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 f or 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 zygo sity and gender. Sixty-one twin pairs where at least one member had been di agnosed with dementia of Alzheimer's type were included in the analyses. Ad ditionally, both members of the twin pair had to be alive at the time the i ndex proband was diagnosed with dementia, The probability of remaining cogn itively 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 r emaining intact was low (0.34, 95% CI 0.16, 0.52), Age of onset of the inde x 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 (chi(2) = 3.86, 1, P < 0.05), evidencing a genetic component for age of onset for dementia, but there were no significant dif ferences between men and women. (C) 1999 Wiley-Liss, Inc.