What Is a Concussion? How Doctors Define It

You know it when you see it, yet it’s still hard to define.

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While many people can recognize a concussion when they see it, it can actually be a challenge to define exactly. That’s partially because a concussion is another name for a “mild” traumatic brain injury (TBI)—and it’s not always easy to determine what counts as mild. 

“There are a lot of definitions of what a concussion is. It's kind of mind-boggling; it sort of adds to the confusion of what concussion is,” says Steven Flanagan, MD, chair of the Department of Rehabilitation Medicine at NYU Langone Health. 

The Centers for Disease Control and Prevention (CDC) defines a TBI as a “bump, blow or jolt to the head … that disrupts the normal function of the brain,” and if it’s mild, it simply results in “a brief change in mental status or consciousness.” This still leaves some room for interpretation: What is brief? What is normal?

“If there's loss of consciousness, by definition, it has to be less than 30 minutes,” says Dr. Flanagan, “and the symptoms generally resolve within hours, days, weeks, [or] sometimes a little bit longer.”

When loss of consciousness is prolonged, then it’s considered a moderate or severe TBI. These are life-threatening emergencies and they can cause long-term effects on the body. Thankfully, severe TBIs are much less common than mild ones, or concussions. (Here are warning signs that a TBI is an emergency.) 

A concussion is often defined by its symptoms, such as memory and thinking problems, mood changes, headaches, sleepiness, and sensitivity to light or noise. (Learn more symptoms of concussion here.) However, it helps to understand what causes a concussion. While a severe TBI may include penetration of the skull and brain—an obvious injury—a concussion can be a lot more subtle.   

“The brain is actually very soft. It has the consistency of formed Jello,” says Dr. Flanagan. “When your head gets jerked backwards and forwards and sort of to the side, the brain can actually compress on itself, stretch, [and] twist a little bit.”

This unnatural twisting and stretching doesn’t just hurt the brain tissue, but it stretches out a part of your nerve cells called axons. “The axons are like telephone wires, or like wires in a computer network. If they're stretched, there's going to be a period of time where they're just not gonna work properly, and they're not gonna transmit information properly,” says Dr. Flanagan.

These injuries to the axons is what leads to the telltale signs of concussion. For example, if the coordination between your eyes and ears are off because of axon malfunction, your balance is going to be off.

But the good news about a concussion is that most people can recover without any major issues. Learn more here about concussion recovery here.