Blackout Vision and Redout Vision

Blackout vision, or simply "blackout," refers to the temporary darkening or loss of vision that can occur during flight when a pilot abruptly pulls up the aircraft, increasing vertical acceleration in contrast to Earth's gravitational acceleration. To experience blackout vision, the body must be subjected to acceleration forces between 4 g and 7 g, meaning four to seven times the gravitational acceleration on Earth's surface. Therefore, it occurs in high-performance aircraft, such as fighter jets or aerobatic planes.

The opposite effect is redout vision, erythropsia, or "redout": the appearance of a reddish tint in vision due to the application of negative g-forces, meaning acceleration in a sudden downward maneuver. Exposure to these accelerations is much less common, and a person's tolerance level is significantly lower than for positive accelerations (symptoms appear with accelerations between -2 g and -3 g).

Symptoms

The symptoms of blackout vision can develop within seconds, depending on the magnitude of the g-forces and the pilot's tolerance to them. The symptoms include:

• Dizziness and disorientation.
• A sensation of pressure on the body due to g-forces.
• Difficulty breathing due to chest pressure.
• Loss of vision, usually preceded by blurry vision, reduced visual field, and grayish vision.

On the other hand, redout vision presents the following symptoms:

• Intense and sometimes persistent headache.
• Facial congestion.
• Palpebral edema: swelling of the eye due to abnormal fluid accumulation in the inner eyelid tissues.
• Red-tinted vision.
• Tearing.
• Conjunctival or nasal hemorrhages.
• Difficulty breathing.
• Petechiae on the skin: small reddish spots on the face and neck.

Causes

Blackout vision is caused by orthostatic hypotension, meaning a drop in blood pressure due to a sudden and violent change in posture. In aviation, when a pilot rapidly ascends, the principle of inertia causes blood flow to remain at rest instead of moving upward to follow the body's vertical motion. This causes blood to pool in the lower extremities, reducing blood supply to the head. If blood pressure drops below 20 mmHg, which is the internal pressure of the eye, blood flow to the central retinal artery gradually stops, leading to blurry vision, concentric reduction of visual fields (tunnel vision), and ultimately, blackout vision.

During a sudden descent, the force of inertia shifts blood flow toward the head, neck, and upper torso, decreasing blood pressure in the abdomen and upper extremities. The sudden increase in blood volume and pressure in the head leads to hyperperfusion of the central retinal artery. Redout vision occurs because the lower eyelid, engorged with blood, intrudes into the visual field due to negative g-forces.

Risk Factors

Factors that increase a pilot's risk of experiencing orthostatic hypotension or erythropsia include:

• Exposure to accelerations above 4 g or negative accelerations.
• Age: Younger pilots have higher tolerance to g-forces.
• Dehydration.
• Poor physical condition.
• Fatigue.
• Absence or incorrect use of protective suits.
• Lack of specific training.
• Underlying medical conditions: cardiovascular or neurological disorders such as hypertension or fragile blood vessels.
• Caffeine and other stimulant consumption.
• Alcohol, drug use, and certain medications.

Complications

If high upward accelerations are sustained, the drop in blood pressure cuts off blood flow to the brain, leading to loss of consciousness (orthostatic syncope), which can last several seconds until the pilot is able to respond to visual and auditory stimuli. This poses a significant flight risk because, during unconsciousness and the pilot's recovery process as g-forces decrease, the aircraft may descend and crash.

In the case of redout vision, loss of consciousness is uncommon, but excessive intracranial pressure may lead to permanent retinal damage or a hemorrhagic stroke.

Prevention

To prevent blackout vision, pilots require specific techniques and training to withstand g-forces:

• Intensive physical training.
• Breathing and muscle contraction techniques to control blood pressure reduction.
• High-fidelity simulator training. Gravitational acceleration tolerance is trained in centrifuge chambers.
• Anti-gravity or anti-G suits: specially designed suits incorporating inflatable chambers that inflate with increasing vertical acceleration, compressing the pilot's legs and abdomen to prevent blood from shifting downward and ensuring proper cerebral circulation.

Regarding redout vision, there are no specific techniques to mitigate its symptoms, and, in fact, the anti-G suits, breathing techniques, or muscle contraction techniques used against blackout vision may be highly detrimental under negative g-forces. Therefore, such maneuvers are avoided as much as possible. However, pilots can train in simulators to improve their tolerance to these forces and determine their individual limit.

Which Doctor Treats Blackout and Redout Vision?

These conditions are studied in the aeronautical medicine unit.