Continuing on the theme of the Insular Cortex, the paper reviewed here is ‘Relationship between Blood Pressure and Subcortical Lesions in Healthy Elderly People’. The authors write about the previously established relationship between elevated systolic blood pressure (even when it is within the ‘normal’ range) and white matter hyperintensities which in itself suggests the importance of keeping a very close eye on blood pressure.
The aim of the study was to investigate the relationship between white matter hyperintensities and blood pressure. The method was to perform ambulatory blood pressure measurements on subjects a week apart and scan them in an MRI scanner for hyperintensities. Subjects were recruited from a newspaper ad. This means there was already some selection bias in that the subjects would be readers of that paper and also likely to respond to these particular ads. The difference between responders and non-responders to ads wasn’t clear. The idea was that the subjects would be healthy and in the age range 55 to 80. Establishing that they were healthy was done in two steps. The first step involved a telephone interview in which the subjects provided details of their medical history. They were excluded if there was any serious medical history (although it’s not clear what the definition of serious was in this instance), hypertension, head injury, obesity, drug abuse, alcohol abuse, psychiatric illness or medications with effects on the cardiovascular system or central nervous system. There is a difficulty here in that this depends on retrospective memory. Additionally the term ‘psychiatric illness’ is so broad that it would be unusual if the subjects hadn’t experienced one (e.g. adjustment reactions). At the second stage, subjects underwent an examination by the doctor together with an ECG and blood tests. There was a more comprehensive set of exclusion criteria used at this stage which seemed to address a good majority of the more common serious age-related illnesses. Given the age range of subjects, this makes them even less representative of the general population. Indeed an obvious question at this stage is why wasn’t a younger age group chosen – after all they would be less likely to have white matter hyperintensities caused by any of these illnesses. Perhaps they would be less likely to have white matter hyperintensities. However as the subjects are relatively elderly, it’s more likely that they could at one stage have had an undiagnosed illness e.g. transient hypertension which could have led to white matter lesions and then resolved. If this is the case, a better study option might have been a prospective study which would however have picked up less positives (since none would have had WMH to begin with) although they would then be able to exclude any non-bp related aetiological illnesses.
The Accutracker II was used for measurement of 24-hour blood pressure as well as an activity monitor. A 1.5 Tesla MRI scanner was used to produce sagittal sections of the brain with prior scans used for the purposes of alignment. The images were assessed by two independent raters with good inter-rater reliability (r=.90, F=18.6).
144 subjects were entered into the study covering a range of ethnic groups. Just over 1/3 were employed and averaged roughly 10 hours per week, high levels of education with only 5 smokers in the group as well as over half earning over $50,000 per year.
There were three broad groups of findings
(a) Amongst other findings, for white matter hyperintensities there were significant associations with systolic blood pressure during casual, awake and sleeping measurements.
(b) For subcortical gray matter, hyperintensities were significantly associated with systolic blood pressure during casual and awake measurements.
(c) Insular subcortical hyperintensities were significantly associted with systolic and diastolic blood pressure during casual measurements (p=0.0001 for both). For awake and asleep measurements these were also significant. However and quite curiously in the table the awake and sleep systolic values have no significant relationship with insular subcortical hyperintensities after controlling for age and sex. I couldn’t find any reference to this in the discussion.
The authors drew two interesting conclusions in the discussion. The first was to point to the importance of the Insular Cortex in terms of blood pressure and they reference other studies showing that infarctions in the insular cortex are associated with higher noradrenaline levels and little or no reduction in BP at night. They also cite a reference suggesting a pathway from the Insula to the heart via the lateral hypothalamic area.
From this study however, which is cross-sectional in nature, we can’t really draw any conclusions about causality. Firstly the insular cortex values for awake and sleep systolic BP seem to disappear after controlling for age and sex (although the casual values are not mentioned). Secondly, the Insular Cortex is rather vulnerable in terms of its blood supply to cerebrovascular events and so we must also ask whether this is the reason that hyperintensities show up here in people with elevated systolic BP.
The other interesting discussion the authors have is about the general implications of the blood pressure results and they cite research suggesting that decreasing diastolic blood pressure by 2mmHg would lead to a 17% reduction in risk of cerebrovascular events.
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Iris Goldstein, Goerge Bartzokis, Darwood Hance and David Shapiro. Relationship between Blood Pressure and Subcortical Lesions in Healthy Elderly People. Stroke. 1998. 29. 765-772.