What is an Atheroma?
An atheroma is an intimal lesion characterized by a fibrofatty plaque that protrudes into the arterial lumen, weakens the underlying media, and is often associated with calcification. 1
Definition and Characteristics
- Atheroma (also called atheromatous plaque) is the basic lesion of atherosclerosis, developing within the innermost layer (intima) of arterial walls 1
- Histologically, atheromas can be broadly classified into two types 2:
- Fibrous plaques: Appear whitish macroscopically and are composed of smooth muscle cells and collagen fibers
- Lipid-rich (atheromatous) plaques: Appear yellow macroscopically with a white fibrous cap covering the lipid-rich core
Structure and Composition
- Atheromas typically consist of two main components 2, 3:
- A lipid-filled core (the "athero" component - from Greek "athéré" meaning gruel or porridge)
- A fibrous cap or shell (the "sclerosis" component - meaning hardening)
- The fibrous cap covering the lipid-rich core can be thin and prone to rupture, especially in vulnerable plaques 2, 3
- Advanced atheromas may contain 1:
- Necrotic core (appears as a signal-poor region with poorly delineated borders in imaging)
- Calcium deposits (appear as signal-poor or heterogeneous regions with sharply delineated borders)
- Cholesterol crystals
Pathophysiology and Development
- Atheroma formation begins with endothelial dysfunction and injury 4, 5
- The evolution involves 3:
- Extensive lipid deposition in the arterial intima
- Exacerbated immune responses (both innate and adaptive)
- Proliferation of vascular smooth muscle cells
- Remodeling of the extracellular matrix
- Inflammation plays a crucial role in all stages of atheroma development 6, 3
- Monocytes adhere to the endothelium, migrate into the intima, and transform into lipid-laden macrophages (foam cells) 4
- With aging, presence of risk factors, and genetic predisposition, atheromas progress to more complicated lesions with surface defects, hemorrhage, and/or thrombosis 1
Clinical Significance
- High-risk (vulnerable) atheromas have 3:
- Large acellular lipid-rich necrotic core
- Thin fibrous cap infiltrated by inflammatory cells
- Diffuse calcification
- Neovascularization (formation of fragile new blood vessels)
- Rupture or erosion of atheromas exposes plaque components to the circulation, triggering 2, 3:
- Activation of tissue factor and the coagulation cascade
- Platelet recruitment, activation, and aggregation
- Formation of a thrombus (atherothrombosis)
- Atherothrombosis can lead to acute cardiovascular events including myocardial infarction and stroke 2, 3
- Thoracic aortic atherosclerosis is less common than abdominal aortic atherosclerosis but remains clinically important 1
Anatomical Distribution
- Atheromas develop predominantly at branches, bends, and bifurcations in the arterial tree 5
- These locations are exposed to low or disturbed blood flow, which exerts low/oscillatory shear stress on the vessel wall 5
- Regions with uniform, unidirectional blood flow and high shear stress are relatively protected from atheroma formation 5
Imaging and Measurement
- Various imaging modalities can detect and characterize atheromas, including 1:
- Intravascular optical coherence tomography (IVOCT)
- Computed tomography (CT)
- Positron emission tomography (PET)
- Measurements that can be made of atheromas include 1:
- Plaque area/cross-sectional area
- Maximum and minimum plaque thickness
- Plaque burden (plaque area divided by the internal elastic membrane area)
- Plaque components (fibrous tissue, lipid pool, calcium, etc.)