Concern about what has been termed, “second impact syndrome” (SIS) is a major factor
determining return-to-play decisions after concussion. However, definitions of SIS vary. We
used Scopus to conduct a systematic review and categorize the definitions used to describe
SIS. Of the 91 sources identified, 79 (87%) clearly specified that SIS involved either cerebral
edema or death after a concussion when a prior concussion had not resolved. Twelve
articles (13%) could be interpreted as merely the events of two consecutive concussions.
Among the articles that listed mortality rates, nearly all (33/35, 94%) said the rate of death
was “high” (e.g., 50% to 100%). Our review found that most articles define SIS as a
syndrome requiring catastrophic brain injury after consecutive concussive episodes. Given
that it is unclear how common it is to have a second concussion while not fully recovered
from a first concussion, the actual mortality rate of SIS is unknown.