• Also Known As:
  • AD
  • Alzheimer Dementia
  • Presenile Dementia
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What is Alzheimer disease?

Alzheimer disease (AD) is an irreversible form of dementia characterized by memory loss, a progressive decline in intellectual ability, deteriorating language and speech skills, and personality and behavioral changes that eventually interfere with daily living. Currently, Alzheimer disease has been estimated to affect more than 5.5 million Americans over the age of 65 and about 200,000 under the age of 65.

Although some aspects of AD mimic changes found in the brain as we age, AD is not a normal part of the aging process. Nerve cell injury and death occur due to the build-up of abnormal protein structures in the brain called amyloid plaques, also known as senile plaques, and neurofibrillary tangles. The destruction of nerve cells also results in decreased levels of substances called neurotransmitters (the most important being acetylcholine) that help transmit brain signals. Over time, AD results in decreased interaction between different areas of the brain.

Relationship with Aging
The risk of having AD and other forms of dementia increases greatly with age. About 10% to 12% of the population will have dementia by the time they are 65 years old, with the risk increasing to 50% for those who reach the age of 100. According to the Alzheimer’s Association, with the U.S. population aging and living longer, it is estimated that the number of those living with AD may be up to 16 million by 2050.

In general, “late onset” AD begins after the age of 65 and is not thought to be inherited. “Early onset” AD, starting before the age of 65, accounts for about 5% to 10% of all AD cases and is more likely to be caused by inherited mutations (disease-causing variants) in one of several genes.

Genetic Relationship
While the more common, late onset Alzheimer disease appears to be caused by a variety of factors that are still not well understood, three genes have been identified in association with a certain form of early-onset familial Alzheimer (also known as Alzheimer disease type 3) disease: PSEN1, PSEN2, and APP. A mutation in any one of these genes can lead to abnormal protein production, which contributes to the senile plaques that cause progressive dementia. Only one copy of one of these genes needs to be altered for it to cause AD, which means that there is a 50% chance of inheriting this mutation from an affected parent. Genetic testing of these genes may be recommended when a person has several generations of individuals who are all diagnosed with AD prior to age 65.

No causative genes have been identified for late onset AD. However, a few genes have been found to be associated with the disorder, meaning that people who have variants in these genes aren’t guaranteed to develop late onset AD, but their risk is increased compared to others in the general population who do not have these variants. These “susceptibility” genes may explain why there is an increased risk of developing late onset AD in individuals who have other affected family members.

The APOE gene is the most clearly established susceptibility gene for late onset AD. This gene directs the production of apolipoprotein E, a protein that helps transport lipids (fats and cholesterol) in the blood. The APOE gene exists in three different forms (alleles): e2, e3, and e4. Everyone inherits a pair of APOE genes that contains some combination of these three alleles. The e3 allele is the most common. It is present in approximately 60% of the general population. The e4 allele has been established as a risk factor (particularly in familial versus sporadic) for AD.

Most individuals with Down syndrome (DS), caused by the abnormal trisomy of chromosome 21, will eventually show some of the mental changes associated with AD, usually by the time they are in their 40’s or 50’s. The pathology found in the brains of these adults with DS is very similar to that found in AD. The extra copy of chromosome 21 causes an increased production of the protein that accumulates and forms senile plaques similar to that seen in AD. Relatives of individuals with DS do not have an increased risk of AD since they do not have the extra copy of chromosome 21.

Other Risk Factors
The risk of AD appears to vary by ethnicity. Compared to Caucasians, individuals of African American ancestry have a risk for AD that is approximately four times greater and individuals of Hispanic ancestry have a risk approximately two times greater. In addition to ethnic background, other risk factors that are associated with AD include: type 2 diabetes and milder forms of insulin resistance, obesity, high blood pressure, unhealthy lipid levels, and high levels of inflammatory markers such as C-reactive protein (CRP).

In 2011, new criteria and guidelines for the diagnosis of Alzheimer disease were released by three expert workgroups spearheaded by the Alzheimer’s Association and the National Institute on Aging (NIA) of the National Institutes of Health (NIH).

Their articles expand the definition of Alzheimer disease to include three stages:

  1. Preclinical Alzheimer disease – measurable changes in biomarkers, including brain imaging and certain proteins in the cerebrospinal fluid, but no outward symptoms; the purpose of defining a preclinical stage is to help guide research.
  2. Mild cognitive impairment (MCI) – mild changes in memory and thinking abilities that do not compromise day to day activities; people with MCI may or may not progress to AD-related dementia.
  3. Dementia due to Alzheimer disease – memory, thinking and behavioral symptoms that impair a person’s ability to function independently in daily life

These guidelines are intended to provide more information about the current understanding of Alzheimer disease, provide a framework for future research, and to propose a stronger role for biomarker use in the future.


About Alzheimer Disease

Signs and Symptoms

According to the Alzheimer Association, there are 10 early signs and symptoms of Alzheimer disease. While some memory problems are a normal part of aging, those associated with Alzheimer disease usually are more serious and become more frequent. They include:

  • Loss of memory that affects daily life—forgetting information that was recently learned. This can occur with normal aging, but the information is usually remembered later. This includes forgetting important dates or events, having to rely on memory aids, and asking for the same information again and again.
  • Difficulty planning or problem solving, such as keeping track of bills and payments
  • Problems completing usual tasks, such as forgetting how to get to a familiar location
  • Confusion about place or time—losing track of time, forgetting where you are or how you got there
  • Increasing difficulty reading or judging distances
  • Problems speaking or writing—forgetting words, repeating the same thing, struggling with vocabulary
  • Losing things more frequently and not being able to logically retrace steps to find them
  • Impaired judgment, such as giving away unusually large amounts of money
  • Increasing withdrawal from activities, including social, work or family events
  • Changes in mood and personality, such as increasing anxiety, fear, suspicion and depression


There are no laboratory tests available that can determine whether or not a person has Alzheimer disease (AD) during life. Currently, the only definitive way to establish a diagnosis of AD is to microscopically examine a section of the person’s brain tissue after death. Pathologists look for senile plaques and neurofibrillary tangles characteristic of AD. Since plaque and tangle formation is also seen in normal aging, the sample must be compared to a control sample of normal, non-AD brain tissue from a person of the same age.

Healthcare practitioners can make a reasonably accurate clinical diagnosis of AD by using a variety of tests and procedures to rule out other causes of dementia. When someone presents with symptoms of dementia, the healthcare practitioner will evaluate the individual’s personal and family history (preferably of several generations); perform a physical exam; determine the age of onset, and give the person neuropsychological tests to measure memory, language skills, and other cognitive functions.

Healthcare practitioners may use a range of general laboratory tests to rule out nutritional deficiencies and other diseases and conditions that could be affecting the person’s memory. They will also look for overmedication as a cause of symptoms. In addition, they may use imaging tools, such as computed tomography (CT) and magnetic resonance imaging (MRI) scans, to look for evidence of trauma, tumors, and stroke that could be causing dementia and to look for brain atrophy, shrinkage that may be present later in the Alzheimer disease progression.

These tests may be used to rule out causes of potentially reversible dementia other than Alzheimer disease.
Test Sample Associated with
Vitamin B12 Blood B12 deficiency
T4 Blood Thyroid disorder
TSH Blood Thyroid disorder
CBC Blood Anemia, infection
Electrolytes Blood Na+, K+, Cl-, CO2 and pH imbalance
CRP, ESR Blood Inflammation
HIV antibody Blood HIV/AIDS
Syphilis test Blood Syphilis
Drug screen Urine Illicit drug use

If a healthcare practitioner suspects AD, other less common laboratory tests (see Table below) may be recommended to help differentiate between AD and other forms of dementia, and to check for genetic risk factors.

Other Tests Used to Help Categorize Dementia

Test Type Name Test Use Associated With
Imaging tests CT (computed tomography) Body scan Rule out AD or with late stage AD Stroke and brain atrophy (shrinkage associated with late stage AD)
MRI (magnetic resonance imaging) Body scan Rule out AD or with late stage AD Stroke and brain atrophy


Test Type Name Test Use Associated With
Less common lab tests Amyloid Beta 42 peptide and Tau protein correlation (Tau/Aß42) CSF Used in research settings to establish presence of “tangles” or “plaque burden”; may help differentiate AD from other forms of dementia; Aβ42 used in some treatment trials In symptomatic people, decreased Aß42 level along with an elevated Tau protein level indicates an increased likelihood of AD, regardless of the cause.
APOE genotype Blood Determine APOE genotype to provide risk factor assessment; adjunct test to confirm/rule out probable AD Roughly 65% of AD patients have at least one APOE e4 allele; persons with 2 APOE e4 alleles have a much higher risk for AD, but this genotype is much less prevalent.
PSEN1 Blood Test for genetic mutation Associated with about half of the cases of early onset familial AD
PSEN2 Blood Test for genetic mutation; available in only a few labs Early onset familial AD; mutation very rare, identified in only a few family lines
APP Blood Test for genetic mutation; available in only a few labs Early onset familial AD; mutation very rare, identified in only a few family lines

Research is ongoing and new biomarkers, in addition to those listed in the table above, are being studied.


There is currently no prevention or cure for Alzheimer disease (AD). People may live with AD for up to 25 years, but the average is 8 to 10 years. Treatment consists of attempting to maintain function, easing symptoms, managing behavioral issues, and providing the affected person and caregivers with support and education. Early in the disease, those with AD may be able to live fairly independent lives with some assistance, such as memory aids and a structured environment. This is the time when the person can make plans and participate in decisions about future care.

Early diagnosis of AD may allow some people to receive moderate benefit from cholinesterase inhibitors. These medications may help to maintain intellectual ability by preserving the function of the neurotransmitter acetylcholine.

Memantine may be prescribed to improve memory and attention, and antidepressants and other drugs may also be used in small quantities throughout the progression of AD to moderate personality and behavioral issues such as depression, agitation, paranoia, and violence.

Whenever possible, the affected person’s other medications are evaluated and the individual is taken off drugs that may worsen confusion. These may include central nervous system depressants, antihistamines, sleeping pills, and analgesics.

Environmental modifications may be made to make the person’s home safer and more familiar, and to make the person more comfortable.

While current research into the protective and therapeutic influences of certain substances is promising, specific agents cannot yet be recommended. They each carry their own associated risks and side effects. Further studies are needed to determine their actual effectiveness and long-term safety.

Common Questions

  1. Besides Alzheimer disease, what are some other causes of confusion, memory lapses, and cognitive decline?
    Occasional forgetfulness is a normal part of aging and should not be a cause for concern unless it increases in frequency or interferes with daily living. However, some of the causes of cognitive decline, besides AD, include:

  2. Is there a way to get involved in research efforts related to Alzheimer disease?
    Yes. Both affected and unaffected individuals can participate in clinical trials. In addition, people can donate the brain tissue of a family member after their death. For scientists to continue to study the causes of AD and to potentially find a cure, samples are needed of brain tissue from people who had AD as well as from elderly people who did not. You can find more information on clinical trials involving AD at the ClinicalTrials.gov website.
  3. Is there a link between mad cow disease and Alzheimer disease?
    There is no evidence at this time of any connection between bovine spongiform encephalopathy, commonly called mad cow disease, and Alzheimer disease even though some of the symptoms may appear similar.

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