Second-impact syndrome (SIS) is a condition in which the brain swells rapidly and catastrophically after a person suffers a second concussion before symptoms from an earlier one have subsided. This deadly second blow may occur days, weeks or minutes after an initial concussion,[1] and even the mildest grade of concussion can lead to SIS.[2] The condition is often fatal, and almost everyone who is not killed is severely disabled. The cause of SIS is uncertain, but it is thought that the brain's arterioles lose their ability to regulate their diameter, and therefore lose control over cerebral blood flow, causing massive cerebral edema.[2]
Most cases of SIS have occurred in young people, who are thought to be particularly vulnerable. Young athletes are most at risk. In order to prevent SIS, guidelines have been established to prohibit athletes from returning to a game prematurely. For example, professionals recommend that athletes not return to play before symptoms of an initial head injury have resolved.
Due to the very small number of recorded cases of SIS, there is doubt about whether it is a valid diagnosis. However, the syndrome is recognized by physicians.[3]
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Though the incidence of second impact syndrome is unknown,[4] the condition is rare; very few cases have been confirmed in medical literature.[1] In the 13 year period from 1980–1993, 35 football-related cases of SIS were recorded, but only 17 of these were confirmed by necropsy or surgery and magnetic resonance imaging to be due to SIS, and 18 cases were found to be probably SIS-related.[2] Additionally, the initial trauma commonly goes unreported, adding to the confusion about how often the syndrome occurs.[5]
By one estimate, the syndrome kills four to six people under the age of 18 per year.[6] According to the Centers for Disease Control, about 1.5 people die each year from concussion in the US; in most of these cases, the person had received another concussion previously.[7]
In part due to the poor documentation of the initial injury and continuing symptoms in recorded cases,[8] some professionals think that the condition is over-diagnosed,[1] and some doubt the validity of the diagnosis altogether.[9]
Second-impact syndrome shares all the risk factors of concussion; that is, those who are at increased risk for concussion are also at higher risk for SIS. Thus, people who participate in sports such as boxing, soccer, gridiron football, baseball, rugby, basketball, ice hockey, and skiing (especially Alpine) are at increased risk.[7] The condition most commonly occurs in American football.[10]
Second-impact syndrome disproportionately affects teenagers. All documented cases occurred in people younger than 20 except in boxing.[11] As of 2000, the syndrome had never been reported in the medical literature in children younger than adolescent age.[12] Young athletes have been found to be both more susceptible to concussions and more likely to get second-impact syndrome than their older counterparts.[13] However, SIS is also a concern for adult athletes.[14] Adolescent and young adult males who play football or hockey, or who box or ski are the most common sufferers of the condition.[5] Every documented case of SIS has occurred in males, but it is not known whether this is due to a special vulnerability or to a greater exposure of males to second impacts.[15]
Studies have found that people who have received one concussion are at a higher risk to receive a concussion in the future.[16]
SIS is the most serious potential complication that could result from an athlete returning to a game before symptoms from a minor head injury have subsided.[17] Such symptoms include headache, cognitive difficulties, or visual changes.[1]
The initial injury may be a concussion, or it may be another, more severe, type of head trauma, such as cerebral contusion.[18] However, the first concussion need not be severe for the second impact to cause SIS.[19] Also, the second impact may be very minor, even a blow such as an impact to the chest that causes the head to jerk, thereby transmitting forces of acceleration to the brain.[20] Loss of consciousness during the second injury is not necessary for SIS to occur.[21][22] Both injuries may take place in the same game.[11]
The athlete may continue playing in the game after the second concussion, and may walk off the field without assistance, but symptoms quickly progress and the condition can rapidly worsen.[23] Neurological collapse can occur within a short period,[12] with rapid onset of dilating pupils, loss of eye movement, unconsciousness, and respiratory failure.[23] Failure of the brain stem frequently occurs between two and five minutes after the second impact,[1][2][23] and death can follow shortly.[21]
SIS is sometimes associated with a small subdural hematoma.[11]
Concussion temporarily changes the brain's function.[16] It is believed that the brain is left in a vulnerable state after concussion and that a second blow is linked to SIS.[25] The actual mechanism behind the catastrophic brain swelling is controversial.[8][26] A second injury during this time is thought to unleash a series of metabolic events within the brain.[27] Changes indicative of SIS may begin occurring in the injured brain within 15 seconds of the second concussion.[28] Pathophysiological changes in SIS can include a loss of autoregulation of the brain's blood vessels,[12][29][30] which causes them to become congested.[4][12] The vessels dilate, greatly increasing their diameter and leading to a large increase in cerebral blood flow.[1] Progressive cerebral edema may also occur.[29][31] The increase of blood and brain volume within the skull causes a rapid and severe increase in intracranial pressure, which can in turn cause uncal and cerebellar brain herniation, a disastrous and potentially fatal condition in which the brain is squeezed past structures within the skull.[12]
Studies on animals have shown that the brain may be more vulnerable to a second concussive injury administered shortly after a first.[32] In one such study, a mild impact administered within 24 hours of another one with minimal neurological impairment caused massive breakdown of the blood brain barrier and subsequent brain swelling.[25] Loss of this protective barrier could be responsible for the edema found in SIS.[25]
Animal studies have shown that the immature brain may be more vulnerable to brain trauma; these findings may provide clues as to why SIS affects only people under age 18.[32]
Measures that prevent head injuries in general also prevent SIS. Thus athletes are advised to use protective gear such as helmets,[1] though helmets do not entirely prevent the syndrome.[27]
Experts advise that athletes who have suffered one concussion and still complain of concussion-related symptoms be prohibited from returning to the game due to the possibility of developing SIS.[2][33] Athletes are also discouraged from returning to play until after they have been evaluated and approved for resuming the sport by a healthcare provider skilled in evaluating concussion.[6] Some athletes may deny concussion symptoms because they do not wish to be prevented from rejoining the game. Thus athletic trainers may prohibit return to play for any athlete who has suffered a concussion, regardless of the speed with which the symptoms reportedly resolve.[34] An initial head injury may impair an athlete's judgment and ability to decide to refrain from participating in risky activity, so some healthcare providers encourage family members and other acquaintances to pressure an athlete not to return to play.[28]
Several different sets of return-to-play guidelines exist for athletes who have suffered minor head trauma. These exist in part to prevent the player from developing SIS.[35] A variety of concussion grading systems have been devised, in part to aid in this determination. Every return-to-play guideline recommends that an athlete not return to competition until all concussion symptoms are absent during both rest and exercise.[12] The American Academy of Neurology recommends that young athletes be prohibited from returning to play for at least a week in most cases of concussion.[36]
The current return-to-play guidelines may not be strict enough to protect young athletes from SIS.[37] On the other hand, they may be too strict for professional football players; evidence that the syndrome exists in this population is lacking.[38]
Magnetic resonance imaging and computed tomography are the most useful imaging tools for detecting SIS.[2] The congestion in the brain's blood vessels may be visible using CT scans.[29]
SIS is distinct from repetitive head injury syndrome, in which a person suffers a series of minor head injuries over time and experiences a slow decline in functions such as cognitive abilities.[1] Unlike SIS, repetitive head injury syndrome may still occur even when symptoms from prior injuries have completely resolved.[1] SIS is thought to be more severe than repetitive head injury syndrome in both the short- and long-term.[1]
Treatment of the loss of autoregulation of the brain's blood vessels may be difficult or impossible.[4] When SIS occurs, surgery does not help and there is little hope for recovery.[8] Treatment requires immediate recognition and includes administration of osmotic agents and hyperventilation[8] in order to lower intracranial pressure.
The mortality rate for SIS approaches 50%,[34] and morbidity (disability) is almost 100%.[39] Since the condition is so rare, the connection between SIS and future disability has been difficult to establish and is therefore poorly understood.[1]
When SIS is not fatal, the effects similar to those of severe traumatic brain injury can occur, including persistent muscle spasms and tenseness, emotional instability, hallucinations,[21] post-traumatic epilepsy, mental disability, paralysis,[5] coma, and brain death.[40]
The condition was first described in 1973[17] by R.C. Schneider,[10] and the term second-impact syndrome was coined in 1984.[1] In 1984, R.L. Saunders described the death of a football player who had died after a second, unremarkable concussion[41] and hypothesized that the second blow caused a catastrophic rise in ICP, possibly through loss of vasomotor tone, because the brain was in a vulnerable state.[42] Between 1984 and 1991, only four cases were documented.[4] Between 1992 and 1998, reports of the condition began to be made more frequently than they had before, a fact is thought to be due to wider recognition of the syndrome by clinicians.[1] In 1991, J.P. Kelly and others reported another football death after repeated concussions[43] and coined the term "vascular congestion syndrome".[42]
In 2002, a high-school football player from Texas named Will Benson suffered two concussions within two weeks; he collapsed shortly after the second one and died six days later as the result.[27] He is thought to have died from SIS.[44] Will's Bill, a law for which the father of the player had advocated after his death, was passed in Texas in June 2007.[27] The law mandates basic emergency and safety training for sports officials in high schools, including education about second-impact syndrome,[27] with a special focus on concussion symptoms.[44]
By 2003, 21 cases of SIS had been reported in medical literature.[42]
The existence of SIS is in question and is somewhat controversial.[32] The sudden collapse seen in sufferers may be due to a type of cerebral edema that can follow an initial impact in children and teenagers, rather than to SIS.[9] This type of edema, referred to as 'diffuse cerebral swelling' may be the real reason for the collapse which young people sometimes experience and which is commonly thought to be due to SIS.[45] Those who doubt the validity of the diagnosis cite the finding that diffuse cerebral swelling is more common in children and adolescents as an explanation for the greater frequency of SIS diagnoses in young people.[45] One group found that of 17 previously identified cases of SIS, only 5 met their diagnostic criteria for the syndrome,[15] with some cases not clearly involving a second impact.[9] They found that diagnoses of SIS were frequently based on bystander accounts of previous injuries, which they showed to be unreliable.[10] Teammates of players who are thought to have SIS may overreport the initial concussion, giving the appearance of a greater number of second impacts than actually exist.[15] Thus critics argue that the small number of reported cases leaves the question of whether SIS really causes the brain to swell catastrophically unanswered.[22]
Whether a second impact is really involved in the diffuse cerebral swelling that occurs on rare occasions after a mild traumatic brain injury is controversial, but experts agree that such catastrophic brain swelling does occur after a very a small number of mild brain injuries and that young age is associated with increased risk.[30] It is also agreed that some people may be particularly vulnerable to catastrophic brain swelling as the result of multiple head injuries.[10]
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