Types of Alzheimer's disease

Early-onset Alzheimer's: This is an uncommon form in which individuals are diagnosed with the disease before age 65. Fewer than 10 percent of all Alzheimer's disease patients have this type. Because of their genetic abnormality, people with Down syndrome are particularly at risk for a form of early-onset Alzheimer's disease. Adults with Down syndrome often are in their mid- to late 40s or early 50s when symptoms first appear.

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Late-onset dementia: The most common form of Alzheimer's disease, late-onset dementia usually strikes after age 65. It occurs in almost half of all people over the age of 85 and may or may not be hereditary. Late-onset dementia is also called sporadic Alzheimer's disease.

Familial Alzheimer's disease (FAD): This form of Alzheimer's disease is known to be entirely inherited. FAD is extremely rare, accounting for fewer than 1 percent of all cases of Alzheimer's disease. It has a much earlier onset (often in the 40s) and follows clear patterns of inheritance.

In addition to age of onset, there are other differences among the types of Alzheimer's.

Younger people who develop Alzheimer's disease have more of the microscopic changes found in the brains of people with Alzheimer's disease. These changes include the twisted nerve cell fibers, known as neurofibrillary tangles, and a sticky protein called beta amyloid, which forms structures called plaques. Plaques and tangles are associated with damage to healthy brain cells and result in brain shrinkage or atrophy. It may be, though, that younger brains simply need to be more damaged before a person starts to show symptoms, so this may not reflect a true difference in the nature of the disease.

A condition called myoclonus--muscle twitching and spasm--is more commonly seen in early-onset than late-onset Alzheimer's.

Some research suggests that people with early-onset Alzheimer's disease decline at a faster rate than do those with late-onset Alzheimer's disease.

Younger people who are diagnosed with the disease tend to be more physically fit and active, and many still have family and career responsibilities. As a result, they tend to feel more powerless, frustrated, and depressed upon diagnosis.