Table 1: Key findings related to the natural history data.

Risk factorKey findingsReferences

Size(i) Larger UIAs have greater RR
(ii) Aneurysm size is a significant independent predictor of RR
(iii) Defining a critical size threshold for aneurysm rupture remains difficult
ISUIA investigators 1998 [7]
Wiebers et al. 2003 [8]
Ishibashi et al. 2009 [9]

Enlargement(i) In most cases, IAs are larger at time of rupture than at initial diagnosis
(ii) Larger UIAs are more likely to grow
(iii) Larger UIAs → greater growth risk → increased RR
Yasui et al. 1996 [10]
Burns et al. 2009 [1]
Matsubara et al. 2004 [11]

Previous SAH(i) Prior history of aneurysmal SAH increases future RR
(ii) Aneurysms <7 mm have an increased RR with prior history of SAH
ISUIA investigators 1998 [7]
Wiebers et al. 2003 [8]

Location(i) Posterior circulation aneurysms are widely considered to be more hazardous
(ii) Include basilar artery, posterior cerebral artery, and vertebrobasilar distribution
(iii) Intracavernous IAs are more benign
Weir et al. 2002 [12]
Wermer et al. 2007 [13]
Kupersmith et al. 1992 [14]

Morphology(i) Multiple lobulations or loculations increases RR
(ii) High dome : neck ratio increases RR
(iii) Aneurysm angle, undulation index, and nonsphericity index are all predictors of aneurysm rupture
Hademenos et al. 1998 [15]
Beck et al. 2003 [16]
Dhar et al. 2008 [17]
Raghavan et al. 2005 [18]

Patient characteristics(i) Age, sex and comorbidities will influence aneurysmal RR
(ii) Female sex and cigarette smoking are independent predictors of both UIA formation and growth
(iii) These factors are especially important when making decision on whether to treat
Nahed et al. 2005 [19]
Juvela et al. 2001 [20]