Ask Smithsonian: What Happens When You Get a Concussion?

It’s scary what we don’t know about the lasting effects after a knock to the noggin

Oodles of studies suggest that banging your head—maybe even a single time—causes short-term damage to the brain, but more recently, it’s become alarmingly clear that there may also be profound long-term consequences.

The process of exactly what happens to the brain after a knock on the noggin is still barely understood, as is the question of who is most vulnerable to long-lasting effects, or who is most at risk for permanent damage. There’s also no clear indicator for how long it takes for any one person to recover from a concussion.

If it seems like there are more questions than answers, that’s because until recently “there really wasn’t a lot of research dollars spent on understanding concussion,” says Patrick SF Bellgowan, program director for repair and plasticity at the National Institute of Neurological Disorders and Stroke.

In the past, it was thought that if you hit your head, you felt bad for a few weeks or a few months, and then you went on with your life. But recently, growing interest in brain science, along with new tools to delve into its mysteries—as well as enhanced public awareness about sports-related concussions and brain injuries sustained in war—has brought a sharpened focus to the issue.

According to the Centers for Disease Control and Prevention, a concussion is “a type of traumatic brain injury (TBI) caused by a bump, blow or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth.” The sudden movement—and there’s no particular threshold for how much is too much, says Bellgowan—can cause the brain to bounce around or twist inside the skull. The trauma stretches and damages the brain cells, or neurons, and also results in chemical changes.

The damage can lead to breaks in the blood-brain barrier—the semi-permeable wall that allows beneficial substances to flow in and out, but also blocks foreign and potentially harmful substances. Inflammation is the response to the leak, which is good initially because it plugs the gap. But chronic inflammation may be linked to longer-term damage, perhaps even chronic traumatic encephalopathy, a brain disease that’s been seen with increasing frequency in aging athletes.

The damage to neurons changes the communication within the brain’s circuitry, which is why people get confused or think more slowly after a concussion, says Bellgowan. And there’s lots of "secondary injuries and multiple pathways that can be disrupted," Bellgowan adds, "so no two concussions are the same, even within the same person.”

Who’s more at risk for concussion? It’s easy to say that people involved in sports that involve a lot of contact or direct blows to the head, like football, boxing, soccer, hockey, basketball and lacrosse, are at risk just by participating. That’s why the National Football League, the National Hockey League and the National Collegiate Athletic Association have all been involved in class action lawsuits alleging that athletes sustained lasting damage from multiple head blows. There’s some data from population-wide studies suggesting that women may be more susceptible and have longer-lasting injuries, but that could be just because they are more likely to report concussions, said Bellgowan.

It’s possible that some people may be genetically more likely to sustain an injury, or for others, that their brains float more in their skulls, heightening the risk of damage, but there’s little scientific evidence to support those theories, he says.

Even diagnosing a concussion is not exact. A physician might order a CT scan or an MRI to make sure there is no bleeding in the brain, but otherwise, a diagnosis is primarily based on the symptoms—nausea, headache, sensitivity to light, difficulty thinking and clumsiness, among others. The NIH and many other institutions are trying to come up with a quick test that would identify substances in the blood associated with a concussion or a brain injury. Brown University, for instance, reported in March of this year that its scientists had come up with a blood test for concussion that could be used in the emergency room, and that they’d already filed for a patent with the aim of commercializing the diagnostic.

Blood tests could also be used after a person was injured to help determine if their brain had healed enough for them to go back to work or to the playing field, says Bellgowan.

Concussion is considered a mild form of traumatic brain injury. But that’s a misnomer, says Bellgowan, because “it’s only mild relative to someone who’s going to be in the intensive care unit for a month and a half.” At least two million Americans a year, says the CDC, receive a TBI diagnosis at the emergency room. Forty percent are due to falls, which are the main cause of a TBI in older people and children. Being hit in the head is the second leading cause of TBI, accounting for 15 percent of emergency room diagnoses.

Figuring out how to treat a concussion and its multiple complications is no easy task. There’s no approved drug therapy or other medical intervention. Generally, people are told to limit activities and then to return to them gradually. But there’s little scientific rationale for how that may help the brain heal, says Bellgowan. It’s just thought that if you aggravate the symptoms, you’ll slow down healing, he adds.

Scientists are focusing more now on how concussion affects children. With their still-developing brains, they are considered to be potentially more vulnerable to longer-lasting problems, but that’s not a given, says Bellgowan.

It’s still not clear if the brain ever truly heals from a concussion—whether it’s one or many. The answers to that and other TBI-related questions may come from eight projects the NIH is funding, thanks to a $30-million grant that the NFL donated in 2012.

But with so little data on concussion and TBI, “we’re in a field that’s in an infant stage,” says Bellgowan.

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