Organic brain syndromes are conditions that can be traced to brain diseases, injuries, other factors affecting brain activity, or diseases of other organs or organ systems. The DSM-IVTR places organic brain syndromes in three groups: delirium, dementia, amnestic disorders, and other cognitive disorders; mental disorders due to a general medical condition; and substance-related disorders. The essential feature of dementia is memory disturbance, although debilitation may be observed in many other cognitive functions. According to the DSM-IV-TR definition, amnestic disturbances are attributable either to the direct physiological consequences of a general medical condition, the continued use of intoxicants or drugs, or exposure to toxic substances, or the cause may remain entirely unresolved. The postconcussional syndrome that arises from traumatic brain injury (TBI) typically involves memory and attention symptoms. The most significant diseases occurring concurrently with Tourette syndrome are obsessive-compulsive disease (OCD), attention-deficit hyperactivity disorder (ADHD), and learning difficulties.