Concussion

01What the measurements show

The Numbers Behind Concussion in Sport

Eight findings that describe the injury, the tool that screens for it, and the clock it runs on.

02The injury defined

Concussion Is a Traumatic Brain Injury, and the Consensus Says So in Its First Clause

A concussion is a traumatic brain injury, and the international consensus definition opens with exactly that phrase. The Amsterdam statement came out of the sixth International Conference on Concussion in Sport and was published in 2023 Patricios 2023. It defines a sport-related concussion as a traumatic brain injury caused by a direct blow to the head, neck or body.

Read the second half of that sentence, because it carries the athletic meaning. The blow transmits an impulsive force to the brain. Nothing in the definition requires the contact to land on the skull. A hit taken through the shoulder, the chest or the hip can whip the head and produce the same injury.

Nor does the definition require a blackout. Across 8,905 high school and collegiate athletes followed prospectively in seven sports, 9 percent of 375 concussions involved loss of consciousness and 30 percent involved amnesia Marshall 2015. An athlete who never went out is the ordinary case, not the mild one.

Why the scan reads normal

The consensus definition states that no abnormality is seen in standard structural neuroimaging such as computed tomography or T1 and T2 weighted magnetic resonance imaging Patricios 2023. That is not a loophole in the diagnosis. It is part of the diagnosis, written into the sentence that defines the injury.

A CT scanner maps density, which is why it finds blood and fractured bone so quickly. An MRI scanner maps water in tissue. Both photograph what the brain is made of. Neither photographs what the brain is doing. The brain is hurt at the level of the signal.

The definition names what is actually happening instead: a neurotransmitter and metabolic cascade, with possible axonal injury, changes in blood flow and inflammation. In plainer terms, biomechanical force sets off disordered neuronal firing and a metabolic energy crisis inside the cell. Networks that normally keep time with one another stop doing so.

A concussion is not a bruise to walk off. It is the brain knocked out of time with itself, and recognizing that the damage is invisible to a scanner is what separates competent concussion care from guesswork. Concussion and the Nervous System carries the cascade in full, from the ionic flux through the energy crisis to the blood flow that lags behind the symptoms. This page stays with the athlete, the sideline and the ladder back.

One more fact belongs here, stated plainly. That conceptual definition did not reach full consensus at the Amsterdam conference, because the American Congress of Rehabilitation Medicine had just rewritten its own diagnostic criteria for mild traumatic brain injury Nakayama 2024. Concussion still has no operational test. The diagnosis is a clinical determination made by a health care professional, and no instrument makes it for them.

03Recognize and remove

The First Decision Is Removal, and No Diagnosed Athlete Returns That Day

The most consequential decision in concussion care is made in the first minute, and it is a removal decision. The SCAT6 states the rule without qualification. Any athlete with a suspected concussion is removed from play, medically assessed, and monitored for signs, symptoms and deterioration of their clinical condition SCAT6 tool 2023.

The second rule is equally flat. No athlete diagnosed with concussion returns to play on the day of injury SCAT6 tool 2023. There is no same-day return and no judgment call left to the athlete, the coach or the score. Concussion is an evolving injury in the acute phase, and the tool instructs the examiner to keep monitoring for symptoms that develop later.

The five steps of the immediate assessment

The immediate assessment on the SCAT6 runs in a fixed order, and the order encodes priority SCAT6 tool 2023. Step one records observable signs: lying motionless, falling unprotected, balance or gait difficulty, disorientation, a blank or vacant look, facial injury after head trauma, an impact seizure, and a high-risk mechanism.

Step two is the Glasgow Coma Scale. Step three is the cervical spine assessment, which asks about neck pain at rest, tenderness to palpation, active pain free range of motion, and limb strength and sensation. Step four screens coordination and ocular motor function with finger-to-nose, and asks whether the athlete can look side to side and up and down without double vision. Step five is the Maddocks questions, scored out of five.

The decision rules for the neck itself belong to Cervical Trauma and the Systematic Read, which carries the NEXUS and Canadian C-spine criteria in full. The athletic cervical spine belongs to The Cervical Spine. The SCAT6 is explicit that a patient who is not lucid or fully conscious is assumed to have a cervical spine injury until proven otherwise.

What the red flags change

Eleven findings on the SCAT6 stop the sideline assessment and start an ambulance SCAT6 tool 2023. Neck pain or tenderness. Weakness or tingling in more than one arm or in the legs. Loss of consciousness, a deteriorating conscious state, or a Glasgow Coma Scale under 15. Seizure or convulsion. Double vision. Vomiting. A severe or increasing headache. Growing restlessness or agitation. Visible deformity of the skull.

Those are not concussion findings. They point at bleeding, swelling or a spinal injury, and a concussion assessment would waste time on them. Emergency and Field Care carries the field protocols that follow. The concussion screen resumes only once the emergency has been excluded.

One line on the tool deserves to be quoted in every locker room. The SCAT6 should not be used by itself to make or exclude the diagnosis of concussion, and an athlete can have a concussion even when the assessment reads within normal limits SCAT6 tool 2023. The screen can take a player out. It cannot clear one in.

04The SCAT6 and its bands

The SCAT6 Covers Ages 13 and Up, and Its Useful Window Is About 72 Hours

The Sport Concussion Assessment Tool 6 is the sideline standard, and it is age banded. The SCAT6 is used for athletes aged 13 years and older SCAT6 tool 2023. Children aged 8 to 12 are assessed with the Child SCAT6. Below age 8 neither tool is validated, so assessment goes to a clinician experienced with young children. Anyone who is not a health care professional uses the Concussion Recognition Tool 6 instead.

Amsterdam narrowed the pediatric band deliberately. The Child SCAT5 covered ages 5 to 12, and the Child SCAT6 covers ages 8 to 12, because validated data in younger children are scarce Nakayama 2024. The office versions follow the same split, with the SCOAT6 for ages 13 and up and the Child SCOAT6 for ages 8 to 12 Davis 2024.

The tool also has an expiry. Apart from the symptom scale, the SCAT6 is intended for the acute phase, ideally within 72 hours and up to seven days after injury SCAT6 tool 2023. Past seven days the examiner moves to the office tool. A correct administration takes 10 to 15 minutes and cannot be done faster.

What the SCAT6 actually scores

Four scored domains sit inside the tool. The symptom scale lists 22 symptoms, each rated 0 to 6, which gives a symptom number out of 22 and a symptom severity score out of 132 SCAT6 tool 2023. That severity score is the number an athlete watches fall, and it is the number this page will come back to.

The cognitive screen is built on the Standardized Assessment of Concussion. Orientation is scored out of 5. Immediate memory uses a 10-word list read over three trials, out of 30. Concentration combines digits backward and months in reverse order, out of 5. Delayed recall is out of 10, and the total cognitive score is out of 50.

That 10-word list is new. The systematic review behind the SCAT6 found ceiling effects on the older five-word list and recommended harder tasks Echemendia 2023. A test that healthy athletes ace cannot detect the athlete who has slipped.

Balance is scored with the modified Balance Error Scoring System, three stances of 20 seconds each with errors counted out of 30. A timed tandem gait follows on a 3 meter line, with an optional dual task of counting backward by sevens while walking heel to toe.

Where the evidence thins

The review that rebuilt the tool found empirical data limited in pre-adolescents, women, para athletes and populations outside North America Echemendia 2023. The instrument is strongest where it has been studied most, which is collegiate and high school athletes in the United States.

One line in the tool corrects a common belief about baselines. Preseason baseline testing is helpful for interpreting post-injury scores and is not required for that purpose SCAT6 tool 2023. A team without preseason numbers is not disarmed. A team with them reads the same scores against the athlete rather than against a population, and The Functional-Neurology Workup builds that argument in full.

05The four symptom families

Concussion Presents Across Four Families, and the Ocular One Is the Easiest to Miss

No two concussed athletes present the same way, and the symptom scale is built to catch that. The 22 items on the SCAT6 sort into four families SCAT6 tool 2023. The somatic family brings headache, pressure in the head, neck pain, nausea, dizziness, blurred vision, balance problems and sensitivity to light and noise, which is nine of the 22.

The cognitive family brings the classic fog: feeling slowed down, not feeling right, difficulty concentrating, difficulty remembering and confusion, which is six. The emotional family brings irritability, sadness, anxiety and being more emotional, which is four. The last three cover sleep and energy: fatigue or low energy, drowsiness, and trouble falling asleep.

In the field the families arrive together. The three most common complaints across 375 concussions were headache at 87 percent, balance problems or dizziness at 77 percent, and feeling in a fog at 62 percent Marshall 2015. Those sit in three different families, which is why one question about headache is not a screen.

Why the vestibular and ocular findings decide the picture

Two systems show up often enough that the consensus built them into the office tool. Beyond 72 hours the Amsterdam recommendations call for a multimodal assessment covering symptoms, balance, cognition, oculomotor and vestibular function, mental health and sleep Davis 2024. Dizziness, gaze instability and trouble tracking a moving target are the signature of a brain struggling to hold the world still during motion.

The athlete who reports only a headache may have a vestibulo-ocular problem driving it. Probing those systems turns a vague complaint into a target with a number attached, which is the whole reason the office tool exists.

Convergence, measured with a ruler

The cheapest of those numbers is near point of convergence, the distance at which a slowly approaching target breaks into two. Among 78 athletes examined a mean of 5.8 days after concussion, 33 had a near point beyond 5 centimeters Pearce 2015. Measurement reliability across three trials was high, with intraclass correlations from 0.95 to 0.98.

The detail that matters most is what happened on repetition. In the athletes with convergence insufficiency, the near point on the first trial differed from trials two and three. In the athletes with normal convergence it did not. The deficit appeared when the system was asked to do the same job again.

Those athletes also scored worse on verbal memory, visual motor speed and reaction time. Near point distance contributed significantly to the reaction time model after controlling for age and symptom score. A ruler held to the nose predicted a computerized cognitive result.

Saccade latency and pursuit gain belong to Sport Vision and Eye Movements. The vestibulo-ocular reflex and postural sway belong to The Vestibular System and Balance, and the assembled battery belongs to The Functional-Neurology Workup.

06Rates and recovery

The Symptom Clock and the Physiology Clock Do Not Run Together

Concussion rates in sport are measured per 1,000 athlete exposures, where one exposure is one athlete in one practice or one game. Across three seasons of boys and men's lacrosse, the concussion rate was 0.7 per 1,000 exposures at youth level against 0.3 at high school and 0.3 in the NCAA Kerr 2019.

That ordering runs against the assumption that bigger athletes produce more brain injury. In the same data the time loss injury rate was lowest in youth at 2.0 per 1,000 exposures, against 2.9 in high school and 3.3 in the NCAA. The youngest players lost the least time overall and took the most concussions.

Across seven sports, concussion incidence was highest in football, then women's lacrosse, men's lacrosse, men's soccer and women's soccer, and the most common mechanism was collision with another player Marshall 2015.

Who takes longer

The largest prospective trajectory study followed 18,531 athlete seasons and identified 570 concussions, or 3.1 percent McCrea 2013. Symptom recovery took longer than seven days in 57 of them, which is 10 percent. Those numbers describe what a cohort recorded. They are not a schedule any individual athlete is owed.

Three acute features predicted the slower course. Unconsciousness carried an odds ratio of 4.15 for prolonged recovery, with a 95 percent confidence interval of 2.12 to 8.15. Post-traumatic amnesia carried an odds ratio of 1.81. More severe acute symptoms predicted it as well. The severity of the first hours forecasts the length of the weeks.

Amsterdam also changed the label for the long tail. Persisting symptoms once meant more than two weeks in adults and more than four weeks in children. The consensus now sets a single threshold of more than four weeks at every age Nakayama 2024.

Two clocks, not one

Here is the finding that reorganizes everything else on this page. In the same trajectory study, the prolonged recovery group at 45 to 90 days after injury still reported elevated symptoms while showing no deficits on cognitive or balance testing McCrea 2013. The felt state and the measured state had separated.

The separation runs in the other direction too, and that direction is the more dangerous one. Medial-lateral sway regularity in concussed collegiate athletes was still 0.268 of an approximate entropy unit below their own preseason value at 48 to 96 hours Cavanaugh 2006. The standard error was 0.072, and conventional postural stability had already normalized.

Changes in that entropy measure showed few significant relationships with changes in symptoms, which is the point. The Vestibular System and Balance carries the sway complexity evidence in full, including what approximate entropy measures and why a more regular sway path is a worse one. An athlete can look steady, report nothing, and still be organizing balance differently than they did in August.

07The return-to-sport strategy

The Return-to-Sport Strategy Runs Six Steps, Each a Minimum of 24 Hours

Amsterdam replaced the old graduated protocol with a six-step return-to-sport strategy, deliberately split into the first three steps and the last three Nakayama 2024. The break sits where medical clearance sits. Everything before it is the athlete rebuilding tolerance, and everything after it is contact.

Step one is symptom-limited activity and can begin within 24 hours of injury. The rest prescribed in the first 48 hours is relative rather than absolute, which means normal daily activities continue while screen time is reduced. From 24 to 48 hours the athlete may return to light-intensity activity such as walking, provided it does not provoke symptoms.

Step two is aerobic exercise, and Amsterdam split it into light aerobic work and moderate aerobic work, which makes the six-step ladder effectively seven. Where exercise testing equipment is available, aerobic exercise can be prescribed within 2 to 10 days of injury, held below the heart rate that mildly exacerbates symptoms on the test.

What counts as a mild symptom increase

The consensus put a number on tolerable. A mild exacerbation is a rise of no more than 2 points on a 0 to 10 scale, lasting less than an hour Nakayama 2024. The comparison value is whatever the athlete reported before the activity started. That is the width of the band the injured system is allowed to be pushed into.

If symptoms rise by more than 2 points during steps one to three, or last longer than an hour, the athlete stops and tries again the next day. If symptoms appear during steps four to six, the athlete returns to step three. Step three is individual sport-specific exercise, and medical clearance is required before step four.

That clearance depends on the full resolution of symptoms, cognitive abnormalities and clinical findings from the current concussion, including the absence of symptoms during and after exertion. Each step typically takes a minimum of 24 hours, the full program takes at least a week, and unrestricted return typically arrives up to a month after injury.

This is not bureaucracy. It is biology, and the pace is set by a measured tolerance rather than by a calendar. The reason to respect it is recorded, because athletes with two or more concussions in the previous 24 months carried 5.5 times the rate of a new one Marshall 2015. Return to Play carries the return-to-play decision frameworks in full.

08Active recovery

Rest Was Tested Against Activity, and Activity Won

The instruction to sit in a dark room until symptoms clear was tested and it failed. Ninety-nine patients aged 11 to 22 were randomized within 24 hours of concussion Thomas 2015. One arm got five days of strict rest. The other got usual care, meaning one to two days of rest followed by stepwise return to activity. Eighty-eight completed the study.

The strict rest group attended less school, as designed, at 3.8 hours against 6.7. There was no clinically significant difference in neurocognitive or balance outcomes. That group reported a higher total symptom score across 10 days, 187.9 against 131.9, and slower symptom resolution.

The consensus followed the data. Rather than strict rest, early return to light physical activity and reduced screen time are what the Amsterdam pediatric recommendations name as facilitating recovery Davis 2024. Cervicovestibular rehabilitation is recommended for adolescents with dizziness, neck pain or headaches lasting more than 10 days.

What active recovery means

Recovery from concussion is active and neurological rather than a dark room and a waiting game. A brief period of relative rest is followed by progressive sub-symptom-threshold activity that nudges the system without overloading it. The targets are specific: graded aerobic loading below the measured threshold, gaze stabilization work for the vestibulo-ocular reflex, and the neck.

The mechanism claim underneath that is neuroplasticity. Nervous tissue reorganizes in response to precise, graded, repeated input, which is why the dose and the specificity of the input matter more than the total amount of rest. Gaze stabilization belongs to The Vestibular System and Balance, and the cervical work belongs to The Cervical Spine.

The goal is not merely a symptom-free athlete. It is reaction time, balance and autonomic control restored to the athlete's own preseason values, which is a target that can be measured rather than asserted. Nothing on this page claims that any form of care treats a concussion or shortens its course.

Who is in the room

Concussion care is explicitly collaborative, and the consensus names the collaborators. When symptoms do not improve across the first two to four weeks, Amsterdam calls for a multidisciplinary team and a multimodal assessment Nakayama 2024.

The clinical network it lists includes sports physicians, athletic trainers and therapists, physiologists, physiotherapists, occupational therapists, sports chiropractors, neurologists, neurosurgeons, neuropsychologists, ophthalmologists, optometrists, rehabilitation physicians, psychologists and psychiatrists. A team physician, an athletic trainer and a chiropractic neurologist reading the same athlete is the standard the consensus describes, not an exception to it.

09What we corrected

Three Claims Removed From This Page

This page previously said that the return ladder exists to prevent second-impact syndrome, the rare and catastrophic swelling that follows a second blow before the first has healed. That claim is downgraded. A review of the published evidence found the case for it as a complication of repeat concussion unconvincing McCrory 2001.

The author proposed that the clinical picture is diffuse cerebral swelling, a recognized complication of traumatic brain injury. It is more common in children and adolescents, which is exactly the demographic the reported cases came from. The ladder stands on its own footing: graded exposure below a measured tolerance, and medical clearance before contact.

The page also said that heart rate variability metrics help track the return of autonomic balance. Two systematic reviews put that in question. A review of 15 studies found impairment during exercise, and fewer than half of the results showed a significant difference between concussed individuals and controls Charron 2021. Heart rate variability stays on this page as a research measure, not as a recovery tracker.

Third, the page carried a quotation attributed to Dr. Jason Dulberg that was not drawn from anything he said or wrote. It has been removed. Every claim here is either sourced to the literature or named explicitly as the model's.

10The model's claim

Concussion Is an Injury to the Range, and the Range Is Measurable

Two layers run through this page. The established science is the Amsterdam definition and the SCAT6, the epidemiology, the trajectory data, the convergence findings, the sway entropy result and the failed rest trial. Every one of those belongs to the investigators who gathered it.

The Unified Model of Tone reads concussion as an injury to regulation. It holds that the master pathology is the loss of the adaptive range within which values should fluctuate, and that this shows up as the inability to modulate between excitation and inhibition as context demands. Concussion is that injury in its clearest athletic form. Nothing a scanner can photograph is torn. What is lost is the width of the band.

Read the protocol as a measurement

Look again at what the Amsterdam strategy actually measures. The 2-point rise on a 0 to 10 scale is a bedside reading of how far the system can be pushed before it stops organizing. The heart rate threshold in step two is the same width, recorded in beats per minute. Neither number describes tissue damage. Both describe available range.

That reading explains why removing input failed. Five days of strict rest produced a higher total symptom score than usual care Thomas 2015. Rest does not widen a range. It shrinks the demand the system is asked to meet, and a system that is never asked stops learning what it can do.

The prediction this page makes

The model predicts that the physiological readouts normalize after the symptoms do. An athlete reports feeling fine while the regulation has not come back. This is a claim about how recovery is organized rather than a claim about what any treatment does, and sport can settle it.

The pieces are already visible. Medial-lateral sway entropy sat 0.268 below preseason after conventional stability had normalized Cavanaugh 2006. A systematic review of heart rate variability in concussed athletes reports the disturbance appearing under submaximal exercise rather than at rest Flores 2023. One of its included studies proposed that symptom resolution does not necessarily reflect autonomic recovery.

The model expects that pattern, because a resting value is the least informative reading of a regulator. A range shows itself when the system is challenged and then asked to come back.

The design is a season and one squad. Record preseason baselines, then repeat four measures at every step of the return. RMSSD after a standardized submaximal exercise challenge. Near point of convergence in centimeters. Medial-lateral sway approximate entropy during quiet standing. Time to return to the pre-activity symptom value after the step two threshold test.

Log the SCAT6 symptom severity score out of 132 alongside them. The model expects the four physiological readings to still sit away from preseason on the day the symptom score reaches zero.

If RMSSD after exercise, near point of convergence, medial-lateral sway entropy and time to return to the pre-activity symptom value are shown to move together within one squad, the unification claim is confirmed.

11The tone reading

Concussion as a Narrowed Range

Three signatures of tone carry this injury, and each one already has a number attached to it in the Amsterdam protocol.

The rest of the library carries the same logic through its other foundations. Coupling is what tracking a ball while the head turns demands of the eyes, the neck and the balance organs together, and near point of convergence is the reading that fails on the second look. Gain is light and noise sensitivity, two of the 22 items on the symptom scale, stadium input answered at the wrong volume. Load is the collision itself, and in lacrosse it landed hardest on the youngest players, at 0.7 concussions per 1,000 exposures against 0.3 at the two higher levels. Oscillation is the rhythm of a timed tandem gait held across 3 meters while counting backward by sevens. Prediction is why clearance before contact requires no symptoms during and after exertion rather than only at rest. Set-point is the step two heart rate threshold, found on a test because no two concussed athletes tolerate the same number. The full framework is set out in the Unified Model of Tone.

12Where this sits

How This Page Relates to the Rest of the Library

Seven places this argument continues, each with the claim that earns the link.

13Questions athletes ask

Questions Athletes Ask

14The sources

References

14 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.