In cases of neurocardiogenic syncope acute sympathetic activity (induced by excitement) provokes a strong reflex vagal response that results in bradycardia. Neurocardiogenic (vasovagal) reflex resulting in syncope is less common in animals than in people, however there are reports of syncope that occur secondary to sudden bradycardia following bouts of tachycardia in especially small breed dogs with advanced valvular disease. Coughing may also induce reflexive bradycardia by stimulation of the vagal nerve. It also decreases intracranial pressures both of which may cause a decease in cerebral perfusion if severe enough coughing occurs. Coughing results in increased intrathoracic pressure which decreases venous return (preload) and cardiac output. This form of situational syncope occurs most often in bracheocephalic dogs however is also common in dogs with airway disease, tracheal collapse or those with severe left atrial (LA) enlargement causing compression of the left maintstem bronchus. Some like to use the term 'cough drop' to describe syncope induced in this manner. Hypoxia as a result of right to left shunts, severe acute anemia or pulmonary disease can result in insufficient cerebral oxygen delivery and syncope.Īnother common occurrence that we see is cough (tussives) syncope. While the majority of animals with severe hypoglycemia will present with weakness or seizures, a fair number may present with syncope or CVE while maintaining normal cardiac output. Non-cardiogenic diseases such as those that result in increased intracranial pressure can result in syncope also by reducing cerebral perfusion pressure by compressing intracranial vessels. Poor myocardial contractility, impaired filling as with pericardial disease or outflow obstructions can all result in an inability of the heart to maintain sufficient cardiac output to meet increased demand during excitable states even under normal cardiac rhythms. Underlying cardiac functional or structural abnormalities exacerbate the negative effect of arrhythmias on cardiac output. Most of these are related to rhythm disturbances which are secondary to inherent cardiac disease. Two-thirds of dogs and cats with syncope also have a cardiac disease. The most common causes we see in our patients are cardiogenic in nature. ![]() A sudden decrease in cardiac output (CO) or vascular resistance reduces mean arterial pressure may both result in reduction of cerebral blood flow. The vast majority of syncopal events in veterinary medicine are due to a transient reduction in brain blood flow. There are numerous diseases that can result in any one or a combination of these mechanisms. These mechanisms usually involve reduced cardiac output resulting from arrhythmias or decreased cardiac filling, obstruction of blood flow from the heart, hypoxia or hypoglycemia (with normal cerebral flow) or severe decreased vascular resistance related to neurocardiogenic reflexes. The mechanisms underlying syncope are usually rather acute in nature. Typically CSE are preceded by loss of muscle tone whereas seizure activity is usually preceded by atypical limb or facial movement or even staring spells prior to the loss of body tone. ![]() What may often confuse one who witnesses a syncopal event is that on occasion some animals may have "convulsive syncopal episodes" (CSE) that results from severe hypotension or asystole. However it is uncommon to see persistent facial fits, persistent tonic/cloned motion, defecation, postictal dementia and neurologic deficits with cardiovascular mediated syncope. This number may be due to the fact that one it is a referral database and two that often to both the trained and untrained eye it may be difficult to distinguish syncope from seizure activity.ĭuring a syncopal event, animals will usually collapse into lateral recumbency and may have concurrent stiffening of the limbs, opisthotonous, urination and vocalization. ![]() The true incidence of syncope is unknown however it has been reported to occur in a referral database in 0.15% of dogs and 0.03% of cats. Syncope is the sudden temporary loss of consciousness that is associated with loss of postural tone as a result of an abrupt decrease in cerebral perfusion or decreased delivery of essential nutrients (i.e.
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