Friday January 6 2012
Cognitive decline does not definitely signal dementia
“Memory and other brain skills begin to decline at the age of 45 – much earlier than previously thought,” the Daily Mail has today reported.
The news is based on a large UK study looking at the rate of cognitive decline in different age groups. Between 1997 and 2007 it assessed 7,390 participants aged between 45 and 70 years, looking at how their performance changed when given simple tests of mental reasoning. The researchers found the greatest rate of cognitive decline was in older subjects, but that all age groups showed some decline. For example, men aged 65-70 when they started the study experienced a 9.6% decline in mental reasoning over 10 years, but men initially aged 45-49 experienced a decline of 3.6%.
This sort of study is interesting for looking at patterns of cognitive decline across ages, and suggests that cognitive decline may begin prior to the age of 60 as the researchers originally theorised. However, it cannot tell us whether this decline actually leads to any significant loss of daily functioning or any increased risk of conditions such as dementia. Additionally, as the youngest people in the cohort were 45 years old, it is not possible to say whether cognitive decline starts at 45 years or even earlier.
Where did the story come from?
The study was carried out by researchers from French and British research bodies, including University College London and the French health and research body Inserm. The study was published in the peer-reviewed British Medical Journal.
The news stories generally reflect the findings of this study accurately. However, this study does not show that cognitive decline starts at 45 years, as the youngest people included in the study were 45. Without including younger people it is not possible to tell whether there is any evidence of decline at younger ages.
The Independent featured a rather pessimistic headline, saying “Life ends at 45... Study reveals when our mental powers start to diminish”. Given the limitations of the study we would urge middle-aged people not to give up on life just yet.
What kind of research was this?
This was a cohort study designed to look at how different age groups experienced cognitive decline over a 10-year period. To do so the researchers divided people into different age categories at the start of the study and repeatedly assessed their cognitive function over the period. For example, they compared whether people who were aged 55-59 at the start of the study experienced a different rate of cognitive decline from those who were aged 45-49 at the start of the study. In particular, drawing on the results of previous research, they were interested in investigating the theory that cognitive decline might begin prior to the age of 60.
This sort of study is interesting for looking at patterns or rates of cognitive decline across ages, but it cannot tell us much more than this about the causes of cognitive decline or dementia, or provide further insight into potential treatments. As the youngest people at the start of the study were 45, it cannot tell us whether cognitive decline starts at 45 as some of the newspapers have suggested.
What did the research involve?
This study used a large sample of middle-aged adults from an ongoing, multipurpose research project called the Whitehall II cohort study. This study was set up in 1985 and enrolled 10,308 British civil servants in order to understand various issues relating to their employment, health and wellbeing. During the first assessment phase, between 1985 and 1988, researchers assessed these people using clinical examination and self-report questionnaires. This 10-year cognition study is based on the study phases that included cognitive testing during the clinical examinations: 1997-99, 2002-4 and 2007-9.
At the first cognitive assessment participants were aged 45-70. The researchers divided them into five-year age bands to allow age-based comparisons of cognitive decline over the next 10 years. These were:
Cognitive analysis at each of the three assessments was based on a series of tests:
- The Alice Heim 4-I (AH4-I) test, which is composed of a series of 65 verbal and mathematical reasoning items of increasing difficulty.
- Tests of two measures of verbal fluency: phonemic (writing as many words beginning with ‘S’ as possible) and semantic (recalling as many animal names as possible).
- The Mill Hill vocabulary test, which is a multiple-choice test that asks participants to choose the word that is the opposite or most closely related to another word, for example, ceiling and floor.
The researchers used statistical tests to compare the cognitive abilities across the five different age groups at each assessment (a cross-sectional analysis) and to assess each group’s rate of cognitive decline over the 10-year follow-up (a longitudinal analysis).
What were the basic results?
For this cognition study, which was carried out between 1997 and 2007, they included 7,390 members of the original full Whitehall II cohort recruited in 1985. In this analysis 70% of subjects were male. Only 4,675 participants (63%) had data available for all of the three assessments (carried out at 1997-1999, 2002-2004 and 2007-2009). Those participating in this study were more likely to be younger and to have had a university education than those in the original 1985 cohort who had either died by this time or been lost to follow-up.
During the 10-year follow-up, 305 of 7,390 (4%) died. Mortality was higher among those with poorer cognitive scores at first assessment.
The main findings:
Ability in all cognitive tests, with the exception of vocabulary, declined over time in all five age categories, with evidence of faster decline in older participants.
- In men, there was a 3.6% decline in reasoning ability in those aged 45-49 at the start of the study, compared with a 9.6% decline in those initially aged 65-70.
- In women, rates of decline over 10 years were comparable: a 3.6% decline in reasoning ability in those aged 45-49 at the start of the study, compared with a 7.4% decline in those initially aged 65-70.
In addition to assessing how each person’s test score changed from their earliest assessment to their last assessment 10 years later, the researchers also compared the test scores of the older groups with those of the younger groups at the first assessment in 1997. They found that in 1997 women who were aged 55-59 had 11.4% lower test scores than women who were 45-49.
The researchers found that this was because educational level was having an influence on cross-sectional assessments. This means that women who were older at the start of the study were less likely than younger women to have had higher education. The researchers say that education, rather than age, was having an effect on the differences in cognitive ability in these cross-sectional analyses.
How did the researchers interpret the results?
The researchers conclude that cognitive decline is already evident in middle-aged people (age 45-49).
This study following a large group of people aged 45-70 at the start of the study suggests that cognitive ability declines over 10 years in all age categories, even the youngest, although there is a greater rate of decline in older age groups. This study lends support to the theory that cognitive decline may begin prior to the age of 60.
However, the study does have limitations:
- Though it included a large sample size, all the participants were British civil servants and hence results may only be applicable to this population group. For example, they may differ from people working in different environments.
- Also, the assessments included only about 70% of the original Whitehall II cohort, and there were differences in educational ability, and possibly other medical and lifestyle factors, between those who were included in this study and those who had died or were lost to follow-up.
- The study can also only provide insight into patterns of cognitive decline over 10 years in different age groups. It cannot tell us much more than this about the causes of cognitive decline or dementia, or provide further insight into ways to combat the condition.
- While cognitive tests are useful tools, a decline in test scores may not necessarily equate to a decline in real-world functioning.
- The research did not feature any participants aged less than 45, and so it is not strictly accurate to assume that cognitive decline begins at 45. Equally, we cannot rule out the possibility that those aged 45 might perform better than younger people, which could be taken to mean cognition actually peaks at 45.
As the researchers suggest, future research needs to identify factors that influence the rate of cognitive decline, and to determine whether some of these factors can be modified in the individual to slow this rate of decline. As they also say, determining the age window at which potential interventions are likely to be most beneficial is also “a crucial next step”.