All About Atheroma

Definition

Webster’s Dictionary defines Atheroma as “An encysted tumour containing curdy matter. A disease characterised by thickening and fatty degeneration of the inner coat of the arteries.”
An encysted tumour containing curdy matter. [quote]A disease characterised by thickening and fatty degeneration of the inner coat of the arteries.:Webster’s Dictionary[/quote]

This disease is the largest cause of death in western countries and hence one of the most important topics in such societies. It causes death from heart diease, cerebral infarction, and ischaemia of the lower limbs, acting by narrowing of the lumen within arteries.

It features lesions on the wall of arteries, which are luminal thickenings due to the depositino of lipids and formation of fibrous tissue, hence athere (porridge, i.e. soft lipid rich part) with sclerotic (hard, i.e. fibrous component.

Apperance

In childhood these are present as fatty streaks, yellowigh spots in the lumina serface. In time these Smooth Muscle Cell (SMC) rich areas enlarge and become increasingly full of lipids. These are present in all children, and many disappear with time. Some however persist and may trigger atheroma.
The start of a lesion is a small oval yellow disc with a smooth fatty surface. With time, these enlarge as more lipid deposition into the intima takes place. Fibrosis forms on the surface of the atheroma. The early atheroma are at a variety of stages in development, which shows they are not triggered by a single event.

Atheroma

Predominatly the abdominal aorta is affected, and patches may become confluent. A lipid rich soft paste is present in the deepest regions, and the fibrosis overlying this is thick and white.[quote]This disease is the largest cause of death in western countries and hence one of the most important topics in such societies.:V F Murphy[/quote]
In time, the fibrosis may break down and cause ulceration, lading to mural thrombosis. Calcification may take place, which reduces vessel compliance. These redefine the atheroma as a complicated atheroma. Aneurism may develop as a result of thinning of the media, and extension of the plaque into the media.

Only in arteries greater than 2mm diamter. Similar to the aorta, but plaques are smaller, often covering the whole intima. This can result in total occlusion. Unfortunately, atheroma favours arteries supplying the heart, brain, and abdominal viscera. There are large variations in distribution.

Effects

Uncomplicated artheroma of large arteries may have no clinical effects. Advanced cases however may lead to aneurysm as mentioned before, thrombi on ulcerated plaques, and even virtual occlusion. Thrombi may break off and form emboli in the arteries of the lower limbs and abdominal organs.
Small vessels have the greatest effects. They are narrowed much easier, and are often totally occluded by thrombosis. These are common in the coronary arteries, leading to ischaemic heart disease, the largest cause of death in the west. Ischaemic brain damage is also common. Aneurysms are not present is smaller arteries. Ischaemia may cause gangrene of the legs. The arms are rarely affected.

Aetiology

Predisposing factors include:
Lipid Accumulation
- Most of the cholesterol in human aortic intima is derived from teh plasma
- Increased permeability at the endothelial lining at bifurcations
- Animals fed cholesterol-rich diet develop similar lesions
- LDL accumulates in the intima
Cellular Proliferation
- The intimal cells that accumulate lipids are modified SMC cells
- LDL promotes proliferation of SMC in culture
- Platelets adhering to injured vessel walls stimulate proliferation of SMC
Fibrosis
- Subendothelial, superficial part
- Provided by SMC
- Monoclonal
- Possibly important in the generation of atherogenesis
Necrosis
- In lipid rich depths
- Due to ischaemia
- Also due to damage from some lipids
- Does not explain the growth of the plaque

Risk Factors

Blood Lipids
Hyperlipidamia as mentioned before. Ischaemic heart disease is generally an indication of sever atheroma, and risk factors are linked.
Age and Gender
Men more severely affected, and slow growth of plaques means problems increase with age. Gender difference may be due to lipid metabolism changes with oestrogens.
Hypertension
Necropsy studies have shown this to be a risk factor. The mechanism is unclear, although raised shear stress is possible. This is backed up by the lack of atheroma in the pulmonary arteries in the absence of pulmonary hypertension.Cigarette somoking – At least doubles the incidence of ischaemic heart disease:Muir’s Textbook of Path
Cigarette somoking
At least doubles the incidence of ischaemic heart disease, and related to cigarette intake. Cigarette smoke causes vascular endothelial injury in rabbits. The role of smoking in atheroma is complex.
Physical activity
Exercise is of great benefit. Possibly by means of using up lipids as carbohydrates, by means of reducing hypertension, or other mechanisms as yet unknown.
Psychological factors
Emotional stress predisposes to atheroma, however the mechanism is unclear. It is possible that output of catecholamines is the cause, or that secondary effects, such as smoking or eating habits may account for observed results.

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