching enzyme deficiency
Considered to be inherited as an autosomal recessive trait, GBED affects foals or aborted fetuses of Quarter horses or Paint horses and is the result of GBE1 gene mutation. In addition to skeletal and cardiac muscles, liver and brain are affected by cytoplasmic storage of basophilic globular or crystalline material. The material reacts positively with periodic acid-Schiff (PAS). Genetic testing for GBE1 mutation can be done on mane or tail hair of foals. Affected foals are born weak, may have flexural deformities, and usually die before the age of 8 weeks.
Figure 9.1. Horse. Skeletal Muscle. Glycogen-Branching Enzyme Deficiency (GBED). Spherical or oval basophilic inclusions are present in the sarcoplasm of myofibers. (H&E) (Courtesy Dr. N. Allison, RPL, Inc., North Carolina.)
Figure 9.2. Horse. Skeletal Muscle. GBED. These inclusions stain positive with periodic acid-Schiff before and after digestion with diastase. (PAS) (Courtesy Dr. N. Allison, RPL, Inc., North Carolina.)
2. Polysaccharide storage myopathy
Abnormal carbohydrate metabolism may lead to excess polysaccharide storage in the sarcoplasm and rhabdomyolysis in affected horses. The condition occurs in Quarter horses, Morgans, warmbloods, and draft horses. Type-1 PSSM is the result of a mutation in the glycogen synthase 1 gene and dominantly inherited. The genetic abnormality is not known for type 2 polysaccharide storage myopathy (PSSM), which, in addition to the breeds just listed, may also occur in Arabians and Thoroughbreds. Clinical signs are gait stiffness, muscle atrophy and generalized weakness, exercise intolerance, and reluctance to move. Serum muscle enzyme levels are usually increased with PSSM. Differential diagnoses include exertional rhabdomyolysis and postanesthetic myopathy. The condition may be inherited as an autosomal recessive trait.
In a skeletal-muscle biopsy of PSSM, the diagnosis is made by microscopically demonstrating accumulated PAS-positive, amylase-resistant glycogen in the sarcoplasm of mainly type 2A and type 2B myofibers. Sometimes there is evidence of fiber size variation.
Figure 9.3. Horse. Skeletal Muscle. PSSM. Intracytoplasmic material has accumulated in the myocyte sarcoplasm as vacuoles and of eosinophilic, hyaline clusters. There is evidence of fiber size variation. (H&E) (Courtesy Dr. B. Valentine, Oregon State University.)
Figure 9.4. Horse. Skeletal Muscle. PSSM. Staining with PAS for glycogen shows that the eosinophilic myocyte clusters stain positive. (PAS) (Courtesy Dr. B. Valentine, Oregon State University.)
Figure 9.5. Horse. Skeletal Muscle. PSSM. After exposure to amylase, affected fibers resisted amylase treatment and react positive for glycogen. (PAS) (Courtesy Dr. B. Valentine, Oregon State University.)
3. Hyperkalemic periodic paralysis
Figure 9.6. Horse. Body. Hyperkalemic Periodic Paralysis (HYPP). Multiple decubital skin ulcers are present due to multiple episodes of involuntary recumbency.
The biochemical disorder is the result of mutation in the sodium channel gene and affects Quarter horses, American Paint horses, and Appaloosas with episodes of generalized muscular fasciculation, laryngospasm, dysphagia, weakness, and emaciation. Clinical signs last from 15 to 60 minutes. Serum potassium concentrations are elevated during clinical manifestations (5 to 9 nmol/L; normal: 3–5.6 nmol/L). Doubling of or prominent muscles and minimal histopathologic changes such as vacuoles in type 2 fibers can be morphologically encountered in HYPP. Carriers of the mutant gene can be identified by DNA testing of blood or hair samples.
Figure 9.7. Horse. Skeletal Muscles. HYPP. Doubling of muscles and muscular prominence are present in the hind limb.
4. Malignant hyperthermia
The inherited disorder occurs in Quarter horses and Paints during anesthesia with volatile anesthetics and can be determined by genetic testing. Malignant hyperthermia (MH) is the result of a single point mutation in the ryanodine receptor 1 gene (RyR1). It leads to rhabdomyolysis, metabolic acidosis, and sudden death in individual cases. Hyperthermia may reach 43°C. Specific staining abnormalities are not observed in myofibers of affected horses.
5. Recurrent exertional rhabdomyolysis
RER has been described in 5–7% of Thoroughbreds, Standardbreds, and Arabians. Painful muscle cramping, stiff gait, and reluctance to move are clinical signs. Episodes are intermittent and associated with the elevation of muscle enzyme serum levels. Diagnosis is made by observing microscopically segmental necrosis and central location of the myofiber nucleus in skeletal muscle biopsies and by ruling out PSSM. Other differential diagnoses are MH and tetanus.
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