Chapter 4 Discussion In this study we found reduced mCBFV and correspondingly elevated CVRi in AD patients compared to matched controls. Also compared to MCI CVRi is significantly elevated in AD. It has been argued that reduction in CBF in AD could be explained by loss of brain tissue due to the neurodegenerative processes, or by a reduction in brain metabolic demand that parallels cognitive decline 23. We found increased medial temporal lobe atrophy in AD compared to MCI, which could support the relation between loss of brain tissue and reduced cere- bral blood flow. Also, recent fMRI data in MCI and AD are beginning to reveal relationships between abnormalities of functional activity in the medial temporal lobe memory system and in functionally connected brain regions 6. Since inclusion of AD and MCI subjects in our study was from an existing cohort of the Memory Clinic in our hospital, MRI data was sometimes acquired at an earlier stage compared to the autoregulation tests. Only in nine subjects autoregulation tests were performed within one year from the MRI acquisition date. While disease progresses this could have blurred the correlation between atrophy and autoregulation data. From the group of MCI patients, during the course of this study, five converted to AD on average two years with a standard deviation of one year after the cerebrovascular measurements were performed. The results for this subgroup compared to the non-converted MCI patients showed mCBFV to be significantly lower and CVRi to be significantly higher. Although the number of converters is small this strongly suggests the value of these parameters in the diagnosis of AD. It could therefore be worthwhile to evaluate the value of screening elderly and specifically MCI patients for these parameters. No significant differences were found between the groups for dCA. This is in correspondence with findings of Claassenet al 4 for spontaneous fluctuations of blood pressure. However, they did show differences in gain during squat-stand maneuvers. They also showed increased spectral power in blood pressure and CBFV during these maneuvers. It could therefore be necessary to induce stronger challenges to the control of cerebral blood flow for changes in cerebral autoregu- lation to become apparent. By inducing stronger challenges such as squat-stand maneuvers maybe also differences between MCI and C might appear. New extensive and long term prospective research is needed to investigate this theory and to study the development from MCI into AD in relation to dCA parameters. Although we included roughly twice the amount of subjects as Claassenet al 4 did, the study might be underpowered to show differences in autoregulation 72
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