Aetiology
Once thought to be congenital, saccular aneurysms are now regarded as an acquired, haemodynamically induced injury to the vascular wall. Other less common causes include trauma, infection, tumour, arteriovenous malformations/fistulas, and drug abuse.
The well-known association with heritable connective tissue diseases and the familial occurrence support a genetic factor. It is considered specifically in patients with autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome type IV, neurofibromatosis type 1, and Marfan's syndrome.[8]
All population studies have consistently shown that cigarette smoking increases the risk for aneurysmal subarachnoid haemorrhage (SAH).[9][10][11][12]
Moderate- to high-level alcohol consumption is an independent risk factor for aneurysmal subarachnoid haemorrhage (SAH).[12]
Pathophysiology
Blood vessels are composed of three layers: tunica intima, tunica media (muscularis), and tunica externa (adventitia). The intima and media are separated by the lamina elastica interna, and the media and adventitia are separated by the membrana elastica externa. In cerebral vessels the media is thinner and the membrana elastica externa is negligible.
Although the pathophysiology of traumatic and infectious cerebral aneurysms is obvious, that of spontaneous saccular aneurysms is less clear. Hypertension and smoking are thought to contribute significantly to the vascular changes associated with saccular cerebral aneurysms.[8] One hypothesis relates to the effect cigarette smoking has on inhibitors of proteases, which results in degradation of various connective tissues, including arterial walls.[13] In pathology specimens, the tunica media is decreased and the aneurysm sac is therefore reduced to a single layer of endothelial cells and a thin fibrous layer. The lamina elastica interna ends at the entrance to the aneurysm sac.[14] Continuous arterial pressure directed on this abnormal section of the artery leads to aneurysmal out-pouching, particularly at arterial branch points, where the pressure is higher.[8]
With increased availability and improved sensitivity of non-invasive cerebral imaging techniques, more unruptured cerebral aneurysms are being detected. Although they are often discovered incidentally, unruptured aneurysms can cause symptoms through the mass effect on neighbouring cranial nerves or brain parenchyma.
Classification
Morphological types of cerebral aneurysm[1]
Saccular
Often termed a berry aneurysm, as it resembles a berry hanging from a vine
Rounded out-pouching attached by a neck or stem to a brain artery.
Fusiform
Also called atherosclerotic aneurysm
Forms as media damage leads to arterial stretching and elongation
Intra-luminal clots may form.
Dissecting
Often termed pseudo-aneurysm
Forms as blood accumulates within a tear between the layers of the cerebral artery
Depending on which plane is dissected, either luminal narrowing or a sac-like out-pouching can occur.
Classification of cerebral aneurysm based on size[2]
Small: <7 mm in diameter
Medium: 7 to 12 mm in diameter
Large: 13 to 24 mm in diameter
Giant: ≥25 mm in diameter.
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