To our knowledge, this is the first population-based sample used to study risk for incident dementia among married older adults, as a function of whether spouse develops incident dementia. The finding of a six-fold increase in dementia risk, among persons so exposed, compared with married men and women whose spouse had not developed dementia, remained after adjustment for age, gender, education, SES, and APOE genotype, all of which were (as expected) associated with dementia risk. Length of marriage did not modify this association. Our stratified analyses suggest a more deleterious effect on husbands than on wives but, given the overlap in confidence intervals, the difference could be due to chance and needs further study.
The distress of watching one’s spouse suffer from dementia, and the physical and mental burden of providing dementia care, are potential causal factors, given the influence of caregiving on elevated risk for depression 25
and mortality 6
. Having a loved one with dementia is stressful regardless of age, but the burden for spouses may be even greater due to close emotional ties to their partner, their own medical co-morbidity, greater risk for functional limitations, and greater likelihood of fatigue at physical exertion. Neuropsychiatric disturbances that are common over the course of dementia are particularly stressful and are the most common reason for institutionalization. 26
Anticipatory grief (related to loss of pre-dementia relationship as dementia progresses with spouse’s impending death) has been independently associated with caregiver burden among dementia caregivers, beyond effects of caregiver characteristics and behavior problems in the care recipients. 27
It is important to note that this finding may be due to several alternative mechanisms. In the absence of direct measures of subjective stress and caregiving activities, part of the observed effect may be due to shared environmental exposures. Our models adjusted for socioeconomic status, providing at least partial adjustment for an environment conducive to healthier lifestyles shared by both spouses. Nevertheless, adjustment for SES does not provide a complete control for potential confounders, such as access to medical care, smoking, alcohol consumption and diet. However, our random effects models controlled for shared unmeasured exposures without any appreciable change in the findings. A third mechanism may be homogamy, or positive assortative mating, to the extent that shared risk for dementia in couples may be influenced by similarity in proneness to distress or mental illness 28
. Thus, while the overall risk for dementia among married individuals whose spouse has dementia was high, these mechanisms are not mutually exclusive. The relative contribution of each of these potential mechanisms to this overall effect is unknown and needs further study.
The data also show that many exposed spouses are not affected. An important focus for future studies should be to better understand the strategies and contexts that limit the adverse effects of the exposure. Results of such analyses would then guide possible interventions. Study strengths include a large community-based sample that avoids the selection bias of clinical samples, 29
and availability of APOE genotypes. Its high baseline participation rate (90%) reduced concerns about non-responder bias. 30
Additional strengths are the longitudinal follow-up allowing identification of incident cases, using an in-depth clinical assessment protocol to evaluate both spouses for dementia, over four triennial waves.