A greyout is a transient loss of vision characterized by a perceived dimming of light and color, sometimes accompanied by a loss of peripheral 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 Blood pressure (BP) is the pressure of Circulatory system, circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term ...

. Greyouts have a variety of possible causes: * Shock, such as

hypovolemia Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the loss ...

, even in mild form such as when drawing blood. * Standing up suddenly (see orthostatic hypotension), especially if sick, hungover, or experiencing low blood pressure. *

Fatigue Fatigue is a state of tiredness (which is not sleepiness), exhaustion or loss of energy. It is a signs and symptoms, symptom of any of various diseases; it is not a disease in itself. Fatigue (in the medical sense) is sometimes associated wit ...

Hyperventilation Hyperventilation is irregular breathing that occurs when the rate or tidal volume of breathing eliminates more carbon dioxide than the body can produce. This leads to hypocapnia, a reduced concentration of carbon dioxide dissolved in the blo ...

, paradoxically: self-induced hypocapnia, such as in the fainting game or in shallow water blackout. * Overexertion * Severe episodes of coughing or sneezing *

Panic attack Panic attacks are sudden periods of intense fear and Comfort, discomfort that may include palpitations, otherwise defined as a Tachycardia, rapid, Arrhythmia, irregular Heart rate, heartbeat, Hyperhidrosis, sweating, chest pain or discomfort, s ...

Possible symptoms include: * blurred vision * tunnel vision *

dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to Balance disorder, disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a ...

* loss of balance * lightheadedness * sudden feeling of exhaustion * breathing accelerates into short quick breaths *

respiratory acidosis The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gr ...

(“hot breath”) *

heart rate Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...

spikes rapidly *

drops *

oxygen saturation Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is Dissolution (chemistry), dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the g ...

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

g-force The g-force or gravitational force equivalent is a Specific force, mass-specific force (force per unit mass), expressed in Unit of measurement, units of standard gravity (symbol ''g'' or ''g''0, not to be confused with "g", the symbol for ...

s 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. This is the reverse of a

redout A redout occurs when the body experiences a negative g-force sufficient to cause a blood flow from the lower parts of the body to the head. It is the inverse effect of a greyout, where blood flows away from the head to the lower parts of the bo ...

, 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

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

l damage and hemorrhagic

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

. Pilots of high performance aircraft can increase their resistance to greyouts by using a

g-suit A g-suit, or anti-''g'' suit, is a flight suit worn by aviators and astronauts who are subject to high levels of acceleration force ( g). It is designed to prevent a black-out and g-LOC (g-induced loss of consciousness) caused by the blood poo ...

, 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 loss of consciousness are separate events. Another common occurrence of greyouts is in roller coaster riders. Many roller coasters put riders through positive g-forces, particularly in

vertical loop The generic roller coaster vertical loop, also known as a Loop-the-loop, or a Loop-de-loop, where a section of track causes the riders to complete a 360 degree turn, is the most basic of roller coaster inversions. At the top of the loop, rid ...

s and

helices A helix (; ) is a shape like a cylindrical coil spring or the thread of a machine screw. It is a type of smoothness (mathematics), smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as ...

. Roller coasters are unlikely to have high enough negative g-forces to induce redouts, as most low-g elements are designed to simulate weightlessness.

Notes

{{DEFAULTSORT:Greyout (Medical) Visual system Human eye Acceleration

See also

Notes