Causes of Dementia

Written by Wendy Leonard, MPH | Published on September 16, 2014
Medically Reviewed by Kenneth R. Hirsch, MD on September 16, 2014

Causes of Dementia

Dementia has many forms and a variety of causes. A great deal of progress has been made in dementia research, but scientists still do not understand all of the causes of dementia. Below, you’ll find some of the known and suspected causes of some common types of dementia.

Alzheimer’s Disease (AD)

The Alzheimer’s Association estimates that Alzheimer’s disease (AD) causes 50 to 80 percent of all dementia.  The exact cause of AD isn’t known, but there are some hypotheses.

Abnormal Buildup in the Brain

The buildup of two abnormal structures in the brain (amyloid plaques and neurofibrillary tangles) is common in AD and may be part of the cause.

Amyloid plaques are clumps of beta-amyloid. Beta-amyloid is a piece of a protein that is found in the normal brain. However, in AD, these beta-amyloid proteins clump together forming plaques that can disrupt communication between nerve cells and cause brain inflammation. People with AD have an abundance of these plaques in their hippocampus, which is a part of the brain that is involved in memory. One aspect of memory that is frequently disrupted in AD is the transfer of short-term memories into long-term memories.

Neurofibrillary tangles are fibrous tangles of an abnormal protein called tau. Tau is an important fiber-like protein that keeps microtubules in the brain stable. These microtubules transport nutrients, molecules, and information to other cells. When tau is harmfully altered (possibly because of a genetic mutation), the fibers get tangled and twisted up together. This causes the microtubules to become unstable and disintegrate. This, in turn, collapses the whole neuron transport system.

Genetic Mutations

A few people develop AD as young as their early 30s and 40s. This is known as early onset Alzheimer’s disease. There are three known gene mutations (defects) that are believed to be involved in the formation of the amyloid plaques in early onset AD. These three gene mutations do not play a role in the more common type of AD (sometimes referred to as “late onset” Alzheimer’s disease).

Inclusions

AD and other dementias are associated with abnormal structures in the brain called inclusions. These structures are made of various abnormal proteins. It is not clear whether these structures cause the disease or are caused by the disease.

Lewy Body Dementia (LBD)

LBD is a common type of progressive dementia. Abnormal structures in the brain called Lewy bodies are characteristic of this disease. Lewy bodies are found in the brain’s outer layer (cortex). The cortex is responsible for 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. Lewy bodies are also commonly found in the brains of people with Parkinson’s disease.

Vascular Dementia

Any condition that prevents normal blood flow to the brain can cause vascular dementia. The brain requires an uninterrupted supply of oxygen from the bloodstream. In the absence of oxygen, brain cells can’t function properly. If the oxygen flow is interrupted long enough, those brain cells die.

There are several types of vascular dementia. The causes and symptoms for each vary slightly. One type, called multi-infarct dementia (MID), is caused by numerous small strokes in the brain. Dementia is more common when the stroke takes place in the left hemisphere of the brain and when the stroke involves the hippocampus.

Not all people who have a stroke develop vascular dementia.

Frontal Lobe Dementia

Frontal lobe dementia is a group of diseases that cause significant changes in behavior and/or language capability. It is sometimes known as frontotemporal dementia.

These diseases all involve the degeneration of brain cells located in the frontal and temporal lobes of the brain. The frontal and temporal lobes are the areas of brain behind the eyes and above the ears. These areas control personality, judgment, emotions, and language. People with frontal lobe dementia rarely have amyloid plaques but often have neurofibrillary tangles. Frontal lobe dementia often runs in families, suggesting that genetics may be an important causal factor.

Pick’s disease is one type of frontal lobe dementia. Like other forms of frontal lobe dementia, there appears to be a strong genetic component to its causation. The brains of people with Pick’s disease have abnormal structures called Pick bodies — mostly made of the protein tau — inside the neurons. Brain cells loaded with Pick bodies tend to balloon up in size and then die, ultimately leading to the characteristic shrinkage (atrophy) in the frontotemporal regions of the brain.  Presently, there is no specific treatment available for any frontal lobe dementia.

Potentially Reversible Conditions

Many causes of dementia and dementia-like symptoms are treatable. They may be 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 (pellagra) that may include dementia.

Deficiencies of vitamin B12 have been linked to dementia in some cases.

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. This 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 called delirium 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 delirium, which can appear similar to dementia.

Heart and Lung Problems

The brain requires a high level of oxygen in order to carry out its normal functions. Conditions 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 delirium that mimic dementia.

Infections

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, 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.

Other Causes

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.

Anoxia

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

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