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Second-impact syndrome (SIS) is a rare condition in which the brain swells rapidly and catastrophically after a patient suffers a second concussion before symptoms from an earlier concussion have subsided. SIS may develop in patients who receive a second blow days or weeks after an initial concussion.[1] Even the mildest grade of concussion can lead to SIS.[2] The condition is often fatal, and patients that are not killed are almost always severely disabled. No one is certain of the cause of SIS, 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 swelling.[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.


Though the incidence of second impact syndrome is unknown,[3] the condition is rare; very few cases have been confirmed in medical literature.[1] Thirty-five football-related cases of SIS were recorded in the 13 year period from 1980-1993, but only 17 of these were confirmed by necropsy or surgery and magnetic resonance imaging to be due to SIS.[2] Eighteen 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.[4]

Some researchers estimate that the syndrome kills four to six people under the age of 18 per year.[5] 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.[6]

Some professionals think that the condition is overdiagnosed,[1] and some doubt the validity of the diagnosis altogether.[7]


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, association football, American football, baseball, basketball, hockey, and skiing are at increased risk.[6]

Second-impact syndrome disproportionately affects teenagers. Almost all documented cases of SIS have occurred in young people.[8] Most cases occurred in athletes under the age of 18.[9] Young athletes have been found to be both more susceptible to concussions and more likely to get second-impact-syndrome than their older counterparts.[10] Adolescent and young adult males who play football or hockey, or who box or ski are the most common sufferers of the condition.[4] However, SIS is also a concern for adult athletes.[11]

At least one study has found that people who have received one concussion are at a higher risk than those who have not to receive a concussion in the future.[6]


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.[12] Such symptoms of 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.[13] 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.[13] Loss of consciousness during the second injury is not necessary for SIS to occur.[14][15] The patient may appear dazed immediately after the injury, and within minutes may collapse and cease breathing.[13] Death can follow shortly after the second impact.[14]

SIS is distinct from repetitive head injury syndrome, in which a patient 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 short- and long-term.[1]

Magnetic resonance imaging and computed tomography are the most useful imaging tools for detecting SIS.[2]


Concussion temporarily changes the brain's function. It is believed that the brain is left in a vulnerable state for up to several days after a concussion; it is more susceptible to damage from a second blow.[16] The actual mechanism behind the catastrophic brain swelling is controversial.[17]

A second injury during this time is thought to unleash a series of metabolic events within the brain,[18] including a loss of autoregulation of the brain's blood vessels,[8] which causes them to become congested[3] and to dilate, greatly increasing their diameter.[1] This leads to a large increase in cerebral blood flow.[1][18] Progressive cerebral edema may also occur.[19] The increase of blood and brain volume within the skull causes a rapid and severe increase in intracranial pressure, which can in turn cause brain herniation,[1] a disasterous and potentially fatal condition in which the brain is squeezed past structures within the skull.

Changes indicative of SIS may begin occurring in the injured brain within 15 seconds of the second concussion.[20] Failure of the brain stem can occur within five minutes of the second impact.[1][2][20]


Measures that prevent head injuries in general also prevent SIS. Thus athletes are advised to use protective gear such as helmets,[1][20] though helmets do not entirely prevent the syndrome.[18]

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.[1][2][21] Athletes are also discouraged from returning to play until after they have been evaluated by a healthcare provider who is skilled in evaluating patients with concussion and the provider approves return to play.[5] Some athletes may deny concussion symptoms because they do not wish to be prevented from rejoining the game. Thus some professionals advise that trainers prohibit return to play for any athlete who has suffered a concussion, regardless of the speed with which the symptoms reportedly resolve.[13] An initial head injury may impair a patient'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 the patient not to return to play.[20]

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.[22] 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.[23]

Some researchers believe that the current return to play guidelines may not be strict enough to protect young athletes from SIS.[24]


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 patient can experience effects similar to those of severe traumatic brain injury, including persistent muscle spasms and tenseness, emotional instability, hallucinations,[14] epilepsy, mental disability, paralysis,[4] coma, and brain death.[25]

Treatment of the loss of autoregulation of the brain's blood vessels may be difficult or impossible.[3] When SIS occurs, surgery does not help and there is little hope for recovery.

The mortality rate for SIS approaches 50%.[13] Morbidity for patients with SIS is almost 100%.[9]


The condition was first described in 1973,[12] and the term second-impact syndrome was coined in 1984.[1] Between 1984 and 1991, only four cases were documented.[3] Between 1992 and 1998, the condition began to be reported more frequently than it had before, a fact is thought to be due to wider recognition of the syndrome by clinicians.[1]

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.[18] He is thought to have died from SIS.[26] Will's Bill, a law for which the father of the player had advocated after his death, was passed in Texas in June 2007.[18] The law mandates basic emergency and safety training for sports officials in high schools, including education about second-impact syndrome,[18] with a special focus on concussion symptoms.[26]


Some experts have questioned the existence of SIS. Some suggest instead that 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.[7] They say that this type of edema, referred to as 'diffuse cerebral swelling' is the real reason for the collapse which young people sometimes experience and which is commonly thought to be due to SIS.[27] They point to 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.[27] One group found that of 17 previously identified cases of SIS, only 5 met their diagnostic criteria for the syndrome,[28] with some cases not clearly involving a second impact.[7] The small number of reported cases leaves the question of whether SIS really causes the brain to swell catastrophically unanswered.[15] Similarly, 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.[28]

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 diffuse cerebral swelling or malignant cerebral edema does occur and that youth is associated with increased risk.[8]

See also[]


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