Definition
Concussions range in severity from mild to severe, but they all share one common factor � they temporarily interfere with the way your brain works. They can affect memory, judgment, reflexes, speech, balance and coordination.

Usually caused by a blow to the head, concussions don't always involve a loss of consciousness. In fact, most people who have concussions never black out. Many people have had concussions and not realized it.

Concussions are common, particularly if you play a contact sport like football. But every concussion, no matter how mild, injures your brain. This injury needs time and rest to heal properly. Luckily, most concussions are mild and people usually recover fully.

Symptoms
The signs and symptoms of a concussion can be subtle and may not  appear immediately. Symptoms can last for days, weeks or longer.

The two most common concussion symptoms are confusion and amnesia. The amnesia, which may or may not be preceded by a loss of consciousness, almost always involves the loss of memory of the impact that caused the concussion.

Other immediate signs and symptoms of a concussion may include:

• Headache
• Dizziness
• Ringing in the ears
• Nausea or vomiting
• Slurred speech

Some symptoms of concussions don't appear until hours or days later. They include:

• Mood and cognitive disturbances
• Sensitivity to light and noise
• Sleep disturbances

Head trauma is very common in young children. But concussions can be difficult to recognize in infants and toddlers because they can't readily communicate how they feel. Nonverbal clues of a concussion may include:

• Listlessness, tiring easily
• Irritability, crankiness
• Change in eating or sleeping patterns
• Lack of interest in favorite toys
• Loss of balance, unsteady walking

Causes
Your brain has the consistency of gelatin. It's cushioned from everyday jolts and bumps by the cerebrospinal fluid that it floats in, inside your skull. A violent blow to your head can cause your brain to slide forcefully against the inner wall of your skull. Even the sudden stop of a car crash can bounce your brain off the inside of your skull. This can result in bleeding in or around your brain and the tearing of nerve fibers.
 
Risk factors
Any blow to the head can cause a concussion. Falls and traffic accidents often involve concussions, with or without other injuries. Without proper safety equipment and supervision, student athletes may be at increased risk of concussions when playing football and other contact sports. And anyone who has had a concussion in the past is at higher risk of having concussions in the future.