Concussion Symptoms: What the Evidence Shows About Early Signs, Delayed Onset, and Recovery

Key Takeaways: Concussion Symptoms

• In the cited athlete cohorts, headache was the most frequently reported symptom, followed by dizziness or balance problems and cognitive complaints such as feeling “in a fog” or difficulty concentrating.
• Symptoms do not always appear at the moment of impact. Some develop over minutes or hours, and fatigue, drowsiness, light and noise sensitivity, and sleep-related symptoms may become more common in the hours and first day after injury.
• Neck pain, double vision, weakness or tingling in more than one arm or in the legs, seizure, vomiting, a severe or worsening headache, or deteriorating consciousness are among the red flags that call for urgent medical attention.
• Whether head CT is needed depends on findings such as the level of consciousness, repeated vomiting, seizures, neurological deficits and signs of skull fracture; guidelines differ on how much weight they give to loss of consciousness and anticoagulant use.
• Most athletes see their symptoms settle within days to weeks, but symptoms lasting more than 4 weeks are defined as persisting symptoms and warrant a structured clinical evaluation.
• From a clinical perspective, an acute concussion and longer-lasting symptoms call for quite different assessment; in my own clinical work, I regard roughly the first 8 hours after the injury as an especially important observation period.
• In my clinical experience, intoxication, anticoagulant medication, progressive nausea and persistent amnesia are situations that lower my threshold for head imaging, and older adults may develop a chronic subdural haematoma weeks after hitting their head.
• In my view, a knockout caused by blows to the head should be treated as a suspected concussion until it has been properly assessed.

Introduction: Concussion Symptoms

Concussion symptoms are surprisingly easy to miss. In US high school athletes, commonly reported symptoms of sport-related concussion included headache (94.7%), dizziness (74.8%), and difficulty concentrating (61.0%) [1].

In this article, I go through what the peer-reviewed evidence says about which concussion symptoms are most common, when they appear, which signs require emergency care, how long symptoms usually last, and how the first days after injury are managed according to current consensus.

In my clinical work, concussion is a condition I come across roughly once a week. It is not an everyday diagnosis, but it is common enough that hardly any clinician can avoid it. From a clinical perspective, one of the first things to establish is whether the situation is acute or whether the person has symptoms that have lasted for longer, because the two call for quite different assessment.

What Is a Concussion? Why Symptoms Vary So Much

The 2022 Amsterdam International Consensus Statement on Concussion in Sport presents a conceptual definition, accepted by a majority vote of 78.6% but short of the panel’s 80% consensus threshold, that describes sport-related concussion as a traumatic brain injury caused by a direct blow to the head, neck or body, with the impulsive force transmitted to the brain. According to the same statement, symptoms and signs may appear immediately or develop over minutes or hours, commonly resolve within days, but may sometimes last longer [2].

The statement also notes that concussion may or may not involve loss of consciousness, and that no abnormality is seen on standard structural neuroimaging such as CT or conventional MRI [2].

The consensus also stresses that the symptoms attributed to concussion are non-specific. They are commonly reported by healthy people and by people with other conditions, and they can be amplified by many biopsychosocial factors [2].

The Most Common Concussion Symptoms

In the Concussion Prevention Initiative, which followed 8,905 high school and collegiate athletes in football, soccer, lacrosse, and ice hockey and recorded 375 concussions, the most common symptoms at the time of injury were headache (87%), balance problems or dizziness (77%), feeling “in a fog” (62%), and difficulty concentrating (52%) [3].

The same study grouped concussion symptoms into three broad clusters [3]:

• Somatic (physical): headache, balance problems or dizziness, nausea, blurred vision, numbness or tingling, vomiting, drowsiness, fatigue, sensitivity to light, and sensitivity to noise.
• Cognitive: feeling “in a fog,” difficulty concentrating, and difficulty remembering.
• Neurobehavioral: sleeping more than usual, trouble sleeping, irritability, and sadness.

Importantly, no single symptom was present in every concussed athlete, and the authors concluded that concussion symptomatology is very diverse between individuals [3].

Loss of Consciousness Is Not Required

Loss of consciousness and amnesia have historically been regarded as important markers of concussion severity [3]. In the same prospective cohort, however, loss of consciousness was reported in only 9% of concussions, and when it occurred it tended to be brief, with a median duration of 5 seconds. Amnesia was reported in 30% of cases [3].

Signs Others Can See

Some concussions are recognized from signs observed by other players, medical staff or officials, not only from what the athlete reports. The Amsterdam consensus lists signs that warrant immediate removal from play: actual or suspected loss of consciousness, seizure, tonic posturing, ataxia, poor balance, confusion, behavioural changes, and amnesia [2]. The panel also recommended adding further visible signs to the recognition tools, including falling without protective action and a blank, vacant or dazed look [2]. A player showing these signs should not return to the same match or training session that day unless an experienced healthcare professional has assessed them and concluded that the sign was not caused by a concussion [2].

When Do Concussion Symptoms Appear? Immediate Versus Delayed Onset

The consensus statement notes that symptoms and signs may evolve over minutes, hours or days, and recommends that a player with a suspected or confirmed concussion be re-evaluated serially over the following hours and days [2].

The Concussion Prevention Initiative data show this pattern clearly. Additional symptoms developed after the time of injury in 18% of cases; in half of these, the new symptoms appeared within 4 hours, while 29% of them developed new symptoms on day 1 or later. Fatigue, drowsiness, and sensitivity to light and noise became more common in the hours and first day after injury, and most neurobehavioral symptoms, such as sleep disturbance, were more frequent after the game and on day 1 than at the time of injury [3].

A prospective study of 144 young athletes (under 20 years) seen at a multidisciplinary concussion program found that 16.7% experienced delayed symptom onset, defined as symptoms starting 15 minutes or more after the injury. In this group, the median time to symptom onset was 60 minutes [4]. The study found no statistically significant differences between the early- and delayed-onset groups in initial symptom severity or physician-documented recovery time, but athletes with early symptom onset were immediately removed from play in 71.6% of cases, compared with only 29.2% of those whose symptoms started later [4].

In other words, feeling fine immediately after a head impact does not by itself exclude a concussion, because symptoms and signs may evolve over minutes, hours or days [2].

Red Flags That Need Emergency Care

Not every symptom after a head impact is a typical concussion symptom. The Sport Concussion Assessment Tool 6 (SCAT6) notes that head impacts can be associated with serious and potentially fatal consequences, and lists the following red flags [5]:

• Neck pain or tenderness
• Seizure or convulsion
• Double vision
• Loss of consciousness
• Weakness or tingling/burning in more than one arm or in the legs
• Deteriorating conscious state
• Vomiting
• Severe or increasing headache
• Increasingly restless, agitated or combative
• Glasgow Coma Scale score below 15
• Visible deformity of the skull

According to the SCAT6, if there are significant concerns, including any of these red flags, the athlete requires urgent medical attention, and if a qualified medical practitioner is not available for immediate assessment, emergency procedures and urgent transport to the nearest hospital or medical facility should be arranged. The SCAT6 should not be used by itself to make or exclude the diagnosis; a concussion is possible even when SCAT6 results are within normal limits [5].

When Is Head Imaging Needed After a Concussion?

In my clinical work, I regard roughly the first 8 hours after the injury as an especially important observation period; this is my own working practice, not a guideline-defined observation time. Many patients with a suspected concussion come to the emergency department for assessment, and in my experience one of the most important questions is whether there was a primary loss of consciousness, because the answer already guides further care. In practice, emergency head imaging usually means CT, which in my experience is more readily available in the emergency setting than MRI, and its main purpose is to identify intracranial bleeding and other acute injuries such as skull fractures. Depending on the clinical findings and risk factors, a patient may undergo CT, be kept under observation, or, in low-risk situations, be discharged home with instructions [6]. When I keep a patient under observation, their condition is followed, they are woken at intervals, and we check whether they can be roused and whether their condition is getting worse.

Guidelines differ in which findings should lead to CT. The Scandinavian guidelines for adults with minimal, mild and moderate head injury list loss of consciousness, repeated (two or more) episodes of vomiting, and anticoagulant therapy or coagulation disorders as reasons for CT in mild head injury, and they class patients who cannot clearly deny loss of consciousness as having suspected loss of consciousness [6]. The same guidelines did not include amnesia or intoxication as risk factors, citing their limited predictive value [6]. For a subgroup of mild head injury patients — GCS 14 without risk factors, or GCS 15 with loss of consciousness or repeated vomiting and no other risk factors — the guidelines allow the blood biomarker S100B to be sampled if less than 6 hours have passed since the injury, and if S100B is below 0.10 µg/l, the patient may be discharged without CT [6].

The Finnish Current Care guideline on brain injuries (Käypä hoito, last updated in December 2023 and currently under revision) uses a CT flowchart adapted from the 2023 NICE guideline. For adults, it indicates CT within 1 hour for findings such as a GCS below 13 at first assessment, a GCS below 15 two hours after the injury, suspected skull fracture, a post-traumatic seizure, a focal neurological deficit, or more than one episode of vomiting. For adults who have had loss of consciousness or amnesia after the injury, the flowchart indicates CT within 8 hours if any further factor is present: age 65 or over, a known bleeding disorder, a dangerous injury mechanism, or retrograde amnesia lasting more than 30 minutes. For patients without loss of consciousness or amnesia who use anticoagulant or antiplatelet medication other than aspirin, the flowchart leaves head CT to clinical discretion [7]. The Finnish guideline further notes that loss of consciousness can be reliably established only from an eyewitness account, that aspirin as the only bleeding risk factor is not an indication for head CT, and that a normal head CT does not rule out a brain injury [7].

Alcohol is a common complicating factor. In my experience, it is unfortunately quite common for a patient with a head injury to be intoxicated, and then their level of consciousness cannot necessarily be followed reliably. In these situations I have often had to image the patient’s head, because consciousness and clinical status alone did not allow me to judge whether consciousness was reduced. In my experience, nausea is similarly hard to interpret: a patient who is coming down from intoxication or is hungover may have nausea and vomiting for that reason alone, which can look very much like concussion symptoms.

The Finnish guideline takes a similar line, advising that when the need for CT cannot be reliably assessed, for example because of intoxication, imaging should be done readily to rule out a serious intracranial injury [7].

Anticoagulant medication is another factor that, in my practice, usually leads to imaging even when there has been no loss of consciousness. Another essential question is whether nausea is progressive. In my experience, nausea after an ordinary concussion often eases during observation. If it does not ease, or gets worse, particularly together with repeated vomiting, a worsening headache or neurological changes, raised intracranial pressure or another intracranial process may be suspected, and in my clinical practice this usually leads to imaging.

Guidance on anticoagulants is not uniform. The Scandinavian adult guidelines list anticoagulant therapy as a reason for CT [6], whereas the Finnish guideline states that patients whose condition does not change after the injury may, at the clinician’s discretion, be managed without CT even if they use anticoagulant or antiplatelet medication [7]. Repeated vomiting, in turn, is itself among the CT criteria in both guidelines [6][7].

The importance of observation becomes especially clear in children, particularly young children, because the threshold for CT may be higher due to radiation exposure. In my experience, these patients are readily admitted to a ward for observation. MRI is not always easily available, although if imaging is necessary, CT is of course performed.

The Scandinavian guidelines for children explicitly take the potential long-term harm of ionising radiation from CT into account and recommend CT only when clinically justified. For certain risk groups of children with minor head trauma, they recommend either admission for observation or CT, regarding immediate CT and in-hospital clinical observation as equally good strategies in that triage, and they recommend admission for observation for children with post-traumatic amnesia or two or more episodes of vomiting [8]. The Finnish adult guideline does not cover children under 16 [7].

Memory gaps are another feature that some patients develop. During observation, I typically wait until the patient’s memory has returned. The patient may never remember the events immediately after the trauma, but in practice I wait until the anterograde amnesia has ended, and I may keep the patient on the ward until then. In my own practice, persistent amnesia lowers my threshold for imaging, and I tend to image readily when a patient has a clear memory gap after a blow to the head.

Among the Finnish guideline’s criteria for safe discharge from the emergency department are a GCS of 15 and resolved post-traumatic amnesia [7].

In my own practice, I usually go through home observation with patients after a concussion, even when there is no clear progressive nausea or drop in the level of consciousness. I prefer that the patient is not alone during the following night and that someone is with them to carry out the observation. If that is not possible, the observation is often done in the emergency department, on a ward or in an emergency observation unit, at least in Finland. If no worsening symptoms appear during the observation period or the following night, the situation can in many cases simply be followed, although even this does not rule out that something may develop later.

Older adults deserve particular attention here. They can develop chronic subdural haematomas that appear only after a delay, sometimes weeks after the injury. These can be difficult to detect, because the symptoms may appear late and may be quite mild but progressive, such as balance problems or a decline in cognition. In my experience, a fairly typical scenario is that I happen to read in the patient records that, a few weeks earlier, the patient had visited the emergency department, where only an 8-hour concussion observation was carried out without head imaging and the patient was discharged. A chronic subdural haematoma then developed with a delay and eventually led to surgical drainage by trepanation. I have seen such cases, and for that reason I have often imaged older patients who hit their head weeks earlier, knowing that a chronic subdural haematoma may in principle be the explanation. In my view, recognising these cases often requires an experienced clinician’s eye.

A literature review describes chronic subdural haematoma as frequently seen in older people, notes a latent period between the injury, which is usually minor, and the onset of symptoms, and describes the haematoma as developing slowly over weeks or months [9].

Why Athletes Miss or Hide Symptoms

Recognising concussion symptoms depends heavily on honest self-reporting. In a survey of 797 former collegiate athletes, 214 (26.9%) recalled sustaining at least one sport-related concussion, and of these, 71 (33.2%) reported not disclosing at least one of them [10]. The most common reasons were not wanting to leave the game or practice (78.9%), not wanting to let the team down (71.8%), not knowing it was a concussion (70.4%), and not thinking it was serious enough (70.4%) [10].

In a small prospective study of 69 adolescent athletes seen at a concussion clinic, those who continued to play after their concussion took longer on average to recover than those removed immediately (mean 44.4 vs 22.0 days) and were more likely to have a protracted recovery of 21 days or more; as an observational study, it shows an association rather than proof of cause and effect [11]. A systematic review and meta-analysis prepared for the Amsterdam consensus likewise found that continuing to play and delayed access to healthcare providers were associated with longer recovery [12].

In combat sports, concussion is something one inevitably encounters. In my view, a knockout caused by blows to the head should be treated as a suspected concussion until an appropriate medical assessment has been completed, even when a loss of consciousness has not been documented. After a knockout, I consider whether head imaging is warranted, mainly to rule out intracranial bleeding or other traumatic injury, alongside further assessment of the concussion; in my own practice I often find imaging reasonable, but the need for acute imaging depends on the clinical findings and the imaging criteria described earlier. Separately, the competition guide of the Finnish Kickboxing Federation (version of 13 September 2026) requires a fighter who has been knocked out by a blow to the head to serve a medical competition ban and, before competing again, to undergo brain imaging (CT or MRI) and obtain a written medical certificate of fitness to compete [13]. That rule concerns eligibility to compete rather than acute care, but it is a clear statement aimed at improving safety in the sport. I have fortunately never been knocked out myself, but I have watched several knockouts from close by. Deaths and serious disabling injuries after knockouts do occur, for example in professional boxing.

A review of neurotrauma in boxing, published online in December 2023, reported that, in a historical collection of 2,076 boxing fatalities, 65.2% occurred after a knockout and nearly 75% during professional bouts, and it described acute subdural haematoma with associated brain swelling as the leading cause of boxing-related death. The authors note that the collection is based on newspaper and magazine reports, so the data may be incomplete or incorrect [14]. These historical figures do not provide a risk of death per bout or per knockout. I discuss combat-sport concussion in more detail in my article on concussions in combat athletes, and the evidence on protective equipment in my article on headgear effectiveness.

How Long Do Concussion Symptoms Last?

For most athletes, concussion symptoms improve relatively quickly. In the Concussion Prevention Initiative, the median time for all symptoms to resolve was 3 days, and symptoms resolved within one week in 86% of cases [3]. The Amsterdam consensus defines symptom resolution and return to sport as separate endpoints, so symptom resolution is not the same as return to sport [2].

A 2023 meta-analysis of 278 studies found a mean of 14.0 days until athletes were symptom free, a mean of 8.3 days until return to learning, and a mean of 19.8 days until return to sport, with very high heterogeneity between studies (I² 98.0–99.3%). In that review, 93% of athletes had fully returned to learning by 10 days without new academic support [12].

When Symptoms Persist

The consensus statement uses the term “persisting symptoms” for symptoms lasting more than 4 weeks in children, adolescents and adults [2]. In a prospective multicentre study of 3,063 children and adolescents aged 5 to under 18 years with acute concussion who presented to one of nine Canadian paediatric emergency departments within 48 hours of injury, persistent postconcussion symptoms — defined as three or more new or worsening symptoms — were present at 28 days in 801 of the 2,584 who completed follow-up (31.0%) [15]. Because this was a paediatric emergency department cohort, the figure should not be applied directly to athletes in general.

A systematic review of predictors of clinical recovery found that the most consistent predictor of slower recovery is the severity of a person’s acute and subacute symptoms. The development of subacute headaches or depression was likely a risk factor for symptoms lasting more than a month, a preinjury history of mental health problems appeared to increase risk, and there was some evidence that the teenage years — with greater risk for girls than boys — may be a particularly vulnerable period [16]. The 2023 meta-analysis likewise identified initial symptom burden as the strongest predictor of a longer time to return to sport [12].

Alongside the acute situation, some patients develop symptoms after a delay or have symptoms that persist. Particularly difficult cases are patients in whom the injury turns out to be more than an uncomplicated concussion, for example a diffuse axonal injury. In my experience, cognitive difficulties in these patients may become clearly apparent only over time, and MRI can show traumatic changes that are not visible on CT. When concussion symptoms persist, further assessment, which may include MRI, in my experience usually takes place at a brain injury outpatient clinic rather than in the emergency department.

Some patients are left with cognitive symptoms for a longer time: for example, they may develop a headache very easily when trying to concentrate, and they may tire quickly. In my experience, some of these patients find an early return to concentration-demanding work difficult, and their symptoms have sometimes become prolonged. I have also met patients who developed work-related burnout after returning to work soon after a concussion, although in an individual case it is difficult to say how much the early return itself contributed. These are often very difficult situations.

Managing the First Days After a Concussion

According to the Amsterdam consensus, recommending strict rest until all concussion symptoms have completely resolved is not beneficial. Instead, relative rest — including normal daily activities and reduced screen time — is recommended immediately and for up to the first 2 days after injury, and light physical activity such as walking is acceptable as long as it does not more than mildly worsen symptoms [2]. The consensus defines a mild and brief exacerbation as an increase of no more than 2 points on a 0–10 symptom scale, lasting less than an hour [2].

In a randomized clinical trial of 103 adolescents aged 13 to 18 years who presented within 10 days of a sport-related concussion, those assigned to individualized subsymptom threshold aerobic exercise recovered in a median of 13 days, compared with 17 days in a placebo-like stretching group [17]. Importantly, early activity does not mean early return to contact: the consensus emphasizes avoiding any risk of contact, collision or falls until a qualified healthcare professional has determined that higher-risk activity is safe [2].

Sleep deserves particular attention. The consensus statement notes that sleep disturbance during the first 10 days after a sport-related concussion is associated with an increased risk of persisting symptoms and may warrant evaluation and treatment [2]. I cover sleep assessment and evidence-based interventions in more detail in my guide on sleep for athletes.

If dizziness, neck pain and/or headaches persist for more than 10 days, the consensus recommends cervicovestibular rehabilitation, and symptoms that persist, worsen or are not progressively resolving 2–4 weeks after injury warrant a multimodal evaluation [2].

Conclusion: Concussion Symptoms

Concussion symptoms are common, varied, and often subtle. Headache, dizziness, and cognitive complaints dominate, loss of consciousness occurs in only a minority of cases, and symptoms can emerge hours after the impact.

Most athletes recover within weeks, and the current consensus favours a short period of relative rest followed by gradual, symptom-limited activity, together with early recognition, removal from play, awareness of red flags, and structured evaluation when symptoms persist.

In my clinical experience, the most challenging situations are often those in which the assessment itself is complicated: an intoxicated patient whose level of consciousness is hard to judge, an older adult whose symptoms appear only weeks later, or a person whose cognitive symptoms linger after returning to concentration-heavy work too soon. From a clinical perspective, a concussion is therefore not only an acute event; in some patients it has a longer course, which is why appropriate follow-up can be an important part of care.

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