A concussion is a “traumatically induced transient disturbance of brain function.” Concussions are a subset of the neurologic injuries known as traumatic brain injuries. Traumatic brain injuries have varying severity, ranging from mild, transient symptoms to extended periods of altered consciousness. Given the usually self-limited nature of symptoms associated with a concussion, the term mild traumatic brain injury (mTBI) is often used interchangeably to refer to a concussion, though concussions are technically a subset of mTBIs. Prognosis is usually good, and most patients experience complete resolution of symptoms.
A concussion occurs as a result of either a direct or indirect injury to the head. Providers often consider a direct, traumatic blow to the head as a significant cause of a concussion. However, indirect traumatic forces elsewhere in the body can lead to an acute acceleration/deceleration injury to the brain, which can also lead to a concussion.
In the United States, each year there are an estimated 1.7 million traumatic brain injuries that prompt presentation to the emergency department. The Center for Disease Control also estimates that when accounting for outpatient visits for TBIs and patients not seeking care for injuries, the actual incidence may range from 1.4 to 3.8 million concussions per year. Frequent causes of concussions are motor vehicle crashes, being struck by an object, assault, and participation in recreational athletics. Although sports-related concussions make up a small percentage of overall concussions, much of the current research surrounding concussions stems from data on sports-related head injuries. Football consistently accounts for the highest number and percentage of athletics-related concussions in high school and collegiate athletes, while soccer is responsible for the highest percentage of concussions in female athletes. Female athletes suffer concussions about twice as often as male participants in the same sport.
The pathophysiologic mechanism of a concussion is complex. The acute symptoms of a concussion are due primarily to a "functional disturbance rather than structural injury." "Neurochemical and neurometabolic events" after an injury to the head result in an alteration of neurologic function. Acceleration, deceleration, or rotation of the head result in acute axonal injury via disruption of neurofilament organization. The release of electrolytes through ion channel depolarization leads to a release of neurotransmitters and subsequent neurologic dysfunction. Changes to glucose metabolism, decreased cerebral blood flow and mitochondrial dysfunction also occur.
Assessment of a patient with a possible concussion should include gathering information on the mechanism of injury, the symptoms the patient is experiencing, the timing of symptom onset, and severity and persistence of symptoms. The symptoms of a concussion can be wide-ranging but often fall into one of four main domains, which are listed below. Some of the most common symptoms seen with concussion within each domain include:
1. Affective/emotional function
2. Cognitive function
3. Physical/Somatic Symptoms
The Sideline Concussion Assessment Tool 5 (SCAT5) has a comprehensive inventory for possible symptoms of a concussion.
Most patients (>90%) diagnosed with a concussion do not have an associated loss of consciousness ; however, loss of consciousness is an important sign of a potentially serious head injury. Additionally, the development of symptoms related to a concussion does not always occur immediately after an injury. Development of symptoms within hours to days after a precipitating injury may still indicate a concussion.
Of particular importance in a patient’s history is whether the patient has a history of and the nature of any prior concussions. “A greater number, severity, and duration of symptoms” with prior concussions can be predictive of longer recovery time. Finally, pre-existing mood disorders, learning disorders, sleep disturbances, and migraine headaches may also impact the management of a concussion.
Formal neuropsychological assessment by a trained neuropsychologist can supplement a clinician's assessment of the multiple domains impacted by concussions.
Diagnosis of a concussion remains an exclusively clinical diagnosis based on history and exam findings. However, there is no single pathognomonic finding or a minimum number of symptoms for the diagnosis of a concussion. Several standardized diagnostic tools can be employed in the pre-hospital setting following an acute head injury to assist in the determination of the presence of a concussion. The Sideline Concussion Assessment Tool 5 (SCAT5) is one of the most commonly used tools for concussion assessment, particularly by athletic trainers and sports medicine providers for assessment of athletes on the sideline after a potential head injury. The Child SCAT-5 exists for assessment of patients between 5 and 12 years of age. The optimal setting for administering these tools is a quiet setting with minimization of surrounding distractions. Monitoring for development of symptoms or any signs of neurologic deterioration after the initial post-injury assessment is necessary because of the potentially delayed presentation of symptoms and objective findings. Signs and symptoms including severe headaches, seizures, focal neurologic deficits, loss of consciousness, deterioration of mental status, and worsening symptoms may indicate a more serious head injury and should prompt referral to an emergency department for further evaluation.
Imaging modalities can be employed to rule out other injuries that may mimic the signs or symptoms of a concussion. In the hospital, a CT scan of the head is the radiographic study of choice for quick evaluation to exclude neurosurgical emergencies. Clinical decision tools, such as the Pediatric Emergency Care Applied Research Network (PECARN) head injury guidelines or the Canadian Head CT Rule, are often utilized to guide the decision about whether head imaging is warranted. However, the majority of patients diagnosed with a concussion do not need head imaging. Additionally, a normal head CT does not mean the patient has not suffered a concussion. Further radiographic investigation with an MRI is a consideration in patients with persistent post-concussive symptoms. The role of other imaging modalities, such as functional MRI or PET scan, is primarily for research rather than clinical practice at this time. Testing for serum biomarkers of concussion is still under development but may have a role in future clinical evaluation.
After the diagnosis of a concussion, outpatient observation by a responsible individual educated on warning signs requiring further evaluation is generally appropriate. Patients with concerning signs or symptoms for more severe head injury may require continued hospital observation. Diagnosis of a concussion should prompt removing a patient from an environment that may lead to a repeat blow to the head (i.e., immediate removal of an athlete from athletic participation).
Treatment of a concussion is primarily supportive. Supportive care of concussion centers around the initial limitation of physical and cognitive activity followed by a gradual return to previous activity levels. There is no longer a role for extended, strict cognitive and physical rest. While reasonable to encourage rest during the acute post-injury period (i.e., the initial 24 to 48 hours), the patient should then undergo a gradual return to activity. However, there is no known optimal amount of time for the initial rest period. The patient should proceed with a stepwise return to activity with careful monitoring for return or worsening of symptoms. Recurrence of symptoms warrants a reduction in activity level until symptoms improve. Each increase in activity should generally take at least 24 hours, but again there is no definitive evidence for the optimal timing of a return-to-activity protocol. An athlete diagnosed with a concussion should be forbidden to return to play until cleared by a medical provider.
There is emerging evidence that early, targeted therapies and interventions aimed at specific clinical profiles of a concussion may be beneficial; however, evidence for identification of these clinical profiles and the efficacy of the therapy is still preliminary. Examples of these interventions include vision training for patients with oculomotor dysfunction or cognitive-behavioral therapy for mood disturbances.
Over-the-counter analgesics aimed at control of headache symptoms are an option, although there is limited evidence as to their efficacy. However, other medications that may alter a patient’s cognitive function, sleep patterns, or mood are not advisable as they may mask symptoms of a concussion. Preventive headache medications should not be initiated after a concussion but can be resumed if the patient was on them prior to an injury.
The differential diagnosis immediately after a head injury should include potentially severe injuries, including cervical spine injury, intracranial hemorrhage, or skull fracture. The differential diagnosis for post-concussive symptoms shifts once outside of the window of the acute injury. Symptoms of a concussion can overlap with other potentially pre-existing chronic conditions such as 
The clinician must distinguish whether a patient's symptoms are a result of a concussion, a product of any other pre-existing conditions, or are of a different etiology.
Prognosis for a patient with a concussion is usually good, with symptom improvement in the first one to two weeks post-injury. Previous research indicated the recovery timeframe after a concussion was typically about 10 days. However, the 5th International Conference on Concussion in Sports recently relaxed the expected recovery timeframe for sports-related concussions, stating that most injured athletes clinically recover within a month. The importance of the recognition of a more variable recovery timeline of concussions also received emphasis in a 2013 expert consensus statement. The severity of symptoms within the first few days after a head injury is the most consistent prognostic indicator. At this time, there is no definitive predictor of recovery time from a concussion, and anticipated recovery timeframe should be individualized for each patient.
The most commonly seen complication of a concussion is post-concussion syndrome (PCS), which is characterized by persistent symptoms lasting weeks to months after the initial injury. The median duration of symptoms in one study was seven months. The transition from a concussion to post-concussion syndrome is "ill-defined and poorly understood." Any of the possible symptoms of concussion can be present with post-concussion syndrome, but PCS characteristically presents with multiple "somatic, emotional, and cognitive symptoms."  The severity of the initial injury does not seem to correlate with the likelihood of developing post-concussion syndrome, but a history of prior concussions does appear to correlate with the likelihood of development of PCS.
One of the most feared and concerning complications of a concussion, although rare, is a second-impact syndrome. Second-impact syndrome (SIS) involves a repeat blow or injury to the head before the complete resolution of the initial concussion, resulting in usually rapid, severe swelling of the brain. SIS has the potential for dangerous neurologic complications, including brain herniation and death, though much of the existing data and research on the condition is anecdotal.
Research on the long-term consequences of a concussion is still limited. Of greatest concern is the potential for the development of chronic traumatic encephalopathy (CTE). This condition is characterized by slow, progressive neurodegeneration due to repeated head trauma and tau protein deposition. Symptoms may include memory disturbances, behavioral or personality changes, and speech or gait abnormalities. The overall incidence and prevalence of CTE are unknown, and at present CTE can only be definitively diagnosed with a neuropathologic examination. Lastly, research has yet to establish a cause-and-effect relationship between concussions and CTE.
Opportunities for concussion prevention in the general population is limited and largely centers around educating patients on the recognition and treatment of concussions. Outside of strategies aimed at preventing falls, there are few options for preventing traumatically induced concussions. Opportunities for prevention of sport-related concussions include strict enforcement of the rules of play, promotion of fair play, and reducing the number of contact practices. Evidence for the reduction of concussions with the use of specific mouth guards or helmets is limited.
No guideline exists regarding the disqualification of a patient from athletic participation after a concussion. The American Medical Society for Sports Medicine suggests an individualized approach for each patient and consideration of factors such as persistent symptoms, the number of lifetime concussions, previous prolonged recoveries from a concussion, and the perceived ease of sustaining a repeat concussion when deciding on continued athletic participation.
Management of a concussion requires a multi-disciplinary approach involving the patient, family members, coaches, therapists, athletic trainers, and medical providers. Multidisciplinary care can have a positive impact on both patient care as well as resource utilization. Therapists and neuropsychologists play a particularly important role in the management of a patient with severe symptoms or post-concussion syndrome. In the pediatric population, involving schools and teachers to facilitate the patient's re-integration into academics is of great importance as well.