Second-impact syndrome (SIS) occurs when the brain swells rapidly, and catastrophically, after a person has a second concussion before symptoms from an earlier one have subsided. This second blow may occur minutes, days, or weeks after an initial concussion, and even the mildest grade of concussion can lead to second impact syndrome. 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.
Most cases of SIS have occurred in children, who are thought to be particularly vulnerable. 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.
Signs and symptoms
SIS is a potential complication from an athlete returning to a game before symptoms from a minor head injury have subsided. Such symptoms include headache, cognitive difficulties, or visual changes.
The initial injury may be a concussion, or it may be another, more severe, type of head trauma, such as cerebral contusion. However, the first concussion need not be severe for the second impact to cause SIS. 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. Loss of consciousness during the second injury is not necessary for SIS to occur. Both injuries may take place in the same game.
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. Neurological collapse can occur within a short period, with rapid onset of dilating pupils, loss of eye movement, unconsciousness, and respiratory failure. Failure of the brain stem frequently occurs between two and five minutes after the second impact, and death can follow shortly.
SIS is sometimes associated with a small subdural hematoma.
Risk factors
Second-impact syndrome shares all the risk factors of a concussion; that is, those who are at increased risk for a concussion are also at higher risk for SIS. Thus, people who participate in sports such as boxing, Association football, American football, baseball, rugby, basketball, ice hockey, pro wrestling, horse riding, and skiing (especially Alpine) are at increased risk. The condition most commonly occurs in American football.
Second-impact syndrome disproportionately affects teenagers. All documented cases occurred in people younger than 20 except in boxing. As of 2000, the syndrome had never been reported in the medical literature in children younger than adolescent age. Young athletes have been found to be both more susceptible to concussions and more likely to get second-impact syndrome than their older counterparts. However, SIS is also a concern for adult athletes. Adolescent and young adult males who play American football or hockey, or who box or ski are the most common individuals with the condition. Most documented cases of SIS have 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.
In Canada, a law mandating concussion education for coaches, athletes, and medical practitioners was passed as a result of the death of Rowan Stringer, a female high school athlete who died after receiving three blows to the head in the course of one week in 2013. She showed only mild symptoms of headache following the first two impacts, but lost consciousness and never regained it following the third. Medical examiners found that SIS was the cause of her death.
Studies have found that people who have received one concussion are at a higher risk to receive a concussion in the future.