Loss of Consciousness: What to Do and What Causes It

What to do if someone loses consciousness

1. Check for danger, then check if they respond: shout and tap their shoulders.
2. Check breathing for up to 10 seconds. If they are not breathing, or only gasping, call 911 or your local emergency number, start CPR (hard, fast chest compressions in the center of the chest) and send someone for an AED (defibrillator).
3. If they are breathing, lay them on their back and raise their legs about 30 cm (12 inches) if a simple faint is likely. Most people who faint wake within seconds.
4. If they stay unconscious but breathe, roll them into the recovery position on their side to keep the airway clear, and call 911.
5. During a seizure, do not hold them down or put anything in their mouth. Clear the space, cushion the head, time it, and turn them on their side once the jerking stops.
6. Do not give food or drink until they are fully alert. If they have diabetes and are awake enough to swallow, give a sugary drink.
7. Stay with them and note what happened. Your account is often the most useful piece of the diagnosis.

Causes of loss of consciousness

Doctors separate transient loss of consciousness (brief, with full recovery, usually syncope or seizure) from prolonged unconsciousness (coma or a reduced level of consciousness).

Fainting (syncope)

Syncope is a brief blackout from a temporary drop in blood flow to the brain. It is by far the most common cause.

• Vasovagal (reflex) syncope: the heart slows and blood vessels widen in response to triggers such as standing for a long time, heat, pain, fear, the sight of blood or needles, coughing or straining. People usually feel warm, sweaty, nauseous and see their vision gray out first. See vasovagal syncope.
• Orthostatic hypotension: blood pressure falls on standing. Causes include dehydration, blood pressure medicines, diabetes, Parkinson's disease and older age. See orthostatic hypotension.

Heart-related (cardiac) causes

These are less common but the most important to find, because they can cause sudden death:

• heart rhythm problems: very fast or very slow rhythms, heart block, long QT syndrome
• structural heart disease: aortic stenosis, hypertrophic cardiomyopathy
• heart attack, pulmonary embolism or aortic dissection
• cardiac arrest, when the heart stops pumping effectively

Warning clues: no warning before collapse, blackout during exercise or when lying down, palpitations or chest pain beforehand, or known heart disease.

Seizures

An epileptic seizure can cause loss of consciousness with stiffening and jerking, a bitten tongue (especially the side), wetting, and confusion or sleepiness for minutes to hours afterward. Brief jerks can also happen in a faint, so a witness description matters.

Brain causes

• concussion or a more serious head injury
• stroke or bleeding in the brain, especially a subarachnoid hemorrhage with a sudden "worst ever" headache
• brain infections such as meningitis or encephalitis

Metabolic and toxic causes

• low blood sugar, mainly in people taking insulin or sulfonylureas
• alcohol, opioids, sedatives and other drugs or overdoses
• carbon monoxide poisoning (headache, several people affected, faulty heater)
• severe dehydration, heat stroke or sepsis

Conditions that mimic blackouts

Functional (psychogenic) attacks, panic with overbreathing, and sudden falls without loss of awareness (drop attacks) can look like blackouts.

Faint vs seizure vs cardiac blackout

How doctors find the cause

Expect detailed questions about what you were doing, how you felt beforehand, what witnesses saw, how long it lasted and how you felt afterward. Then:

• Blood pressure lying and standing, pulse and a full examination
• 12-lead ECG for everyone with a blackout. It can reveal heart block, long QT and other rhythm risks
• Blood tests: blood sugar, blood count for anemia or bleeding, electrolytes, sometimes troponin
• Heart rhythm monitoring over days to weeks, or an implantable loop recorder for rare episodes
• Echocardiogram if a structural heart problem is suspected
• Tilt-table testing for recurrent unexplained faints
• EEG and brain MRI if a seizure is likely, and a CT scan after head injury

Treatment

Treatment targets the cause:

• Reflex faints: learn your triggers, drink more fluid, and at the first warning sign sit or lie down, or use counterpressure moves (crossing the legs and tensing, gripping the hands). Your doctor may suggest more salt if your blood pressure allows.
• Orthostatic hypotension: stand up slowly, review blood pressure medicines, compression stockings.
• Heart rhythm problems: medicines, a pacemaker, an implantable defibrillator or a catheter ablation.
• Seizures: anti-seizure medicine after diagnosis.
• Low blood sugar: fast-acting sugar, and adjusting diabetes medicines.

After any unexplained blackout, ask your doctor about driving. Most places restrict driving after a blackout until the cause is known and treated.

Frequently asked questions

What is the most common cause of loss of consciousness?

Vasovagal syncope, the ordinary faint. It is triggered by things such as standing for a long time, heat, pain, fear or the sight of blood, and the person usually recovers within a minute. Heart problems, seizures and low blood sugar are less common but more important causes.

When is passing out an emergency?

Call 911 if the person is not breathing normally, stays unconscious for more than a minute, blacked out during exercise or while lying down, had chest pain or palpitations, hit their head, has diabetes, may have taken drugs, or has stroke signs such as one-sided weakness.

What is the difference between fainting and blacking out?

Fainting is one type of blackout. "Blacking out" covers any brief loss of consciousness, including fainting, seizures and heart rhythm problems. People also use it for memory loss from alcohol, where the person stays awake but does not remember events.

What should you not do when someone faints?

Do not sit them up quickly, give food or drink before they are fully alert, or slap or splash them. Do not leave them alone. If they do not wake within a minute, or are not breathing normally, call 911 instead of waiting.

Should I see a doctor after a single faint?

Yes, at least once, unless it was clearly a typical faint you have had before. A doctor will check your blood pressure and do an ECG to rule out heart rhythm problems. Faints with no warning, during exercise or with chest symptoms need prompt assessment.