A greyout occurs when a person is on the verge of fainting, or syncope. Vision fades and light and color are reduced to a grey, washed-out blur. There will often be a loss of peripheral vision in a sort of tunnel effect which is caused by a drop in blood pressure to the brain, such as when suddenly standing from a reclined position or possibly a serious medical condition. The effect of this event, also called presyncope, is temporary, and lying down will usually cure it.
Syncope has numerous causes, but the primary cause is decreased blood flow to the brain. The problem has generally a short duration because when a person who has fainted falls or reclines, the blood no longer needs to push against gravity to reach the brain and is restored. Pale skin, hyperventilation, and nausea may accompany the greyout, followed by a profound weakness of the limbs. The legs fold and the person collapses unless they are able to get to a chair or lie flat.
Fighter pilots may experience a greyout if they are subject to high-speed maneuvers that exert positive g-forces on their bodies. The pressure forces the blood to the lower extremities and away from the brain, causing presyncope symptoms. Wearing a G-suit to control the flow of blood to the lower body can prevent it. People who ride big roller coasters have suffered the same effects on the tight turns and sharp loops the rides sometimes take.
While fainting is not usually serious, certain cardiac conditions can predispose people to sudden blackouts. Cardiac syncope can be a sign that a life-threatening event is occurring, so all episodes of unexpected syncope should be taken seriously. Other people suffer from situational syncope, where fainting is triggered only in certain circumstances. Coughing and defecating can cause a drop in cranial blood pressure, and in elderly people postprandial syncope can sometimes happen after eating. Situational syncopes often are accompanied by presyncope symptoms such as greyout, dizziness, and tunnel vision.
First aid for presyncope symptoms recommends that the person sit down and put the head between the legs or lie on the floor with legs elevated. Blood will then be directed back toward the brain, relieving the symptoms and possibly preventing loss of consciousness. A visit to the doctor is necessary if a person has repeated fainting episodes or greyouts to rule out any serious medical conditions.