Environmental Factors

There is evidence that environmental factors may interact with the genetic factors. One hypothesis is that repeated head injuries do not increase the risk of developing AD in people without ApoE4. When ApoE4 in present, however, repeated head injuries increase the risk for AD by ten times. An interesting fact is that the known high incidence of fatal myocardial infarction in this population culled out many of the E4 carriers before the age of risk for AD. The E4 allele seems to be associated as a risk factor with unfavorable outcomes as a result of head injury, anesthesia or cardiac bypass and recovery from other stresses such as intracerebral hemorrhage. Following an event requiring extensive rehabilitation, it was found that those people with the apoE4 gene were more psychologically impaired. Without investigating other factors, it was hypothesized that apoE4 may not protect the brain from metabolic stress as apoE3 and apoE2.

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