Impairment is an un-recoverable loss of body function or structure despite of adequate treatments for the injuries or diseases. Disability evaluation is an assessment of the degree and nature of the impairment. The optimal time for the disability evaluation is when the symptoms are fixed. To get the optimal time for disability evaluation, the author tried to assume the average period of symptom fixation after closed head injuries.

The author reviewed the literature on the long-term outcome after traumatic brain injuries. The author investigated factors influencing the long-term outcome in reports describing late outcome (more than 2 years after head injuries) or comparing the short-term (1 or 2-year) outcome and the long-term outcome.

In 60%~80% of the injured patients, the long-term outcome did not significantly differ from the short-term outcome up to 5 years. The long-term outcome can be better in 15~25%, when the initial brain injury was mild, pre-injury education level or work was good, and the cognitive function and behavior were good at the 1-year outcome. It may worse in 5~20%, when the age was below 7 or above 40, the initial brain injury was severe, pre-injury education level or work status was poor, and there was poor cognitive function or behavioral problems at the 1-year outcome. However, the degree of the change after 1 year is not so high, even in selected cases. The changes are usually apparent by some tools to measure minute changes.

The disability evaluation can be done at 1 year after head injuries, since the outcome will not change up to 5 years later, at least in 60~80%. The optimal time for disability evaluation can be individualized according to the Maximal Medical Improvement, when there is a possibility to change the degree of disability significantly.