Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 82. Brain Dysfunction

Topics: Introduction | Dysfunction by Location | Specific Types of Dysfunction

Introduction

When the brain is damaged, it can malfunction in many ways. 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. These disorders include those that produce 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; and very high or 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. These disorders include brain tumors, brain abscesses, and severe head injuries. Certain drugs, such as opioids (narcotics), some sedatives (for example, 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. These disorders include structural abnormalities, such as a brain tumor; disorders that reduce the blood (and thus the oxygen) supply to a specific area, such as a stroke; and certain types of seizure disorders.

See the figure When Specific Areas of the Brain Are Damaged.

The extent and location of brain damage are interrelated in the way they affect the severity of brain dysfunction. Within a relatively large area of the brain such as the cerebral cortex, the degree of dysfunction is relative to the extent of the damage: The more extensive the damage, the more severe the dysfunction is likely to be. However, in a smaller area such as the brain stem (which regulates critical body functions and levels of consciousness), 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 contribute to its ability to compensate and recover after it has been damaged:

• Redundancy (many brain functions can be performed by more than one area)
• Adaptation (areas with somewhat overlapping functions can sometimes compensate for lost functions)
• Plasticity (certain areas can shift to a different function)

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.