While a great deal of progress has been made in dementia research, scientists are still trying to definitively figure out the causes of both the neurodegenerative and non-neurodegenerative causes of the condition. Below, you’ll find some of the known and suspected causes of some common types of dementia

Alzheimer’s Disease (AD)

The build-up of two abnormal structures in the brain, called amyloid plaques and neurofibrillary tangles, likely play an integral role in the development of AD. Additional causal factors may include environmental influences, lifestyle factors, and genetics.

Amyloid plaques are dense, mostly insoluble (not dissolvable) clumps of protein fragments that deposit a highly damaging gunky substance outside and around the brain’s nerve cells. People with AD have an abundance of these destructive plaques in their hippocampus—a structure in the brain that plays a vital role in memory, including the transfer of immediate or short-term memories into long-term memories.

Neurofibrillary tangles are insoluble twisted up fibers that form and build up inside the brain’s nerve cells—which essentially clogs up the brain works from the inside out.

Early Onset Alzheimer’s Disease

For those few people who develop AD as young as their early 30s and 40s there are three known defective genes (called mutations) that are believed to be involved in the production and processing of the amyloid protein that forms the amyloid plaques in early onset AD. Of note, these three particular gene mutations do not play a role in the more common type of AD (sometimes referred to as “late onset” Alzheimer’s disease).

Alzheimer’s Disease, Parkinson's Dementia, & Pick's Disease

These (and other dementias) are associated with a person having abnormal structures in the brain called, inclusions—which are made of various abnormal proteins. However, it’s still unclear at to whether the inclusions are a side effect of the disease process (that lead to the dementia in the first place), causal, or a combination of the two.

Vascular Dementia

There are several types of vascular dementia, which vary slightly in their causes (and symptoms). One type, called multi-infarct dementia (MID) is caused by numerous small strokes in the brain. Of note, dementia is more common when the stroke takes place in the left hemisphere of the brain, and when the stroke involves the hippocampus—a structure in the brain that plays a vital role in memory, including the transfer of immediate or short-term memories into long-term memories.

That said, not all people who suffer strokes develop vascular dementia; and, vascular dementia may or may not get worse over time. Additionally, any condition that prevents normal blood flow to the brain can cause vascular dementia. This is because the brain requires an uninterrupted supply of oxygen, which it gets from the bloodstream. In the absence of oxygen, brain cells can’t function properly; and if the oxygen flow is interrupted long enough, those brain cells die.

Frontal Lobe Dementia (FTD, also known as Frontotemporal dementia)

Accounting for two to 10 percent of all cases of dementia, FTD describes a group of diseases (including Pick’s disease). These diseases all involve the degeneration of brain cells located in the frontal and temporal lobes of the brain—the areas of brain behind the eyes and above the ear—that control judgment and social behavior. People with FTD rarely have amyloid plaques, but often have neurofibrillary tangles. Also, as FTD often runs in families, this suggests there’s also a genetic component.

Pick’s Disease

Pick's disease is one type of FTD. One of hallmark of Pick's disease is having abnormal brain cells (neurons) that balloon up in size and then die. The brains of people with Pick's disease also have abnormal structures called Pick bodies—mostly made of the protein tau—inside the neurons. Pick’s disease also tends to run in families, suggesting that a genetic component is likely involved.

What is tau? Think of it this way: Brain cells (neurons) have a specialized transport system called microtubules, which act like railroad tracks, safely guiding and transporting nutrients, molecules, and information to other cells. Tau is a critically important fiber-like protein responsible for keeping those microtubules stable. However, when tau is harmfully altered (possibly because of a genetic mutation), the fibers get tangled and twisted up together—which causes the microtubules to become unstable and disintegrate. This, in turn, collapses the whole neuron transport system.

Lewy Body Dementia (LBD)

LBD is a common type of progressive dementia. Abnormal structures in the brain called Lewy bodies are the hallmark of this disease. Lewy bodies are found in the brain’s outer layer, called the cortex—which is responsible for such key processes as thinking, perceiving, producing, and understanding language. Lewy bodies are also often located in various parts of the brain stem—particularly the substantia nigra. Here, nerve cells release essential neurotransmitters that help control movement and coordination. As an aside, Lewy bodies are also commonly found in the brains of people with Parkinson's disease.

Potentially Reversible Conditions That Cause Dementia

As discussed by the National Institute of Neurological Disorders and Stroke (NINDS) of the National Institutes of Health (NIH), there are also many causes of dementia-like symptoms that are treatable and sometimes potentially reversible if caught early enough, and if appropriate treatment is administered. While not an exhaustive list, here are some of the more common reversible conditions:

Nutritional Deficiencies

  • Deficiencies of thiamine (vitamin B1) often result from chronic alcoholism and can seriously impair mental abilities—particularly memories of recent events.
  • Severe deficiency of vitamin B6 can cause a multisystem illness (called pellagra) that may include dementia.
  • Deficiencies of vitamin B12 have been linked to dementia in some cases.
  • Dehydration (lack of water) can cause mental impairment that can resemble dementia.

Poisoning

  • Exposure to lead, other heavy metals, or other poisonous substances can lead to symptoms of dementia. These symptoms may or may not resolve after treatment, depending upon how badly the brain is damaged.
  • People who have abused substances such as alcohol and recreational drugs sometimes display signs of dementia even after the substance abuse has ended. This condition is known as substance-induced persisting dementia.

Metabolic Problems and Endocrine Abnormalities

  • Thyroid problems can lead to apathy, depression, or dementia.
  • Hypoglycemia, a condition in which there is not enough sugar in the bloodstream, can cause confusion or personality changes.
  • Too little or too much sodium or calcium can trigger mental changes.
  • Some people have an impaired ability to absorb vitamin B12, which creates a condition called pernicious anemia that can cause personality changes, irritability, or depression.

Drug Effects

  • Prescribed medications can sometimes lead to reactions or side effects that mimic dementia. These dementia-like effects can occur in reaction to just one drug or they can result from drug interactions. They may have a rapid onset or they may develop slowly over time.
  • Illicit drugs can also lead to dementia.

Heart and Lung Problems

The brain requires a high level of oxygen in order to carry out its normal functions. Problems such as chronic lung disease or heart problems that prevent the brain from receiving adequate oxygen can starve brain cells and lead to the symptoms of dementia.

  • Many infections can cause neurological symptoms, including confusion or delirium, due to fever or other side effects of the body's fight to overcome the infection.
  • Meningitis and encephalitis, which are infections of the brain or the membrane that covers it, can cause confusion, sudden severe dementia, impaired judgment, or memory loss.
  • Untreated syphilis can damage the nervous system and cause dementia.
  • In rare cases, Lyme disease can cause memory or thinking difficulties.
  • People in the advanced stages of AIDS may develop a form of dementia.
  • People with compromised immune systems, such as those with leukemia (and AIDS), may develop an infection called progressive multifocal leukoencephalopathy (PML). PML is caused by a common human polyomavirus, called the JC virus, and leads to damage or destruction of the myelin sheath that covers nerve cells. PML can lead to confusion, difficulty with thinking or speaking, and other mental problems.

Subdural hematomas

Subdural hematomas, or bleeding between the brain's surface and its outer covering (the dura), can cause dementia-like symptoms and changes in mental function.

Brain Tumors

In rare cases, people with brain tumors may develop dementia because of damage to their brains. Symptoms may include changes in personality, psychotic episodes, or problems with speech, language, thinking, and memory.

Anoxia

Anoxia occurs when the brain is starved of oxygen. Anoxia may be caused by many different problems, including heart attack, heart surgery, severe asthma, smoke or carbon monoxide inhalation, high-altitude exposure, strangulation, or an overdose of anesthesia. Recovery depends upon the severity of the oxygen deprivation.