[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Blood vessels](https://durgical.com/en/general-pathology/areas/vessels/) › Aneurysms: types and pathogenesis

# Aneurysms: types and pathogenesis

- Mechanism

An aneurysm is a permanent bulge of a weak artery or heart wall. The weakness comes from a poor matrix or from a matrix that is being destroyed.

Updated Nov 12, 2026

## Key points

- True aneurysm: all three layers; false aneurysm: blood outside the wall contained by surrounding tissue (often after trauma or catheterisation).
- Shape: saccular (outpouching of part of the wall) vs fusiform (circumferential dilation).
- Marfan syndrome: FBN1 (fibrillin-1) mutation; excess TGF-β activity and weak elastic fibres; aortic root dilation.
- Loeys–Dietz: mutations in the TGF-β receptors; aggressive aneurysms and dissection at small diameters.
- Cystic medial degeneration: loss of smooth-muscle cells, fragmented elastic fibres and pools of proteoglycan in the media.
- Atherosclerosis is the main cause of abdominal aneurysms; hypertension and genetic disorders dominate in the thoracic aorta.

## What it is

An aneurysm is a localised abnormal dilation of a vessel or the heart. A true aneurysm involves all layers of the wall; a false aneurysm (pseudoaneurysm) is a contained leak, a hematoma outside the vessel that communicates with the lumen. They form when connective tissue is poor in quality (Marfan, Loeys–Dietz, Ehlers–Danlos), when matrix breakdown outpaces synthesis (inflammation and metalloproteinases in atherosclerosis), or when the media loses its blood supply (hypertension, syphilis). Thoracic aneurysms are mainly due to hypertension and genetic matrix disorders.

## Structure

- Cystic medial degeneration: elastic stain shows fragmented, lost elastic lamellae; H&E shows pale pools of proteoglycan and loss of smooth-muscle nuclei.
- Thoracic aneurysm: dilated ascending aorta, sometimes with a widened aortic valve ring.

## Step by step

- The aortic media normally balances matrix synthesis (smooth-muscle cells) with controlled breakdown.
- Genetic defects make weak collagen or elastin, or disturb TGF-β signalling (Marfan, Loeys–Dietz, Ehlers–Danlos IV).
- Inflammation in plaques releases metalloproteinases that destroy elastin and collagen faster than they are rebuilt.
- Ischemia of the outer media (thick intima, narrowed vasa vasorum in hypertension or syphilis) kills smooth-muscle cells.
- The weakened wall stretches; by Laplace's law, a wider vessel has higher wall tension and stretches faster.
- The aneurysm grows until it ruptures, dissects, thromboses or compresses its neighbours.

## Why the aortic wall gives way: the main causes

| Cause | Defect in the wall | Typical aneurysm |
| --- | --- | --- |
| [Marfan syndrome](https://durgical.com/en/conditions/marfan-syndrome/) | Abnormal fibrillin-1, excess TGF-β, elastic fragmentation | Aortic root and ascending aorta |
| Loeys–Dietz syndrome | TGF-β receptor mutations | Diffuse thoracic aorta and branches |
| Ehlers–Danlos (vascular type) | Defective type III collagen | Arterial rupture, saccular aneurysms |
| [Hypertension](https://durgical.com/en/conditions/arterial-hypertension/) | Ischemic medial damage, cystic medial degeneration | Ascending and arch aorta |
| [Atherosclerosis](https://durgical.com/en/conditions/abdominal-aortic-aneurysm/) | Plaque inflammation, metalloproteinases, medial thinning | Infrarenal abdominal aorta |
| [Tertiary syphilis](https://durgical.com/en/conditions/syphilis/) | Obliterative endarteritis of vasa vasorum | Ascending aorta and arch |

## Clinical notes

- Thoracic aortic aneurysms cause aortic valve regurgitation, compression of airway, esophagus or recurrent laryngeal nerve (hoarseness), bone erosion and rupture.
- Bicuspid aortic valve and Turner syndrome are linked to ascending aortic dilation.

## Memory aid

Bad bricks (genes), wrecking crews (inflammation) or no supplies to the builders (ischemia): any of the three and the wall bulges.

## Test yourself

Think of the answer, then tap the question to see it.

## What distinguishes a false aneurysm from a true aneurysm?

A false aneurysm is a breach in the wall with an extravascular hematoma contained by surrounding tissue; a true aneurysm involves all layers of an intact wall.

## Which gene is mutated in Marfan syndrome, and what signalling pathway is overactive?

FBN1, encoding fibrillin-1; TGF-β signalling is excessive because fibrillin normally holds TGF-β in the matrix.

## What is cystic medial degeneration?

A loss of medial smooth-muscle cells and elastic fibres with pools of proteoglycan, weakening the aortic wall (Marfan, hypertension, ageing).

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## Elsewhere in DurgiPedia

- [Fibrillin-1](https://durgical.com/en/biochemistry/enzymes/fibrillin-1/)
- [Elastin](https://durgical.com/en/biochemistry/enzymes/elastin/)
- [Collagen](https://durgical.com/en/biochemistry/enzymes/collagen/)
- [Blood vessels: artery, vein and capillary layers](https://durgical.com/en/histology/blood-vessels-artery-vein-and-capillary-layers/)

## Connections

## Related 5

- [Abdominal aortic aneurysm: the lesion, its variants and rupture](https://durgical.com/en/general-pathology/abdominal-aortic-aneurysm-the-lesion-its-variants-and-rupture/)
- [Aortic dissection](https://durgical.com/en/general-pathology/aortic-dissection/)
- [Infectious vasculitis and aortitis](https://durgical.com/en/general-pathology/infectious-vasculitis-and-aortitis/)
- [Vascular anomalies: berry aneurysm, AV fistula, fibromuscular dysplasia](https://durgical.com/en/general-pathology/vascular-anomalies-berry-aneurysm-av-fistula-fibromuscular-dysplasia/)
- [Complications of myocardial infarction](https://durgical.com/en/general-pathology/complications-of-myocardial-infarction/)

## Related conditions 6

- [Thoracic aortic aneurysm and aortitis](https://durgical.com/en/conditions/thoracic-aortic-aneurysm-and-aortitis/)
- [Abdominal aortic aneurysm](https://durgical.com/en/conditions/abdominal-aortic-aneurysm/)
- [Marfan syndrome](https://durgical.com/en/conditions/marfan-syndrome/)
- [Aortic dissection](https://durgical.com/en/conditions/aortic-dissection/)
- [Subarachnoid haemorrhage](https://durgical.com/en/conditions/subarachnoid-haemorrhage/)
- [Syphilis](https://durgical.com/en/conditions/syphilis/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Aortic Aneurysm | MedlinePlus](https://medlineplus.gov/aorticaneurysm.html) (MedlinePlus, US National Library of Medicine, 2025); [Marfan syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/marfan-syndrome/) (MedlinePlus, US National Library of Medicine, 2025); [Loeys-Dietz syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/loeys-dietz-syndrome/) (MedlinePlus, US National Library of Medicine, 2025).

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Source: https://durgical.com/en/general-pathology/aneurysms-types-and-pathogenesis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/aneurysms-types-and-pathogenesis/