Brain damage can cause many types of dysfunction. Such dysfunction ranges from complete loss of consciousness (as occurs in a coma), to disorientation and an inability to pay attention (as occurs in delirium), to impairment of one or several of the many specific functions that contribute to conscious experience.
The type and severity of brain dysfunction depend on
Brain dysfunction may be
Localized brain dysfunction is caused by disorders that occur in a specific area of the brain, including the following:
Diffuse brain dysfunction is caused by disorders that affect large areas of the brain, including the following:
Disorders that affect blood vessels, such as vasculitis (inflammation of blood vessels)
Cancer that has spread through several areas of the brain or to the tissues that cover the brain and spinal cord (meninges)
Disorders that cause progressive degeneration of the brain, such as Alzheimer disease and other dementias
Diffuse brain dysfunction may also result from disorders that occur in a specific area of the brain if they cause swelling of or put pressure on a large area of the brain. These disorders include the following:
Certain drugs, such as opioids (narcotics), some sedatives (such as benzodiazepines and barbiturates), and antidepressants, cause diffuse brain dysfunction if people are sensitive to their effects (as older people are) or if the level of drug in the blood is too high.
The type and severity of brain dysfunction depend on the location of the brain damage, as well as whether it affects the whole brain (diffuse) or only part (localized) of the brain. When the cerebral cortex (the outer layer of the cerebrum, the largest part of the brain) is damaged, the degree of dysfunction is often proportionate to the extent of the damage: The more extensive the damage, the more severe the dysfunction is likely to be. However, damage to some areas of the brain can cause severe dysfunction even when the damaged area is small. When the brain stem (which regulates critical body functions and levels of consciousness) is damaged, a relatively small amount of damage may cause coma and even death.
Disorders that progress rapidly are more likely to cause more noticeable symptoms of brain dysfunction than disorders that progress slowly. For example, a severe stroke is more likely to cause noticeable symptoms than a slow-growing tumor. The brain compensates for gradual changes more easily than for rapid changes.
Also, diffuse brain damage that occurs suddenly tends to affect consciousness, making people drowsy, difficult to arouse (causing stupor), or impossible to arouse (causing coma). However, diffuse brain damage progresses more slowly and affects consciousness less often. So people usually remain conscious and awake. Localized damage tends to affect specific functions.
Two characteristics of the brain help it compensate and recover after it has been damaged:
Consequently, undamaged areas of the brain sometimes take over functions performed by a damaged area, contributing to recovery. However, as people age, the brain becomes less able to shift functions from one area to another. Some functions, such as vision, cannot be performed by other areas of the brain. Direct damage to areas that control such functions may have permanent effects.