Second Impact Syndrome Explained | Shapiro, Washburn & Sharp

Let’s get the medical—and, quite frankly, frightening—medical definition of second impact syndrome (SIS) out of the way right up front. According to the American Association of Neurological Surgeons,

Second-impact syndrome results from acute, often fatal brain swelling that occurs when a second concussion is sustained before complete recovery from a previous concussion. This is thought to cause vascular congestion and increased intracranial pressure, which can occur very rapidly and may be difficult or impossible to control. The risk of second-impact syndrome is higher in sports such as boxing, football, ice or roller hockey, soccer, baseball, basketball and snow skiing.

Saving the life of a SIS victim can be difficult because heavy bleeding in the brain starts almost immediately and respiratory failure sets in soon after that.


SIS appears to be rare, but anyone who suffers a traumatic brain injury (TBI) or concussion should be careful to avoid blows to the head until all their symptoms resolve. This can mean waiting months or years to resume participation in certain athletic pursuits.

Post-concussion syndrome, which develops in as many as half of people who suffer what doctors call a mild TBI, leaves individuals struggling with headaches, dizziness, blurred vision, sleep disturbances and difficulties concentrating far past the point when they expected to return to all of their previous activities. Coaches, parents and student-athletes must be particularly attuned to and respectful of post-concussion syndrome and SIS.

The concerns over post-concussion syndrome and SIS should not stop at the edge of the playing field or the door of the arena, however. Car, truck and motorcycle crashes cause the largest number of TBIs and concussions. Pedestrians and bike riders who get hit by a motor vehicle are especially prone to brain injuries.

Then, falls rank third as the cause of brain injuries, right behind sports. The lesson is that second impact syndrome is a danger for millions of people each year.