1.2 Pathophysiology and Prevalence of Aneurysms A cerebral or intracranial aneurysm is a focal dilation of an artery in the brain that results from a weakening of the inner muscular layer (the intima) of a blood vessel wall. The pathogenesis of intracranial aneurysms remains incompletely understood. Most aneurysms arise sporadically but occasionally they may be: dissecting (resulting from a luminal endothelial tear), traumatic (usually within 2-3 weeks after severe head injury) or mycotic (as a result of embolism of infected material). Aneurysm etiology is multifactorial, including congenital medial arterial wall defects, degenerative changes, and accruing hemodynamic stress, particularly at sites of turbulent blood flow. Contributing factors include connective tissue disorders, hypertension, anatomy variations, atherosclerosis, trauma, mycosis, and tumors. Epidemiological studies have already identified aneurysm-specific risk factors such as size and location, as well as patient-specific risk factors, such as age (higher in adults), sex (higher in females), and presence of medical comorbidities, such as hypertension. In addition, exposure to certain environmental factors such as smoking has been shown to be important in the formation of IA. Furthermore, substantial evidence proves that certain loci contribute genetically to IA pathogenesis. Genome-wide linkage studies using relative pairs or rare families that are affected with the Mendelian forms of IA have already shown genetic heterogeneity of IA, suggesting that multiple genes, alone or in combination, are important in the disease pathophysiology [19].