What is cerebral amyloid angiopathy?

What is Cerebral Amyloid Angiopathy?

Cerebral amyloid angiopathy (CAA) is a cerebrovascular disorder characterized by the accumulation of amyloid-beta (Aβ) protein in the walls of small cortical and leptomeningeal arteries and arterioles, leading to vessel wall weakening, lobar intracerebral hemorrhage, cognitive decline, and other neurological manifestations. 1, 2

Pathophysiology

CAA develops through progressive amyloid-β deposition in cerebral vessel walls, which weakens the vessel structure and increases the risk of blood leakage into adjacent brain tissue. 3, 1

• The disease follows consecutive stages: initial amyloid-β deposition, decreased vascular reactivity, nonhemorrhagic changes, and ultimately hemorrhages 4
• Impaired perivascular clearance is the leading hypothesis underlying CAA pathogenesis, though glymphatic dysfunction also plays a significant role 4
• The pathophysiology involves a two-compartment framework: vessel wall/perivascular space where Aβ40 deposition coincides with endothelial stress, and the brain parenchyma where inflammatory circuits and synaptic vulnerability develop 5
• CAA has a long preclinical phase starting decades before symptoms emerge 4

Clinical Manifestations

Hemorrhagic Presentations

Lobar intracerebral hemorrhage is the most serious and common symptomatic manifestation, typically occurring in the frontal, parietal, temporal, or occipital lobes of elderly patients. 6, 7

• Cortical superficial siderosis, caused by blood leakage into the subarachnoid space, is an important predictor of future hemorrhage 6
• Cerebral microbleeds appear as small lobar hemorrhagic lesions on MRI; approximately 12% of memory clinic patients develop incident microbleeds over 1.9 years of follow-up without attributable clinical symptoms 6
• The presence of microbleeds identifies patients at increased risk for subsequent lobar intracerebral hemorrhage, though microbleeds themselves do not reliably predict immediate neurological symptoms 6
• In individuals who present with an initial lobar hemorrhage, the number of microbleeds increases risk of subsequent lobar hemorrhage 3
• CAA-associated intracerebral hemorrhage has high mortality, morbidity, and recurrence rates 7

Non-Hemorrhagic Manifestations

• CAA can cause progressive dementia even in the absence of typical Alzheimer's disease pathology 6
• Transient focal neurological episodes and convexity subarachnoid hemorrhage can occur 7
• It is not established that microhemorrhages themselves cause neurological symptoms in Alzheimer's disease and presumed CAA 6

Imaging Findings

MRI reveals characteristic patterns that help identify CAA and stratify hemorrhagic risk. 6, 7

• Lobar microbleeds are more often due to CAA, while deep central grey and brain stem microbleeds are more often due to hypertensive angiopathy 3
• White matter hyperintensities, especially in frontal and parietal-occipital regions, are common MRI findings 6
• Superficial siderosis prevalence is much less common than microbleeds (only 0.7% in population-based studies) and is felt to be an indicator of CAA 3
• Amyloid PET imaging cannot differentiate vascular amyloid deposition of CAA from parenchymal amyloid plaques 6

Relationship with Alzheimer's Disease

CAA frequently co-occurs with Alzheimer's disease, but represents a distinct pathological process. 1, 4

• Despite common co-occurrence, CAA can occur independently of AD 4
• Patients with CAA are often excluded from anti-amyloid therapies due to increased risks of cerebral edema and hemorrhage 1
• CAA may be a neuropathologic common denominator for treatment-related amyloid-related imaging abnormalities (ARIA) in immunotherapy trials 3

Disease Heterogeneity

CAA is not a single entity but encompasses multiple phenotypes with variable disease courses. 4, 2

• Some patients develop more microbleeds, while others develop larger hemorrhages, suggesting differentiation in vascular remodeling 4
• Emerging evidence identifies sporadic, hereditary, inflammatory, and iatrogenic CAA forms 4
• Iatrogenic CAA has been identified as occurring decades after neurosurgical procedures involving cadaveric dura mater, representing prion-like transmission of amyloid-β 4
• Some patients develop symptomatic immune responses (CAA-related inflammation), and inflammation may be an important contributor to vascular damage in CAA generally 4

Epidemiology

• CAA is an age-related phenomenon that occurs even in neurologically healthy individuals 3
• Point prevalence of microbleeds in healthy elderly populations is approximately 6%, rising to 50-80% in elderly individuals with cerebrovascular disease 3
• Prevalence increases with age and cardiovascular risk factors 3

Clinical Implications

CAA affects multiple clinical decisions including antithrombotic medication use, safety for anti-amyloid immunotherapy, and need for anti-inflammatory treatment. 7

• There are currently no approved treatments specifically for CAA 1
• The disease has a dynamic natural history with periods of high activity and remission, challenging the traditional view of linearly progressive disease 2
• CSF biomarker profiling may help stratify CAA phenotypes and predict hemorrhagic risk, though further validation is needed 8