Dementia is not a single disease in itself, but a general term to describe symptoms such as impairments to memory, communication and thinking.
While the likelihood of having dementia increases with age, it is not a normal part of aging. Before we had today's understanding of specific disorders, "going senile" used to be a common phrase for dementia ("senility"), which misunderstood it as a standard part of getting old.
Light cognitive impairments, by contrast, such as poorer short-term memory, can happen as a normal part of aging (we slowly start to lose brain cells as we age beyond our 20s3). This is known as age-related cognitive decline, not dementia, because it does not cause the person or the people around them any problems. Dementia describes two or more types of symptom that are severe enough to affect daily activities.
Symptoms that are classed as "mild cognitive impairment" - which, unlike cognitive decline, are not a normal part of aging - do not qualify as dementia either, since these symptoms are not severe enough.For some people though, this milder disease leads to dementia later on.
A number of brain disorders with more severe symptoms are classified as dementias, with Alzheimer's disease being the best known and most common.
An analysis of the most recent census estimates that 4.7 million people aged 65 years or older in the US were living with Alzheimer's disease in 2010.5 The Alzheimer's Association has used this analysis to number-crunch the extent of the disorder in its 2013 report. It estimates that:
The non-profit organization says Alzheimer's accounts for between 60% and 80% of all cases of dementia, with vascular dementia caused by stroke being the second most common type.
All dementias are caused by brain cell death,1 and neurodegenerative disease - progressive brain cell death that happens over a course of time - is behind most dementias.
Nerve cells (neurons) in the brain - loss or damage can cause dementia. Alzheimer's disease is the leading cause.
But as well as progressive brain cell death like that seen in Alzheimer's disease, dementia can be caused by a head injury, a stroke or a brain tumor, among other causes.
Some of the causes are simpler to understand in terms of how they affect the brain and lead to dementia:
Some types of traumatic brain injury - particularly if repetitive, such as received by sports players - have been linked to certain dementias appearing later in life. Evidence is weak, however, that a single brain injury will raise the likelihood of having a degenerative dementia such as Alzheimer's disease.
Alzheimer's dementia is caused by progressive brain cell death. Estimates range between 60% and 80% for the proportion of all cases of dementia being accounted for by Alzheimer's disease.2 In the US, about 5.3 million people are thought to have the disorder among the estimated 6.8 million individuals who have some form of dementia.
Alzheimer's is thought to be caused by "plaques" between the dying cells in the brain and "tangles" within the cells (both are protein abnormalities: a build-up of "beta-amyloid" in plaques and the disintegration of "tau" protein in tangles).
These inclusions in the brain are always present with the disorder but whether they are themselves the cause, or if there is some other underlying process, is not known - and there is some overlap with other disorders that show similar changes in brain cells.
The brain tissue in a person with Alzheimer's has progressively fewer nerve cells and connections, and the total brain size shrinks.
See the Medical News Today Alzheimer's disease page for more detailed information about this specific type of dementia.
Dementia with Lewy bodies is also caused by neurodegeneration linked to abnormal structures in the brain. Here, the brain changes involve a protein called alpha-synuclein.
Mixed dementia refers to a diagnosis of two or three types occurring together. A person may show both Alzheimer's disease and vascular dementia at the same time. Or the combination could be Lewy bodies and Alzheimer's. There can also be a combination of all three types.
Parkinson's disease is also marked by the presence of Lewy bodies. While the part of the brain affected means there are classic movement symptoms, people with Parkinson's can also go on to develop dementia symptoms as the degenerative changes in the brain gradually spread.
Huntington's disease is similar to Parkinson's in the respect of being classically marked by uncontrolled movements yet having dementia as a component. It results in mood changes, too. Huntington's is an inherited condition caused by a single faulty gene. This can produce the disease at any age - as young at 2 years of age and as old as 80, but typically between the ages of 30 and 50 years.