Autopsy studies have shown an occurrence of unruptured intracranial aneurysms (UIAs) incidences in approximately 5 percent of all performed autopsies^6,13). 6 of 100,000 patients will have ruptured aneurysm induced subarachnoid hemorrhage each year in the USA^1,17). UIAs are a cerebrovascular condition that are commonly diagnosed due to improved imaging techniques. While the rupture risk of UIAs leads many neurosurgeons to advise its preventive treatment, the management of patients with UIAs remains controversial. Surgeon preference and experience as well as morphologic and anatomic considerations play important roles in the debates that continue over the controversy about whether to treat UIAs. Many studies have reported that certain patient characteristics increase the risk of rupture, including old age, female gender, and Japanese or Finnish descent. Aneurysms characteristics that increase the risk of rupture are location at the posterior circulation and increasing size^10,22,32,33). Thus, these factors affect the management of UIAs. In this study, we evaluated the association between these risk factors, which are involved in future rupture of UIAs and management decision.

Our study found the following factors as significant between treated and observation group: age, smoke history, aneurysmal height and width, maximum diameter, aspect ratio, aneurysm located in AcoA.

MCA, Cavernous ICA and daughter sac. The significant factors for the treated group were different than those for the observation group and included the following: young mean age (58.49 years vs 61.9 years), more patients with a smoke history (25% patients vs 13% patients), high height (5.13 mm vs 3.37 mm ) and width (5.62 mm vs 4.08 mm), a large maximum diameter (6.27 mm vs 4.27 mm), large aspect ratio(1.46 vs 1.18), more AcoA location (24% vs 12%), MCA (32% vs 19%), less cavernous ICA (16% vs 34%) and more daughter sac (56% vs 5%).

In a multivariate analysis, young age, high height, small aspect ratio, presence of daughter sac, and UIA located in AcoA and MCA were independently associated with the treatment group. The effect of age as a risk factor of the rupture of UIA has been consistently reported in the past. Rinkel et al. and Vlak et al. described a higher incidence rate of SAH among older populations^22,32). In our study, young age was significantly associated with the treated group. This is presumably due to the reason that treatment is recommended and implemented in younger patients because of their better general condition than those of older patients. Several researchers reported that the size and location of aneurysm were predictors of UIA rupture. The International Study of Unruptured Intracranial Aneurysms [ISUIA] in 1998 demonstrated a low rupture rate for an unruptured aneurysm smaller than 7 mm. A larger size and a posterior circulation site were predictors as independent risk factors of aneurysm rupture, and patients with a history of ruptured aneurysms were at higher risk⁹⁾. A Japanese cohort study reported that the following factors increased the risk of rupture: aneurysms of 7 mm or larger, aneurysms located in posterior circulation except for PcoA, and aneurysms with a daughter sac¹⁰⁾. While recent literature recommends treatment of aneurysm that are more than 4 mm in size and multilobular or aneurysms with bleb changes⁴⁾, our study showed that the most commonly treated location was the MCA and the maximum diameter was 6.27±5.26 mm. Further, the location of the aneurysm in the AcoA and MCA is significantly associated with the treated group in the multivariate analysis. MCA was easier to access in surgical clipping, and there were more cases of surgery^{3,5,7,18,20,23,24,27,30)}. While the univariate study showed that high aneurysmal height, large aneurysmal width, large maximum diameter and large aspect ratio were significantly related to the treated group, the multivariate study showed that maximum diameter and aneurysmal width did not show any difference and high aneurysmal height, small aspect ratio and daughter sac were significantly associated with the treatment group^2,12,14,21). Contrary to previous studies, our study implies that a low aspect ratio was related to the treatment group^25,29).

Some known risk factors did not show a significantly difference. While a female gender has been reported as risk factor in some studies^10,28), more incidences of ruptured aneurysm are found in women than in men³²⁾, and, in our study the female gender was the dominant gender of total patients. However the gender was not associated with the treated group. Hypertension was a controversial risk factor as hypertension did not significantly increase the risk of rupture¹⁰⁾. However, in other study hypertension is a major risk factor for subarachnoid hemorrhage^19,32). In our study, hypertension did not show a significant difference between the treated and the observation group. Several researchers reported smoking history, alcohol history and previous episode of SAH as risk factors for UIA rupture^{8,10,11,15,31)}. In our analysis, the smoke history was significantly related to the treated group in the univariate study but not in the multivariate study, and alcohol use was not statistically significant in both analyses. In our study, we excluded patients who underwent previous episodes of SAH and symptomatic aneurysm. Further investigations of a larger number of patients is warranted to determine any significant relationships.

The indication for UIA in our institution is for patients with a tolerable whole body condition, with an aneurysm height of 5 mm or more, and with the UIA located in AcoA or MCA with or without a daughter sac. Furthermore, treatment of UIA located in MCA was usually performed by surgical clipping. Conversely, Cavernous ICA or posterior circulation aneurysms are performed mainly by endovascular treatment.

Our study has some limitations. Patient data were collected and analyzed at a single center, as a consequence, the potential for bias in treatment choice and conservative management may exist. The number of patients who were enrolled in our study was relatively small. Especially for the group of PcoA and posterior circulation, which are known to have a high risk of rupture, there was no difference between the treatment group and the observation group. It is estimated that the bias was generated fromthe limit that the subject was relatively small. We did not consider statistical errors or differences in the tendency of the operator.

Management of UIA has never been straight forward. Decisions on treatment of UIA should be carefully made after investigation of many risk factors for rupturing. In the present study, we conclude that a young age, a small aspect ratio, a large aneurysmal height, a daughter sac, and some locations of aneurysm (AcoA, MCA) are associated with the treatment group. Prospective studies should be performed in larger sample sizes and more concise evaluation to make better decisions for the treatment of UIA.