What is syncope?
Syncope (or fainting) is a temporary loss of consciousness that occurs when the brain does not receive enough oxygen. The most common reason for decreased oxygen in the brain is reduced blood flow caused by a heart abnormality. Most dogs who experience syncope spontaneously recover once appropriate levels of oxygen reach the brain.
What are the clinical signs of syncope?
A typical syncopal episode will start suddenly, with no warning, often during physical activity and exertion. The dog may initially appear weak or wobbly and this period will be short-lived. When the dog collapses, they will suddenly go limp. They may urinate or defecate during the episode.
A syncopal dog may move their legs but these movements are typically associated with the dog trying to get back up. Recovery occurs rapidly, within seconds to minutes.
What causes syncope in dogs?
Syncope may be caused by problems in the heart, outside the heart, or in the nervous system. Heart-related causes of syncope include:
- Bradycardia (an abnormally slow heart rate). If there is prolonged time between beats, the brain doesn’t receive enough oxygen.
- Tachycardia (an abnormally fast heart rate). When the heart beats too fast, it doesn’t have time to fill with enough blood to send to the brain.
- Other arrhythmias (irregular heart rate). Most commonly, arrhythmias cause bursts of abnormal heart contractions that don’t pump blood effectively.
- Low cardiac output(the heart ejects less blood than normal). Low cardiac output may be caused by a variety of factors:
- The heart muscle may be weakened (cardiomyopathy, heart failure).
- The heart valves may have deteriorated over time.
- The dog may have been born with a heart valve defect that caused abnormal narrowing of the vessels leading out of the heart.
- Pulmonary hypertension (high blood pressure in the lung arteries, making it difficult for enough blood to get from the right side of the heart to the left side and to the rest of the body.
- Heartworm disease may have clogged the heart chambers and the surrounding blood vessels, interfering with the passage of blood.
- Blood clots in the heart chambers or the lungs, called thromboemboli, may interfere with blood flow.
- Tumors in the heart can obstruct blood flow and cause decreased cardiac output.
Nervous system-related causes of syncope include:
- Vasovagal syncope: When a dog feels very stressed or excited, its nervous system can make its heart beat very fast for a short time, causing a temporary rise in blood pressure. If the dog’s body reacts to the increased blood pressure by lowering the heart rate and widening the blood vessels at the same time, it can cause reduced blood flow, resulting in less oxygen going in the brain.
- Situational syncope: Situational syncope may occur with deep coughing, swallowing, or an abdominal press associated with urination or passage of stool.
- Carotid sinus hyperactivity: The carotid sinus is in the carotid artery, which leads to the head, and it helps to regulate heart rate and blood pressure. A pull on a dog’s collar may stimulate the carotid sinus, causing low blood pressure (hypotension) or bradycardia.
Other causes of syncope include:
- Medications that affect blood pressure.
- Low blood sugar (hypoglycemia).
- Low levels of calcium (hypocalcemia) or sodium (hyponatremia) in the blood.
- Decreased red blood cell numbers leading to decreased circulating oxygen levels. Can be caused by blood loss, blood cell destruction, or low production of red blood cells by the bone marrow.
Are some dogs more likely to develop syncope than others?
Dogs that experience arrhythmias (irregular heart rhythm) are more likely to have syncope. A condition called “sick sinus syndrome” may occur in Cocker spaniels, miniature schnauzers, pugs, and dachshunds, in which heart rate varies from very slow to very fast. Boxers and German shepherds may develop an irregular heart rate called ventricular arrhythmia. Syncope is more common in older dogs, and the signs that develop depend on the underlying cause of the fainting spells.
How is syncope diagnosed?
Your vet may suspect syncope based on your dog’s clinical signs and a physical examination. However, additional testing is required to determine the underlying cause so it can be treated. Additional testing may include:
- Lab testing, including a complete blood count (CBC), biochemistry profile (including calcium and glucose), and urinalysis
- Blood pressure evaluation
- ECG: An electrocardiograph assesses your dog’s heart rhythm over a few minutes
- Holter monitor: A device that your dog wears to monitor their ECG over 24 hours or more
- Echocardiogram (heart ultrasound) that assesses your dog’s heart function and screens for pulmonary hypertension
- Chest X-rays (radiographs) to assess heart size and pulmonary vessels, and screen for masses
Your veterinarian may also recommend referral to a cardiologist or neurologist for further assessment.
How is syncope treated?
Syncope is treated based on the underlying cause of the fainting spells:
- If a dog is taking a medication that can cause syncope as a side effect, then your veterinarian will look for an alternative medication.
- If the dog has heart disease, then it should be treated.
- If the heart rate is too fast or too slow, your veterinarian may prescribe medication to help to regulate the heart rate. Dogs with severe heart disease may require hospitalization.
- Dogs with decreased cardiac output may need to decrease their activity level.
- Dogs with vasovagal syncope can be prescribed medication.
- Some dogs with sick sinus syndrome may benefit from having a pacemaker implanted.
What follow-up care is required for dogs with syncope?
As with treatment, follow-up care for dogs with syncope depends on the underlying cause. For a dog with heart-related syncope, your veterinarian may prescribe a holter monitor for periodic or continuous monitoring of an electrocardiogram (ECG). It is wise to minimize any stimuli that may precipitate episodes, including minimizing activity for dogs with decreased cardiac output.
Most non-heart-related syncope is not life-threatening. Heart-related syncope can often be treated, but the risk of death is higher in these patients. In any case of canine syncope, it is important to get an accurate diagnosis to understand how best to proceed and to understand the potential outcomes.
