The Science Behind Second Impact Syndrome and Chronic Traumatic Encephalopathy

Second impact syndrome (SIS) can have in-an-instant, tragic consequences. Chronic traumatic encephalopathy (CTE), on the other hand, can have slow and heartbreaking consequences.

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Second Impact Syndrome involves two mild head injuries or concussions, mild traumatic brain injuries are also referred to, in close proximity to each other. Typically, the literature suggests that Second Impact Syndrome happens when an athlete has been injured early in a game and then maybe later in the game. We did not see that the person had that injury originally and they get an injury on top of it. It also can happen, though, within days of the first injury. So if you've had one injury and then 4 days later have another mild concussion, you may end up with Second Impact Syndrome. Second Impact Syndrome is a catastrophic reaction in the second exposure. What that really means is we have a massive intracranial pressure, and that involves autoregulatory systems in the vascular system. When that happens, a player, in fact, can die or have a very, very catastrophic outcome due to this intracranial pressure. So it looks like a very severe head injury, even though it's been only two mild head injuries. This is different, though, than what we talk about with regard to the cumulative effects in the brain. The cumulative effects, we believe at this point, can relate to a degenerative neurologic condition, which is called Chronic Traumatic Encephalopathy. In this, there is a change in brain tissue that is not related to intracranial pressure or bleeding or any autoregulatory problem, but it is, rather, a degeneration of tissue over time based on the fact that the person has had multiple concussive or subconcussive events. The difficulty, of course, with studying this is you can't do an experimental design to give some group of athletes a lot of concussions and have other players that don't get concussions and compare them 30 years down the line as to what their brain looks like. The interesting research that's going on with Dr. McKee and others shows us, though, that some professional athletes that have died at early ages of unrelated causes, their brains, in fact, look like they have a severe degenerative neurologic condition, much like Alzheimer's disease, but it is different histologically in some respects.
Posted on BrainLine October 20, 2010. Reviewed December 20, 2017.

Produced by Victoria Tilney McDonough and Brian King, BrainLine.