Syncope and seizure both can cause a loss of consciousness, but there are important differences.

Published: January 8, 2024

Syncope and seizure both can cause a loss of consciousness, but there are important differences.

Syncope — commonly referred to as passing out or fainting — often results from a drop in blood pressure.

Seizures, on the other hand, are caused by abnormal brain activity. The reason behind the abnormal activity may be epilepsy, stroke, cancer or an infection in the brain.

People who pass out as a result of syncope generally recover on their own, and there may be no lasting symptoms. Seizures tend to last longer and often are followed by confusion and extreme fatigue.

“If you lose consciousness, you should talk to your health care provider. Both seizures and syncope can have serious causes and should be checked out,” said Ambica M. Tumkur, M.D., neurologist, Norton Neuroscience Institute.

The difference between syncope and seizure often can be detected by looking at brain waves using an electroencephalogram (EEG). What happened just before the loss of consciousness also can offer clues about the cause.

To function properly, your brain needs blood pumping from the heart as well as controlled electrical signals. When a seizure or syncope occurs, one of these processes is not working the way it should.

About half of syncope cases are caused by a drop in blood pressure. Fainting like this can be caused by stress, such as exposure to injury or blood, fatigue, standing for too long, or being in a hot or crowded place. The loss of consciousness often is preceded by lightheadedness, nausea, feeling warm or cold, sweating, weakness, dizziness, blurred vision or changes in hearing. Syncope also can have more serious causes. These include an irregular heartbeat or issues with blood flow to the brain. 

You should call 911 if the person is not breathing, does not wake up after one minute, has difficulty moving or speaking after waking up, has chest pains or heart palpitations, is injured or fainted while exercising or lying down.

Unlike syncope, seizures happen not from a drop in blood pressure, but because of uncontrolled activity in the brain. The underlying cause can be epilepsy, infection, brain injury, fever, stroke, withdrawal from some drugs, electrolyte imbalances, low blood sugar or sleep deprivation.

Seizures do not always result in a loss of consciousness. They may cause someone to make a noise or cry out when the seizure begins, to bite their tongue or suddenly urinate. Some people report a strange feeling just before the seizure begins, which is sometimes referred to as an “aura.” Seizures can cause convulsions, which usually begins with muscles stiffening and then enters a phase of uncontrolled rhythmic movement. Someone who has a seizure usually has no memory of the experience.

With epilepsy, seizures happen more than once and occur spontaneously, without any apparent cause.

If you see someone having a seizure, get them to the floor if they are not already lying down.  Move hard or sharp objects away from them. Do not try to restrain them and don’t put anything in the person’s mouth.

You should call 911 if it is a person’s first seizure, if the seizure lasts for more than five minutes, seizures happen one after another, the person appears to be choking or having difficulty breathing, the person is injured or the seizure happens in water.