A greyout is a temporary visual disturbance characterized by a dimming of sight. This phenomenon occurs when the body struggles to maintain adequate blood flow to the head, often due to rapid changes in internal body dynamics, such as intense acceleration or sudden shifts in posture. The visual symptoms serve as an important warning sign that the brain is experiencing a temporary reduction in oxygen supply. Understanding the science behind a greyout involves examining the cascade of events initiated by a sudden drop in blood pressure at head level.
Defining the Visual and Cognitive Experience
The subjective experience of a greyout is defined by a distinct loss of visual fidelity, most notably the perceived dimming of the surroundings. The world often appears to lose its color, a temporary state known as achromatopsia, as the retina’s function begins to falter. This visual disturbance is typically accompanied by the sensation of “tunnel vision,” where peripheral sight constricts inward, leaving only a small, central field of view.
As the condition progresses, light perception decreases, and the central visual field continues to shrink. Despite this profound visual impairment, a person experiencing a greyout usually maintains full consciousness and motor control, which separates it from a complete blackout. The temporary lack of oxygen can also lead to subtle cognitive clouding, manifesting as mild disorientation or dizziness.
The symptoms are generally rapid in onset and equally fast in resolution, often clearing completely within seconds once the precipitating force or posture change is reversed. This rapid recovery is a direct result of blood flow being quickly restored to the eyes and brain. The symptoms act as a clear physiological indicator that the body is nearing the threshold of a more serious event, such as a complete loss of consciousness.
The Physiological Mechanism of Retinal Ischemia
The underlying cause of a greyout is retinal ischemia, a transient reduction of blood flow to the retina. The retina, the light-sensing tissue at the back of the eye, is among the most metabolically active tissues in the body and is highly sensitive to oxygen deprivation. Because the retina’s needs are so high, visual symptoms begin before the brain’s main structures suffer sufficient oxygen loss to cause unconsciousness.
In situations of high positive acceleration directed from head to foot, known as +Gz, the body’s fluid dynamics are dramatically altered. The increased force pulls blood volume toward the lower extremities, directly reducing the arterial pressure in the vessels supplying the head and eyes.
For vision to function, the blood pressure supplying the retina must exceed the intraocular pressure (the pressure inside the eyeball). When the head-level arterial pressure drops significantly, the difference between the supply pressure and the intraocular pressure shrinks. This leads to inadequate perfusion, starving the retinal cells of oxygen and triggering the visual symptoms. If the pressure continues to drop, the oxygen-sensitive retina ceases to function, resulting in the greyout. The brain’s main structures are the next in line to fail, which then leads to a blackout.
Primary Situational Triggers
Greyouts are most frequently associated with environments that induce high levels of positive acceleration. The primary context is high-performance aviation, where pilots executing tight turns or abrupt pull-up maneuvers experience significant positive G-forces (+Gz). These forces displace the blood column downwards, initiating the physiological mechanism.
Beyond aerospace, the phenomenon can be experienced on high-speed amusement park rides, such as roller coasters with deep vertical loops that subject riders to high G-loads. The rapid onset of the force overwhelms the body’s natural compensatory mechanisms, causing the temporary visual loss.
A more common trigger is orthostatic hypotension, a rapid drop in blood pressure upon standing up quickly. This change in posture uses gravity to pull blood to the lower body, leading to momentary cerebral hypoperfusion that can cause a fleeting greyout, particularly in individuals who are dehydrated. Other triggers include shock, certain medications, or intense physical exertion followed by a sudden stop.
Differentiating Greyout from Related Conditions
A greyout exists on a spectrum of visual disturbances caused by altered blood flow to the head, alongside blackout and redout. A blackout represents the progression of a greyout, marked by a complete loss of vision while the person remains conscious. It occurs when the reduction in blood flow to the retina is complete, but the brain still receives just enough oxygen to maintain awareness and control.
If the pressure drop continues past the blackout stage, it results in G-force induced Loss of Consciousness (G-LOC). In G-LOC, the brain is fully deprived of sufficient oxygen and the individual passes out entirely.
Unlike a greyout, which is caused by positive G-forces (+Gz) pushing blood away from the head, a redout is the inverse. A redout is caused by negative G-forces (-Gz), which push blood toward the head, leading to excessive blood pooling and congestion.
The result of a redout is a reddish tint to the vision, a phenomenon known as hyperemia, possibly caused by the engorged lower eyelid entering the visual field. Redouts are less common than greyouts because pilots rarely sustain high negative G-forces due to the lower human tolerance for blood pressure increases in the head, which can lead to serious conditions like retinal damage or hemorrhagic stroke.