Causes of Fainting

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The symptoms of a faint are caused by a temporary shortage of blood supply to the brain. The blood carries oxygen, which the brain needs to function. A fall in blood pressure can be caused by a number of different things, including:

low blood pressure, particularly if you faint after a few days in bed, or as you stand up from a sitting or lying position;
over-stimulation of an important nerve called the vagus nerve. This causes the heart rate to slow down and lowers the blood pressure. It is called a vaso-vagal attack and is triggered by intense stress, fear or pain;
anaemia;
a fall in blood sugar levels, which may happen if you are diabetic, or if you have not eaten anything for a long time;
fast, shallow breathing (hyperventilation);
heat exhaustion and dehydration;
unusually strenuous exercise;
constipation (straining can cause a reduction in blood supply to the brain);
menstruation; and
pregnancy.

After a faint, most people should return to normal fairly quickly, although they may feel nauseous and disorientated for a short while. If a person feels very weak, or has difficulty speaking, this may indicate a more serious underlying problem.

Fainting can sometimes be a symptom of a more serious disorder, such as:

temporary abnormal heart rhythms (paroxysmal arrhythmia, Stokesâ€“Adams attacks);
epilepsy;
transient ischaemic attack (TIA or â€˜mini-strokeâ€™);
heart problems; including heart attack and aortic valve stiffening (stenosis);
micturition and cough syncope (fainting when passing urine or during a severe coughing fit); and
narcolepsy (sleeping disease).