Saccular Aneurysm: Definition and Clinical Significance
A saccular aneurysm is an abnormal focal dilation of an artery that forms a sac-like outpouching with a well-defined neck, representing the most common type of cerebral aneurysm. 1
Characteristics of Saccular Aneurysms
Anatomical Features
- Definition: An abnormal focal dilation of an artery with attenuation of the vessel wall that forms a distinct sac-like outpouching 1
- Morphology: Typically has a well-defined neck connecting to the parent vessel 1
- Structure: Consists of a dome (fundus) and neck region 1
- Contrast to other types: Different from fusiform, dolichoectatic, dissecting, serpentine, traumatic, or mycotic aneurysms 1
Size Classification
Saccular aneurysms are categorized by their maximum diameter:
- Small aneurysms: ≤5 mm
- Medium aneurysms: >5 mm and <15 mm
- Large aneurysms: ≥15 mm and <25 mm
- Giant aneurysms: ≥25 mm 1, 2
Neck Characteristics
- Small neck: ≤4 mm
- Wide neck: >4 mm 1
- Dome-to-neck ratio: Important measurement that influences treatment decisions and rupture risk 2
Clinical Significance
Prevalence and Location
- Saccular aneurysms are the most common type of cerebral aneurysm 1
- Most frequently occur at arterial branching points, particularly in the circle of Willis 3
- Can be found in various locations including the internal carotid artery, anterior cerebral artery, and middle cerebral artery 2, 4
Rupture Risk Factors
- Size: The most consistent predictor of rupture 3
- Aneurysms <5 mm have lower rupture rates
- Aneurysms >10 mm have significantly higher rupture risk
- Location: Certain locations (e.g., communicating segment) carry higher rupture risks 2
- Morphology: Irregular shape and presence of daughter sacs increase rupture risk
- Venous drainage patterns: Abnormalities in venous drainage appear to be associated with hemorrhagic presentation 1
Clinical Presentation
- Many saccular aneurysms are asymptomatic and discovered incidentally 3
- Ruptured aneurysms typically present with:
- Subarachnoid hemorrhage
- Severe headache ("worst headache of life")
- Nausea, vomiting, photophobia
- Nuchal rigidity
- Altered level of consciousness
- Focal neurological deficits 1
Pathophysiology
Formation Mechanism
- Develops from a combination of:
- Congenital or inherited defects weakening the arterial wall
- Acquired degenerative vascular disease 3
- Hemodynamic stress at arterial bifurcations contributes to formation 5
- Maximum biaxial stresses and strains are most often at the fundus, where rupture tends to occur 5
Risk Factors for Development
- Hypertension
- Tobacco use
- Familial predisposition
- Female sex (women are more susceptible than men) 1
- Associated conditions: fibromuscular dysplasia, polycystic kidney disease, arteriovenous malformations 1
Diagnostic Evaluation
Imaging Modalities
- Digital subtraction angiography: Highest spatial resolution for evaluating dome-to-neck ratio and exact dimensions 1, 2
- CT angiography: High sensitivity (>90%) for initial detection 2
- MR angiography: Useful for follow-up and screening high-risk populations 2
Clinical Implications
The identification and proper characterization of saccular aneurysms is critical for risk stratification and treatment planning. Understanding the specific morphology, size, and location helps determine the risk of rupture and guides the decision between observation, surgical clipping, or endovascular treatment.
Proper measurement and documentation of aneurysm characteristics (size in three dimensions, dome-to-neck ratio, and relationship to parent vessels) are essential for clinical decision-making and research purposes 1.