The popularity of sports exposes a large number of people daily to the risk of brain injury. Each year in the United States there are an estimated 500,000 sport-related brain concussions. Second Impact Syndrome” (SIS) occurs when a second concussion occurs within hours, days, weeks following a prior concussion. Despite its debilitating effects, many players, parents, and coaches believe that concussion should not restrict participation from competition. From Return to Contact Sports Following Concussion, By Dan Mendonca, MEDS 2000. Children/Youth Head Injury.