Vascular Effects
In Alzheimer's disease, vascular injury and parenchymal inflammation perpetuate the cycle of protein aggregation and oxidation in the brain; damage from strokes and white-matter lesions contribute greatly to cognitive decline. Ischemic disease affects 60 to 90% of patients with Alzheimer's disease, with major infarctions representing one third of vascular lesions in autopsy cases. Conversely, one third of putative cases of vascular dementia have coincidental pathological features of Alzheimer's disease. Although clinically and radiographically “pure” cases of vascular dementia are recognized,110,111 most cases of dementia are in fact mixed. Pervasive pathological changes include cerebral amyloid angiopathy,112 affecting more than 90% of patients with Alzheimer's disease, capillary abnormalities, disruption of the blood–brain barrier, and large-vessel atheroma.113 None of these changes alone explain the symmetric reductions of cerebral blood flow in patients with Alzheimer's disease, which are more likely to reflect regional energy underutilization. 114,115
Another hypothesis holds that clearance of Aβ along diseased perivascular channels and through the blood–brain barrier is impeded in Alzheimer's disease. The source of vascular Aβ (mostly 40 amino acid form) is heterogeneous, comprising neurons, degenerating myocytes, and the circulation. Amyloid deposition in the arteriolar wall enhances vasoconstriction in ex vivo studies.116 Aβ is also cytotoxic to endothelial117 and smooth-muscle118 cells, conferring a predisposition to lobar hemorrhage in advanced age. The “neurovascular uncoupling” hypothesis proposes that deregulation of Aβ transport across the capillary blood–brain barrier is caused by the imbalanced expression of low-density lipoprotein receptor–related proteins and receptors for advanced glycation end products, which mediate Aβ efflux and influx, respectively119 (Figure 5FIGURE 5
Inflammation and Mechanisms of Aβ Clearance.
).
Short of prophylaxis against stroke, there are few specific therapies for the vascular changes in Alzheimer's disease. Centrally acting angiotensin-converting–enzyme inhibitors were associated with reductions in yearly cognitive decline in one observational study.120 Patients with hypertension who are receiving medication have fewer neuropathologic features of Alzheimer's disease.121 Folic acid reduces homocysteine levels and may lower the risk of Alzheimer's disease, but it does not improve cognition in established Alzheimer's disease.122,123 A phase 2 study of inhibitors of receptors for advanced glycation end products in mild-to-moderate Alzheimer's disease (NCT00566397) is under way. Concern has been expressed about the safety of Aβ immunotherapy because of the possibilities of increased vascular amyloid, microhemorrhages, and vasogenic edema as the efflux of Aβ into vascular compartments is stimulated.12