Vascular dementia and intelligence

“Researchers have found a link between dementia and childhood intelligence”, the Financial Times reported today. It said that a study has found that having a lower IQ as a child increases the risk of developing vascular dementia, which is caused by problems in blood supply to the brain. The Daily Telegraph also covered the story and said that the risk of developing vascular dementia later in life increased by 40% if intelligence levels were lower.

The study was a case control study, and studies of this type are prone to biases that need to be considered when their results are interpreted. In addition, there are other factors linked to both “mental ability” and dementia outcomes that the researchers did not take into account; these include alcohol use, smoking, physical health problems, diet, genetics, and other factors.

Given the complexity of vascular pathology, it is unlikely to be a single factor such as intelligence that causes this condition in the elderly. In an effort to prevent vascular disease, it may be more helpful to address the downstream effects of personality and intelligence, such as behaviour. This study does not diminish the importance of tackling the well-known risk factors for vascular disease, of which smoking is generally the most important.

Where did the story come from?

Dr Brian McGurn and colleagues from the Royal Victoria Hospital in Edinburgh carried out the research. He was supported by a clinical research training fellowship, while a second author was in receipt of the Wolfson Research Merit Award from the Royal Society. The study was published in the peer-reviewed medical journal: Neurology .

What kind of scientific study was this?

The authors of this case control study looked at the cognitive abilities of people when they were young in order to investigate the effect on late-onset dementia.

To do this, they used local health records to identify people born in 1921 who had developed dementia (Alzheimer’s disease, vascular dementia or unspecified dementia) after they were 65 (i.e. they had late-onset dementia). This birth date was used so that people who had also taken part in the Scottish Mental Health Survey of 1932 were included.

In the 1932 survey, Scottish schoolchildren born in 1921 participated in a large survey of their general mental ability using the Moray House Test (MHT) No 12. This test consists of 71 questions  and tests a child’s ability in the following areas; determining same from opposites, analogies, reasoning, arithmetical, spatial awareness and interpreting mixed sentences and proverbs.

The researchers identified 297 people born in 1921 who went on to develop vascular dementia. Of these, 173 had taken part in, and had data available from the 1932 mental health survey. These were included as “cases” in this study.

Researchers then matched each case to four control children who were identified through the local birth registers. In the first control group, each case was matched to two controls by age, gender and the district in which their birth was registered. In the second control group, each case was matched to two controls on age, gender, district in which the birth was registered and father’s occupation (as a reflection of social class). The researchers then looked for associations between results on the mental ability tests in the survey and the different types of dementia. They considered other factors that may have influenced the results including the mother and father’s age, length of marriage, and the age of the subject when they undertook the mental health survey.

What were the results of the study?

Of the 173 dementia cases, 50% had Alzheimer’s disease, 19% had vascular dementia, and 25% had unspecified dementia. There was no link between the father or mother’s age and any diagnosis of dementia. When the researchers looked at the results by dementia type, they found that people with vascular dementia had significantly lower mental ability scores than both control groups.

They said that a 10-point increase in MHT score reduced the odds of vascular dementia by 40%. This difference was not evident with Alzheimer’s disease or for unspecified dementia.

What interpretations did the researchers draw from these results?

The researchers concluded that their study shows that lower cognitive ability before disease onset increased the risk of vascular dementia but not of Alzheimer’s disease. This association was independent of the factors assessed by the study.

What does the NHS Knowledge Service make of this study?

There are several points to bear in mind when reading these news stories:

• Case-control studies, by their very design, are prone to bias. There could also be other factors that are responsible for the association between vascular dementia and cognitive ability seen in this study. The study could not control for the effects of education and occupation on the risk of dementia. These are important factors and others such as smoking, high blood pressure, high cholesterol, diabetes or other heart problems were also not taken into account. These may increase the risk of vascular dementia in people. Although the researchers said that other research has shown that the risk of vascular disease is independent of socioeconomic status and smoking, this study does not assess this.
• The study found no association between Alzheimer’s disease (the most common form of dementia in the UK) and this measure of mental ability. The news headlines may be misleading by implying that “dementia” is linked to childhood intelligence.
• The researchers said that “a number of controls may have developed dementia”. Using a case control design, it is not possible to estimate the incidence (rate of new cases) of the illness in a population. If many controls went on to develop dementia, but were not captured as “cases” in this study, the results will not represent the true differences between these groups.
• The researchers also acknowledged that there might have been some inaccuracies in establishing the diagnoses of the different  dementias.

The relationship between vascular pathology and dementia is a complex one and it is unlikely that a single factor such as low IQ causes vascular dementia. It seems more plausible that there may be an intermediate lifestyle factor that is linked both to a lower mental ability and to vascular disease in later life. For example, lower mental ability in childhood could be associated with poor diet, alcohol misuse or other lifestyle factors that were not measured here.

Sir Muir Gray adds...

Low intelligence usually leads to low income and low income is known to increase the risk of vascular disease.