Greyout

 ( Visual System ) 

A greyout is a transient loss of vision characterized by a perceived dimming of light and color, sometimes accompanied by a tunnel vision. It is a precursor to fainting or a blackout and is caused by hypoxia (low brain oxygen level), often due to a loss of blood pressure.

Greyouts have a variety of possible causes:

• Shock, such as hypovolemia, even in mild form such as when drawing blood.
• Standing up suddenly (see orthostatic hypotension), especially if sick, hangover, or experiencing hypotension.
• Fatigue
• Hyperventilation, paradoxically: self-induced hypocapnia, such as in the fainting game or in shallow water blackout.
• Overexertion
• Severe episodes of coughing or sneezing
• Panic attack

Possible symptoms include:

• blurred vision
• tunnel vision
• dizziness
• loss of balance
• lightheadedness
• sudden feeling of exhaustion
• breathing accelerates into short quick breaths
• respiratory acidosis (“hot breath”)
• pulse spikes rapidly
• blood pressure drops
• oxygen saturation plummets

Recovery is usually rapid. A greyout can be readily reversed by lying down as the cardiovascular system does not need to work against gravity for blood to reach the brain.

A greyout may be experienced by aircraft pilots pulling high positive as when pulling up into a loop or a tight turn, which forces blood to the lower extremities of the body and lowers blood pressure in the brain.Braithwaite MG, Durnford SJ, Crowley JS, Rosado NR, Albano JP. "Spatial disorientation in U.S. Army rotary-wing operations." Aviation, Space, and Environmental Medicine 69(11):1031-7 (November 1998). This is the reverse of a redout, or a reddening of the vision, which is the result of negative g-forces caused by performing an outside loop, that is by pushing the nose of the aircraft down. Redouts are potentially dangerous and can cause damage and hemorrhagic stroke. Pilots of high performance aircraft can increase their resistance to greyouts by using a g-suit, which controls the pooling of blood in the lower limbs, but there is no suit yet capable of controlling a redout. In both cases, symptoms may be remedied immediately by easing pressure on the flight controls. Continued or heavy g-force will rapidly progress to g-LOC (g-force induced Loss of Consciousness). Untrained individuals can withstand approximately 4g, while fighter pilots with g-suits are trained to perform 9g maneuvers.

Surprisingly, even during a heavy greyout, where the visual system is severely impaired, pilots can still hear, feel, and speak. Complete greyout and g-LOC are separate events.

Another common occurrence of greyouts is in roller coaster riders. Many roller coasters put riders through positive g-forces, particularly in and helix. Roller coasters are unlikely to have high enough negative g-forces to induce redouts, as most low-g elements are designed to simulate weightlessness.

• Blackout (disambiguation)
• Brownout (disambiguation)
• Whiteout (disambiguation)