An athlete who is recovering from a concussion, but who has not yet fully recovered, is at risk for second impact syndrome (SIS).
Typically, the athlete suffers post-concussion symptoms  after the first head injury, such as headache, visual, motor or sensory changes or mental difficulty, especially with the thought and memory  process. Before these symptoms have cleared, which may take minutes, hours, days or weeks, the athlete returns to competition and receives a second blow to the head, which can cause massive swelling in the brain. Since the brain is contained inside the rigid bone of the skull, this swelling causes compression of the brain. In severe cases, the brain is squeezed through small holes within the skull. This squeezing of the brain through these small holes is known as "herniation." Herniation can lead to decreased blood flow to the brain, and ultimately, to the athlete's death.
The second blow may be unremarkable, perhaps only involving a blow to the chest that jerks the athlete's head and indirectly sends accelerating forces to the brain. Affected athletes may appear stunned, but do not suffer loss of consciousness (LOC)  and often complete the play. They usually remain alert on their feet for 15 seconds to 1 minute or so but seem dazed. Often, affected athletes remain on the playing field or walk off under their own power. Usually within seconds to minutes of the second impact, the athlete - conscious but stunned - suddenly collapses to the ground, semi-conscious with rapidly dilating (widening) pupils and loss of eye movement, and stops breathing.
Every year during football season, it seems, there is an article in the newspaper about an athlete who returned to football before he had recovered completely from a concussion. Often, the athlete told his doctor, athletic trainer, and coach that he was better but confided to friends and teammates that he still had lingering symptoms, such as headaches or nausea. And, although no one remembers any major blows to the head or collisions, the athlete develops massive brain swelling and dies.
As Dr. William P. Meehan III , Director of the Sports Concussion Clinic at Children's Hospital Boston writes in his 2011 book, Kids, Sports, and Concussion , "[a]though second impact is rare, it has a devastating effect on those involved. Those who do survive second impact syndrome are neurologically devastated. They often spend many weeks to months in a coma. Many times they have a hole cut in their neck through which a permanent breathing tube is inserted. They are hooked up to a machine to help them breathe. They receive all of their nutrition intravenously as they are not awake enough or alert enough to eat or drink."
He notes that the effect on the athletic trainer, team physician, primary care physician, and other medical personnel involved in the athlete's care can also be devastating. Many:
As Dr. Meehan notes, "second impact syndrome is perhaps hardest on the teammates of the injured player":
Concussion experts agree that, in general, the younger the athlete, the longer it takes for the symptoms of a concussive event to clear. The brains of young athletes are still developing, making them particularly susceptible to catastrophic injury if the brain has not healed before a second blow to the head. Indeed, the vast majority of the victims of Second Impact Syndrome (95% by some estimates) are under the age of eighteen, almost all of them males. Second impact syndrome (or what some see as a form of malignant cerebral oedema, or swelling) is known to occur only in young children and adolescents, and does not occur in the adult population.
Dr. Meehan suspects, however, that "girls, women and older men are equally susceptible and cases [of second impact syndrome in these populations] may be seen in the future. Since the majority of athletes playing contact and collision sports are young men and boys, and since second impact syndrome is rare, it is statistically more likely to occur in young male athletes." But with the number of female athletes participating in contact and collision sports increasing every year, Dr. Meehan says that it "is possible that a case of second impact syndrome will occur in a female athlete in the future."
May 21, 2015 Update: Dr. Meehan's prediction may have come true: While the results of a coroner's inquest in Canada into the death of a 17-year-old female rugby player are not yet known, the testimony thus far strongly suggests that she died as a result of second impact syndrome, when she returned to play days after suffering a severe concussion and while admitting to friends, but not to her coach or medical personnel, that she was still experiencing concussion symptoms.
In an editorial in the September 2012 Clinical Journal of Sports Medicine,1 Christopher Randolph, PhD of the Department of Neurology at Loyola University Medical Center in Maywood, Illinois, reminds parents that "the risk of catastrophic outcome after sport-related concussion is very low. Each year, tens of millions of young people participate in organized sports, and death from sport-related head trauma, although devastating in every case, occurs less than death by a host of other causes, including sport-related cardiovascular events and lighting strikes."
As a result, Randolph says, "all athletes and parents need to be counseled that catastrophic outcomes from head injuries are an extremely rare, but inherent, risk of contact sports, and he argues that to suggest "that such an outcome would be eliminated by simply preventing repeat concussions or 'second impact syndrome' is grossly misleading."
1. Kirkwood MW, Randolph C, Yeates KO. Sport-Related Concussion: A Call for Evidence and Perspective Amidst the Alarms. Clin J Sports Med 2012;22(5):383-384.
2. Davis GA, Purcell LK. The evaluation and management of acute concussion in young children. Br J Sports Med. 2014;48:98-101. doi:10.1136/bjsports-2012-092132.
3. McCrory P, Davis G, Makdissi M. Second impact or cerebral swelling after sporting head injury. Curr Sports Med Rep. 2012;11:21-23.
Revised and updated May 21, 2015