Alzheimer's disease is a neurological disorder in which the death of brain cells causes memory loss and cognitive decline.

It is the most common type of dementia, accounting for 60 to 80 percent of cases of dementia in the United States.

In 2013, 6.8 million people in the U.S. had been diagnosed with dementia. Of these, 5 million had a diagnosis of Alzheimer's. By 2050, the numbers are expected to double.

Alzheimer's is a neurodegenerative disease. At first, symptoms are mild, but they become more severe over time.

Fast facts on Alzheimer's disease

• Alzheimer's disease is the most common type of dementia.
• It happens when plaques containing beta amyloid form in the brain.
• As symptoms worsen, it becomes harder for people to remember recent events, to reason, and to recognize people they know.
• Eventually, a person with Alzheimer's is likely to need full-time assistance.

Symptoms

To receive a diagnosis of Alzheimer's, the person must have experienced a decline in cognitive or behavioral function and performance compared with how they were previously. This decline must interfere with their ability to function at work or in usual activities.

The cognitive decline must be seen in at least two of the five symptom areas listed below:

1. Reduced ability to take in and remember new information, which can lead, for example, to:

• repetitive questions or conversations
• misplacing personal belongings
• forgetting events or appointments
• getting lost on a familiar route

2. Impairments to reasoning, complex tasking, and exercising judgment, for example:

• poor understanding of safety risks
• inability to manage finances
• poor decision-making ability
• inability to plan complex or sequential activities

3. Impaired visuospatial abilities that are not, for example, due to eye sight problems. These could be:

• inability to recognize faces or common objects or to find objects in direct view
• inability to use simple tools, for example, to orient clothing to the body

4. Impaired speaking, reading and writing, for example:

• difficulty thinking of common words while speaking, hesitations
• speech, spelling, and writing errors

5. Changes in personality and behavior, for example:

out-of-character mood changes, including agitation, apathy, social withdrawal or a lack of interest, motivation, or initiative

• loss of empathy
• compulsive, obsessive, or socially unacceptable behavior

If the number and severity of symptoms confirm dementia, the following factors can then confirm Alzheimer's.

• a gradual onset, over months to years, rather than hours or days
• a marked worsening of the individual's normal level of cognition in particular areas

If symptoms begin or worsen over the course of hours or days, you should seek immediate medical attention, as this could indicate an acute illness.

Alzheimer's is most likely when memory loss is a prominent symptom, especially in the area of learning and recalling new information.

Language problems can also be a key early symptom, for example, struggling to find the right words.

If visuospatial deficits are most prominent, these would include:

• inability to recognize objects and faces
• difficulty comprehending separate parts of a scene at once
• difficulty with reading text, known as alexia

The most prominent deficits in executive dysfunction would be to do with reasoning, judgment, and problem-solving.

Other early signs

In 2016, researchers published findings suggesting that a change in sense of humor might be an early sign of Alzheimer's.

Recent research suggests that the features of Alzheimer's, such as brain lesions, may already be present in midlife, even though symptoms of the disease do not appear until years later.

Early-onset Alzheimer's disease

Early-onset familial Alzheimer's disease can affect younger people with a family history of the disease, typically between the ages of 30 and 60 years.

It accounts for under 5 percent of all Alzheimer's cases.

Stages

The progression of Alzheimer's can be broken down into three main stages:

• preclinical, before symptoms appear
• mild cognitive impairment, when symptoms are mild
• dementia

In addition, the Alzheimer's Association describes seven stages along a continuum of cognitive decline, based on symptom severity.

The scale ranges from a state of no impairment, through mild and moderate decline, eventually reaching "very severe decline."

A diagnosis does not usually become clear until stage four, described as "mild or early-stage Alzheimer's."