Types of Dementia

What Is Dementia?

Dementia is the loss of mental functions, such as thinking, memory, and reasoning, that is severe enough to interfere with a person's daily life. Dementia is not a disease itself, but rather a group of symptoms that may accompany certain diseases or conditions. Symptoms may involve changes in personality, mood, and behavior.

Dementia develops when the parts of the brain that are involved with learning, memory, decision-making, and language are affected by injury or disease. The most common cause of dementia is Alzheimer's disease, which is considered responsible for at least half of all cases of dementia. However, there are as many as 50 other known causes of dementia, but most of these causes are very rare.

Although many diseases that cause dementia are not curable, some forms of dementia may improve greatly when the underlying cause is treated. For instance, if dementia is caused by vitamin or hormone deficiencies, the symptoms may resolve once the problem has been corrected. Therefore, dementia symptoms require comprehensive evaluation, so as not to miss potentially reversible conditions. The frequency of "treatable" causes of dementia is believed to be about 20%.

What Causes Dementia?

The most common causes of dementia include:

• Degenerative neurological diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease (a rare inherited disorder), and some types of multiple sclerosis
• Vascular disorders, such as multi-infarct dementia, which is caused by multiple strokes in the brain
• Traumatic brain injury caused by motor vehicle accidents, falls, etc.
• Infections of the central nervous system such as meningitis, HIV, and Creutzfeldt-Jakob disease, a quickly progressing and fatal disease that is characterized by dementia and muscle twitching and spasm
• Chronic alcohol or drug use
• Depression
• Certain types of hydrocephalus, an excess accumulation of fluid in the brain that can result from developmental abnormalities, infections, injury, or brain tumors

Types of Dementia

Dementia can be split into two broad categories -- the cortical dementias and the subcortical dementias -- based on which part of the brain is affected.

• Cortical dementias arise from a disorder affecting the cerebral cortex, the outer layers of the brain that play a critical role in thinking abilities like memory and language. Alzheimer's and Creutzfeldt-Jakob disease are two forms of cortical dementia. People with cortical dementia typically show severe memory loss and aphasia -- the inability to recall words and understand language.
• Subcortical dementias result from dysfunction in the parts of the brain that are beneath the cortex. Usually, the forgetfulness and language difficulties that are characteristic of cortical dementias are not present. Rather, people with subcortical dementias, such as Parkinson's disease, Huntington's disease, and AIDS dementia complex, tend to show changes in their speed of thinking and ability to initiate activities.

There are cases of dementia where both parts of the brain tend to be affected, such as multi-infarct dementia.