Cerebrovascular Reactivity and Cognitive Decline in Patients With Alzheimer Disease

Decreased vasomotor reactivity in Alzheimer's disease

Background

Reduced cerebral blood flow and microvascular abnormalities have been suggested as the vascular pathogenesis of Alzheimer's disease (AD). Transcranial Doppler sonography (TCD) can be used as a noninvasive method for measuring cerebral vasomotor reactivity (VMR) which represent the capability of arterioles to dilate and constrict in order to maintain cerebral blood flow.

Objective

The objective of this study was to determine whether VMR is decreased in AD patients. Methods: Seventeen consecutive patients who met NINDS-ADRDA criteria for AD, and 17 age- and sex-matched controls were included in this study. MRI and MRA were performed for the grading of white-matter lesions. Patients with cerebral infarct or stenosis of the middle cerebral artery (MCA) were excluded. The fixed TCD probe was used to monitor the mean flow velocity (MFV) in the MCA. A 6-L rebreathing bag was applied to patients for at least 5 minutes to elevate the CO₂ concentration, which was continuously monitored with a capnometer. VMR was calculated as the percentage change in the MFV.

Results

Baseline characteristics - including cerebrovascular risk factors, grades of white-matter lesions, baseline MFV, and pulsatility index - did not differ between the two groups. Mini-Mental State Examination score was significantly low in AD group (20.5 vs. 27.5, p<0.05). VMR was significantly reduced in AD group both in the right-side (24.5% vs. 36.6%, p<0.05) and left-side (20.7% vs. 34.1%, p<0.05) MCAs.

Conclusions

Our finding that VMR is reduced in AD may be suggestive of underlying microangiopathic mechanism in AD patients. Future studies should check the validity of these experimental and hypothesis-generating pilot results.

Haemodynamic and neural responses to hypercapnia in the awake rat

The relationship between localized changes in brain activity and metabolism, and the blood oxygenation level-dependent (BOLD) signal used in functional magnetic resonance imaging studies is not fully understood. One source of complexity is that stimulus-elicited changes in the BOLD signal arise both from changes in oxygen consumption due to increases in activity and purely 'haemodynamic' changes such as increases in cerebral blood flow. It is well established that robust cortical haemodynamic changes can be elicited by increasing the concentration of inspired CO(2) (inducing hypercapnia) and it is widely believed that these haemodynamic changes occur without significant effects upon neural activity or cortical metabolism. Hypercapnia is therefore commonly used as a calibration condition in functional magnetic resonance imaging studies to enable estimation of oxidative metabolism from subsequent stimulus-evoked functional magnetic resonance imaging BOLD signal changes. However, there is little research that has investigated in detail the effects of hypercapnia upon all components of the haemodynamic response (changes in cerebral blood flow, volume and oxygenation) in addition to recording neural activity. In awake animals, we used optical and electrophysiological techniques to measure cortical haemodynamic and field potential responses to hypercapnia (60 s, 5% CO(2)). The main findings are that firstly, in the awake rat, the temporal structure of the haemodynamic response to hypercapnia differs from that reported previously in anaesthetized preparations in that the response is more rapid. Secondly, there is evidence that hypercapnia alters ongoing neural activity in awake rats by inducing periods of cortical desynchronization and this may be associated with changes in oxidative metabolism.

Considerations of the ischemic basis and therapy of Alzheimer Disease

A recently presented concept of Alzheimer disease (AD) is based on a primarily ischemic (rather than degenerative) type of brain disease. Etiologically, this new concept is presumed to be related to the human upright gait along with individual predisposition. The proposed treatment--head-down therapy (HDT) --is the centerpoint of this presentation: a simple and generally accessible type of therapy, with monitoring by neuropsychological questioning, electroencephalography, and transcranial Doppler. As a treatment of AD (limited to stage 1), HDT is expected to be helpful. It is possible, however, that its prophylactic use may be of even greater importance.

Interaction of cardiovascular disease and neurodegeneration: transcranial Doppler ultrasonography and Alzheimer's disease

OBJECTIVE

Recent post-mortem studies have reported that the severity of atheromatous deposits in the circle of Willis is significantly greater, relative to non-demented (ND) elderly persons, in subjects with neuropathologically diagnosed Alzheimer's disease (AD). Additionally, the severity of intracranial atherosclerosis correlates significantly with the densities of neuritic plaques and neurofibrillary tangles. In this study, we examine the arteries of the circle of Willis by transcranial Doppler (TCD) ultrasonography.

METHODS

TCD was used to measure, in 25 AD patients and 30 ND elderly subjects, mean flow velocities and pulsatility indices in 16 different segments of the circle of Willis. The data were compared with and without adjustment for age, gender and systolic blood pressure.

RESULTS

The AD patients had systematically higher pulsatility indices (p<0.005) than the ND group. Incremental increases of pulsatility indices in these segments had odds ratios ranging from 1.8 to 48 for the presence of AD when adjusted for age, gender and systolic blood pressure. The left internal carotid artery siphon and the left posterior cerebral artery were the two vessels that were strongly associated with AD diagnosis. Mean flow velocities were generally lower in patients with AD but the differences did not reach the significance level.

DISCUSSION

The pulsatility indices of the arteries of AD patients were generally greater than those of similarly-aged ND subjects. This difference is most likely due to increased arterial wall rigidity imposed by atherosclerotic changes. Atherosclerotic disease of intracranial arteries may be a risk factor for AD.

Methodology for measuring cerebrovascular disease burden

Biological aging of the brain is partly attributable to aging of the cerebrovascular circulation and the effects of these vascular changes on the brain. A variety of techniques ranging from simple, clinical scores to complex radiological techniques have been used in an attempt to understand, describe and quantify this process. Simultaneously attempts have been made to relate these changes to cognitive and physical changes and the risk of dementia and stroke associated with brain aging. The most frequently used clinical scores are the Framingham Stroke Risk Profile and the Hachinski Ischemic Score for vascular dementia. Radiological techniques to estimate cerebrovascular burden include many varieties of ultrasonographic, computerized tomography (CT), magnetic resonance imaging (MRI) and nuclear medicine techniques. The radiological techniques evaluate the nature and extent of disease in the vessels supplying the brain and the pattern and extent of radiological evidence of damage to the brain both on static and dynamic imaging and are briefly outlined in this review. There are several studies using these techniques to study 'normal' aging populations, and the techniques used in the most widely known of these studies are briefly highlighted in this review.