G-force acceleration and loss of consciousness (G-LOC)

A 28-year-old pilot trains aerobatics in a two-seat civil aircraft, with an instructor in the back seat. He performs a maneuver that begins with a forward push of the stick, which generates negative acceleration, and right after that he pulls hard to make a tight turn. His vision goes gray and narrow. The instructor notices that the pilot has let go of the controls, that his head has fallen to the side and that his arms had rapid jerks for a few seconds. The instructor takes over the aircraft. The pilot wakes up confused, not knowing where he is for a moment, and regains clarity in about a minute. They land without problems. The next day, the pilot seeks care, and the first doctor asks whether it was a seizure. The scene is fictional and was written for teaching.

This text uses the scene as a setting for clinical reasoning: what acceleration-induced loss of consciousness is, known by the acronym G-LOC, why it causes jerks that resemble an epileptic seizure, how to separate syncope from seizure, which tests help and how it is prevented. The text does not replace medical assessment or the rules of each country's aviation authority. There are no medication doses here, because the prevention and management of G-LOC are not pharmacological. If you play the case of the day on NEXO, you will recognize the challenge: a very striking sign, such as the jerks, can lead to a wrong diagnosis when the context is left aside.

Key points

• G-LOC is the loss of consciousness caused by the sudden drop in blood flow to the brain during positive acceleration along the head-to-foot axis, called +Gz.
• Gray vision and narrowing of the visual field usually come before loss of consciousness.
• Myoclonic jerks are common in syncope and are not enough to diagnose an epileptic seizure.
• A negative acceleration maneuver right before the pull reduces tolerance to +Gz, in what is called the push-pull effect.
• The prevention described in the sources includes training, muscle contraction, hydration, conditioning and planning.

Why G-LOC is a reasoning setting

Burton defined G-LOC as a state of altered perception in which awareness of reality is absent because of a sudden and critical reduction in cerebral circulation caused by increased G force (Burton, 1988). The phenomenon was identified in Britain in World War I as "fainting in the air" and, in the United States, in 1922, in an air race. The ICD-10 from the World Health Organization includes abnormal G forces under code T75.8, among the other specified effects of external causes.

The setting is useful because it brings together a rare context, aerial aerobatics, with a doubt common in any emergency department: did the person who lost consciousness and had jerks have syncope or an epileptic seizure? Copeland and Whinnery recall that G-LOC, altered consciousness and visual effects play a role in aerobatic and agricultural aviation accidents, and not only in high-performance aircraft (Copeland and Whinnery, 2023).

What the scene teaches

The scene has five clues. The first is the maneuver: negative acceleration followed by a hard pull. The second is the visual symptom that precedes loss of consciousness. The third is the short loss of consciousness. The fourth is the brief jerks of the arms. The fifth is the rapid recovery, with confusion lasting about a minute, as in the G-LOC described in a centrifuge. The post about signs, symptoms and history shows how the sequence of events guides the diagnosis.

Pathophysiology: the blood that does not reach the brain

The FAA booklet on acceleration describes the +Gz force as the one acting from head to feet, felt when the pilot pulls out of a dive or enters an inside loop. Faced with it, the heart beats harder and faster and vessel tone increases, to keep blood rising to the head. If this response of the heart and vessels does not keep up with a force that arrives quickly, the pilot's performance drops, possibly to the point of loss of consciousness and of the ability to fly (FAA, 2021).

The same booklet separates three axes. The Gx force acts from chest to back, as in the takeoff run, and from back to chest, as in landing. The Gy force acts from one shoulder to the other, as in rolls. The Gz force acts on the vertical axis of the body, and it is there that the greatest risk for the pilot lies (FAA, 2021).

Why vision fails first

According to the FAA, the eyes are very sensitive to low blood flow. When blood pressure in the eye falls below what the retina needs, the pilot may notice loss of peripheral vision, tunnel vision, then an even smaller field, gun-barrel vision, and then greyout and blackout. If the force continues, the result can be G-LOC (FAA, 2021).

The rate at which the force sets in changes the picture. When the force rises slowly, about 0.1 G per second, visual symptoms come before G-LOC. When it rises quickly, 1 G per second or more, G-LOC can happen without visual warning (FAA, 2021). In negative force, -Gz, the FAA describes loss of vision through redout, and loss of consciousness follows soon if the pilot does not ease the control.

A model of cerebral flow

Copeland and Whinnery developed a computer model based on cerebral blood flow that takes into account sex, cardiovascular conditioning, anti-G suits, pressure breathing and muscle contraction, and that considers fatigue, medications and dehydration as risk factors (Copeland and Whinnery, 2023).

The push-pull effect

The maneuver in the scene is the most important clue. When the pilot pushes the stick, he generates negative force. The FAA describes that, in -Gz, blood has trouble descending through the jugular veins to the heart, while arterial flow to the head increases (FAA, 2021). Right after comes the pull, with positive force.

Xu and colleagues simulated this sequence in a centrifuge, with 5 seconds at -1 Gz followed by 10 seconds at +Gz, in 20 fighter pilots and 6 volunteers. After 5 seconds at -1 Gz, tolerance to +Gz fell in a statistically significant way in all participants, both pilots and volunteers (Xu and colleagues, 2012). The authors recall that the push-pull effect has already been pointed out as a factor in fatal accidents.

What this changes in the case

In the pilot's case, the sequence of the maneuver explains why the loss of consciousness happened at an acceleration he might have tolerated in another situation. The post on pretest probability shows how this kind of context greatly increases the probability of a hypothesis before any examination.

The G-LOC episode step by step

Whinnery proposed measuring G-LOC by video recording, with attention to the time of exposure to acceleration, the period of incapacitation and myoclonic convulsions (Whinnery, 1989). The literature usually divides the episode into absolute incapacitation, when the person is unconscious, and relative incapacitation, when they have woken up but are still confused and without normal performance.

Recovery takes longer than it seems

Wilson and colleagues recorded the EEG and cerebral oxygenation in G-LOC episodes in a centrifuge. Brain oxygen returned to the previous level in about 15 seconds, but the EEG and performance on tasks took about 60 seconds to recover (Wilson and colleagues, 2005). The authors also described large-amplitude slow waves before the loss and before the return of consciousness.

This interval explains the scene. The pilot woke up, but for about a minute he was confused. In a single-pilot aircraft, this period of incapacitation can be as dangerous as the loss of consciousness.

Hallucinations and the witness

In syncope induced in volunteers, 60 out of every 100 participants reported visual or auditory hallucinations (Lempert et al., 1994). Whinnery argues for video recording precisely because only it accurately describes the exposure, the incapacitation and the myoclonic convulsions (Whinnery, 1989). In the scene, the instructor plays this witness role.

Syncope or epileptic seizure: the differential diagnosis

The jerks are what most often lead to error. Lempert et al. induced syncope in healthy volunteers and recorded the episodes on video. They provoked syncope in 56 of 59 volunteers with a sequence of hyperventilation, standing and the Valsalva maneuver, filmed by two cameras. Among 42 complete syncopes, which lasted on average about 12 seconds, 38 had myoclonic jerks, almost always in several parts of the body and without rhythm. Head movements, oral automatisms and movements to sit up appeared in 79 out of every 100, the eyes stayed open in most and the initial upward deviation of gaze was common. The 13 incomplete syncopes were shorter and generally without jerks (Lempert et al., 1994).

In other words, jerks are the rule, not the exception, in a complete syncope. Sheldon et al. applied a 118-item questionnaire to 671 people with loss of consciousness. Among the 539 with a defined cause, 102 had epilepsy and 437 had syncope. A score built only from the symptoms around the episode correctly classified 94 out of every 100 patients, with sensitivity and specificity of 94 out of every 100 for epileptic seizure, and adding the number of episodes did not improve the result (Sheldon et al., 2002). What separates syncope from seizure is the whole of the history.

Source clue	What the source describes	Source
Force rises slowly, about 0.1 G per second	Visual symptoms before G-LOC	FAA, 2021
Force rises fast, 1 G per second or more	G-LOC without visual warning	FAA, 2021
Negative force, -Gz	Red out and loss of consciousness if the command is not relieved	FAA, 2021
-1 Gz for 5 seconds before the pull	Drop in tolerance to +Gz in all participants	Xu, 2012
Induced complete syncope	Duration of about 12 seconds, jerks in 38 of 42	Lempert, 1994
Eyes during syncope	Open in most, initial upward deviation common	Lempert, 1994
Symptoms around the episode	Score is right 94 out of every 100 between syncope and seizure	Sheldon, 2002
Recovery in G-LOC	Cerebral oxygen in about 15 seconds, EEG and performance in about 60	Wilson, 2005

What the table does not resolve

The table gathers only what the sources read measure. It does not bring the individual items of the Sheldon score, which do not appear in the study abstract, and so it does not list isolated signs in favor of epileptic seizure. The post on differential diagnosis shows how to weigh clues together.

Assessment after the episode

The sources read for this text do not define a list of tests after a G-LOC. What they show is the value of the history and of recording the episode.

The witness account

Sheldon et al. showed that the symptoms around loss of consciousness separate syncope from seizure in the great majority of cases (Sheldon et al., 2002). Lempert et al. and Whinnery used video to describe the episodes accurately (Lempert et al., 1994; Whinnery, 1989). It is worth recording the maneuver, the duration, the movements, the eyes and the time until the person returns to talking normally. The post on clinical decision-making helps choose tests by what they change in management.

The pilot in the scene

For the pilot in the scene, with a well-described push-pull maneuver, visual symptoms before the episode, short jerks and recovery of about one minute, the history matches what the sources describe for G-LOC. The choice of additional tests is up to the physician assessing the case.

Management and return to flight

The management described in the sources does not involve medications. The FAA leaflet notes that, if the aircraft has enough altitude, the pilot can relieve the G force and recover vision or consciousness (FAA, 2021).

Return to flight

The sources reviewed do not define a time frame or tests for returning to flight after a G-LOC. That decision follows the rules of each country's civil aviation authority and the assessment of the medical examiner.

Correcting the risk factors

The FAA states that G-force tolerance drops with dehydration, fatigue, hunger and medications, and that lack of conditioning, a sedentary life and smoking also increase susceptibility. It also notes that each pilot's tolerance can vary from one day to the next (FAA, 2021). The Copeland and Whinnery model includes fatigue, medications and dehydration as risk factors.

Hydration and conditioning

According to the FAA, a rested, hydrated and fit pilot withstands higher G forces. Hydration increases the volume of circulating blood. A program that combines aerobic exercise with strength training increases G-force resistance, and aerobic training alone can lower the heart rate so much that it does not speed up enough to compensate. The leaflet recommends that any exercise program be approved by a doctor before starting (FAA, 2021).

Prevention: training, muscle contraction and planning

G-LOC prevention has three fronts. The first is training: knowing the visual symptoms that come before loss of consciousness and easing the pull when they appear. The second is the muscle contraction maneuver against G force. The third is planning the maneuvers, to avoid push-pull sequences and very rapid accelerations without preparation.

The contraction maneuver

The FAA describes the anti-G maneuver as pushing air from the lungs against a closed glottis while contracting the muscles of the calves, thighs and shoulders. The maneuver increases G-force tolerance, but, to avoid injury, it should not be attempted at 1 G with maximum effort (FAA, 2021). Military pilots do a warm-up before flying high-performance aircraft to assess how they will tolerate G force that day. Behm and colleagues, in a scoping review, note that recovery breathing between cycles can lower blood pressure and that contraction strength can drop with fatigue, and they discuss forms of training to improve tolerance (Behm and colleagues, 2026).

For people without aeronautical training, a simpler approach also helped. Pollock and colleagues tested, in a centrifuge that simulated suborbital flights, early contraction of the legs and abdomen, without the breathing part. It was well tolerated, prevented greyout in all participants on the simulated launch and in 54 out of every 100 on reentry, and delayed the onset of symptoms when they appeared (Pollock and colleagues, 2024).

What operational experience shows

Lyons and colleagues analyzed two decades of G-LOC records and showed that G-LOC accidents decreased after training programs were introduced, while a technical pressure-breathing resource did not reduce those accidents (Lyons and colleagues, 2004). The finding suggests that training and pilot behavior carry great weight in prevention. The FAA suggests that pilots include G force in flight planning, alongside time, fuel and distance (FAA, 2021).

Who is exposed: aerobatics, agricultural flying and spaceflights

G-LOC is often associated with high-performance aircraft, but exposure to acceleration goes beyond them. Aerobatic, competition and agricultural pilots make tight turns and repeated pulls, sometimes close to the ground, where there is little time to regain control. Copeland and Whinnery cite precisely aerobatic and agricultural aviation among the areas where acceleration effects appear in accidents (Copeland and Whinnery, 2023).

Suborbital flights with passengers

A new group is passengers on commercial suborbital flights. Pollock and colleagues note that, in centrifuge simulations of these flights, launch and reentry often cause visual symptoms and that G-LOC can happen. The authors suggest that the effects may be more intense on reentry after a period without gravity, and the study participants were 34 to 82 years old (Pollock and colleagues, 2024). For these people, without aeronautical training, a simple muscle contraction maneuver is more likely to be used than the classic maneuver with forced breathing.

What changes in care

For the physician seeing patients outside an aerospace medicine center, the key is to ask about context. A patient who lost consciousness on a roller coaster, in a training centrifuge or during an aerobatic flight may have had the same mechanism as the pilot in the scene. The question about the activity at the time of the episode is simple and changes the reasoning.

Who can tolerate less

The FAA lists medications, dehydration, fatigue, hunger, a sedentary lifestyle and smoking among the factors that reduce tolerance, and Copeland and Whinnery include sex and cardiovascular conditioning in the model. For these people, the conversation before the activity has value. The post on clinical decision-making reminds us that the best approach is sometimes preventive.

Reasoning: the sign that draws too much attention

The scene shows a predictable error: seeing jerking movements and thinking of an epileptic seizure. Croskerry described how the first striking finding can anchor reasoning and make the physician ignore the rest of the history (Croskerry, 2003). Here, the question that changes everything is: what was he doing one second before?

The post on examples of heuristics discusses useful shortcuts that sometimes mislead. The one on calibration of confidence shows how to adjust certainty to what is known, and the one on the diagnostic pause proposes a final question before closing the case: is there anything in this history that does not fit?

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before each guess, ask what the person was doing when the symptom began. When a sign draws a lot of attention, list the diseases in which it also appears. After the result, note which clue you underestimated.

Three exercises with cases

Reconstruct the sequence. Write down what happened before, during and after the episode.

Question the striking sign. Ask whether the most eye-catching finding also appears in other conditions.

Use the witness. Look in the case for the information that only someone who saw the episode could give.

The post on learning based on clinical cases shows how cases help fix these clues.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is G-LOC caused by the drop in blood flow to the brain?

Yes. +Gz acceleration pushes blood to the legs and reduces the pressure reaching the brain.

Does vision usually fail before consciousness?

Yes. Grayout and narrowing of the visual field usually come first, except in very rapid accelerations.

Can jerking movements happen in syncope?

Yes. In a video study, most complete induced syncopes had myoclonic jerking.

Does the push-pull effect reduce acceleration tolerance?

Yes. A negative acceleration just before the pull reduces tolerance to +Gz.

Does full recovery take longer than the return of consciousness?

Yes. In a centrifuge study, the EEG and performance took about a minute to recover.

Does contracting the legs and abdomen help prevent the symptoms?

Yes. It delayed or prevented grayout in centrifuge studies.

Final checklist

• I asked what the person was doing before the episode.
• I identified the maneuver and the sequence of accelerations.
• I asked about visual symptoms before the loss of consciousness.
• I heard the witness about the duration and the movements.
• I compared the recovery with that of an epileptic seizure.
• I recorded the witness account right after the episode.
• I looked for risk factors such as dehydration, fatigue, hunger and medications.
• I asked about the rate of onset of the G force.
• I remembered that the return to flying follows the aviation authority and the medical examiner.
• We discussed training, muscle contraction and planning of the maneuvers.

Want to practice reasoning with progressive clues? Play the case of the day.

References

1. Behm DG, Lawson ED, Martin BCW. A century of anti-G straining maneuvers. Are there further changes in AGSM physical training that can improve +Gz tolerance? A scoping review. Front Physiol. 2026. PMID 42519539.
2. Burton RR. G-induced loss of consciousness: definition, history, current status. Aviat Space Environ Med. 1988. PMID 3281645.
3. Copeland K, Whinnery JE. Cerebral Blood Flow Based Computer Modeling of G(z)-Induced Effects. Aerosp Med Hum Perform. 2023. PMID 37069756.
4. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
5. Federal Aviation Administration. Acceleration in Aviation: G-Force. Brochure OK-21-0375, Civil Aerospace Medical Institute, 2021. Document.
6. Lempert T, Bauer M, Schmidt D. Syncope: a videometric analysis of 56 episodes of transient cerebral hypoxia. Ann Neurol. 1994. PMID 8053660.
7. Lyons TJ, Davenport C, Copley GB, et al. Preventing G-induced loss of consciousness: 20 years of operational experience. Aviat Space Environ Med. 2004. PMID 14960050.
8. World Health Organization. International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD-10), 2019 version. Chapter XIX, T75. Document.
9. Pollock RD, Britton JK, Green NDC, et al. Prevention of G-Induced Effects on Vision and Consciousness During Simulated Suborbital Spaceflight. Aerosp Med Hum Perform. 2024. PMID 39711414.
10. Sheldon R, Rose S, Ritchie D, et al. Historical criteria that distinguish syncope from seizures. J Am Coll Cardiol. 2002. PMID 12103268.
11. Whinnery JE. Methods for describing and quantifying +Gz-induced loss of consciousness. Aviat Space Environ Med. 1989. PMID 2673203.
12. Wilson GF, Reis GA, Tripp LD. EEG correlates of G-induced loss of consciousness. Aviat Space Environ Med. 2005. PMID 15672982.
13. Xu Y, Li BH, Zhang LH, et al. A centrifuge simulated push-pull manoeuvre with subsequent reduced +Gz tolerance. Eur J Appl Physiol. 2012. PMID 22083535.