Fainting (syncope) is a brief, sudden loss of consciousness, typically followed by a complete recovery. Fainting occurs when the brain is starved for oxygen because of temporary inadequate blood flow. It occurs most often when a person has low blood pressure and the heart cannot pump a normal supply of oxygen to the brain.

Fainting may be a symptom of an underlying disorder, such as a heart condition or stroke. In about a third of cases, however, episodes of syncope may prove impossible to explain. Episodes of fainting are more common with age, perhaps due to an increased prevalence of certain diseases or an inability to respond to stressors that would not have caused fainting at a younger age.

A person with diabetes may faint as a result of hypoglycemia (low blood glucose) or autonomic neuropathy. Low glucose (blood sugar) first affects the autonomic nervous system. As hypoglycemia goes untreated, the brain begins to suffer from this lack because the brain needs glucose for energy. Low blood glucose can also impair regulation of blood pressure, which can lead to low blood pressure.

Fainting may also be due to hyperglycemic emergencies that can result in diabetic coma, such as diabetic ketoacidosis or hyperosmolar hyperglycemic nonketotic syndrome (HHNS).

Because of its link to glucose irregularities and cardiovascular problems, it is important for anyone experiencing fainting to report the episode to a physician as soon as possible.