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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 how extensive brain damage is, where the damage is, and how quickly the disorder causing it is progressing.

Brain dysfunction may be widespread (diffuse) or limited to a specific area (localized). Diffuse dysfunction is caused by disorders that affect large areas of the brain, including the following:

• Disorders that cause metabolic abnormalities, such as low levels of sugar in the blood (hypoglycemia) or low levels of oxygen in the blood (usually due to a lung or heart disorder)
• Infections, such as meningitis and encephalitis
• Very high or very low blood pressure

Diffuse brain dysfunction may also result from disorders that cause swelling of or put pressure on a large area of the brain, including the following:

• Brain abscesses
• Large brain tumors
• Severe or blunt head injuries

Certain drugs, such as opioids (narcotics), some sedatives (such as benzodiazepines and barbiturates), and antidepressants may 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.

Localized brain dysfunction is caused by disorders that affect a specific area of the brain, including the following:

• Brain tumors that affect a relatively small area of the brain
• Disorders that reduce the blood (and thus the oxygen) supply to a specific area, such as a stroke
• Penetrating head injuries that affect only a relatively small area of the brain
• Certain types of seizure disorders

When Specific Areas of the Brain Are Damaged Different areas of the brain control specific functions. Consequently, where the brain is damaged determines which function is lost.

Diffuse damage tends to affect consciousness, making people difficult to arouse (causing stupor) or impossible to arouse (causing coma). Localized damage tends to affect specific functions. However, the severity of brain dysfunction depends on the extent of brain damage as well as the location. When the cerebral cortex (the outer layer of the cerebrum, the largest part of the brain) is damaged, the degree of dysfunction is proportionate to the extent of the damage: The more extensive the damage, the more severe the dysfunction is likely to be. However, when the brain stem (which regulates critical body functions and levels of consciousness) is damaged, a relatively small amount of damage may cause complete loss of consciousness and even death.

Disorders that progress rapidly are more likely to cause noticeable symptoms of brain dysfunction than disorders that progress slowly—for example, a fast-growing brain tumor versus a slow-growing one. The brain compensates for gradual changes more easily than for rapid changes.

Three characteristics of the brain help it compensate and recover after it has been damaged:

• Redundancy: More than one area can perform the same function.
• Plasticity: Nerve cells in certain areas can change so that they can perform a different function.
• Adaptation: Areas with somewhat overlapping functions can sometimes compensate for lost functions.

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.

Last full review/revision March 2008 by Juebin Huang, MD, PhD