Concussion

Concussion has many different meanings to patients, families, and physicians. ^{[9, 10]} One definition of concussion is a condition in which there is a traumatically induced alteration in mental status, with or without an associated loss of consciousness (LOC). ^[11]

The American Medical Society for Sports Medicine (AMSSM) defines a sports concussion as a traumatically induced transient disturbance of brain function that involves a complex pathophysiological process. It is a subset of mTBI that is classified based on acute injury characteristics at the less severe end of the brain injury spectrum. The clinical signs and symptoms of concussion cannot be otherwise explained by drug, alcohol, or medication use or by other injuries (such as cervical injuries or peripheral vestibular dysfunction) or other comorbidities (psychological or medical conditions). ^[5]  

The 2022 6th International Conference of Concussion in Sports defines concussion as "a traumatic brain injury caused by a direct blow to the head, neck or body resulting in an impulsive force being transmitted to the brain that occurs in sports and exercise-related activities. This initiates a neurotransmitter and metabolic cascade, with possible axonal injury, blood flow change and inflammation affecting the brain. Symptoms and signs may present immediately, or evolve over minutes or hours, and commonly resolve within days, but may be prolonged." ^[4]  

For many physicians, even those who specialize in mTBI, this area is confusing owing to the paucity of scientific evidence to support much of the clinical decision making that is faced in the office. ^{[9, 12]} The amount of good scientific research in the area of mTBI is due to problems with ambiguous definitions of concussion, inconsistent criteria when selecting patients to study, variability of injury mechanisms and locations, and differing means of measuring cognitive function. ^[13]  The purpose of this article is to review the epidemiology and diagnosis (but not necessarily the classification) of mTBI, as well as the role of imaging studies, issues regarding return to play, and complications surrounding mTBI.

United States statistics

The incidence of concussion varies with the sport and the age of participants. Many head injuries are likely unreported owing to their supposed mild nature; mild concussions may go unnoticed by teammates, coaches, and even the athletes themselves. An athlete's fear of medical disqualification may also lead to underreporting.

Data from emergency department (ED) visits, office visits, and a high school injury surveillance system estimated 1.0-1.8 million sports related concussions (SRCs) per year in the 0-18 years age range and a subset of about 400,000 SRCs in high school athletes. ^[1]

Among National Collegiate Athletic Association (NCAA) soccer players, the rate of injury has been reported as 0.4-0.6 per 1000 athlete exposures ^[3] ; 72% of these injuries were described as mild and were almost always secondary to direct contact with an opponent. None of the injuries in this group of Atlantic Coast Conference (ACC) soccer players was noted to be a direct result of heading the ball. In contrast, boxing is the sport with the highest rate of head injuries and has more deaths than any other organized athletic activity. At the professional level, many of the boxing bouts end with a technical knockout (ie, brain injury).

Sports activities that place the athlete at high risk for a head injury include boxing, football, ice hockey, wrestling, rugby, and soccer. Physicians and other allied health professionals who are responsible for the medical care of such contact or collision sports participants should be adept at evaluating, treating, and making playability decisions related to the short- and long-term consequences of an injury to the brain.

The mechanisms of concussion may differ among sports activities. Possible mechanisms of injury include compressive forces, which may directly injure the brain at the point of contact (coup); tensile forces produce injury at the point opposite the injury (contrecoup) because the axons and nerves are stretched; finally, rotational forces may result in a shearing of axons. Therefore, the direct force at the point of contact may not be solely responsible for the severity of an injury if a high rotational component with a significant shear effect occurs.

All of the different mechanisms may result in biochemical changes related to perfusion, energy demand, and utilization at the site of injury that are not well understood. It remains unclear whether any experimental animal model or human studies on more severe brain-injured patients accurately reflect the pathophysiology of the typical mild traumatic alteration in brain function.

A previous concussion is a significant risk factor for sustaining a subsequent concussion. ^{[14, 15, 16]}

One study reported that the risk of sustaining a concussion was 4-5 times higher in patients who had at least 1 concussion in the past. Another study reported that athletes with a history of 3 or more previous concussions were 3-fold more likely to have a concussion than players who had no history of concussion. ^[16]

Other risk factors for sustaining a concussion that have been suggested but not proven include not wearing mouth guards, poor fitting helmets, and genetic predisposition. ^{[17, 18]}  Research in all of these areas continues.

Most patients with a concussion are able to return to full competition without complication. Because many patients may not report minor head injuries to the athletic trainer, ED, or primary care physician, the overall prognosis of many head injuries is unclear.

A study including male high school football players noted that dizziness at the time of injury is associated with an increased risk of protracted (≥21 d) recovery. Another study involving athletes aged 9-23 years with a diagnosed protracted concussion found that those who have vestibular symptoms after concussion may have slower reaction times than those who do not and thus may be at greater risk for new injury. ^[19]

A study by Ling et al indicated that at least 4 months after an mTBI, the brain continues to display signs of damage, even if the clinical symptoms of injury have subsided. Evaluation of patients with mild brain injury, however, revealed no evidence of cortical or subcortical atrophy. ^[20]

In a prospective cohort study of 280 patients aged 11 to 22 years who presented to an ED with acute concussion, repeated concussions increased the risk for prolonged recovery. ^[21]  Patients with a history of previous concussions had symptoms that lasted twice as long (24 days) as those who did not have such a history (12 days).

An analysis of children presenting to the ED with concussion showed that the patients were still had a significant burden of symptoms 1 week after injury. ^[22]  Headache was the most common initial symptom; by day 7, 69.2% were still experiencing headaches. Fatigue persisted in 59.8% of children at day 7, and poor concentration persisted in 56.8% 1 week later. Emotional symptoms (eg, depression, frustration, irritability, and restlessness) also developed and increased by day 7 but were largely resolved by day 90.

A retrospective study by Kontos et al showed that adolescent athletes with concussion who received clinical care within 7 days of the injury recovered in a mean of 20 days more quickly than athletes who received care 8-20 days after the injury. The researchers suggest that the earlier initiation of active rehabilitation strategies may explain the more rapid recovery. ^[23]

Chronic symptoms can be quite severe, with the most dramatic presentation including dementia pugilistica, which is associated with boxing. This Alzheimer-like condition has a reported incidence of 15% among professional boxers. Fortunately, this condition is rare in most other sports. Hopefully, more frequent, detailed neuropsychologic testing will decrease the frequency of prolonged symptoms among elite and professional athletes by detecting more subtle injuries earlier.

Complications

Chronic traumatic encephalopathy (CTE) 

Persons with a history of repetitive brain trauma, including boxers and football players, are at risk for developing chronic traumatic encephalopathy (CTE), a progressive degenerative disease. Degenerative changes, which can begin months to decades after the patient’s last brain trauma, include atrophy of the cerebral hemispheres, medial temporal lobe, thalamus, mammillary bodies, and brainstem. The condition is also characterized by ventricular dilatation and by fenestration of the cavum septum pellucidum, as well as the accumulation of phosphorylated tau in the brain, with deposits of the protein being found in the sulci and in perivascular areas of the cerebral cortex. Symptoms of CTE include memory loss, confusion, impaired judgment, reduced impulse control, aggression, explosive anger, depression, and progressive dementia. ^{[24, 25]}

The incidence and prevalence of CTE in the general population and former athletes are unknown. Postmortem CTE changes and antemortem behavioral and cognitive manifestations have not demonstrated causality, with asymptomatic players having autopsy confirmed CTE pathology. ^[5]

According to a report from the US Department of Veterans Affairs and Boston University, 87 of 91 deceased former players for the National Football League (NFL) (96%) who donated their brains for study were found to have changes consistent with CTE. These finding need to be tempered by the fact the donors had, prior to death, expressed concern that they might have CTE and so may have had a higher proportion of the disease than does the overall population of former NFL players. In addition, these individuals had not necessarily had clinical symptoms of CTE but felt they might be at risk. ^{[26, 27]}

A study by Mez et al diagnosed CTE in 177 (78%) of 202 samples from deceased American football players. The samples included 111 former NFL players of which, 110 (99%) were diagnosed with CTE. The study also found that among the 26 participants diagnosed with mild CTE, 96% had behavioral or mood symptoms or both, 85% had cognitive symptoms, and 33% had signs of dementia. In the 84 participants diagnosed with severe CTE, 89% had behavioral or mood symptoms, 95% had cognitive symptoms, and 85% had signs of dementia. ^[28]

A study by Alosco et al showed that a distinct pattern of frontal-temporal atrophy on MRI may suggest CTE. Compared with persons with normal cognition, those with CTE had significantly greater atrophy in several brain regions, including the orbital-frontal cortex, dorsolateral frontal cortex, superior frontal cortex, anterior temporal lobes, and medial temporal lobe. ^[29]

Other complications

The following complications are unlikely to be caused by a concussion; however, healthcare professionals should be acutely aware of these pathologies when evaluating an athlete with a head injury.

A subdural hematoma is a rare injury in the athlete who presents with a presumed concussion. The classic presentation of a subdural hematoma is an acute and persistent LOC associated with the initial injury.

No association between epidural hematoma and brain injury exists. This condition classically presents with a brief period of unconsciousness, followed by a lucid period, and then a subsequent deterioration over 15-30 minutes. Tearing of the middle meningeal artery secondary to an associated temporal skull fracture is the usual cause of an epidural hematoma.

Subarachnoid bleeding may also occur with a head injury of any type. Worsening headache and other signs of increasing intracranial pressure will gradually grow after the initial event.

Second impact syndrome has been described. ^[30] In this condition, fatal brain swelling occurs after minor head trauma in individuals who still have symptoms from previous minor head trauma. Thus far, all cases of second impact syndrome have been described in relatively young patients (aged < 23 y). Significant controversy exists over the etiology of this condition, although it is thought to be secondary to loss of autoregulation of cerebral blood flow in an already injured brain.

Authors have questioned the validity of second impact syndrome due to problems with the documentation of the (1) initial event, (2) persistent symptoms, and (3) severity of the second impact. Despite these problems, practitioners should be aware of this possible complication, especially when treating the relatively immature brain of a young athlete. Treatment of second impact syndrome requires immediate recognition and immediate treatment with hyperventilation and osmotic agents. Surgical treatment for this condition is ineffective. The overall prognosis is usually grim.

Persistent postconcussive symptoms

Postconcussion syndrome is a diagnosis that has been frequently referenced to describe patients with lingering symptoms; however, a more preferred term is persistent postconcussive symptoms (PPCS). This is defined as symptoms that persist beyond the expected recovery time frame (>2 weeks in adults, >4 weeks in children). ^[5]  Common symptoms can be incorrectly attributed to concussion, which is why it is crucial to be aware of preexisting or coexisting symptoms.

A retrospective case-control study indicated that children with a personal or family history of mood disorders who sustain a sports-related concussion have a significantly increased risk for developing postconcussive syndrome. ^[31]

A study that included 2413 participants by Grool et al reported a lower risk of persistent postconcussive symptoms in those who participated in early physical activity compared to those with no physical activity (24.6% vs 43.5%; absolute risk difference, 18.9% [95% CI,14.7%-23.0%]). However, further clinical studies are needed to examine this association. ^[32]   

A double-blind study by Miller et al indicated that hyperbaric oxygen (HBO) is no better than sham therapy in the treatment of postconcussive syndrome. The study involved 72 persons, 94% of whom were enlisted in military service, with participants experiencing ongoing postconcussion symptoms for a period of at least 4 months after sustaining an mTBI. Patients received HBO treatment, sham air-compression therapy, or routine care alone. ^[33]

A study of retired professional football players (average age, 53.8 ± 13.4 y) by Guskiewicz et al reported significant memory changes in those players with a history of recurrent concussions. ^[34]  Another report by the same authors of these retired football players suggested a link between recurrent sports-related concussions and an increased risk of clinical depression. ^[15]

It is important to educate allied health professionals, coaches, school administrators, families, and athletes about the recognition and acute management of a concussion, the difficulties involved with a concussion, the difficulty in managing and treating concussions, and the subtle problems with long-term complications. Understanding and recognition of these issues may help prevent recurrent concussion problems. Inexperienced healthcare professionals may find the use of published guidelines helpful when initially managing these injuries.

It is also crucual to understand that no two concussions are alike, even in the same indivicual. Each concussion should be treated with a tailored plan specific to the current injury.