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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Myopathy causing chronic exertional rhabdomyolysis characterized by increased skeletal muscle glycogen and amylase-resistant polysaccharide inclusions.
  • Type 1 PSSM and type 2 PSSM recognized

Pathophysiology!!navigator!!

  • PSSM1 has a genetic basis.
  • Unknown PSSM2 cause(s)

Systems Affected!!navigator!!

  • Endocrine/metabolic.
  • Neuromuscular.
  • Renal

Genetics!!navigator!!

  • PSSM1—autosomal dominant mutation in GYS1 gene.
  • PSSM2—possibly heredity in Quarter Horses

Incidence/Prevalence!!navigator!!

PSSM1 breed prevalence—North American Belgian, Percheron (high); Shire, Clydesdale (low); Quarter Horses (6–10%); Paint, Appaloosa (6–8%); halter Quarter Horses (28%); racing Quarter Horses (low); Warmblood, Irish Draught, Cob, Connemara (low); Arabian, Standardbred, Thoroughbred (very low).

Geographic Distribution!!navigator!!

  • Worldwide

Signalment!!navigator!!

  • 2–4-year-old Quarter Horses.
  • ±Foal

Signs!!navigator!!

Quarter Horses

  • “Tying-up” episodes shortly after exercise characterized by muscle stiffness, sweating, reluctance to move.
  • Tachypnea, tachycardia, muscle fasciculations, tucked-up abdomen, camped-out stance, firm painful lumbar and gluteal muscles, gait asymmetry, hindlimb stiffness.
  • Subclinical to severe with recumbency and renal failure.
  • Pawing or rolling, ± resembles colic.
  • ±Rapid muscle atrophy after concurrent respiratory disease

Draft Horses

  • Variable—normal to weakness, recumbency.
  • Muscle soreness.
  • Generalized muscle atrophy.
  • Exertional rhabdomyolysis.
  • Hindlimb weakness, difficulty rising

Causes!!navigator!!

Hereditary ± environmental influence.

Diagnosis ⬆ ⬇

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DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Sporadic or chronic exertional rhabdomyolysis.
  • Non-exercise-associated rhabdomyolysis, e.g. infectious and immune-mediated myopathies (Clostridium, influenza, Streptococcus equi ssp. equi, Sarcocystis), nutritional myodegeneration, traumatic myopathy, idiopathic pasture myopathy, and toxic muscle damage, e.g. monensin, white snake root ingestion.
  • Colic.
  • Laminitis.
  • Pleuropneumonia.
  • Aortoiliac thrombosis.
  • Tetanus.
  • Hyperkalemic periodic paralysis.
  • Recumbent neuropathy

CBC/Biochemistry/Urinalysis!!navigator!!

  • CK, AST elevations (>10 000 U/L) after exercise.
  • Persistent CK elevation.
  • Myoglobinuria.
  • ±Azotemia.
  • ±High potassium and low sodium and chloride serum concentrations.
  • ±CK, AST elevation in draft horses

Other Laboratory Tests!!navigator!!

  • Serum vitamin E and whole-blood selenium to rule out deficiency.
  • Fractional excretion of urine electrolytes.
  • Genetic testing for GYS1 mutation

Other Diagnostic Procedures!!navigator!!

  • Muscle (semimembranosus/tendinosus) biopsy.
  • Submaximal exercise test—>3–4-fold increased CK 4–6 h after 15 min walk/trot

Pathologic Findings!!navigator!!

Skeletal muscle—subsarcolemmal vacuoles, increased glycogen staining, amylase-resistant polysaccharide accumulation.

Treatment ⬆ ⬇

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TREATMENT

Appropriate Health Care!!navigator!!

  • Acute episode—oral and/or IV balanced polyionic fluids ± electrolytes, analgesia, rest.
  • Appropriate diet and exercise

Activity!!navigator!!

Daily turnout, gradual return to consistent exercise.

Diet!!navigator!!

  • Forage 1.5–2% of body weight.
  • Low-starch (<10% daily energy) diet.
  • Eliminate grain, replace with fat (corn or soy oil).
  • ±Commercial made low-starch, high-fat ration

Client Education!!navigator!!

Appropriate diet and exercise management to control clinical signs.

Medications ⬆ ⬇

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MEDICATIONS

Drug(s) of Choice!!navigator!!

  • NSAIDs—phenylbutazone (4.4 mg/kg IV or PO every 12 h for 1 day then 2.2 mg/kg PO every 12 h) or flunixin meglumine (0.5–1.1 mg/kg IV or PO every 12–24 h). Care if dehydrated or myoglobinuric
  • Acepromazine (0.04–0.11 mg/kg IV or IM every 8–12 h).
  • ±Detomidine (0.005–0.02 mg/kg IV or IM).
  • ±Methocarbamol (40–60 mg/kg PO daily).
  • ±IV DMSO (<10% solution)

Contraindications!!navigator!!

  • NSAIDs with dehydration or myoglobinuria.
  • Acepromazine with dehydration

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor signs, especially during abrupt training changes.
  • Serum CK (resting or 4 h post exercise) prior to restarting exercise.
  • If myoglobinuria, monitor renal parameters.
  • If depressed or inappetent after acute episode, suspect acute renal failure and treat promptly

Prevention/Avoidance!!navigator!!

  • Low-starch, high-fat diet.
  • Daily exercise and turnout careful obesity does not occur

Possible Complications!!navigator!!

  • Severe rhabdomyolysis and muscle damage, recumbency, death.
  • Renal failure

Expected Course and Prognosis!!navigator!!

With proper diet and exercise, good prognosis for athletics.

Miscellaneous ⬆ ⬇

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MISCELLANEOUS

Associated Conditions!!navigator!!

Severe rapid muscle atrophy after concurrent respiratory disease in Quarter Horses.

Age-Related Factors!!navigator!!

  • ±Signs and rhabdomyolysis in 1-day-old foal.
  • Muscle biopsy abnormalities after 1 year of age

Pregnancy/Fertility/Breeding!!navigator!!

Heritable disorder, counsel breeders.

Synonyms!!navigator!!

  • EPSM, EPSSM (equine polysaccharide storage myopathy).
  • Azoturia.
  • Monday morning disease

Abbreviations!!navigator!!

  • AST = aspartate aminotransferase.
  • CK = creatine phosphokinase.
  • DMSO = dimethylsulfoxide.
  • GYS1 = glycogen synthase 1.
  • NSAID = nonsteroidal anti-inflammatory drug.
  • PSSM = polysaccharide storage myopathy

Suggested Reading ⬆ ⬇

Valberg SJ. Diseases of muscle. In: Smith BP, ed. Large Animal Internal Medicine, 5e. St. Louis, MO: Elsevier Mosby, 2015:1299–1304.

Author(s) ⬆

Author: Anna M. Firshman

Consulting Editor: Elizabeth J. Davidson