A cognitive disorder is defined as a significant decline in one’s ability to process and recall information. The DSM IV (American Psychiatric Association 1994) describes three major types of cognitive disorder: delirium, dementia and amnestic disorder. A delirium develops over a short period of time and is characterized by an impairment of short-term memory, disorientation and perceptual and language problems. Amnestic disorders are characterized by impairment of memory such that sufferers are unable to learn and recall new information. However, no other declines in cognitive functioning are associated with this type of disorder. Both delirium and amnestic disorders are usually due to the physiological effects of a general medical condition (e.g., head injuries, high fevers) or of substance use. There is little reason to suspect that occupational factors play a direct role in the development of these disorders.
However, research has suggested that occupational factors may influence the likelihood of developing the multiple cognitive deficits involved in dementia. Dementia is characterized by memory impairment and at least one of the following problems: (a) reduced language function; (b) a decline in one’s ability to think abstractly; or (c) an inability to recognize familiar objects even though one’s senses (e.g., vision, hearing, touch) are not impaired. Alzheimer’s disease is the most common type of dementia.
The prevalence of dementia increases with age. Approximately 3% of people over the age of 65 years will suffer from a severe cognitive impairment during any given year. Recent studies of elderly populations have found a link between a person’s occupational history and his or her likelihood of suffering from dementia. For example, a study of the rural elderly in France (Dartigues et al. 1991) found that people whose primary occupation had been farm worker, farm manager, provider of domestic service or blue-collar worker had a significantly elevated risk of having a severe cognitive impairment when compared to those whose primary occupation had been teacher, manager, executive or professional. Furthermore, this elevated risk was not due to differences between the groups of workers in terms of age, sex, education, drinking of alcoholic beverages, sensory impairments or the taking of psychotropic drugs.
Because dementia is so rare among people younger than 65 years, no study has examined occupation as a risk factor among this population. However, a large study in the United States (Farmer et al. 1995) has shown that people under the age of 65 who have high levels of education are less likely to experience declines in cognitive functioning than are similarly aged people with less education. The authors of this study commented that education level may be a “marker variable” that is actually reflecting the effects of occupational exposures. At this point, such a conclusion is highly speculative.
Although several studies have found an association between one’s principal occupation and dementia among the elderly, the explanation or mechanism underlying the association is not known. One possible explanation is that some occupations involve higher exposure to toxic materials and solvents than do other occupations. For example, there is growing evidence that toxic exposures to pesticides and herbicides can have adverse neurological effects. Indeed, it has been suggested that such exposures may explain the elevated risk of dementia found among farm workers and farm managers in the French study described above. In addition, some evidence suggests that the ingestion of certain minerals (e.g., aluminium and calcium as components of drinking water) may affect the risk of cognitive impairment. Occupations may involve differential exposure to these minerals. Further research is needed to explore possible pathophysiological mechanisms.
Psychosocial stress levels of employees in various occupations may also contribute to the link between occupation and dementia. Cognitive disorders are not among the mental health problems that are commonly thought to be stress related. A review of the role of stress in psychiatric disorders focused on anxiety disorders, schizophrenia and depression, but made no mention of cognitive disorders (Rabkin 1993). One type of disorder, called dissociative amnesia, is characterized by an inability to recall a previous traumatic or stressful event but carries with it no other type of memory impairment. This disorder is obviously stress-related, but is not categorized as a cognitive disorder according to the DSM IV.
Although psychosocial stress has not been explicitly linked to the onset of cognitive disorders, it has been demonstrated that the experience of psychosocial stress affects how people process information and their ability to recall information. The arousal of the autonomic nervous system that often accompanies exposure to stressors alerts a person to the fact that “all is not as expected or as it should be” (Mandler 1993). At first, this arousal may enhance a person’s ability to focus attention on the central issues and to solve problems. However, on the negative side, the arousal uses up some of the “available conscious capacity” or the resources that are available for processing incoming information. Thus, high levels of psychosocial stress ultimately (1) limit one’s ability to scan all of the relevant available information in an orderly fashion, (2) interfere with one’s ability to rapidly detect peripheral cues, (3) decrease one’s ability to sustain focused attention and (4) impair some aspects of memory performance. To date, even though these decrements in information-processing skills can result in some of the symptomatology associated with cognitive disorders, no relationship has been demonstrated between these minor impairments and the likelihood of exhibiting a clinically diagnosed cognitive disorder.
A third possible contributor to the relationship between occupation and cognitive impairment may be the level of mental stimulation demanded by the job. In the study of rural elderly residents in France described above, the occupations associated with the lowest risk of dementia were those that involved substantial intellectual activity (e.g., physician, teacher, lawyer). One hypothesis is that the intellectual activity or mental stimulation inherent in these jobs produces certain biological changes in the brain. These changes, in turn, protect the worker from decline in cognitive function. The well-documented protective effect of education on cognitive functioning is consistent with such a hypothesis.
It is premature to draw any implications for prevention or treatment from the research findings summarized here. Indeed, the association between one’s lifetime principal occupation and the onset of dementia among the elderly may not be due to occupational exposures or the nature of the job. Rather, the relationship between occupation and dementia may be due to differences in the characteristics of workers in various occupations. For example, differences in personal health behaviours or in access to quality medical care may account for at least part of the effect of occupation. None of the published descriptive studies can rule out this possibility. Further research is needed to explore whether specific psychosocial, chemical and physical occupational exposures are contributing to the aetiology of this cognitive disorder.