The research followed more than half a million Norwegian, Swedish and Austrian people for an average of about 10 years, looking at how several factors related to their risk of developing a brain tumour. After dividing people into five bands according to their blood pressure, the researchers found that people with the highest 20% of blood pressure readings were between 45% and 84% more likely to have a brain tumour. However, they found that having high blood pressure while the heart is at rest was only associated with an 18% risk increase once adjustments were made to account for other factors, such as age, gender and smoking status. After these adjustments, there was no increased risk for people who had higher systolic blood pressure (pressure while the heart contracts and pumps blood).
While some news sources have suggested that high blood pressure is associated with a doubling in risk for brain tumours, most of the study’s results suggested the associated risk was much lower. Brain tumours were also still extremely uncommon in the group, regardless of the subject’s blood pressure. This study has various other limitations and is a single study, which means that further study is warranted.
Where did the story come from?The study was carried out by researchers from the Innsbruck Medical University, Austria and researchers from other institutes in Norway, Sweden and the US. It was funded by the World Cancer Research Fund International and published in the peer-reviewed Journal of Hypertension.
News sources were correct to highlight that this study did not show that high blood pressure causes brain tumours, although some of the statistics they quoted may be misinterpreted. For example, some reports quoted figures suggesting that the risk of a certain type of tumour called meningioma more than doubled, but the risk increase was actually much lower than this. The researchers also produced a model adjusting their results to account for important factors such as age, smoking status and gender. It would have been more appropriate for the newspapers to quote these adjusted figures.
The research also separately analysed two types of blood pressure measurements (diastolic and systolic), which were each associated with different risks. Systolic measurements express blood pressure at the point the heart is contracting and forcing blood out into the body, while diastolic is the blood pressure between beats, when the heart is at rest.
What kind of research was this?This was a prospective cohort study that assessed whether there was an association between the risk of brain tumour and metabolic syndrome. Metabolic syndrome is a combination of medical conditions (such as raised cholesterol, raised blood pressure, obesity and high blood sugar) that increases the risk of heart disease and diabetes.
Cancer Research UK reports that there are around 8,000 brain tumours each year in the UK. As brain tumours are relatively rare, the researchers needed to follow a large number of people over time to see which factors were associated with developing a brain tumour. This type of study can only show an association between a factor and brain tumours. It cannot determine whether the factor caused the tumour to develop.
What did the research involve?The cohort study involved is called the Metabolic Syndrome and Cancer Project. It included 578,462 participants with ages ranging from 15 to 99 at the point at which they entered the study, known as the “baseline”. Participants were recruited between 1972 and 2005. The study population was from Austria, Norway and Sweden. When each person entered the cohort, information about their height, weight, blood pressure, blood glucose, cholesterol and blood fats were recorded. Each participant’s smoking status was also noted: whether they had never smoked or were a former smoker or current smoker.
The researchers used nationwide cancer and cause-of-death registries to identify patients who had developed both benign and cancerous brain tumours. In their analyses, the researchers adjusted for sex, birth year, baseline age and smoking status. They did this in a way that took into account how certain factors, such as smoking, influence both blood pressure and cancer.
What were the basic results?The average age of the cohort at baseline was 41. Nearly half of the participants were overweight and nearly a third had hypertension. People in the cohort were followed for 9.6 years on average, and in this time there were 1,312 diagnoses of primary brain tumours (where the cancer originated in the brain rather than spreading from another part of the body affected by cancer). The average age of diagnosis with a brain tumour was 56.
A third of the tumours were classified as a type called a 'high grade glioma', and 8% were 'low grade gliomas'. In the Swedish and Norwegian cohorts, further diagnostic details were available and in these groups 29% of people with brain tumours had a 'meningioma', which is a cancer of the meninges (a membrane that envelops the brain).
The researchers used the participants’ baseline data to divide people into five groups of the same size. Group allocation was dependent on body mass index (BMI), so people with BMIs in the top 20% would be in the top group (or 'quintile'), and people with BMIs in the lowest 20% would be in the bottom quintile. They also grouped the participants into quintiles according to cholesterol levels, fat content in the blood, blood pressure (both systolic blood pressure and diastolic blood pressure) and blood glucose levels to analyse how these factors were associated with tumour risk.
The researchers found that when they compared the risk of brain tumours in the top quintile with the bottom quintile, BMI, cholesterol and blood fat levels were not associated with a risk of developing a brain tumour.
The researchers then looked at blood pressure and found that the group with the highest systolic blood pressure measurements (average 157mmHg) were 45% more likely to have a brain tumour than people in the quintile with the lowest blood pressure measurements (average 109mmHg) [hazard ratio (HR) 1.45; 95% confidence interval (CI) 1.01 to 2.09].
People in the quintile with the highest diastolic blood pressure measurements (average 95mmHg) were 84% more likely to have a brain tumour than people in the quintile with the lowest blood pressure measurements (average 65mmHg) [HR 1.84, 95% CI 1.24 to 2.72].
The researchers repeated the same analysis but this time they looked at whether there was an association between blood pressure and the risk of developing a particular type of brain tumour. They found that:
- Relative to the lowest quintile, the highest quintile systolic blood pressure was associated with a fourfold increase in the risk of meningioma (HR 4.26, 95% CI 1.98 to 9.17).
- Relative to the lowest quintile, the highest quintile diastolic blood pressure was associated with a twofold increase in the risk of meningioma (HR 2.33, 95% CI 1.13 to 4.85).
- There was no association between blood pressure and low-grade gliomas.
- There was no association between systolic blood pressure and high-grade gliomas.
- Relative to the lowest quintile, the highest quintile diastolic blood pressure was associated with an almost threefold increase in the risk of high-grade gliomas (HR 2.67 to 5.50).
How did the researchers interpret the results?The researchers said that increased blood pressure was related to the risk of primary tumour, particularly of meningioma and high-grade glioma.
ConclusionThis large prospective cohort study comprising more than 500,000 people from Austria, Norway and Sweden suggested an association between high blood pressure and some types of brain tumour. It should be noted, however, that even among the group of people with highest blood pressure the overall incidence of brain cancers was low.
Furthermore, there were several limitations to this study:
- Data was only available for the three types of tumour: meningioma and high- and low-grade glioma. Other types of tumour accounted for around 32% of the tumours in the study population.
- The researchers did not collect information on whether the participants had used medication, particularly on whether they were taking medication for lowering their blood pressure. These may have had an impact on the association between blood pressure and risk of brain tumour.
- The researchers suggested that people with high blood pressure may be expected to undergo more neurological investigations such as brain imaging, which may mean that tumours are more likely to be diagnosed earlier in this group.
- Although there was an association between blood pressure and tumours, it is not possible to say that high blood pressure causes brain tumours to develop.
- The researchers used data on blood pressure, cholesterol, BMI and other metabolic measurements gathered at the start of the study. These may have changed over the follow-up period, which was nearly 10 years on average. For example, a person who was overweight at the start of the study may have lost weight over this period.
- This study only included people from Sweden, Norway and Austria. It is not clear whether this population would share similar demographics to a UK population and therefore it is not clear the extent to which these findings could apply to Britain.