Alzheimer’s Disease

Alzheimer’s disease (AD), also referred to simply as Alzheimer’s, is the most common cause of a neurocognitive disorder, accounting for 60–70% of its cases. Alzheimer’s is a progressive disease causing problems with memory, thinking, and behavior. Symptoms of Alzheimer’s (AD) usually develop slowly and get worse over time and eventually become severe enough to interfere with daily life functioning and tasks. ^[1]

The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation (including easily getting lost), mood swings, loss of motivation, not managing self-care, and behavioral issues. In the early stages of Alzheimer’s disease (AD), memory loss is mild but in people with late-stage Alzheimer’s, individuals will lose the ability to carry on conversations and respond to their environment.

Figure 1. Alzheimer’s disease is not simply part of the aging process. It is a disease with physiological symptoms and decay in the brain.

Alzheimer’s is the sixth-leading cause of death in the United States. On average, a person with Alzheimer’s will live for four to eight years after diagnosis, but can live as long as 20 years, depending on other health factors. It is important to note that increasing age is the greatest known risk factor, but Alzheimer’s is not a normal part of aging. The majority of people with Alzheimer’s are 65 and older, though approximately 200,000 Americans under the age of 65 have younger-onset Alzheimer’s disease (also known as early onset Alzheimer’s). (What is Alzheimer’s? Alzheimer’s Association. Retrieved from https://www.alz.org/alzheimers-dementia/what-is-alzheimers.)

Etiology

The cause of Alzheimer’s disease is poorly understood. About 70% of the risk is believed to be inherited from a person’s parents with many genes usually involved. Other risk factors include a history of head injuries, depression, and hypertension. The disease process is associated with plaques and neurofibrillary tangles in the brain.

The genetic heritability of Alzheimer’s disease (and memory components thereof), based on reviews of twin and family studies, ranges from 49% to 79%. Around 0.1% of the cases are familial forms of autosomal (not sex-linked) dominant inheritance, which have an onset before age 65. This form of the disease is known as early onset familial Alzheimer’s disease. Most autosomal dominant familial AD can be attributed to mutations in one of three genes: those encoding amyloid precursor protein (APP) and presenilins 1 and 2.

A probable diagnosis is based on the history of the illness and cognitive testing with medical imaging and blood tests to rule out other possible causes. Initial symptoms are often mistaken for normal aging, but an examination of brain tissue, specifically of structures called plaques and tangles, is needed for a definite diagnosis. Though qualified physicians can be up to 90% certain of a correct diagnosis of Alzheimer’s, currently, the only way to make a 100% definitive diagnosis is by performing an autopsy of the person and examining the brain tissue. In 2015, there were approximately 29.8 million people worldwide with AD. In developed countries, AD is one of the most financially costly diseases.

Epidemiology

Two main measures are used in epidemiological studies: incidence and prevalence. Incidence is the number of new cases per unit of person at risk (usually the number of new cases divided by the persons at risk at a given time). For example, a study could look at the number of new cases of neurocognitive disorder over a five-year period, then divide that by the total time each individual was observed to find the incidence, or person-year rate. Prevalence is the total number of cases of the disease in the population at any given time.

Regarding incidence, cohort longitudinal studies (studies where a disease-free population is followed over the years) provide rates between 10 and 15 per thousand person-years for all dementias and five to eight per thousand for Alzheimer’s Disease, which means that half of the new dementia cases each year are AD. Advancing age is a primary risk factor for the disease and incidence rates are not equal for all ages: every five years after the age of 65, the risk of acquiring the disease approximately doubles, increasing from three to as much as 69 per thousand person-years. There are also sex differences in the incidence rates, women having a higher risk of developing AD particularly in the population older than 85. In the United States, the risk of dying from Alzheimer’s disease is 26% higher among the non-Hispanic White population than among the non-Hispanic Black population, whereas the Hispanic population has a 30% lower risk than the non-Hispanic White population.