Brain injury and concussion
Brain injuries and concussion are invisible, physical injuries. Acquired brain injury is grouped into traumatic brain injury (TBI) / concussion, and non-traumatic brain injury.
Non-traumatic brain injury is caused by an internal event (e.g. stroke, aneurysm, poisoning, anoxia, brain tumour, infection).
This categorization can be misleading, because many would equate an injury labelled "traumatic" as serious, as one labelled as "non-traumatic" as minor. This is not necessarily so. The labelling comes from the root meaning of the word trauma (rub, turn, twist, pierce) and is not an indicator of the severity of the injury.
An external event caused the injury.
An internal event caused the injury.