Causes and signs of a concussion
TBI, also known as a Traumatic Brain Injury could be caused by a blow or hit to the head or a hit to the body where even the head and the brain get affected and move back and forth rapidly. Concussion is what happens as a result when the brain moves rapidly in the skull. This also creates chemical changes in the brain at times and can lead to brain cell damage.
Though medically concussions are known to be common and harmless, there could be serious effects. Causes of a concussion are not completely inevitable as they are also external, but one can always take precautions.

Our brain is cushioned by cerebrospinal fluid inside the skull to protect it from jolts and bumps. This cushioning is great for protecting the brain from day to day head bumps that are common, but when there is a bump or a blow that is more intense than the regular and harmless bumps, the skull cannot provide 100% protection at that time and it causes the brain to move inside the skull concussion.
When there is a sudden movement and head gets involved in that extremely sudden movement, it could cause concussion to happen. Car crash, a violent shake are examples of a sudden movement.
Some common signs of a concussion include:
Fatigue 
 Unconsciousness 
 Nausea 
 Dizziness 
 Ringing in the ears 
 Vomiting 
 Confusion 
 Vision sensitivity 
 Hearing sensitivity 
 Lack on concentration 
 Lack of interest 
 Sleepiness 
 Inattentive 
 Taste and smell disorder 
 Seizures 
Medical attention must be given when the signs of a concussion are there for longer period.
You will see the signs of a concussion appearing more than once when the it is the third grade of concussion, which is a serious grade and one must visit the doctor immediately.
If you are not able to diagnose a concussion despite paying close attention to all the signs of a concussion, you can take the person to the hospital and let the doctors diagnose the problem. In the hospital, doctors perform an overnight observation (more when it is a serious case), conduct a verbal or written checklist test that helps doctors to identify if the patient has the signs of a concussion or not based on his replies to the question.

