Aging of the brain, entropy, and Alzheimer disease.

Sporadic Alzheimer disease (AD) is related to advancing age far more than to any other risk factor and ultimately affects almost half of the population over age 85. Despite its remarkable prevalence among the elderly, it has been regarded as a specific disease, distinct from "normal aging." This view is supported in large part by clinical and pathologic similarities to early-onset, dominantly inherited familial AD, where genetic mutations related to beta-amyloid have been identified. There is much evidence that sporadic AD overlaps with normal aging in many clinical and pathologic features. Some of the many molecular age-related changes (ARCs) affecting the brain, both intrinsic (programmed) and extrinsic (stochastic), are reviewed, with discussion of the effects they have singly and collectively on neuronal viability and vulnerability. The effect of ARCs on the brain is seen as the biologic manifestation of increasing entropy, an approach that helps to explain the progressive decline of neural and cognitive function over time; the ability of multiple, varied ARCs to summate as individuals age; the transitional relationship between normal aging, mild cognitive impairment, and AD; and the apparent differences between normal aging and AD. Increasing entropy, manifest through a complex network of interacting ARCs, is seen as the fundamental driving cause of neural and cognitive decline in the elderly, as well as the overriding etiologic principle in further transition to sporadic AD. Research on sporadic AD has largely focused on finding a single causal metabolic disorder or genetic mutation. Multiple ARCs contribute to declining function and increased frailty in the aging brain, however, and to the catastrophic disintegration of sporadic AD. Effective prevention or treatment will depend on recognizing the contributions of a multiplicity of ARCs to AD and reducing the burden of as many as possible. The role of amyloid is seen as one element in the larger network of senescent changes involving the aging brain.