In industrialized countries, the number of elderly people continues to grow.

The health care systems must be prepared to support this fast-growing part of the population, whose needs are specific: the elderly population is vulnerable to a high occurrence of organic diseases and to cognitive impairments that will affect the quality of life.

The prevalence of mild to important cognitive deficits is usually set between 4% and 10% in the elderly population living in the community (1,2,3). Obviously, individuals with severe cognitive disorders related to dementia are rarely found in the community because they are quickly losing the autonomy necessary to live in the community.

Several longitudinal studies have put forward an increased risk of mortality in non-demented elderly individuals with cognitive impairments (4-8). The adjusted relative risks (for age, sex, health status) of mortality was found to be 1.7 to 3.6 times higher in elderly with mild to severe cognitive impairments (4-12). Some longitudinal studies showed that the mortality rate was not statistically different in any age group among subjects with good cognitive performance, and that the preservation of cognitive functions was associated with better survival in old individuals (11,13,14).

The decline in cognitive performance has been associated with several factors, including neurological diseases, vascular diseases, depression (16,17,18) and diabetes (15) but not always (19).

However, other factors significantly accounted for this decline in the aging process; namely, the educational level (20,21) and social disengagement (22).

More recently, excessive daytime sleepiness has been associated with poor cognition and dementia (23). Moreover, an increased mortality risk of 1.73 was found in elderly individuals with cognitive impairments napping most of the time (24).

This study aimed to determine to which extent EDS is predictive of cognitive impairment in an elderly population.

METHODS

1,026 subjects 60 years of age or older, representative of the general population living in the metropolitan area of Paris (France), were interviewed by telephone using the Sleep-EVAL expert system. To find these subjects, 7,010 randomly selected households were called: 1,269 of them had at least one household member in this age range (participation rate: 80.9%).

In addition to DSM-IV and ICSD diagnoses, the system administered to participants the Psychological General Well-Being Schedule; the Cognitive Difficulties Scale (Mac Nair-R) and an independent living scale.

RESULTS

EXCESSIVE DAYTIME SLEEPINESS

EDS was reported by 13.6% of the sample with no significant difference among age groups. Compared to non-sleepy subjects, those with EDS were at increased risk of cognitive impairment on all the dimensions of the Mac Nair-R scale after controlling for age, gender, physical activity, occupation, organic diseases, use of sleep or anxiety medication, sleep duration and psychological well-being. The odd ratios were: 2.1 for attention-concentration deficits; 1.7 for praxis; 2.0 for delay recall; 2.5 for difficulties in orientation for persons; 2.2 for difficulties in temporal orientation and 1.8 for prospective memory.

NAPPING

The proportion of subjects taking a nap at least two days per week was 30.1%. The percentage of nappers increased with age: 22.4% of subjects between 60 and 64 years napped. This was the case with 29.3% of individuals between 65 and 69 years of age; 34.3% of those between 70 and 74, and 33.8% of those 75 or older (c2 = 11.053; df = 3; p = 0.01). Most of them were taking only one nap per day (90.4%). Among the nappers, 14.6% had non-intentional naps; i.e., they were not planned. The proportion of non-intentional nappers was comparable in each age group.

Intentional napping was associated with a higher score on difficulties in orientation for persons as compared to subjects who never napped. Subjects who took non-intentional naps had higher scores on attention-concentration deficits, delay recall, difficulties in orientation for persons and difficulties in temporal orientation (Table 3).Intentional nappers reported slightly but not significantly more frequently to feeling sleepy during the daytime (14.0% vs. 9.5%; p=.06). The association between intentional naps and excessive daytime sleepiness was significant only in the two younger groups. In subjects between 60 and 64 years old, 19.5% of intentional nappers reported excessive daytime sleepiness as compared with 8.7% of the non-nappers (p < 0.05). In subjects between 65 and 69 years, 15.8% of intentional nappers also reported having excessive daytime sleepiness as compared with 5.6% in the non-nappers (p = 0.02).

ORGANIC DISEASES

Overall, 27% of the subjects in the study were suffering from an organic disease. The most frequently reported diseases were arthritic diseases (20.2%), hypertension (17.3%) and heart diseases (10.5%). The prevalence of organic diseases was higher in the two oldest groups (70-74 y.o.: 34.6%; ≤ 75 y.o. 31.7%) when compared with the two younger groups (60-64 y.o.: 18.7%; 65-69 y.o.: 25.2%; c2=18.833; df=3; p < 0.001).These subjects achieved higher scores on five of the six dimensions of the Cognitive Difficulties scale: attention-concentration deficits, praxis, difficulties in orientation for persons, difficulties in temporal orientation and prospective memory (Table 4).

OBSTRUCTIVE SLEEP APNEA SYNDROME

OSAS was found in 2.6% of the sample with no significant difference among age groups. Generally speaking, OSAS subjects obtained comparable cognitive difficulties scores except on prospective memory, where the scores were significantly higher (Table 4).

DISCUSSION

To our knowledge, this is the first study that aimed to verify whether excessive daytime sleepiness is an independent predictive factor for cognitive difficulties in elderly individuals living in the community.

Our results clearly showed that excessive daytime sleepiness is a strong predictive factor for cognitive difficulties in elderly even after controlling for possible confounding effects of age, gender, physical activity, occupation, organic diseases, use of sleep or anxiety medication, napping and mental diseases.

In our sample:

• 14.1% of individuals 65 years and older had moderate to severe daytime sleepiness.
• Napping at least two days per week was reported by 32.6% of the subjects aged 65 years and over;
• 14.5% of the elderly were taking a nap daily.

Excessive daytime sleepiness should be distinguished from intentional naps. The latter may not reflect the presence of a sleep disorder and can be a healthy habit in the elderly. The increasing rate of napping with age is probably due greatly to the fact that older people have little or no life constraints preventing them from napping whenever they feel the urge, and rarely does this constitute a social problem. There was no association between intentional napping and excessive daytime sleepiness in the elderly in this study. The association was, however, significant in younger subjects of our sample. These younger subjects were mostly still working and therefore could not nap when they felt the need. If intentional napping is rather harmless, excessive daytime sleepiness can be the expression of a more important underlying disorder; for example, a sleep-related breathing disorder or depression (16-18,41-44).

SIGNIFICANCE OF EXCESSIVE DAYTIME SLEEPINESS

In this study, we found that excessive daytime sleepiness is an independent predictive factor of a variety of cognitive difficulties that may impair the quality of life. Elderly subjects who participated in this study were non-demented and had enough autonomy to still live in the community. However, excessive daytime sleepiness and cognitive difficulties were both related in our study to a decreased ability to perform activities of daily living. There was also the possibility that elderly individuals who were retired or who had fewer domestic obligations received less stimulation and therefore were more sleepy during the day. This could be deduced by the increasing number with age of subjects who took a nap during the day. However, from the results of the logistic regressions that we performed, it appears that age was not a major factor for the presence of cognitive difficulties and napping was not related at all. Older age was an independent predictive factor on only two of the seven cognitive measures: praxis and memory deficits on the Mini-Mental State.

An independent predictive factor, that may be surprising at first for difficulties in orientation for persons and prospective memory, was the presence of an occupation.

However, it is not so astonishing: the cognitive difficulties scale is based on a self-report of daily living difficulties. An individual who is still active is more likely to be confronted with a decline in his cognitive abilities than a person who has retired from active life. This is confirmed by the fact that this factor was not predictive of memory deficits when we used the results on memory from the Mini-Mental State examination, where memory is more objectively assessed. Furthermore, active individuals at any age in our study less frequently reported being limited in their travels and in their capacity to do their shopping, two activities that are narrowly related to the items assessed in the prospective memory (need of a list when shopping, forgetting the things that s/he was planning to buy).What can be the mechanism that explains the predictive value of daytime sleepiness for cognitive difficulties?

• A possible explanation is that other disorders such as a mental, an organic pathology or an obstructive sleep apnea syndrome (OSAS) caused daytime sleepiness. Recently OSAS, for which daytime sleepiness is a cardinal symptom, has been found to cause cognitive deficits because of the repeated anoxia provoked by breathing pauses during sleep (41,42). However, this explanation does not fully explain our findings. Indeed, the multivariate models controlled for the effects of these three types of pathologies, and daytime sleepiness still emerged as a strong independent predictor of cognitive difficulties.
• Another possibility is that daytime sleepiness may be due to a lack of cognitive and/or social stimulations. Several studies have shown that when elderly people receive cognitive stimulation and are kept socially active, the likelihood of cognitive decline decreases (22,48,49).
• Another possible explanation is that daytime sleepiness is an early indicator that may predict subsequent cognitive decline. However, longitudinal studies are needed to confirm this hypothesis.

CONCLUSIONS

In summary, these data from a community-based sample indicated that excessive daytime sleepiness is a good predictor of cognitive difficulties. Physicians who treat elderly patients with such complaints should be aware that these patients are at greater risk to have cognitive deficits. As shown in longitudinal studies, it is possible to delay or to prevent these cognitive deficits by keeping up intellectual stimulation and by promoting social engagement in these elderly individuals.

REFERENCE