What causes a person to lose consciousness?

In this brief guide we are going to answer the question ‘’What causes a person to lose consciousness?’’ We will explain the clinical causes of loss of consciousness and the first aid in case a person loses consciousness.

What causes a person to lose consciousness?

Loss of consciousness (syncope), is caused by a lack of blood supply to the brain. By definition, syncope starts quickly, lasts a short time and is fully recovered within a few seconds or minutes without sequelae.

It is usually preceded by various symptoms, such as dizziness, pallor, sweating, weakness and blurred vision (presyncope), which allow the patient to sit up or lie down before losing consciousness. Presyncope is common, especially in the elderly, and has the same causes as syncope. However, in some circumstances, syncope may occur without warning.

What are the causes of syncope?

Syncope can be due to:

Syncope of neurological cause (reflex syncope).

They are due to alterations, generally transitory, of the reflexes responsible for maintaining sufficient blood pressure to provide a normal blood supply to the brain. As a consequence of these alterations, the blood pressure drops suddenly, the heart slows down and the blood supply to the brain is reduced. They include:

Vasovagal syncope (the typical fainting spell).

It occurs in situations of fever, intense heat, charged environments, fear, anxiety, nervousness, etc. It is usually preceded by pallor, dizziness, sweating, nausea and weakness. Sometimes, after losing consciousness, abnormal muscle movements and urine leakage may occur, which may lead to confusion with an epileptic seizure. In contrast to seizures, patients recover quickly without subsequent grogginess.

Syncope associated with various situations:

  • Coughing (cough syncope). It can also be produced when blowing, lifting a weight, etc.
  • Urination (Micturition syncope).
  • Swallowing, defecation or manipulation of the gastrointestinal tract (e.g. endoscopy).
  • Neck massage.
  •  Manipulation of the eyes (squeezing the eyes, surgical intervention on the eyes).

Orthostatic hypotension

For example, a sudden drop in blood pressure when standing up (getting up from a chair or bed). It is due to the same neurological causes referred to above but presents in a chronic form. They are usually presyncope, i.e. situations of dizziness, pallor, sweating and weakness without complete loss of consciousness. It may occur in:

Neurological diseases

Neuro System Diseases like:

  • Parkinsons
  • Amyloidosis.
  •  Diabetes.
  •  HIV infection.
  •  Sjögren’s syndrome.

Other diseases

  • It can also occur due to certain diseases such as:
  • Hypotension after getting up from a meal.
  • People taking medicines that can affect blood pressure control, such as high blood pressure medicines or prostate medicines.
  • Due to low fluid intake (e.g. in people with fever, diarrhea, vomiting or dehydration in general).
  • Syncope of cardiological origin:
  • Arrhythmias.
  • Cardiac diseases that prevent adequate pumping of blood from the heart (heart failure, obstruction of a valve, myxoma, etc.).

How is the disease diagnosed?

The first thing to do is to differentiate syncope from other diseases that also produce transient loss of consciousness but are not due to the global interruption of blood supply to the brain such as:

  • Seizures.
  • Transient ischemic attack.
  • Hyperventilation (very rapid breathing) is seen in some anxiety attacks.
  • Hypoglycemia (low blood sugar).
  • Hypoxia (lack of oxygen to the brain).
  • Intoxications.

It should also be differentiated from diseases that may appear similar but do not alter the level of consciousness:

  • Accidental falls.
  • Cataplexy.
  • Functional syncope, i.e., feigning fainting.

To differentiate it, it is necessary to take an appropriate clinical history that identifies the circumstances in which the loss of consciousness occurs and whether or not it is preceded by suggestive symptoms. 

In addition, an examination should be performed to assess blood pressure, heart rate and brain function. Finally, an electrocardiogram should always be performed.

What Should You Do When Someone Faints?

Fainting is generally benign, except for those due to cardiac causes; in this type of loss of consciousness, the affected person does not have time to react and loses consciousness completely. In addition there is an absence of pulse and respiration. In fainting, the heart does not stop and the person never stops breathing. That is why the first action to be performed will be:

  1. Check for breathing and pulse. This way you will be able to know if it is fainting or if it is a cardiorespiratory arrest. To see the pulse, if you can’t feel it in the wrist, you can take it in the neck. If you see the absence of both, call for help and call an ambulance immediately. The action guideline will be cardiac resuscitation.
  2. In case of fainting, it is important that the body adopts a horizontal position to improve the supply of oxygen to the brain. You should also make sure that there is space in the place and that the person is not wearing any clothing that cuts off breathing (corsets, belts or ties).
  3. Try to lift the affected person’s feet and legs. Ideally, lift above head height. You can also place a jacket under the affected person’s feet. This will improve blood flow to the head and speed up the healing process.
  4. Control the time, fainting lasts only a few minutes. If you see that the affected person does not respond, you should immediately call a medical assistance service.
  5. When the affected person starts to react, try not to get up in a hurry. Once the person recovers from fainting, it is best to lie down on the floor for 10 to 15 minutes. The incorporation should be slow.
  6. The affected person should take deep breaths. Breathing exercises should be done. Inhale and exhale a couple of times deeply and soothingly while lying on the floor.
  7. It is important to drink plenty of fluids some time after fainting. Dehydration is a possible cause.

If you feel any symptoms of fainting, it is best to sit or lie down immediately. Many people are seriously injured not because of fainting, but because of the fall to the floor that occurs after losing consciousness.

What is the treatment of syncope?

It depends on the cause of syncope:

  • Syncope of neurological cause does not usually require treatment. At the moment of loss of consciousness, the patient should lie down and raise the legs to allow blood to flow to the brain. 

To avoid further episodes it is important to be well hydrated. In cases of similar repeated syncope, treatments with drugs that increase blood pressure, such as fludrocortisone, have been used, although the scientific basis for their use is questionable. In exceptional situations, pacemaker placement may be considered.

  • Syncope due to orthostatic hypotension improves if the patient remains well hydrated and gets out of bed slowly, remaining seated on the edge of the bed for a while before getting up. Substitution of those drugs that could favor syncope episodes should be considered.
  • Syncope of cardiological cause should be treated according to the cardiac disease responsible. Many are due to the presence of sinus node dysfunction (bradycardia-tachycardia syndrome). 

It should also be assessed whether any treatment received by the patient may favor a slow heart rhythm (bradycardia) and whether or not a pacemaker or defibrillator should be inserted.


Any loss of consciousness without a clear trigger requires a consultation with the primary care physician.

The emergency department should be called when there is an unexplained loss of consciousness in a person with known heart disease, when the loss of consciousness is accompanied by chest pain, shortness of breath, headache, abnormal movements or other symptoms, or when the loss of consciousness is very prolonged. Emergency care should also be sought if the patient is groggy, disoriented or drowsy after waking up.

In this brief guide we are going to answer the question ‘’What causes a person to lose consciousness?’’ We explained the clinical causes of loss of consciousness and the first aid in case a person loses consciousness.

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