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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • A form of severe nonexertional rhabdomyolysis due to eating plant material containing toxin hypoglycin A; aka “seasonal pasture myopathy”
  • Horses ingest toxin-containing seeds from box elder (Acer negundo, midwest USA and Canada) or sycamore maple (Acer pseudoplatanus, UK and northern Europe)
  • Metabolized hypoglycin A inhibits acyl-coenzyme A dehydrogenase, resulting in massive aerobic metabolism disruption and severe type I muscle fiber damage
  • Necrosis of skeletal, respiratory, and cardiac muscles results in myoglobinuria, weakness, recumbency, respiratory difficulty, renal damage, and death in a high proportion of affected horses

Signalment!!navigator!!

  • Typically ≤3 years of age
  • Any breed or gender

Signs!!navigator!!

  • Weakness, stiffness, rapid progression to recumbency and death in 72 h
  • Discolored (red/brown) urine
  • Tachycardia, respiratory difficulty
  • ±Bright and appetent while recumbent
  • ±Subclinical disease in pasture mates

Causes and Risk Factors!!navigator!!

  • Consumption of seeds containing hypoglycin A
  • >12 h of pasture access
  • Young age, dietary indiscretion
  • Competitive feeding situations
  • Lack of supplemental feeding, particularly during cold or inclement weather
  • Overgrazed pastures

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Conditions causing weakness, stiffness, recumbency, and/or discolored urine such as laminitis, tetanus, colic, viral encephalitides, acute exertional rhabdomyolysis, selenium deficiency, red maple (Acer rubrum) toxicity, and other disorders causing intravascular hemolysis/methemoglobinemia.

CBC/Biochemistry/Urinalysis!!navigator!!

  • Elevated serum creatine kinase (>100 000 U/L in many), aspartate aminotransferase, and lactate dehydrogenase
  • Hypochloremia, hypocalcemia, hyponatremia, hyperkalemia
  • Azotemia
  • Hyperglycemia, hyperlipemia, hyperlactatemia
  • Myoglobinuria

Other Laboratory Tests!!navigator!!

  • Cardiac troponin I or T
  • Methylenecyclopropylacetic acid conjugates (hypoglycin A metabolite) in serum or urine

Imaging!!navigator!!

Echocardiography (in survivors).

Other Diagnostic Procedures!!navigator!!

Muscle biopsy—excessive lipid accumulation in type I skeletal muscle fibers.

Pathologic Findings!!navigator!!

  • Massive skeletal muscle necrosis
  • Cardiac muscle necrosis in ~50% of cases
  • Acute tubular necrosis ± pigmenturia

Treatment ⬆ ⬇

TREATMENT

  • Reduce discomfort, prevent further muscle necrosis, restore fluid volume and acid–base and electrolyte status, reverse shock, and prevent renal damage
  • Prompt, intensive supportive care. Hospitalization to provide appropriate fluid therapy and to manage recumbency
  • IV or oral fluid therapy with balanced electrolyte solutions until myoglobinuria resolves
  • Deep bedding, head and eye protection, and limb wraps are indicated for recumbent horses
  • ±Slinging

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Analgesics—butorphanol (0.01–0.04 mg/kg IM or IV; or as a CRI at 23.7 μg/kg/h after a loading dose of 17.8 μg/kg IV); lidocaine (CRI at 30–50 μg/kg/min after a loading dose of 1.3 mg/kg IV slowly); and flunixin meglumine (0.5–1.1 mg/kg IV or PO)
  • Antioxidant drugs—DMSO (1 g/kg IV or PO as a 10% solution), vitamin C (30 mg/kg IV diluted in fluids), vitamin E (2000–5000 IU PO every 24 h)
  • Muscle relaxants—methocarbamol (5–22 mg/kg IV slowly, every 6–12 h) and dantrolene (4–6 mg/kg PO every 12 h). Dantrolene prevents additional muscle necrosis and may speed return of function, but is poorly absorbed in horses that have not eaten within 4 h prior to administration
  • Insulin (0.2–0.4 U/kg insulin glargine IV or SC every 24 h) for hyperlipemia

Contraindications/Possible Interactions!!navigator!!

  • Avoid NSAIDs in horses with azotemia, myoglobinuria, or profound shock
  • Avoid dantrolene in horses with hyperkalemia

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Close monitoring of clinical and clinicopathologic variables is essential in affected horses, including serial monitoring of serum muscle enzymes, cardiac troponin, creatinine, and other relevant variables.

Prevention/Avoidance!!navigator!!

  • Prevent access to causative tree species via fencing or tree removal
  • Provide supplemental feeding during inclement weather or remove horses from pasture

Possible Complications!!navigator!!

  • Acute or chronic renal insufficiency secondary to myoglobinuria
  • Acute or chronic cardiac dysfunction
  • Atrophy of skeletal musculature

Expected Course and Prognosis!!navigator!!

  • Guarded to grave prognosis with a 75–80% fatality rate. Surviving horses can have chronic complications
  • Survivors require months of rest. Reevaluation including assessment of cardiac function is recommended before return to exercise

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Heart failure
  • Renal insufficiency

Age-Related Factors!!navigator!!

Highest prevalence in young horses.

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Abbreviations!!navigator!!

  • CRI = constant rate infusion
  • DMSO = dimethyl sulfoxide
  • NSAID = nonsteroidal anti-inflammatory drug

Suggested Reading ⬆ ⬇

Valberg SJ, Sponseller BT, Hegeman AD, et al. Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree. Equine Vet J 2013;45:419–426.

Author(s) ⬆

Author: Erica C. McKenzie

Consulting Editor: Elizabeth J. Davidson