Excerpt from Repetitive Head Injury Syndrome

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Background

Primary head injury can be catastrophic, but the effects of repetitive head injuries must also be considered. Second-impact syndrome (SIS), a term coined in 1984, describes the situation in which an individual sustains a second head injury before the symptoms from the first head injury have resolved.

The second injury may occur from days to weeks following the first. Loss of consciousness is not a requirement of this condition, the impact may seem relatively mild, and the athlete may appear only dazed initially. However, this second impact causes cerebral edema and herniation, leading to collapse and death within minutes. Only 17 cases of confirmed SIS have been reported in the medical literature. Thus, the true risk and pathophysiology of SIS has not been clearly established.

Importantly, even if the effects of the initial brain injury have already resolved (6-18 mo post injury), the effect of multiple concussions over time remains significant and can result in long-term neurologic and functional deficits. These multiple brain insults can still be termed repetitive head injury syndrome, but they do not fit the classification of SIS. True SIS would most likely have a devastating outcome.

A study of American high school and college football players demonstrated 94 catastrophic head injuries (significant intracranial bleeding or edema) over a 13-year period.¹ Of these, only 2 occurred at the college level. Seventy-one percent of high school players suffering such injuries had a previous concussion in the same season, with 39% playing with residual symptoms. On the other hand, results from a study of concussion by the National Football League demonstrated no cases of SIS or catastrophic head injury in players returning to play in the same game after resolution of symptoms.²

The outcome of multiple minor head injuries over a prolonged period has not been well studied and is not well understood. The preponderance of data assessing the impact of repetitive head injuries on short- and long-term neurologic (cognitive) performance has been focused on the sports of boxing and American football.^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}

Numerous studies of professional boxers have shown that repeated brain injury can lead to chronic encephalopathy, termed dementia pugilistica.^{12, 13, 14, 15} Likewise, the autopsies of 2 former professional football players with a history multiple concussions demonstrated changes that were consistent with chronic encephalopathy.^{5, 6}
Another investigation of retired professional football players showed a 3-fold increase of depression in players with a history of 3 or more concussions.³ Older studies of American and Australian rules football showed no effect from repetitive mild head injuries.¹¹ However, more recent studies of collegiate football players showed an association between multiple concussions and reduced cognitive performance, prolonged recovery, and the increased likelihood of subsequent concussions.

Evidence has also been gleaned from other sports that involve head impact. Nonrandomized studies of soccer players who have had multiple minor concussions have demonstrated that these individuals performed worse on neuropsychologic tests compared with a control group.^{16, 17, 18, 19}

Neuropsychologic testing is the standard for monitoring cognitive recovery after concussion. However, 2 studies suggest that abnormalities in visual motor and motor cortex function persist after neuropsychologic testing has normalized.^{8, 20} Slower recovery in patients with a second concussion was also seen.

Basic science research is also ongoing. Experiments in concussed rats demonstrated prolonged abnormalities in metabolic markers of brain activity when a second impact was administered at 3 days^{21, 22} This implies there may be a metabolic window of vulnerability to a second impact that leads to chronic or prolonged symptoms. Clinically useful biomarkers for brain injury are also being investigated.

Certainly, more research is needed to better understand the chronic and catastrophic effects of repetitive head injuries.

For excellent patient education resources, visit eMedicine's Back, Ribs, Neck, and Head Center and Dementia Center. Also, see eMedicine's patient education articles Concussion and Dementia in Head Injury.

Frequency

United States

The National Center for Catastrophic Sports Injury Research in Chapel Hill, NC, reported 35 cases of SIS among American football players from 1980-1993. Seventeen were confirmed by necropsy, surgery, or magnetic resonance imaging (MRI) findings. Eighteen were probable cases of SIS, despite inconclusive necropsy findings.

The number of reported SIS cases increased from 1992-1998, but this increase is thought to be due to more frequent recognition and reporting. Some clinicians believe that SIS is overreported. Boden et al reported an average of 7.08 catastrophic head injuries per year in high school football, compared with 0.15 for college football from 1989-2002.¹ The incidence was 0.67/100,000 for high school players and 0.21/100,000 for college players. Thirty-nine percent of the affected athletes reported playing with residual symptoms.¹ There were 8 fatalities, of which 1 individual had cerebral edema as the only radiographic finding. It was unclear as to whether a second impact occurred in this case.

With the advent and improvement of the helmet in American football and with the introduction of new rules that make spearing illegal, the incidence of head-injury fatalities has decreased from 2.64 cases per 100,000 persons in 1968 to 0.20 cases per 100,000 persons since 1977. The US Centers for Disease Control and Prevention estimates a 20% rate of concussion from football brain injuries (predominantly high-school and college level), which equates to an estimated 300,000 concussions per year.

Collins et al showed that 20% of the college football players they studied had 2 or more concussions during their career.⁷ Furthermore, a study by Daniel et al found that the symptoms of an estimated 60,000 football players who suffer concussion may persist for 4 or more months in up to 24% of these individuals.²⁰

Schulz et al reported on a prospective cohort study of North Carolina high-school athletes followed from 1996–1999.²³ Subjects were clustered by school and sport, and the sample included 15,802 athletes, with 1–8 seasons of follow-up per athlete. Concussion rates ranged from 9.36 concussions per 100,000 athlete-exposures in cheerleading to 33.09 concussions per 100,000 athlete-exposures in football, where "athlete-exposure" is 1 athlete participating in 1 practice or game. The overall rate of concussion was 17.15 concussions per 100,000 athlete-exposures.

Cheerleading was the only sport for which the practice rate of concussions was greater than the game rate.²³ Almost two thirds of cheerleading concussions involved 2-level pyramids. Concussion rates were elevated for athletes with a history of concussion, and they increased with the increasing level of body contact permitted in the sport.

Powell and Barber-Foss reported a 2-year review of 235 US certified athletic high-school training records. The authors estimated a total of 62,816 cases of mild traumatic brain injury (TBI) annually among high-school varsity athletes, with football accounting for approximately 63% of these cases and a varied incidence among 10 other popular sports.²⁴

Matser et al showed that 23% of the amateur soccer players they studied had 2-5 concussions during their career.¹⁶ Boden et al found that the overall prevalence of college soccer-related concussions was 0.6 cases per 1000 athlete-exposures for men and 0.4 cases per 1000 athlete-exposures for women.¹⁷ The authors reported that the vast majority (72%) of these concussions were grade 1, and none were grade 3.¹⁷

The actual number of athletes who may be affected by repeated minor head injuries is largely unknown.

Functional Anatomy

SIS is thought to occur because of a loss of autoregulation of the cerebral blood flow, which leads to vascular engorgement, increased intracranial pressure (ICP), and eventual herniation. This herniation may involve the medial temporal lobe and may occur medially across the falx cerebri or inferiorly through the tentorium. Herniation can also force the cerebellar tonsils to move inferiorly through the foramen magnum. The athlete's condition rapidly worsens, and brainstem failure occurs in 2-5 minutes.

Sport-Specific Biomechanics

The brain is protected by bone and is cushioned by tough meninges and cerebrospinal fluid. Despite these protective surroundings, blunt-force trauma to the head can cause injury to the site of impact (coup injury) and the site immediately opposite of the impact (contrecoup injury). Factors that dissipate the force (eg, equipment, neck muscle strength) can minimize this trauma.

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