Brain damage can cause many types of dysfunction. Such dysfunction ranges from complete loss of consciousness (as occurs in a coma Stupor and Coma Stupor is unresponsiveness from which a person can be aroused only by vigorous, physical stimulation. Coma is unresponsiveness from which a person cannot be aroused and in which the person's... read more ), to disorientation and an inability to pay attention (as occurs in delirium Delirium Delirium is a sudden, fluctuating, and usually reversible disturbance of mental function. It is characterized by an inability to pay attention, disorientation, an inability to think clearly... read more ), 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 brain damage is
How quickly the disorder causing it is progressing
Brain dysfunction may be
Localized (focal): Limited to a specific area
Diffuse (global): Widespread
Causes of Brain Dysfunction
Localized brain dysfunction is caused by disorders that occur in a specific area of the brain, including the following:
Disorders that reduce blood flow (and thus the oxygen supply) to a specific area, such as a stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more
Diffuse brain 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 Hypoglycemia Hypoglycemia is abnormally low levels of sugar (glucose) in the blood. Hypoglycemia is most often caused by drugs taken to control diabetes. Much less common causes of hypoglycemia include other... read more ) or low levels of oxygen in the blood (hypoxia—usually due to a lung or heart disorder or, often, to respiratory or cardiac arrest Cardiac Arrest and CPR Cardiac arrest is when the heart stops pumping blood and oxygen to the brain and other organs and tissues. Sometimes a person can be revived after cardiac arrest, particularly if treatment is... read more )
Infections, such as meningitis Introduction to Meningitis Meningitis is inflammation of the layers of tissue that cover the brain and spinal cord (meninges) and of the fluid-filled space between the meninges (subarachnoid space). Meningitis can be... read more and encephalitis Encephalitis Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved... read more
Very high blood pressure High Blood Pressure High blood pressure (hypertension) is persistently high pressure in the arteries. Often no cause for high blood pressure can be identified, but sometimes it occurs as a result of an underlying... read more or very low blood pressure Low Blood Pressure Low blood pressure is blood pressure low enough to cause symptoms such as dizziness and fainting. Very low blood pressure can cause damage to organs, a process called shock. Various drugs and... read more
Disorders that affect blood vessels, such as vasculitis Overview of Vasculitis Vasculitic disorders are caused by inflammation of the blood vessels (vasculitis). Vasculitis can be triggered by certain infections or drugs or can occur for unknown reasons. People may have... read more (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 Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more 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:
Large brain tumors
Severe or blunt head injuries
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.
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.
Symptoms of Brain Dysfunction
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.
Prognosis of Brain Dysfunction
Two characteristics of the brain help it compensate and recover after it has been damaged:
Redundancy: More than one area can perform the same function. Areas with somewhat overlapping functions can sometimes compensate for lost functions.
Plasticity: Nerve cells in certain areas can change so that they can perform new functions.
Did You Know...
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.