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Mental challenge in the workplace and risk of dementia in old age: is there a connection?
  1. R S Wilson
  1. Correspondence to:
 Dr R S Wilson
 Rush Alzheimer’s Disease Center, Rush University Medical Center, Armour Academic Center, 600 South Paulina, Suite 1038, Chicago, IL 60612, USA; rwilsonrush.edu

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Commentary on the paper by Seidler et al (Occup Environ Med, December 2004)*

Dementia is among the most feared consequences of growing old, and with the aging of the generation born after World War II, its prevalence is expected to increase in most industrialised nations in the coming decades. Cognitive decline and dementia in late life are the result of multiple environmental and genetic risk factors interacting with multiple age related pathological changes taking place in the brain. The good news is that recent studies have identified a range of lifestyle factors associated with risk of dementia, including characteristic levels of cognitive, social, and physical activity and proneness to experience negative emotions, suggesting that some proportion of risk may be modifiable.

In the December 2004 issue of Occupational and Environmental Medicine, Seidler and colleagues report an association between psychosocial features of the workplace environment and risk of dementia, with decreased risk in persons with more cognitively and socially challenging working conditions.1 The finding is noteworthy because few previous studies have examined the relation of workplace activities to risk of dementia in old age.

Perhaps the first question we should ask about the finding is whether it is valid. Seidler and colleagues1 selected dementia cases from medical records, which could bias results because many older persons with dementia do not come to medical attention, and those who do appear to differ from those who do not. In addition, although the cross-sectional case-control design of the study is a relatively cost-effective way to identify potential risk factors for dementia, a longitudinal cohort design has the advantage of collecting exposure data before dementia onset, so that results do not depend on the comparability of cases with controls or of informant-report (of job history) with self-report. Prospective studies have not examined psychosocial working conditions in detail like Seidler and colleagues,1 but global ratings of the cognitive demands and occupational prestige of work have found decreased risk of dementia associated with more cognitively demanding and prestigious jobs,2 consistent with their findings.

How might a challenging occupation help protect against the development of dementia in old age? Seidler and colleagues1 suggest several possibilities. One is that decades before the appearance of clinically evident dementia, symptoms of the underlying disease might be affecting behaviour in the workplace. The onset of Alzheimer’s disease, the most common cause of dementia, is indeed insidious. A prodromal period characterised by mild cognitive impairment precedes the appearance of manifest dementia, but the duration of this period is probably only a few years,3 and almost certainly not a few decades. Because most dementia develops after the age of 70, very early preclinical manifestations are unlikely to explain the relation of occupational history to dementia risk.

Occupations differ in risk of exposure to potentially neurotoxic agents, but Seidler and colleagues1 rightly point out that there is currently little evidence that such exposures can account for the relation between occupation and risk of dementia. The authors also consider the possibility that differing levels of work related stress might affect risk of cardiovascular disease and ultimately of dementia. Secondary analyses did not support this explanation, however.

The leading causes of dementia in old age are the accumulation of amyloid-beta and tau-postive neurofibrillary tangles in the brain and cerebral infarctions. These pathological lesions are commonly found in the brains of older persons who die without dementia, however. This suggests that some risk factors for dementia may work by affecting the accumulation of these lesions, whereas others may work by affecting the brain’s capacity to tolerate these lesions, sometimes referred to as brain reserve. Extensive animal research has shown that environmental complexity is related to individual differences in adulthood in neurogenesis and synaptogenesis in brain regions critical for memory and thinking. According to the brain reserve theory, therefore, engagement in activities that are cognitively, socially, and physically stimulating somehow enhances brain reserve so that selected neural systems are more difficult to disrupt and more age related pathology is needed to impair the skills mediated by those systems. Support for this idea comes from a clinicopathological study of older Catholic clergy members in which the deleterious impact of Alzheimer’s disease pathology on cognitive function was reduced in persons with more education relative to those with less.4 The results of Seidler and colleagues1 are consistent with the brain reserve hypothesis, as they note.

Finally, are we to conclude that persons whose work does not routinely involve cognitive challenges and social stimulation are destined to face a heightened risk of dementia in old age in exchange for their labours? A recent prospective study suggests not: the association of occupational prestige with risk of incident dementia was eliminated after controlling for late life engagement in cognitively stimulating activities,5 implying that it is the overall level of stimulating activity that is important, not whether it occurs during work or during leisure time and underscoring the particular importance of maintaining an active lifestyle in old age.

Acknowledgments

This research was supported by National Institute on Aging Grant R01 AG17917.

Commentary on the paper by Seidler et al (Occup Environ Med, December 2004)*

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