The methods that are most helpful in assessing severity are different at different stages of recovery. Predictions made early on are often not very accurate but they get more accurate as time passes.
The level of consciousness (measured using ‘The Glasgow Coma Scale’) and the duration of unconsciousness (minutes, hours or days) immediately after the injury allow some general predictions to be made. Injuries that do not cause unconsciousness from the moment of impact do not usually cause significant physical damage to the nerve cells of the brain.
However, sometimes the injury damages blood vessels that leak and form a blood clot within the brain, triggering a sequence of changes in the brain that causes the level of consciousness to fall. Then the outcome will depend on how quickly medical and surgical treatment can be started. In such cases, although the effect of the initial impact may have been mild, the overall eventual damage may be severe.