Etiology of Alzheimer's Disease

Overview

Alzheimer's disease is characterized by a progressive loss of memory and cognitive function. Although autopsy shows characteristic brain pathology, Alzheimer's etiology is unknown. For a small percent of persons with early-onset Alzheimer's, there is a strong genetic association but this is not the case for most. Some lifestyle factors or health conditions are associated with greater or lesser risk of Alzheimer's.

Background

Alzheimer's is a neurodegenerative brain disorder with no known cure. According to the Centers for Disease Control and Prevention, Alzheimer's was the sixth leading cause of death in 2007. The Alzheimer's Organization notes that Alzheimer's clinical symptoms include memory loss, language disorders, and behavioral disturbances. Approximately half of dementias are due to Alzheimer's.

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Pathology

According to the National Institute on Aging, several changes occur in the brains of persons with Alzheimer's. Characteristic are plaques, which are microscopic deposits between the neurons consisting of insoluble proteins, neuron fragments and other materials. Tangles, or clumps of a protein called tau protein, are also present inside the neurons. Tangles represent a breakdown in the infrastructure of neurons. The plaques and tangles lead to a loss of connections between neurons and cell death. Associated with all of the neuronal degradation is inflammation of the brain tissue.

Genetics

According to the Alzheimer's Association, genetic links are either strong or weak.



Between 5 and 10 percent of persons with Alzheimer's have a strong family history of early-onset Alzheimer's. In these cases having a specific gene essentially determines you will develop Alzheimer's.



For late-onset Alzheimer's, having one of three genes is associated with an increased risk of developing the disease. However, not everyone with one of these genes will get Alzheimer's and many who develop Alzheimer's have none of these genes.

Risk and Protective Factors

According to the National Institute on Aging, Alzheimer's is associated with several risk or protective factors. First, age and Alzheimer's are strongly associated. Although only 5 percent of persons over 60 have Alzheimer's, the percentage doubles for each decade. By 85, almost half the population shows evidence of Alzheimer's.



In addition, research suggests an association between excess belly fat, type II diabetes and cognitive decline. Having a gene associated with late-onset Alzheimer's also increases the risk.



Estrogen replacement therapy in postmenopausal women, given early, may be protective. Given after diagnosis, however, it does not prevent or treat the evolution of the disease.

Conclusion

According to the Alzheimer's Association, significant challenges in studying Alzheimer's include the lack of a clinical diagnostic test, the inability to identify Alzheimer's early in the illness, difficulties in recruiting people into clinical trials, and inadequate funding. In addition, although genetics plays strong role the small percent of early-onset Alzheimer's, the genetic associations discovered so far have been weak and it is likely that other factors are important. Given the aging population and the fact that Alzheimer's is a leading cause of death, finding its cause is a high priority.