Application of life table analysis to the onset of dementia in a genetically informative design

The primary objective of this study was to estimate the survival function for time to onset of dementia in initially unaffected twins from when their partner (index proband) was diagnosed with dementia of the Alzheimer's type. Survival functions generated by life table analyses were compared by zygosity and gender. Sixty-one twin pairs where at least one member had been diagnosed with dementia of Alzheimer's type were included in the analyses. Additionally, both members of the twin pair had to be alive at the time the index proband was diagnosed with dementia. The probability of remaining cognitively intact within the first three years after the proband was diagnosed was high (0.93, 95% CI 0.89, 1.0), but after 15 years the probability of remaining intact was low (0.34, 95% CI 0.16, 0.52). Age of onset of the index proband was a significant covariate in the survival functions. There were significant differences in the survival functions for monozygotic (MZ) and dizygotic (DZ) co-twins (chi2 = 3.86, 1, P < 0.05), evidencing a genetic component for age of onset for dementia, but there were no significant differences between men and women.