Age at onset of Alzheimer's disease: clue to the relative importance of etiologic factors?

Clues to the relative importance of possible etiologic factors for dementia of the Alzheimer type may be gained by examining the fit of case series to Sartwell's model of the distribution of incubation periods. If age at disease onset is used as the incubation period of this disease, a genetic or environmental factor acting during the prenatal period is suggested if the distribution of these ages fits the lognormal curve; otherwise, environmental factors acting after birth are implicated. Case series were identified from the literature. Four case series were found which contained sufficiently detailed data to permit this secondary analysis; only one case series was population-based. The distribution of age at disease onset for each series was graphically and statistically assessed for fit to the logarithmic normal distribution. Each case series fit the lognormal curve well. This suggests that research into the etiology of dementia of the Alzheimer type should focus on the prenatal experiences of patients with this disease.