Alzheimer’s disease (AD)
Alzheimer’s disease (AD) is the most common and fastest growing cause of dementia, accounting for an estimated 60 to 80 percent of all cases. Irreversible and progressive, AD slowly destroys memory and thinking skills and, eventually, the ability to carry out the simplest tasks of daily living. Although treatment can help manage the symptoms of AD, there is no cure for the disease.
According to the Alzheimer’s Association, an estimated 6.2 million Americans were living with Alzheimer’s dementia in 2021. By 2025, that number is expected to increase to more than 7 million. Eighty percent of those currently living with AD are age 75 or older. Out of the total U.S. population, one in 10 people (10%) age 65 and older has Alzheimer’s dementia. The percentage of people with Alzheimer’s dementia increases with age: 3% of people age 65-74, 17% of people age 75-84, and 32% of people age 85 and older have Alzheimer’s dementia.
People younger than 65 can also develop Alzheimer's dementia, but it is much less common and prevalence is uncertain.
What causes AD?
Several brain changes are believed to contribute to the development of AD. The hallmark changes of AD are the accumulation of the protein beta-amyloid outside neurons (called beta-amyloid plaques) and the accumulation of an abnormal form of the protein tau inside neurons (called tau tangles). Plaques and tangles may make healthy neurons in the brain work less efficiently and eventually die. Neurons in the hippocampus — the brain’s learning and memory center — are among the first to be damaged and die. Neuronal damage eventually spreads to other parts of the brain, causing it to shrink with the loss of neurons. Scientists believe the brain changes of AD begin 20 years or more before the clinical symptoms appear.
According to Alzheimer's Association, the progression of Alzheimer’s disease ranges from brain changes that are unnoticeable to the person affected, to brain changes that cause problems with memory and eventually physical disability is called the Alzheimer’s disease continuum. On this continuum, there are three broad phases:
- preclinical Alzheimer’s disease,
- mild cognitive impairment (MCI) due to Alzheimer’s disease, and
- dementia due to Alzheimer’s disease
The Alzheimer’s dementia phase is further broken down into the stages of mild, moderate and severe, which reflect the degree to which symptoms interfere with one’s ability to carry out everyday activities. While we know the continuum starts with preclinical Alzheimer’s and ends with severe Alzheimer’s dementia, how long individuals spend in each part of the continuum varies. The length of each phase of the continuum is influenced by age, genetics, gender and other factors.
The three stages are characterized by specific biological states, such as decreased glucose metabolism, accumulation of beta-amyloid plaques, and brain atrophy (shrinkage). These “biomarkers” can be detected and monitored by blood and cerebrospinal fluid testing and by positron emission tomography (PET) and magnetic resonance imaging (MRI).
For a detailed report on Alzheimer's disease and treatment options in the US, download 2021 Alzheimer's Disease Facts and Figures.
*Image: Detection of Alzheimer's Disease using PET. (A) 11C-FIB PET scans and (B) 18F-FDDNP PET scans between a healthy patient and one with AD. Patient average age is 67 years old.