Concussions occur when someone experiences a blow to the head or a violent movement that causes the brain to shift within the skull. If that movement is forceful enough, the brain tissue may twist, stretch, or rupture or the brain itself may hit the inside of the skull, possibly causing a concussion. There's a lot doctors still don't know about concussions. For example, no one knows whether—or by how much—wearing a bike helmet reduces the risk of concussion, but it likely helps to some degree. But one thing is for sure: concussions can be serious. Here are four important brain-safety facts to remember.

1. It’s not “blacking out.” While losing consciousness was once considered a hallmark of a concussion, in the “overwhelming majority” of concussions, the victim does not lose consciousness, according to Robert Cantu M.D., Clinical Professor of Neurology and Neurosurgery at Boston University Medical School.

Symptoms of concussions include confusion, dizziness, headache, nausea/vomiting, sensitivity to light, and trouble with memory. If you experience any of these symptoms after an accident or you have any question about the severity of your injury, seek emergency medical attention.  

2. The next concussion can be worse. If you’ve had a concussion and have not fully healed, subsequent head trauma will likely result in more symptoms which will take much longer to subside.  

3. Or much worse. Second impact syndrome is a difficult to diagnose but very dangerous complication that can occur when the victim of a concussion suffers another blow to the head before the first one is fully healed. The rapid and severe swelling of the brain can be fatal if not treated immediately. Even less understood is a potential long-term link between multiple concussions and increased risk of chronic traumatic encephalopathy (CTE), a progressive degenerative disease of the brain.

4. All whacks are created equal. Your brain doesn’t care how it got whacked. If you’ve suffered a recent concussion in a bicycle accident or any other sport, take special precautions to avoid a subsequent head injury whatever the cause.