Background: This study compares the relative importance of heredity and environment in the development of Alzheimer disease and vascular dementia. The relationship between apolipoprotein E and dementia is also tested.
Methods: A total of 23,000 cognitively impaired subjects from Norwegian institutions for the elderly were identified, and their files were checked against the records of 26,000 twin pairs from the Norwegian Twin Register. A sample of 72 twin pairs was selected and thoroughly investigated clinically. The mean age of the sample was 80 years.
Results: The pairwise concordance rate for Alzheimer disease was 78% (7/9) among monozygotic and 39% (9/23) among dizygotic twin pairs. The probandwise concordance rate was 83% (10/12) among monozygotic and 46% (12/26) among dizygotic twin pairs. There was no significant difference in the rate of apolipoprotein E epsilon 4 allele between twin pairs concordant and discordant for Alzheimer disease. By using tetrachoric correlations, the estimated heritability was approximately 0.6. Environmental factors shared by cotwins seemed to explain most of the remaining variance. In vascular dementia, there was no significant difference in pairwise concordance rates among monozygotic (1/6 [17%]) and dizygotic (4/16 [25%]) twin pairs or in probandwise concordance rates among monozygotic (2/7 [29%]) and dizygotic (5/17 [29%]) twin pairs.
Conclusion: Heredity is the major causal factor in late onset Alzheimer disease, whereas environmental factors dominate in vascular dementia.