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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Myocardial disease includes myocardial degeneration, ischemia, necrosis, inflammation, fibrosis, fibrofatty infiltration, or a combination of these
  • It may be focal or generalized; clinical signs are generally more severe when widespread

Pathophysiology!!navigator!!

  • Focal myocardial disease often leads to arrhythmias, which, if rapid, reduce diastolic filling time and compromise cardiac output
  • Generalized myocardial disease leads to decreased systolic function, reduced cardiac output, and poor perfusion of vital organs/tissues, including the myocardium
  • Lack of forward flow can lead to pulmonary congestion, edema, and signs of acute left-sided heart failure

Systems Affected!!navigator!!

  • Primary—cardiovascular
  • Secondary—renal, gastrointestinal, hepatobiliary, musculoskeletal

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

Uncommon

Geographic Distribution!!navigator!!

N/A

Signalment!!navigator!!

No specific breed, age, or sex predilections.

Signs!!navigator!!

General Comments

The severity of clinical signs generally reflects the nature and extent of myocardial pathology.

Historical Findings

  • Poor performance
  • Collapse and distress
  • Possible fever

Physical Examination Findings

  • Tachycardia
  • Arrhythmias
  • Weakness
  • Weak peripheral pulses
  • Pale mucous membranes
  • Pulse deficits
  • Respiratory distress, tachypnea, cough, and frothy nasal discharge
  • Moist crackles in the lungs

Causes!!navigator!!

  • Focal fibrosis/fibrofatty infiltrate—incidental or found in horses with persistent arrhythmias; etiology unknown
  • Bacterial infection (localized to the myocardium or by extension from endocardial or pericardial lesions)
  • Viral infection
  • Fungal infection
  • Aberrant parasite migration
  • Immune-mediated disease
  • Toxins (ionophores, snake venom, hypoglycin A, and others)
  • Neoplastic infiltration, e.g. lymphosarcoma, hemangiosarcoma
  • Myocardial failure occurs with MODS and SIRS, principally due to dysregulation of systemic vascular function and accompanied by microthrombosis. A direct myocardial depressant effect mechanism may also occur
  • Streptococcal toxic shock
  • Brain–heart syndrome
  • White muscle disease (linked to selenium deficiency, primarily in young animals)
  • Amyloidosis
  • Coronary artery disease; not well documented but may occur

Risk Factors!!navigator!!

Grazing seeds of Acer pseudoplatanus or Acer negundo during late fall or early spring or grazing marsh mallow weed.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Secondary causes of arrhythmias—hypoxia, toxemia, septicemia, or metabolic disturbances
  • Valvular heart disease including infective endocarditis—murmurs usually present, differentiate echocardiographically
  • Pericarditis—muffled heart sounds and/or friction rubs, differentiate echocardiographically
  • Severe skeletal myopathies—increases in serum activities of creatine kinase and myoglobinuria
  • Pneumonia—differentiate with thoracic US and radiography
  • Hemoperitoneum or hemothorax—differentiate with US

CBC/Biochemistry/Urinalysis!!navigator!!

  • Increased serum creatinine concentration and blood urea nitrogen suggest prerenal or, if marked, concurrent renal dysfunction
  • Neutrophilic leukocytosis and hyperfibrinogenemia may be present

Other Laboratory Tests!!navigator!!

  • Elevated cardiac troponin I or cardiac troponin T possible early but usually quickly returns to normal; often normal with focal myocardial disease. Marked increases indicative of myocardial disease; mild increases are nonspecific
  • Blood lactate may be increased, reflecting poor tissue perfusion
  • Blood culture and viral serology in selected cases
  • Transtracheal aspirates for bacterial or fungal culture in selected cases
  • Serum selenium and glutathione peroxidase concentrations if white muscle disease suspected

Imaging!!navigator!!

ECG

Paroxysmal or sustained supraventricular and/or ventricular arrhythmias may be present.

Echocardiography

  • With focal myocardial disease, the echocardiogram may be normal or focal abnormalities are observed
  • With generalized myocardial disease abnormalities could include:
    • Ventricular dilation (left and/or right); ventricular apices may be rounded
    • Regional or generalized hypokinesis or dyskinesis
    • Decreased fractional shortening
    • Marked spontaneous contrast
    • Increases in the mitral E point–septal separation
    • Increased preejection period and decreased left ventricular ejection period
    • Flattening of the aortic root, reduced aortic root diameter
    • Mild, usually anechoic, pericardial effusion
  • With myocardial neoplasia, nodular masses with mixed echogenicity may be visible

Thoracic US

With pulmonary edema, peripheral pulmonary irregularities (B-lines—previously known as comet-tail artifacts) are visible.

Thoracic Radiography

Possible pulmonary edema.

Other Diagnostic Procedures!!navigator!!

Radiotelemetric ECG Monitoring

For real-time monitoring of unstable cardiac rhythms.

Continuous 24 h Holter Monitoring

For identifying intermittent or paroxysmal cardiac arrhythmias, quantifying numbers of isolated premature complexes, and assessing response to therapy.

Exercise ECG

For characterization of exercise-induced cardiac arrhythmias and their clinical significance.

Noninvasive Blood Pressure Measurement

For monitoring horses with generalized myocardial disease.

Toxicology

Analyze stomach contents, heart muscle, and feedstuffs for ionophores, particularly in group outbreaks of myocardial disease.

Endomyocardial Biopsy

Safety in clinical cases unknown.

Pathologic Findings!!navigator!!

  • Grossly, there may be focal or diffuse areas of discolored, pale myocardium
  • Histologically, there may be focal or diffuse myocardial degeneration, ischemia, necrosis, inflammation, fibrosis, and/or fibrofatty infiltrate
  • Neoplastic infiltration is generally visible grossly, but must be confirmed histologically
  • Evidence of poor perfusion may be evident on histologic examination of kidneys, liver, and intestine
  • There may be accumulation of frothy pink-tinged fluid in the alveoli and small and large airways

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Restoration of cardiac output and improved tissue perfusion
  • Specific therapy aimed at cause
  • Antiarrhythmic therapy if unstable, life-threatening arrhythmias are present

Appropriate Health Care!!navigator!!

  • Anti-inflammatories, antiarrhythmics, and, if necessary, pressor support may be useful in generalized myocardial disease, in addition to specific measures aimed at the cause, if known
  • With focal myocardial disease, rest with or without corticosteroid therapy is helpful in some cases. If the problem persists, and exercising ECG is normal, the horse may still be able to be used for some level of ridden exercise

Nursing Care!!navigator!!

  • Continuous ECG monitoring indicated if the cardiac rhythm is unstable
  • Horses should be kept quiet and not moved if showing signs consistent with low cardiac output

Activity!!navigator!!

Horses with active focal or generalized myocardial disease should be rested until there is significant improvement in their clinical status, echocardiogram, and ECG.

Diet!!navigator!!

  • In white muscle disease, selenium, vitamin E, and other antioxidants should be added to the diet
  • If feed-derived toxins are suspected, the source of feed should be changed. If associated with grazing, horse should be removed from affected pasture

Client Education!!navigator!!

Clients must be warned of the grave prognosis with generalized myocardial disease.

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Antiarrhythmic drugs—see chapters Supraventricular arrhythmias and Ventricular arrhythmias
  • Broad-spectrum antimicrobials, such as penicillin and gentamicin, are indicated if bacterial myocarditis is diagnosed
  • Furosemide (1 mg/kg IV every 8 h) may relieve pulmonary congestion or torsemide at 0.5–1 mg/kg POP every 12 hours
  • Dobutamine (1–5 μg/kg/min constant rate infusion) may improve cardiac output, and digoxin (0.011 mg/kg PO every 12 h or 0.22 mg/100 kg IV every 12 h) has potentially beneficial positive inotropic and negative chronotropic effects
  • Corticosteroids may be useful in horses with immune-mediated or other forms of inflammatory myocarditis; either prednisolone 1 mg/kg PO every 48 h or dexamethasone 0.05–0.1 mg/kg IV or 0.1 mg/kg PO every 24 h for 3 or 4 days and then continued every 3–4 days in decreasing dosages is recommended
  • Vitamin E supplementation at up to 10 IU/kg PO every 24 h may be beneficial

Contraindications!!navigator!!

  • Digoxin contraindicated if ionophore toxicity suspected
  • Corticosteroids contraindicated with concurrent pituitary pars intermedia dysfunction

Precautions!!navigator!!

  • Use potentially nephrotoxic drugs cautiously in horses with poor tissue perfusion
  • Therapeutic drug monitoring is recommended

Possible Interactions!!navigator!!

Monitor plasma concentrations of digoxin when used concurrently with other drugs with known interactions, such as quinidine.

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor systemic blood pressure and blood lactate to assess early response to therapy
  • If biomarkers are increased, monitor to follow resolution of the active myocardial disease
  • Frequent echocardiographic and 24 h Holter ECG recordings should be obtained in the convalescent period

Possible Complications!!navigator!!

Renal failure and congestive cardiac failure.

Expected Course and Prognosis!!navigator!!

  • Focal myocardial disease can have limited clinical significance, although cardiac arrhythmias may persist
  • Generalized myocardial disease is life-threatening with a grave prognosis. Horses can return to athletic activity if they survive the acute stages of myocardial failure. Persistent echocardiographic evidence of significantly reduced ventricular function and exercising arrhythmias warrant retiring horses from ridden activities

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Myocardial disease can be associated with respiratory infection, SIRS, or MODS.

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

High risk of fetal compromise if mares develop low cardiac output during pregnancy.

Abbreviations!!navigator!!

  • MODS = multiple organ dysfunction syndrome
  • SIRS = systemic inflammatory response syndrome
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Bauquier J, Stent A, Gibney J, et al. Evidence for marsh mallow (Malva parviflora) toxicosis causing myocardial disease and myopathy in four horses. Equine Vet J 2017;49(3):307–313.

Bonagura JD, Reef VB, Schwarzwald CC. Cardiovascular diseases. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 3e. St. Louis, MO: WB Saunders, 2010:372–487.

Decloedt A, DeClerq D, Ven S, et al. Right atrial and right ventricular ultrasound-guided biopsy technique in standing horses. Equine Vet J 2015;48:346–351.

Gilliam LL, Holbrook TC, Ownby CL, et al. Cardiotoxicity, inflammation and immune response after rattlesnake evenomation in the horse. J Vet Intern Med 2012;26:1457–1463.

Verheyen T, Decloedt A, DeClerq D, van Loon G. Cardiac changes in horses with atypical myopathy. J Vet Intern Med 2012;26:1019–1026.

Author(s) ⬆

Author: Virginia B. Reef

Consulting Editors: Celia M. Marr and Virginia B. Reef

Acknowledgment: The author acknowledges the prior contribution of Celia M. Marr.