Causes of Syncope in Canines

The term syncope, pronounced SIN-co-pee, means the same when applied to dogs and humans -- fainting. It's not a medical condition in itself but can be a symptom of underlying disease that causes a temporary reduction in blood flow, and thus oxygen delivery, to the brain. Typically a dog collapses on his side, losing consciousness partially or fully for a few seconds to minutes, sometimes moving his legs, making sounds or losing bladder control. Syncope is most often seen in older dogs of certain breeds, especially cocker spaniels, miniature schnauzers, pugs, boxers, dachshunds and German shepherds. Dogs experiencing fainting spells should always be examined by a veterinarian.

In about two-thirds of dogs and cats affected by syncope, cardiac disease is the underlying cause, veterinary cardiologist Henry Green III of Purdue University writes in "DVM 360" magazine. Syncope can be symptomatic of a wide range of heart problems, including disruptions in heart rhythm, tumors and obstructions or narrowing in cardiac blood vessels. Dogs don't have the human equivalent of strokes as often as people, but they have been reported to occur in small breeds with advanced heart valve disease. Syncope induced by coughing, sometimes known as "cough drop," is common in dogs with some forms of respiratory and heart disease, as well as brachycephalic breeds -- those with wide, flat skulls, such as pugs, bulldogs, Cavalier King Charles spaniels, Boston terriers, shih tzus and Pekinese, Green says. Syncope can also be a symptom of heartworm disease.

In dogs, symptoms of severe hypoglycemia -- low blood sugar -- most often include weakness or seizures, Green writes, but fainting spells in affected dogs aren't uncommon. According to WebMD, hypoglycemia is a particular concern for toy breed puppies between 6 and 12 weeks of age and can be triggered by stress, exhaustion from too much physical activity or missing meals. In diabetic dogs, insulin overdose can cause hypoglycemia. In older dogs with no other obvious risk factors, vets might suspect insulin-secreting pancreatic tumors. Whatever the cause, hypoglycemia, especially when accompanied by syncope, can be life-threatening. Fainting can also be a side effect of some veterinary drugs.

Sometimes canine over-exuberance can result in isolated fainting episodes. A pooch pulling forcefully enough on his leash to reduce blood flow in the arteries to his brain could make himself woozy for a few seconds. Like humans, overexcited dogs can swoon or even pass out from not knowing when to quit pushing their bodies. But in summer, dog owners must be especially careful to keep pets hydrated and provide them with a refuge from the heat, because by the time a heat-exhausted dog reaches the point of collapse, it may already be too late to save his life. Electrolyte imbalances caused by dehydration can bring on cardiac arrhythmia, leading to death.

Seizures aren't caused by a reduction in blood flow to the brain, and therefore aren't syncope, but, as Canine Epilepsy Resources points out, syncope sometimes "looks so much like a seizure it is worth mentioning." Even vets may have difficulty telling them apart solely on the basis of what they see, Green notes. Epilepsy is one reason a dog might have a seizure, a neurological disturbance in the brain causing convulsions, but not the only reason, says Thomas K. Graves, who is both a vet and the owner of an epileptic dog. Poisoning, low calcium or high blood ammonia levels can also trigger seizures. No two seizures look exactly alike, but the biggest difference between syncope and seizures is the dog's state of consciousness, Graves writes. A dog who has fainted may not lose consciousness entirely, while a dog in the throes of a true seizure will always be unconscious and completely unaware of his surroundings.