Introduction

• Syncope is defined as a transient loss of consciousness and muscle tone secondary to temporary deficiencies in oxygen transport or perfusion to the brain. The diagnostic differentiation of syncope vs seizures is important. The most common causes of syncope include structural cardiac disease, arrhythmias, and abnormal vasodilation.
  Print off the owner factsheet on Fainting (syncope) Fainting (syncope) to give to your client.

Pathogenesis

Etiology

• The causes of syncope include:
  • Structural cardiac disease that impedes or reduces cardiac output:
    • Advanced mitral valve disease Atrioventricular valve dysplasia.
    • Severe subaortic stenosis Heart: aortic stenosis.
    • Advanced dilated cardiomyopathy Heart: dilated cardiomyopathy (DCM)).
  • Cardiac arrhythmias Heart: dysrhythmia (sinus arrest, high grade second degree or complete AV nodal block, sick sinus syndrome).
  • Vasovagal or neurocardiogenic, pulmonary hypotension, and rarely, orthostatic (ie postural) hypotension and severe hypoxemia Hypoxemia.

Predisposing factors

General

• Underlying cardiac disease.
• Pulmonary hypertension.
• Exercise.
• Stress.

Pathophysiology

• Obstructive cardiac disease (ie subaortic Heart: aortic stenosis or pulmonic stenosis): 
  • Lesions that impede outflow of blood into the aorta or pulmonary artery, and reduce cardiac output. During times of activity or exercise, cardiac output may not be sufficient to provide cerebral perfusion. 
• Other cardiac diseases associated with low or fixed cardiac output (ie mitral valve disease, dilated cardiomyopathy, pulmonary hypertension):
  • Loss of normal forward cardiac flow due to valve regurgitation, poor cardiac contractility, or downstream hypertension.
• Cardiac arrhythmias Heart: dysrhythmia (ie heart block, sick sinus syndrome): 
  • Extended periods of sinus arrest or ventricular asystole (usually >6 sec), and tachyarrhythmias (ventricular or supraventricular tachycardia) resulting in insufficient cerebral perfusion.
• Reflex-mediated syncope:
  • Reflex bradycardia, vasodilation, and reduced cardiac output secondary to activation of mechanoreceptors within the carotid sinus, aortic arch, and ventricles by stress or increased sympathetic tone.
• Orthostatic hypotension:
  • Rare, failure to maintain adequate blood pressure and cardiac output while standing, associated with severe volume depletion or autonomic system disease.
• Severe hypoxia:
  • Rare, dogs with laryngeal paralysis, airway foreign body.

Timecourse

• Period of unconsciousness and loss of motor function is typically brief (<1 minute). Syncopal events typically do not have a preictal or postictal phase that is common with seizures Seizures.

Diagnosis

Treatment

Prevention

Outcomes