JPC SYSTEMIC PATHOLOGY

MUSULOSKELETAL SYSTEM

April 2025

M-M20

 

Signalment (JPC #2784509): A five-month-old female quarter horse

 

HISTORY: This horse presented with a two-month history of front limb weakness and tiring easily after exercise. Muscle enzymes CK (2820 IU/L) and AST (3580 IU/L) were elevated. 

HISTOPATHOLOGIC DESCRIPTION: SLIDE A: Skeletal muscle: Severely affecting 70% of this section and mildly to moderately affecting the remainder; diffusely separating and surrounding remaining myocytes and replacing myocytes which are lost; are numerous macrophages, eosinophils, lymphocytes, fewer plasma cells, and neutrophils, and plump, reactive fibroblasts with minimal fibrous connective tissue, admixed with abundant lytic necrosis characterized by eosinophilic cellular and karyorrhectic debris as well as mild hemorrhage. Multifocally in less affected areas, myocyte sarcoplasm is either swollen, pale, and vacuolated with occasional internalization of nuclei (degeneration); hypereosinophilic, fragmented, and lacking cross striations with pyknotic or karyorrhectic nuclei (necrosis); or shrunken and rounded with small, hyperchromatic nuclei (atrophy). Degenerate and necrotic myofibers often contain basophilic granular to fragmented material (mineralization). Multifocally, several myocytes contain intrasarcoplasmic, lightly basophilic, 20µm x 40µm amorphous material (polysaccharide) often surrounding a hypereosinophilic center (hyaline crystalline material); this material is also extracellular. There are multifocal areas of adipocyte infiltration.

 

Slides B, C: (PAS; PAS with diastase): Skeletal muscle: Diffusely the intrasarcoplasmic and extracellular material is PAS positive both before and after diastase treatment (diastase resistant; polysaccharide). 

 

MORPHOLOGIC DIAGNOSIS: Skeletal muscle: Myocyte degeneration, necrosis, and loss, chronic, diffuse, marked, with mineralization, intrasarcoplasmic basophilic and crystalline inclusions, fibrosis and moderate lymphohistiocytic and eosinophilic myositis, American quarter horse, equine.

 

CAUSE: Unknown

 

CONDITION: Equine Polysaccharide Storage Myopathy (EPSSM)

 

SYNONYMS: Shivers syndrome, recurrent exertional rhabdomyolysis (ER)

 

GENERAL DISCUSSION: 

• EPSSM is a presumed hereditary autosomal dominant myopathy (gene defect of GSY1 in some, but not all animals) with variable expression, characterized by abnormal accumulations of glycogen, glucose-6-phosphate, and abnormal polysaccharide within type 2 (2A and 2X) skeletal muscle cells – leading to glycogen uptake faster than utilization
• Two types of EPSSM: 
  • Type 1 (PSSM1): GYS1 mutation (SNP gain of function mutation in skeletal muscle glycogen synthase 1 gene); causes the most severe disease; more coarse, granular histologic appearance to the abnormal polysaccharide, usually cytoplasmic location, amylase resistant; also leads to excessive glycogen production; may also have ryanodine receptor 1 (RYR1) mutation which produces more severe clinical disease than GYS1 mutation alone
  • Type 2: Diagnosed with biopsy as EPSSM but NO GYS1 gene mutation (i.e. muscle biopsy required for diagnosis); fine, granular histologic appearance to the polysaccharide which is located under the sarcolemma, amylase sensitive; horses are often younger intact males when diagnosed
• High prevalence in quarter horses, draft horses, warmblood and related breeds; has also been described in mules

 

PATHOGENESIS: 

• Unknown pathogenesis
• Presumed to be a heritable disorder of carbohydrate metabolism; affects primarily skeletal muscle fiber types 2A (oxidative-glycolytic) and 2X (glycolytic); rare cases involving cardiac muscle (some studies have found no evidence of cardiac myocyte injury in PSSM1; GYS1 is highly expressed in both cardiac and skeletal muscle however and increased workload of cardiac myocytes may be protective
• There is no defect in glycolytic or glycogenolytic pathways in affected horses
• Rhabdomyolysis and polysaccharide accumulation appear to be influenced by diet, exercise, and other disease states
• Muscular dysfunction may reflect abnormal muscle energy production; several theories include enhanced uptake of glucose into muscle cells, abnormal regulation of intramuscular calcium concentration, and amylopectinosis due to a glycogen branching enzyme deficiency (glycogen storage disease type IV)
• Polysaccharide inclusions may not be evident for up to 3 years in muscle biopsy samples; clinical presentation is variable and false-negatives are possible in young animals on biopsy alone
• Insulin potentiates effects of GYS1, which worsens accumulation state – this can be offset by feeding higher fat, lower carbohydrate rations and maintaining insulin sensitivity through exercise

 

TYPICAL CLINICAL FINDINGS: 

• Usually first noticed in adult horses at commencement of training or when exercise resumes after a lay-up period
• Generalized weakness and muscle atrophy 
• Exercise intolerance; stiff gait; back pain
• Associated with recurrent exertional rhabdomyolysis (M-M14), neuromuscular weakness with abnormal pelvic limb gait (shivers), and post anesthetic myopathy

 

CLINICAL PATHOLOGY:

• Mild to moderate increases in creatine kinase (CK) typical but may be normal; the half-life of CK is short, so prolonged increases indicate continual muscle damage
• Mild to moderate increases in aspartate aminotransferase (AST) but may be normal
• Massive muscle damage / necrosis may cause hyperkalemia 

 

TYPICAL GROSS FINDINGS: 

•  Varies from no gross findings to severe muscle atrophy
•  Affected muscles are diffusely pale, streaked or have multifocal areas of pallor
  • If there was rhabdomyolysis, muscle is stained red due to myoglobin release
• Most commonly affected muscles (those composed mostly of type 2 fibers) are: semimembranosus, semitendinosus, gluteal, longissimus, and pectoral muscles; semimembranosus and semitendinosus are the muscles of choice for biopsy
• Severe diaphragmatic necrosis may be present (especially in animals dying of respiratory failure following rhabdomyolysis)
• Swollen, dark kidneys in animals with rhabdomyolysis (U-M20)

  

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Numerous myocytes contain pale pink to blue grey, single to multiple, intrasarcoplasmic, segmental inclusions which replace up to 90% or more of the area of affected myofibers
  • Inclusions are composed of complex polysaccharide (PAS positive, amylase-resistant); the complex polysaccharides may be described with the terms amylopectin and polyglucosan
  • Inclusions occur only in type 2X glycolytic and 2A oxidative-glycolytic fibers
  • Inclusions may contain basophilic crystalline structures
• There are also subsarcolemmal rounded vacuoles (in other fibers) which are clear to pale pink and hyalinized; these are PAS positive, amylase-sensitive = glycogen
• Myofibers containing either complex polysaccharide inclusions or glycogen vacuoles are found in aggregates near the edges of fascicles
• Chronic myopathic changes seen in older horses: Increased variation in fiber size (atrophy and hypertrophy) and more internalized nuclei in myofibers
• +/- myofiber necrosis and regeneration
• Scattered necrotic fiber segments and macrophages may be phagocytizing the PAS positive material
• Can see marked replacement of skeletal myofibers with mature adipose tissue (more commonly reported in draft horses)
• Most other tissues are not affected with the exception of rare complex polysaccharide inclusions in the heart

 

ULTRASTRUCTURAL FINDINGS:

• Filamentous masses of complex polysaccharide
• Abundant intracytoplasmic electron dense, granular glycogen at the periphery of filaments; glycogen storage is NOT intralysosomal but scattered throughout the sarcoplasm
• Decreased number of mitochondria
• Crystalline structures are amorphous, electron dense, and have a periodicity of 8.6 nm in each direction

 

ADDITIONAL DIAGNOSTIC TESTS: 

• The presence of periodic acid-Schiff (PAS) positive and amylase resistant inclusions of complex polysaccharides within myofibers is considered pathognomonic; and inclusions occur only in type 2X glycolytic (previously called 2B) and 2A oxidative-glycolytic fibers
• Ubiquitin immunostaining may be useful in diagnosis of EPSSM in horses; however, it is no more sensitive than PAS; ubiquitin may aid in the development of amylase resistance and stains both glycogen and complex polysaccharide

 

DIFFERENTIAL DIAGNOSIS: 

Equine myocyte necrosis:

• Exertional rhabdomyolysis (M-M14): Myonecrosis of type II muscle fibers without intrasarcoplasmic inclusions
• Clostridial myositis (malignant edema) (M-B01): Clostridium septicum is the most common cause, but other clostridial organisms that can cause myositis include Clostridium perfringens types A to E, C. chauvoei, C. novyi, C. fallux; these gram-positive bacilli can cause hemorrhage, edema, necrosis, and emphysema
• Streptococcus-associated myopathy: Similar findings to clostridial myositis; vasculitis causing infarction and coagulative necrosis
• Corynebacterium pseudotuberculosis (pigeon fever): Gram-positive bacilli, intramuscular abscesses, most common in pectoral muscles following penetrating wounds (“pigeon breast”)
• Ionophore (monensin) toxicity (M-T01): Multifocal segmental necrosis of skeletal muscle
• Vitamin E / Selenium deficiency (white muscle disease) (M-M11): Most common in foals, multiphasic myositis of predominately temporal and masseter muscles
• Cassia occidentalis (coffee senna) (M-T02) and Thermopsis spp. toxicosis: Plant toxicosis, polyphasic myonecrosis 

 

COMPARATIVE PATHOLOGY:

 

REFERENCES: 

1. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:205-207.
2. Firshman AM, Valberg SJ, Bender JB, Annandale EJ, Hayden DW. Comparison of histopathologic criteria and skeletal muscle fixation techniques for the diagnosis of polysaccharide storage myopathy in horses. Vet Pathol. 2006;43:257-269.
3. Lewis SS, Nicholson AM, Williams ZJ, Valberg SJ. Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy. Am J Vet Res. 2017;78(11):1305-1312.
4. McCue ME, Armien AG, Lucio M, Mickelson JR, Valberg SJ. Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses. Vet Pathol. 2009;46:1281-1291.
5. Naylor RJ, Luis-Fuentes V, Livesey L, et al. Evaluation of cardiac phenotype in horses with type 1 polysaccharide storage myopathy. J Vet Intern Med. 2012;26(6):1464-1469.
6.  Valentine BA. Skeletal Muscle. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1020-1021

 

 

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