Aneurysms are permanent and abnormal dilation of a part of a blood vessel or heart due to an attenuated intact arterial wall, or thinned out wall of a heart, mainly ventricles. This is also known as a “True aneurysm”. The difference between a true and a false aneurysm is the intact wall in a true aneurysm. A false aneurysm is actually formed due to a defect in vascular wall, which leads to extra-vascular hematoma which freely communicates with the inside of the vessel. This is formed when blood is contained by the surrounding connective tissue after a complete breach in the vascular wall.
Eg :- as an acute complication after a Myocardial infarction, Cardiac rupture can occur. The blood in ventricles will be escaped into the pericardial space and the pericardial adhesions can limit this breach to a single focus.
Due to constant hemaodynamic stress a vessel wall is facing, it is in a continous process of synthesizing, degrading and remodelling/repairing the connective tissue. Any defect in this cycle would disrupt the intergrity of the vessel wall and lead to an aneurysm. All these reasons can be categorized mainly in to 3 main topics, as follows;
- Defective synthesis of connective tissue
- Marfan syndrome —-> defective synthesis of scaffolding protein —> Aberrant TGF – beta activity—-> Weakening of elastic tissue
- Loeytz-Dietz Syndrome —-> Mutation in TGF-beta —-> Defective synthesis of collagen I and III and elastin
- Scurvy—-> Vitamin C deficiency —–> Defective cross linking of collagen
- Increased Degradation of connective tissue.
- Increase degradation normally occurs when there is an inflammation going on in the wall. During atherosclerosis abnormal activity of macrophages will lead to increased levels of Matrix Metello Proteases(MMP) which would accelerate the degradation of vascular wall, over the synthesis. Also Production of certain inflaaotory cytokines will lead to stimulation of macrophages to release MMPs. Vasculitis is another example
- Weakening of the vascular wall due to loss of smooth muscles or disposition of non collagenous/elastic substances. (Explained in Quick look at Aortic aneurysms and dissections)
In contrast to aneurysms, Dissections have a distinct breach in the vessel wall, which leads to accumulation of blood, from the intravascular space between the laminar planes of media. This forms a blood filled channel within the media and blood do not communicate with the outside of the vessel as in false aneurysms.
Although even if we say there is a breach in the vessel wall, aortic dissections sometimes can occur without any intimal tear. Aorta is a large artery. It has a very large Tunica media. This layer is so thick, that small blood vessels called, “Vasa Vasorum”are present to supply blood to this layer to provide their nutritional and oxygen requirements. During hypertension due to medial thickening in the arterioles (Quick look at Pathology in hypertension) and loos of their integrity, these can rupture leading to accumulation of blood within the laminar planes of media of aorta.