A Second Look at Second Impact Syndrome


Source: McCrory PR, Berkovic SF. Second impact syndrome. Neurology 1998;50:677-683.

Catastrophic swelling of the brain after seemingly mild head trauma is a rare but potentially deadly complication of many contact sports and falls, especially in children and young adults. When this occurs following two shortly successive blows to the head, it has been called the "second impact syndrome" (SIS). Awareness of this syndrome has influenced the management of concussion in athletes, although the statistical probability of the phenomenon has not been established. McCrory and Berkovic searched for documentation of SIS and identified only 17 published case reports. They excluded 35 additional cases of SIS cited by the U.S. National Center for Catastrophic Sport Injury Research because they lacked confirmatory details. They designed their own criteria to retrospectively differentiate definite, probable, and possible cases of SIS. In addition, they carried out a study in 102 Australian football players to test whether retrospective reports of head trauma during an athletic event are reliable.

Of the 17 reviewed cases, only five met criteria for probable SIS, with most of the others considered possible. None was considered definite SIS. Neuropathologic and/or neuroimaging evidence of diffuse cerebral swelling was present in 13 cases. However, only five cases had undergone a medical examination after the first impact, and only six had a witnessed deterioration after a second blow to the head. Ongoing symptoms from time of first impact to the second were reported in seven instances.

Australian football players were found to overreport episodes of head trauma in their teammates compared to self-report and videotaped episodes. This bias occurred for episodes of head trauma with loss of consciousness (LOC) as well as minor episodes without LOC. The authors conclude that SIS either doesn't exist or has been overdiagnosed. They suggest that the prevention strategies currently followed to avoid cerebral edema in athletes are arbitrary and unproven, and call for case-control studies to further assess whether previous head trauma is a risk factor for diffuse brain swelling.


Contact sports such as boxing, American football, and soccer carry a small but potentially lethal risk of trauma-induced acute brain swelling. Unfortunately, young athletes are rarely educated about such risks and are often inadequately protected from them. Amateur and professional athletes may be motivated to ignore their injuries in order to continue play. Amateur championships sometimes require participation in several events over successive days, a practice that carries with it an increased risk of repeated brain injury. As recently as 1996, a 19-year-old boxer died of diffuse cerebral swelling while participating in his fourth fight in a week as part of a Golden Gloves competition in Texas. While such deaths may be unpredictable, they are conceivably preventable through improved inter-event neurological evaluation.

McCrory and Berkovic may be justified in questioning whether the occurrence of the second impact syndrome has been over-reported. However, even if diffuse cerebral edema after sports-related head trauma is decidedly rare, its predilection to affect children and adolescents makes it worthy of particular attention. We believe that concern about the possibility of SIS should continue to be one of the motivating factors in delaying return to sports after recent concussion. Since head trauma can lead to slowed reaction times and altered information processing, even mild signs and symptoms place the athlete at increased risk for successive injuries and, therefore, deserve careful neurologic attention. Irrespective of whether second impact has been proven to cause massive cerebral edema, greater vigilance should be given to assessing post-concussive changes, both to better understand SIS and to protect the lives and well-being of our young athletes. -nrr