Thursday July 3 2008
Overall prevalence of all-cause dementia in 90+-year-olds was 41.2%
“Study shows women–dementia link,” is the headline on the Channel 4 News website today. Women are “much more likely than men to be suffering from dementia when they reach the end of their lives” the website says. The US study showed that about 45% of women from a group of 911 people aged 90 years or older had dementia compared with 28% of men. The chance of having dementia doubled every five years after 90 years in women, but not in men. Channel 4 News also reports that “women with a higher education were up to 45% less likely to develop dementia than those who were not so well-educated”. The reliable study underpinning these stories provides more data about the rates of dementia in a very old population, and it may help in planning healthcare services.
Where did the story come from?
Dr María M Corrada from the University of California, Irvine and colleagues from around the US carried out this research. The study was funded by a grant from the National Institute of Health, and the Al and Trish Nichols Chair in Clinical Neuroscience. It was published in the peer-reviewed medical journal: Neurology.
What kind of scientific study was this?
This was an analysis of data from a prospective cohort study – The 90+ Study – in which the researchers questioned 941 elderly people from California aged 90 years and over. Dementia was diagnosed using personal examinations as well as telephone and informant questionnaires.
The population for this study had previously been involved in another study – the Leisure World Cohort Study – and are described as white, well-educated, upper middle-class and mostly female (66%). Of the 1,151 participants from the original study, those aged 90 years and older on January 1 2003 were invited to join The 90+ Study. By July 1 2006, 941 participants had been recruited into the study. At the start of the study, participants (or their relatives/informants) were sent a questionnaire with questions about age, sex, medical history and medication use. Different questionnaires were used, depending on whether the participants could be interviewed directly or not.
The researchers determined the diagnosis of dementia by using the available information in the following order:
- A neurological examination.
- A Mini Mental State Examination (MMSE).
- A Cognitive Abilities Screening Instrument (questionnaire).
- Dementia questionnaire.
- Two other types of questionnaire.
If a participant had a neurological examination, cognitive status was determined by the examination only. If the participant had no neurological examination but had an MMSE score, then the diagnosis of cognitive status was based on the MMSE only; and so on, down through the list of questionnaires.
Results were analysed for age and sex groups. Additional analyses were performed to assess the impact of education, and to compare those aged more than 95 years with those between 90 and 94.
What were the results of the study?
Of the 941 people recruited for the study, a dementia diagnosis was available for 911 patients. The overall rate (prevalence) of dementia from all causes was higher in women (45%) than in men (28%). Among women, the prevalence increased with age after age 90, “essentially doubling every 5 years.” A lower prevalence of dementia (36–45% lower) was significantly associated with higher education in women, but not in men.
What interpretations did the researchers draw from these results?
The researchers conclude that dementia prevalence increases past age 90 for women, but remains stable for men. This is consistent with most other studies, which have also shown a higher prevalence in women than in men, and increasing prevalence with age. They suggest that because of the high rates of dementia, and as the number of people in this age group increases, “dementia will become a greater public health problem in terms of the number of people with the disease, and the amount of money necessary for their care.”
What does the NHS Knowledge Service make of this study?
This is a reliable study with a few limitations acknowledged by the authors.
- The different methods for diagnosing dementia, by examination and questionnaire, were not ideal. The researchers suggest that the more stringent examination methods may have underestimated the rate of dementia compared with the rate reported by relatives.
- The researchers tested for differences between the methods of diagnosis and found that in 81% of the participants who who were diagnosed using both methods there was agreement between them. Where there were discrepancies, the “in-person” diagnosis including an examination gave higher rates of dementia than the questionnaire or telephone interviews.
- All-cause dementia was reported, so it is not possible to say if there was a difference between sexes for different types of dementia.
- Surprisingly, half the people who died with dementia and had post-mortem examinations did not show enough of the typical features of dementia to account for their symptoms. This confirms the importance of making an accurate diagnosis for this condition.
Overall, this study provides more data on the rates of dementia in a very old population, and will help in planning healthcare services. Future studies might consider including imaging with CT or MRI scans in the battery of tests to increase the reliability of the diagnosis of dementia.
Sir Muir Gray adds...
Not new but still alarming, for women and society.