Dementia occurs when neurons (nerve cells) in the brain stop working, lose connections with other brain cells, and die.
The result is cognitive impairment and other changes that make it difficult for people to function in their daily lives.
The most common cause of dementia is Alzheimer’s disease, which is responsible for 60 to 80 percent of all cases. The other main forms of dementia are vascular dementia, Lewy body dementia, and frontotemporal disorders (also known as frontotemporal dementia).
Researchers believe that all dementia has a genetic link but that, with the exception of rare dementias caused by gene mutation, environment and lifestyle also play an important role. (1)
After Alzheimer’s disease, vascular dementia is the most frequent cause of dementia, accounting for about 10 percent of all cases. Still, many experts believe that vascular dementia, like Alzheimer’s disease, is under-diagnosed. (2)
Vascular dementia often coexists with other forms of dementia. Autopsy studies reveal that 50 percent of people with Alzheimer’s also had another form of dementia, most commonly vascular dementia. This is especially true of the “oldest-old” — people who are 85 and above.
Vascular dementia is caused by impaired blood flow to the brain, which prevents brain cells from getting the oxygen and nutrients they need.
A stroke that blocks an artery in the brain has the potential to cause permanent damage and dementia. The severity of the stroke and its location in the brain determines its impact on thinking and reasoning.
Some strokes, called “silent brain infarctions," may not cause any obvious symptoms. However, the more strokes one has, the higher one’s dementia risk.
Risk factors for vascular dementia overlap with those for heart disease and stroke. They include:
Lewy bodies are abnormal clumps of the protein alpha-synuclein that accumulate in neurons in the brain. Lewy body dementia can develop when these clumps impede normal brain function, affecting thinking, movement, behavior, and mood.
Researchers are still trying to understand what causes Lewy body dementia. They’ve begun to correlate the accumulation of alpha-synuclein with the loss of neurons that produce two important neurotransmitters: acetylcholine, which plays a key role in learning and memory, and dopamine, which is important for behavior, cognition, mood, movement, and more.
The brains of people with Lewy body dementia can also exhibit the beta-amyloid (protein) plaques and tangled tau (protein) strands found in the brains of people with Alzheimer’s.
Risk factors include:
Frontotemporal disorders, commonly called frontotemporal dementia, develop when the frontal (front) and temporal (side) lobes of the brain atrophy.
Usually, there’s no clear explanation for why this happens; the only known risk factor is family history. Typically this dementia affects people younger than 65. Most are between 40 and 45.
The frontal and temporal lobes are the parts of the brain that are responsible for personality, behavior, and language. This explains why some people with these disorders undergo extreme personality changes and become impulsive, emotionally indifferent, or socially inappropriate. Other people lose their capacity to use language.
There are two main subtypes of frontotemporal disorders. One involves the build-up in the brain of the protein tau; the other, build-up of the protein TDP-43.
In some cases, atrophying parts of the brain contain microscopic tau-protein-filled structures, called Pick bodies, within the brain cells.
Researchers have recently identified genetic and molecular overlaps between these disorders and amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. Investigating those similarities may help scientists better understand and treat both conditions. (5)
While dementia often goes unreported and undiagnosed, depriving people who need help from getting it, there are situations in which people who appear to have dementia actually don’t.
The reason is that certain conditions — such as depression, delirium, side effects from medications, thyroid problems, certain vitamin deficiencies, and excessive alcohol use — can cause dementia-like symptoms.
One meta-analysis found that 9 percent of people who appeared to have dementia did not but were instead experiencing other potentially treatable or reversible conditions. (1)