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. 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, sailing, 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.
Pathophysiology
A concussion temporarily changes the brain's function. It is believed that the brain is left in a vulnerable state after a concussion and that a second blow is linked to SIS. The actual mechanism behind the catastrophic brain swelling is controversial. A second injury during this time is thought to unleash a series of metabolic events within the brain. Changes indicative of SIS may begin occurring in the injured brain within 15 seconds of the second concussion. Pathophysiological changes in SIS can include a loss of autoregulation of the brain's blood vessels, which causes them to become congested. The vessels dilate, greatly increasing their diameter and leading to a large increase in cerebral blood flow. Progressive cerebral edema may also occur. 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. Studies on animals have shown that the brain may be more vulnerable to a second concussive injury administered shortly after a first. 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. Loss of this protective barrier could be responsible for the edema found in SIS. Animal studies have shown that the immature brain may be more vulnerable to brain trauma; these findings may provide clues as to why SIS primarily affects people under age 18.
Diagnosis Magnetic resonance imaging and computed tomography are the most useful imaging tools for detecting SIS. The congestion in the brain's blood vessels may be visible using CT scans. SIS is distinct from repetitive head injury syndrome, in which a person has a series of minor head injuries over time and experiences a slow decline in functions such as cognitive abilities. Unlike SIS, repetitive head injury syndrome may still occur even when symptoms from prior injuries have completely resolved. SIS is thought to be more severe than repetitive head injury syndrome in both the short- and long-term.
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