There are many different potential causes of fainting. A sudden decrease in blood flow to the brain is the most common cause, because it momentarily deprives the brain of an adequate flow of oxygen. Bleeding, severe dehydration or merely rising too quickly from a lying to a standing position can cause a drop in blood pressure that results in fainting. Actions that reduce oxygen flow to the brain can also lead to fainting, including standing for long periods of time, dehydration, coughing, urination and straining to have a bowel movement. Other causes of fainting include fear, emotional distress and hyperventilation (all symptoms potentially associated with mental disorders such as panic attacks and specific phobias), and severe pain.

Some people faint when their head is turned to the side. This may indicate that bones in the neck are pinching one of the blood vessels leading to the brain. A drop in blood sugar can cause fainting in both people with diabetes and in people who have not eaten for a long period of time (such as those with certain eating disorders). Use of both prescription drugs (including antipsychotics and antidepressants) and recreational drugs such as alcohol, marijuana and cocaine can cause fainting.

More serious causes of fainting include seizures and heart problems (e.g., arrhythmias, or irregular heartbeats, which may result from a number of conditions, including alcoholism [alcoholic cardiomyopathy]) or abnormalities in the blood vessels leading to the brain.

Several tests may be performed to reveal the cause of fainting including:

• Electrocardiograph (ECG). Machine that records the electrical impulses of the heart.
  
  
• Holter monitor. Machine that creates a record of the heart rhythm over 24 hours.
  
  
• Chest x-ray. A form of electromagnetic radiation that creates an image of the chest, lungs, heart, large arteries, ribs and diaphragm.
  
  
• Echocardiograph. Machine that creates an ultrasound image (produced by sound waves) of the heart.
  
  
• Electroencephalograph (EEG). Machine that creates a graphic record of brainwaves.
  
  
• Blood analysis. In some cases anemia (low red blood cell count) may make people feel lightheaded, or may cause fainting.

After ruling out the possible major physiological causes of fainting, a tilt-table test can also be performed to help determine neurological causes of a patient’s fainting. During this test, the patient lies flat on a table while blood pressure and heart rate are monitored. The table is tilted so the upper part of the body is raised in a fashion similar to going from a lying position to a standing position. The change in angle places stress on the autonomic nervous system (ANS), which regulates heart rate and blood pressure. The physician monitors the body’s reaction to this change, which can reveal important information about potential causes of fainting.

It should be noted that in some cases, the cause of fainting cannot be determined.