Epidemiology
Though the incidence of second impact syndrome is unknown, the condition is rare; very few cases have been confirmed in medical literature. In the 13 year period from 1980–1993, 35 football-related cases of SIS were recorded, but only 17 of these were confirmed by necropsy or surgery and magnetic resonance imaging to be due to SIS, and 18 cases were found to be probably SIS-related. Additionally, the initial trauma commonly goes unreported, adding to the confusion about how often the syndrome occurs.
By one estimate, the syndrome kills four to six people under the age of 18 per year. According to the Centers for Disease Control, about 1.5 people die each year from concussion in the US; in most of these cases, the person had received another concussion previously.
In part due to the poor documentation of the initial injury and continuing symptoms in recorded cases, some professionals think that the condition is over-diagnosed, and some doubt the validity of the diagnosis altogether.
Read more about this topic: Second-impact Syndrome