Post-concussion syndrome
Post-concussion syndrome, also known as postconcussive syndrome or PCS, is a set of symptoms that may continue for weeks, months, or a year or more after a concussion – a mild form of traumatic brain injury (TBI). The rates of PCS vary, but most studies report that about 15% of individuals with a history of a single concussion develop persistent symptoms associated with the injury. A diagnosis may be made when symptoms resulting from concussion last for more than three months after the injury. Loss of consciousness is not required for a diagnosis of concussion or post-concussion syndrome.
Post-concussion syndrome
Not all people with PCS have abnormalities on imaging, however, and abnormalities found in studies such as fMRI, PET, and SPECT could result from other comorbid conditions such as depression, chronic pain, or post-traumatic stress disorder (PTSD). 
Proponents of the view that PCS has a physiological basis point to findings that children demonstrate deficits on standardized tests of cognitive function following a mild TBI. A few studies have shown that people with PCS score lower than controls on neuropsychological tests that measure attention, verbal learning, reasoning, and information processing, but issues related to effort and secondary gain can not be ruled out as contributing to these differences. Recovery as measured by scores on cognitive tests frequently do not correlate with resolution of symptoms; individuals diagnosed with PCS may still report subjective symptoms after their performance on tests of cognitive functioning have returned to normal. Another study found that although children with PCS had poorer scores on tests of cognitive functioning after the injury, they also had poorer behavioral adjustment before the injury than children with no persistent symptoms; these findings support the idea that PCS may result from a combination of factors such as brain dysfunction resulting from head injury and preexisting psychological or social problems. Different symptoms may be predicted by different factors; for example, one study found that cognitive and physical symptoms were not predicted by the manner in which parents and family members coped with the injury and adjusted to its effects, but psychological and behavioral symptoms were.