Common causes of syncope (fainting)

Syncope (fainting) is a common condition that approximately 20% of the U.S. population will experience in their lifetime. Anyone who has a syncopal episode should seek prompt medical attention to rule out dangerous causes of this condition.

Here are some of the most common causes of syncopal episodes.

1) Hypotension

Hypotension (low blood pressure) is defined as a blood pressure of 90/60 or less. When the blood pressure is too low, the vascular system can’t overcome gravity to supply oxygenated blood to the brain. This can cause a patient to collapse and lose consciousness. When the patient is lying flat, the vascular system no longer has to combat gravity, so blood flow is restored to the brain, and the patient regains consciousness.

Hypotension can be caused by dehydration, sepsis, blood loss, certain medications, and underlying cardiovascular conditions. I.V. fluids are often used to reverse hypotension and raise blood pressure. In cases of severe blood loss, a blood transfusion is often necessary.

2) Cardiac arrhythmia

Cardiac arrhythmias happen when the electrical impulses that control the heart’s rate or rhythm are disrupted. Arrhythmias can cause the heart to beat too slow, too fast, or in an abnormal rhythm.

Cardiac arrhythmias are one of the most dangerous causes of syncope. When the heart is in an abnormal rate or rhythm, it’s not able to provide adequate blood supply to the brain. Arrhythmias typically cause sudden loss of consciousness without any warning symptoms like dizziness, nausea, blurred vision or sweating.

An EKG is the most common test performed to detect cardiac arrhythmias. Some arrhythmias are treated with medication; others are treated with defibrillation or cardioversion, which shocks the heart back into a normal rhythm.

3) Micturition syncope

Micturition syncope is a phenomenon that causes people to faint during or shortly after urinating. It’s more common in older men, and in patients who have just woken up from a deep sleep. It’s also more likely to happen in patients who are dehydrated, intoxicated, hungry, or taking blood pressure-lowering medications.

Researchers think micturition syncope happens because the combination of standing up plus relaxing the bladder muscles leads to a sudden drop in blood pressure, resulting in loss of consciousness.

Being well-hydrated and sitting while urinating can decrease the risk of this form of syncope.

4) Vasovagal syncope

Vasovagal syncope is the most common cause of syncope. It typically happens when the body overreacts to stress or fear, which causes a sudden drop in blood pressure. Common triggers of vasovagal syncope in the medical setting include the sight of blood or needles.

Patients will often have presyncopal symptoms, including nausea, warmth, sweating, dizziness, blurred vision, or turning pale. Laying the patient flat or having them put their head between their knees can improve blood flow to the brain and prevent a loss of consciousness.

Vasovagal syncope can often be prevented by adequate hydration, and by laying patients with a history of vasovagal syncope flat during procedures like blood draws or injections.


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Written by Sarah Thebarge, Physician Assistant