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Basics ⬇

Outline

BASICS

Definition!!navigator!!

An inherited muscle disease, affecting Quarter Horses and related breeds, resulting in sporadic episodes of muscle tremors or paralysis.

Pathophysiology!!navigator!!

Caused by a point mutation in skeletal muscle sodium channel.

Systems Affected!!navigator!!

  • Neuromuscular
  • Respiratory
  • Cardiovascular
  • Ophthalmic

Genetics!!navigator!!

  • Autosomal dominant trait
  • Linked to Quarter Horse stallion named “Impressive”

Incidence/Prevalence!!navigator!!

Approximately 4% of Quarter Horses.

Geographic Distribution!!navigator!!

  • Worldwide

Signalment!!navigator!!

  • Quarter Horses or Quarter Horse-related breeds that are descendants of a sire or dam with the genetic mutation
  • Horses with hypertrophied muscles. Clinical signs usually noted by 2–3 years of age

Signs!!navigator!!

  • Asymptomatic to daily muscle fasciculations and weakness
  • In mild disease—muscle fasciculations in flanks, neck, shoulders, and/or facial muscle spasm and generalized muscle tension
  • In severe disease—severe muscle cramping, weakness with swaying, staggering, dog sitting, or recumbency, ± tachycardia
  • Third eyelid prolapse
  • Tachypnea, respiratory stridor, or distress
  • Death

Causes!!navigator!!

Genetic mutation—phenylalanine/leucine substitution in skeletal muscle sodium channel causing excessive inward flux of sodium and outward flux of potassium, resulting in persistent depolarization and muscle weakness.

Risk Factors!!navigator!!

  • Sudden dietary change or ingestion of feeds high in potassium (>1.1%) (e.g. alfalfa hay, molasses, electrolyte supplements, kelp-based supplements), fasting, anesthesia or heavy sedation, trailer rides, and stress
  • Exercise does not stimulate clinical signs, may relieve them

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Differentials for clinical signs—colic, exertional rhabdomyolysis, tetanus, botulism, laminitis, seizures, upper airway obstruction
  • Differentials for hyperkalemia—delay before serum centrifugation, hemolysis, ruptured bladder, chronic renal failure, severe rhabdomyolysis

CBC/Biochemistry/Urinalysis!!navigator!!

  • Hyperkalemia (6–9 mEq/L) during an episode
  • Hemoconcentration
  • Hyponatremia
  • ±Serum creatine kinase mildly increased

Other Laboratory Tests!!navigator!!

DNA testing of mane or tail hair samples (including hair bulb) or whole blood (EDTA).

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

Electromyography between episodes reveals abnormalities.

Pathologic Findings!!navigator!!

  • N/A

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • ±Spontaneous recovery in 20 min
  • Early mild episodes may be halted using low-grade exercise or feeding grain or corn syrup to stimulate insulin-mediated movement of potassium across cell membranes
  • With severe signs, aggressive medical therapy may be needed

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

Regular exercise and/or frequent access to a large paddock.

Diet!!navigator!!

  • Low dietary potassium (0.6–1.1% of total ration)
  • Small meals of low-potassium diets (grass hay—later cuts of Timothy or Bermuda; grains—oats, corn, wheat, barley, beet pulp)
  • Commercial complete feeds for hyperkalemic periodic paralysis

Client Education!!navigator!!

Dietary management as above.

Surgical Considerations!!navigator!!

With severe respiratory obstruction, ± tracheostomy.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • During an episode, epinephrine (3 mL/500 kg 1:1000 formulation IM)
  • In severe cases, IV calcium gluconate (0.2–0.4 mL/kg 23% solution diluted in 1 L of 5% dextrose) or IV dextrose (6 mL/kg of a 5% solution) alone or combined with sodium bicarbonate (1–2 mEq/kg)
  • If clinical signs cannot be controlled with dietary changes, acetazolamide (2–4 mg/kg PO every 8–12 h) or hydrochlorothiazide (0.5–1 mg/kg PO every 12 h)

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

Anesthesia or heavy sedation may precipitate signs.

Possible Interactions!!navigator!!

Glucocorticoids may be contraindicated in susceptible horses as they induce episodes in humans with similar disorders.

Alternative Drugs!!navigator!!

  • N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Serum potassium concentrations are usually normal between episodes
  • Success of therapy is based on the absence of clinical signs
  • Affected horses require frequent monitoring for clinical signs

Prevention/Avoidance!!navigator!!

See Treatment and Medications.

  • Avoid high-potassium feeds (alfalfa hay, brome hay, canola oil, soybean meal or oil, sugar and beet molasses)
  • General anesthesia appears to be a risk
  • Owners should be discouraged from breeding these animals

Possible Complications!!navigator!!

  • Death during acute severe episodes
  • Respiratory distress due to paralysis of upper respiratory muscles
  • ±Aspiration pneumonia due to laryngeal dysfunction, particularly in foals
  • ±Cardiac arrhythmias (ventricular fibrillation)

Expected Course and Prognosis!!navigator!!

  • Prognosis for most is good with low-potassium diet ± diuretic therapy
  • ±Recurrence of clinical signs
  • Severe episodes may be fatal

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Owners should be discouraged from breeding affected horses.

Synonyms!!navigator!!

N/A

Suggested Reading ⬆ ⬇

MacLeay JM. Diseases of the musculoskeletal system. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 3e. St. Louis, MO: WB Saunders, 2010:506–508.

Author(s) ⬆

Author: Anna M. Firshman

Consulting Editor: Elizabeth J. Davidson