Fainting—also called syncope—is a sudden, momentary loss of consciousness with spontaneous recovery. Fainting is common in dogs with heart disease, but can be caused by many other illnesses as well. Fainting is less common in cats. When the fainting is caused by heart abnormalities, it is most likely due to an abnormal heart rhythm which results in either a very slow or very fast heartbeat. Because the heart does not pump enough blood to the brain during these abnormal rhythms, this leads to a brief loss of blood flow to the brain which causes fainting. Fainting can look like a seizure but it is important to determine the exact cause of fainting since the treatment approaches are very different.
In both fainting and seizure, animals will lose consciousness, collapse on their side, and they may urinate or defecate. However, a fainting episode is more likely to occur after exercise or coughing, it usually lasts only a short time (about 30 seconds or less), and the animal recovers quickly (usually within 30 seconds to 2 minutes). Seizures are a bit less likely to have excitement or exertional triggers, seizures are more likely to have excessive salivation or chomping movements during the event, they often last for close to 2 minutes or longer, and seizures usually have a longer recovery period with disorientation after the event. A specific description of the event is very helpful to determine whether a fainting event or seizure has occurred.
A fainting episode may occur only once or they can occur quite frequently. As fainting can be a sign of very serious heart disease, it is important to perform diagnostic tests to evaluate the condition of the heart. When routine testing does not identify the cause for fainting, a 24 hour Holter monitor or an event recorder may be used to record the activity of the heart to help determine the cause of fainting.