A group of disorders caused by or associated with impairment of brain tissue function, and characterized by impairment of (a) orientation, (b) memory, (c) judgment,(d) general intellectual functions such as comprehension, learning, and reasoning, and by (e) lability or shallowness of affect (emotional reactions). These symptoms may be mild, moderate, or severe, depending on the nature and extent of the brain damage. The individual’s reaction to the impaired functioning depends not only on the severity of the precipitating organic disorder, but on his personality characteristics, current emotional conflicts, life situation, and relationships. For this reason, severe brain damage sometimes produces surprisingly slight effects on the personality; and, conversely, mild damage may lead to extreme and even psychotic reactions.Approximately 15 million people in the United States suffer from brain disorders, though the great majority of cases do not involve serious mental disturbance. Nevertheless, disorders associated with brain pathology constitute over one third of all first admissions to mental hospitals. This percentage is constantly increasing due to the lengthening of the life span and the fact that brain disorders reach a peak in old age.Organic brain disorders are classified as acute and chronic. The acute disorders result from temporary, reversible impairment; the chronic disorders involve permanent damage to the nervous system. However, a sharp line cannot be drawn between the two types, since some conditions, such as a blow on the head, may produce what appears to be a temporary disorder but actually cause permanent damage; while other conditions, such as pernicious anemia, may produce effects that last for weeks or months, yet the patient may recover fully in the long run.Acute brain disorders produce symptoms ranging from mild changes in mood to acute delirium with delusions and hallucinations. In some cases the tissue impairment releases latent personality and behavior disturbances. The American Psychiatric Association (1952) classifies these acute disorders into syndromes associated with (1) intracranial infection (encephalitis, meningitis, brain abscess); (2) systemic infection (pneumonia, typhoid fever, acute rheumatic fever); (3) drug or poison intoxication (by bromides, barbiturates, opiates, and such poisons as lead and gas); (4) alcoholic intoxication (delirium tremens, acute alcoholic hallucinosis); (5) brain trauma (head injury due to accident); (6) circulatory disturbance (cerebral embolism, arteriosclerosis, hypertension, cardiac-renal disease, and cardiac disease); (7) convulsive disorder (idiopathic epilepsy); (8) metabolic disturbance (uremia, diabetes, hyperthyroidism, hypocalcemia, vitamin deficiency, hypoglycemic states, etc.; (9) intracranial neoplasm (brain tumor); (10) diseases or conditions of unknown cause (multiple sclerosis, etc.).Chronic brain disorders involve disturbances of the higher functions— memory, judgment, reasoning, comprehension—and in general, the greater the amount of tissue loss, the greater the impairment of function. In some cases neuroses, psychoses, or character disorders are precipitated by the impairment, or superimposed upon it. These disorders are classified by The American Psychiatric Association into syndromes associated with (1) congenital cranial anomalies (congenital spastic paraplegia, mongolism, prenatal infectious disease, birth trauma—resulting in mild, moderate or severe mental deficiency); (2) central nervous system syphilis (meningoen- cephalitic type—general paresis); (3) central nervous system syphilis (meningovascular type); (4) central nervous system syphilis (other types); (5) cranial infections other than syphilis; (6) intoxication by lead, arsenic, mercury, carbon monoxide, illuminating gas, alcohol; (7) brain trauma (permanent head injury); (8) cerebral arteriosclerosis; (9) other circulatory disorders (due to cerebral embolism, hemorrhages, arterial hypertension); (10) convulsive disorder (idiopathic epilepsy); (11) senile brain disease; (12) other disturbances of metabolism, growth, or nutrition (glandular, pellagra, familial amaurosis, complications of diabetes, thyroid, pituitary, and adrenal disorders); (13) intracranial neoplasm; and (14) diseases and conditions of unknown or uncertain cause, such as multiple sclerosis, Pick’s disease, or Huntington’s chorea. For individual topics, see the listing Organic Brain Disorders in the Category Index.