Dementia is the loss of cognitive functioning—thinking, remembering, and reasoning—and behavioral abilities to such an extent that it interferes with a person’s daily life and activities. Dementia ranges in severity from the mildest stage, when it is just beginning to affect a person’s functioning, to the most severe stage, when the person must depend completely on others for basic activities of daily living.
Many conditions and diseases cause dementia. Two of the most common causes of dementia in older people are Alzheimer’s disease and vascular dementia. Others include frontotemporal disorders and Lewy body disease.
In addition, some people have mixed dementia—a combination of two or more disorders, at least one of which is dementia. A number of combinations are possible. For example, some people have Alzheimer’s disease and vascular dementia at the same time.
Other causes of dementia include: Huntington’s disease, Parkinson’s disease dementia, normal pressure hydrocephalus, Creutzfeldt-Jakob disease, and Wernicke-Korsakoff syndrome. For more information about these conditions, visit the National Institute of Neurological Disorders and Stroke.
Vascular dementia, considered the second most common form of dementia after Alzheimer’s disease, results from interrupted blood flow to the brain, often after a stroke or series of strokes. The symptoms can be similar to those of Alzheimer’s, and both conditions can occur at the same time. A common type of vascular dementia is multi-infarct dementia, caused by a series of small strokes.
Recent research has shown that Alzheimer's and vascular disease-associated cognitive impairment are closely intertwined. For example, a large proportion of people diagnosed with Alzheimer's also have brain damage due to vascular disease. In addition, several studies have found that many of the major risk factors for vascular disease may also be risk factors for Alzheimer's. The overlap between the two may be important because drugs and lifestyle modifications known to be effective in preventing vascular disease, such as controlling high blood pressure, lowering cholesterol, and engaging in physical activity, may also help prevent Alzheimer's disease.
Lewy body disease is another common cause of dementia in older people. Lewy body disease happens when abnormal structures (microscopic deposits, or lesions, of abnormally aggregated proteins), called Lewy bodies, build up in areas of the brain. The disease may cause a wide range of symptoms, including: changes in alertness and attention, hallucinations, problems with movement and posture, muscle stiffness, confusion, and loss of memory.
Lewy body disease can be hard to diagnose because Parkinson's disease and Alzheimer's disease cause similar symptoms. Scientists think that Lewy body disease might be related to these diseases, or that they sometimes happen together.
See also: The National Institute of Neurological Disorders and Stroke's Dementia with Lewy Bodies Information Page, Lewy Body Disease page from NIH's MedlinePlus, a Lewy Body Dementias fact sheet from the UCSF Memory and Aging Center, and another Lewy Body Dementia fact sheet from the Emory University Alzheimer’s Disease Research Center.
Frontotemporal disorders are a form of dementia caused by a family of brain diseases known as frontotemporal lobar degeneration (FTLD). These disorders are the result of damage to neurons (nerve cells) in parts of the brain called the frontal and temporal lobes. As neurons die in the frontal and temporal regions, these lobes atrophy, or shrink. Gradually, this damage causes difficulties in thinking and behaviors controlled by these parts of the brain. Many possible symptoms can result, including strange behaviors, emotional problems, trouble communicating, or difficulty with walking and other basic movements.
Frontotemporal disorders can be grouped into three types, defined by the earliest symptoms physicians identify when they examine patients.
See also: Frontotemporal Disorders: Information for Patients, Families, and Caregivers; Frontotemporal Disorders: A Resource List; Frontotemporal Dementia information from the UCSF Memory and Aging Center; What Is Frontotemporal Degeneration (bvFTD)? (PDF, 78K) from Northwestern University; What Is PPA? (PDF, 394K), also from Northwestern; and Primary Progressive Aphasia, from UCSF.
In a study of 349 people who had been diagnosed before death with either Alzheimer's or MCI, scientists at the Rush University Alzheimer's Disease Center in Chicago found evidence of other disease processes in almost half of the Alzheimer's brains and 20 percent of the MCI brains. Most common were microinfarcts; next most common were Lewy bodies in the cortex. Researchers observed that the people whose brains showed "mixed" disease (plaques and tangles plus microinfarcts or Lewy bodies) were more cognitively impaired when they died than those whose brains contained only comparable levels of the plaques and tangles characteristic of Alzheimer's.