Cerebral amyloid angiopathy (CAA) is the deposition of amyloid-β protein in the walls of small and medium-sized arteries and arterioles of the cortex and leptomeninges. It is the most important cause of lobar intracerebral hemorrhage in the elderly, a major contributor to vascular cognitive impairment, and frequently coexists with Alzheimer disease. The neuropathology is distinctive and unforgiving — once vessel walls are riddled with amyloid, they are mechanically fragile and prone to rupture. This page covers CAA pathology, the disease patterns it produces, and the diagnostic features at autopsy and on imaging.

Pathology of CAA

The amyloid in CAA is composed primarily of Aβ40 (the 40-amino-acid form of amyloid-β) — distinguishing it from Alzheimer disease plaques, which are predominantly Aβ42. The peptide is deposited in:

  • Tunica media of small leptomeningeal and cortical arteries.
  • Smooth muscle cell layer: progressively replaced by amyloid.
  • Adventitia and basement membranes.
  • In severe cases, the vessel wall is essentially replaced by amyloid.

Vessel Changes in CAA

  • Eosinophilic, glassy thickening of vessel walls on H&E.
  • Concentric splitting: vessel wall has a layered appearance — “double-barrel” lumen.
  • Fibrinoid necrosis: at sites of severe wall damage.
  • Microaneurysmal dilation: where amyloid has weakened the wall.
  • Perivascular inflammation in some cases — defines the CAA-related inflammation subtypes.

Stains

  • Congo red: amyloid stains red on bright-field; apple-green birefringence under polarized light is pathognomonic.
  • Thioflavin S: fluorescent amyloid staining.
  • Aβ40 and Aβ42 IHC: distinguish CAA-predominant (Aβ40 > Aβ42) from parenchymal plaque pathology (Aβ42 > Aβ40).

Clinical Manifestations

Lobar Intracerebral Hemorrhage

The most clinically important manifestation:

  • Cortical / subcortical white matter location.
  • Often hemispheric (frontal, parietal, occipital, temporal lobes).
  • Multiple bleeds over time in many patients.
  • 10-15% per year recurrence rate after symptomatic ICH.
  • Often in patients over 60.
  • Coexisting hypertension common but the deep distribution typical of hypertensive ICH is absent.

Cerebral Microbleeds

Small (under 5 mm) hemosiderin deposits visible on SWI/GRE MRI in a lobar distribution:

  • Cortical and subcortical white matter.
  • Lobar predominance over deep structures.
  • Often numerous (dozens to hundreds).
  • Asymptomatic; mark increased risk of future ICH.

Cortical Superficial Siderosis

Hemosiderin staining of the cortical pial surface from recurrent convexity SAH. Visible on SWI as low-signal lines following cortical sulci. Highly specific for CAA in elderly patients. Predicts future ICH.

Convexity Subarachnoid Hemorrhage

Localized SAH at the brain convexity (not basal cisterns). Often the first clinical sign of CAA. Important to recognize because the differential includes aneurysm (basal cisterns), trauma, and reversible cerebral vasoconstriction syndrome.

Vascular Cognitive Impairment

CAA contributes to cognitive decline through:

  • Multiple microbleeds and microinfarcts.
  • Chronic white matter ischemia (Aβ-induced vessel dysfunction).
  • Coexisting Alzheimer disease.

CAA-Related Inflammation

A subset of patients develop a subacute encephalopathy with:

  • Cognitive decline.
  • Headache.
  • Seizures.
  • White matter T2/FLAIR hyperintensities on MRI.
  • Perivascular inflammation around amyloid-laden vessels on biopsy.

Two variants:

  • CAA-related inflammation (CAA-I): perivascular lymphocytic infiltration without vasculitis.
  • Amyloid-β-related angiitis (ABRA): granulomatous vasculitis around amyloid-laden vessels.

Both respond to corticosteroids and/or other immunosuppression. Recognition is important — these are treatable forms of CAA.

Diagnostic Criteria

Boston Criteria (Boston Criteria v2.0, 2022)

For probable CAA without pathologic confirmation:

  • Age ≥ 50.
  • Clinical presentation: spontaneous ICH or cognitive decline.
  • MRI: at least 2 lobar hemorrhagic lesions (ICH, microbleed, or cortical superficial siderosis) in cortical/leptomeningeal distribution; or 1 lobar hemorrhagic lesion + 1 white matter feature (severe perivascular spaces, multispot WMH pattern).
  • No other cause of hemorrhage.

Definite CAA requires pathologic confirmation. Probable CAA with supporting pathology requires biopsy showing CAA. The modern Boston criteria v2.0 have improved sensitivity and now incorporate non-hemorrhagic MRI features.

Pathologic Spectrum

Vonsattel Grading

Grades severity of vessel involvement:

  • Grade 0: no amyloid.
  • Grade 1: amyloid confined to vessel wall, normal architecture preserved.
  • Grade 2: replacement of vessel wall components by amyloid.
  • Grade 3: severe — fibrinoid necrosis, vessel wall splitting, microaneurysm, perivascular hemorrhage. Severe CAA.

Higher grades correlate with higher risk of clinical ICH.

CAA Distribution

Most pronounced in:

  • Occipital and parietal lobes.
  • Posterior temporal.
  • Posterior frontal (less than other lobes).
  • Less common in deep gray, brainstem, cerebellum (some involvement in genetic CAA syndromes).

Hereditary CAA Syndromes

Rare genetic forms:

  • Dutch-type (APP gene, E693Q mutation): severe early-onset CAA + ICH.
  • Iowa-type (APP gene, D694N mutation).
  • Italian-type (APP).
  • Flemish-type (APP).
  • Icelandic-type (HCHWA-I): cystatin C mutation; severe ICH.
  • British / Danish-type: BRI2 gene; familial dementia with CAA.

Pathologic features overlap with sporadic CAA but with earlier onset and more severe involvement.

CAA and Alzheimer Disease

The two diseases overlap substantially:

  • About 80-90% of AD patients have some degree of CAA at autopsy.
  • About 30-60% of CAA patients meet pathologic criteria for AD.
  • The Aβ peptide is shared but the predominant form differs (Aβ40 in CAA, Aβ42 in plaques).
  • Coexisting AD pathology accelerates cognitive decline in CAA patients.

Clinical Management

  • BP control (still beneficial despite the cortical mechanism).
  • Avoid anticoagulation when possible — major increase in bleeding risk.
  • Cautious antiplatelet use; sometimes deferred.
  • Acute ICH management as standard.
  • Steroid trial for suspected CAA-related inflammation (CAA-I, ABRA).
  • Cognitive assessment and supportive care.
  • Anti-Aβ monoclonal antibodies (lecanemab, donanemab) for AD increase risk of ARIA (amyloid-related imaging abnormalities) — particularly in patients with CAA. Pre-treatment SWI screening is now standard, and severe CAA may be a contraindication.

🔍 Did You Know?

The advent of anti-amyloid monoclonal antibodies (lecanemab and donanemab) for Alzheimer disease has reshaped the clinical importance of cerebral amyloid angiopathy. These drugs, which clear amyloid from the brain, can produce amyloid-related imaging abnormalities (ARIA) — either ARIA-E (edema/effusion, vasogenic edema in subcortical white matter) or ARIA-H (hemorrhages, both microbleeds and macrohemorrhages). The risk is substantially higher in patients with significant baseline CAA. Pre-treatment screening with susceptibility-weighted MRI is now standard before initiating anti-Aβ therapy: a patient with multiple lobar microbleeds or established cortical superficial siderosis indicates significant CAA and may not be a candidate. The mechanism is thought to be amyloid clearance from vessel walls (where it was structurally integrated, however dysfunctionally) producing transient inflammation and bleeding. This is an unusual situation in medicine: an effective therapy whose benefits in the cortex come at the cost of vascular fragility — and where neuropathology directly informs prescribing decisions. CAA was once an obscure pathology for the autopsy room; it is now a key clinical consideration for every AD treatment decision in 2026.

Pitfalls and Pearls

  • CAA = lobar ICH in the elderly. Different from hypertensive ICH (deep).
  • Aβ40 dominates CAA; Aβ42 dominates parenchymal plaques.
  • Congo red apple-green birefringence: pathognomonic.
  • Multiple lobar microbleeds + cortical superficial siderosis: probable CAA (Boston criteria v2.0).
  • Convexity SAH in elderly: consider CAA.
  • CAA-related inflammation (CAA-I) and ABRA: subacute encephalopathy + WMH; biopsy-confirmed; steroid-responsive.
  • Hereditary CAA: early onset; family history; APP mutations (Dutch type classic).
  • CAA and AD coexist; both contribute to cognition.
  • Avoid anticoagulation in CAA when possible.
  • Anti-Aβ monoclonal antibodies (lecanemab, donanemab): ARIA risk much higher with CAA; pre-treatment SWI screen.
  • Vonsattel grading: structural severity correlates with bleeding risk.
  • Posterior predilection: CAA is most pronounced in occipital and parietal regions.

References

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  2. Charidimou A, Boulouis G, Frosch MP, et al. The Boston criteria version 2.0 for cerebral amyloid angiopathy: a multicentre, retrospective, MRI-neuropathology diagnostic accuracy study. Lancet Neurol. 2022;21(8):714-725.
  3. Greenberg SM, Charidimou A. Diagnosis of cerebral amyloid angiopathy: evolution of the Boston criteria. Stroke. 2018;49(2):491-497.
  4. Vonsattel JP, Myers RH, Hedley-Whyte ET, Ropper AH, Bird ED, Richardson EP Jr. Cerebral amyloid angiopathy without and with cerebral hemorrhages: a comparative histological study. Ann Neurol. 1991;30(5):637-649.
  5. Sperling RA, Jack CR Jr, Black SE, et al. Amyloid-related imaging abnormalities in amyloid-modifying therapeutic trials: recommendations from the Alzheimer’s Association Research Roundtable Workgroup. Alzheimers Dement. 2011;7(4):367-385.
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