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Vol. 40 (2012) No. 3 p. 173-178




The management of patients with small, asymptomatic, unruptured intracranial aneurysms (UIA) is controversial. We assessed the chronological sequences of the prevalence of UIA less than 5 mm in diameter empirically analyzed the factors influencing the change in surgery rate on UIA. We analyzed 132 cases (158 UIA <5 mm in diameter) out of 273 patients (339 UIA) with no history of subarachnoid hemorrhage (SAH). Aneurysms were detected by magnetic resonance angiography or by 3-dimensional CT angiography. All incidentally found UIA <5 mm were classified into two groups. Group I comprised 39 cases (44 UIA) experienced in the period from March 1998 to March 2005. Group II comprised 93 cases (114 UIA) experienced from April 2005 to October 2010.
We assessed inter-period differences in terms of frequency (%) of small UIA <5 mm, aneurysm site, surgery rate, surgical outcome, as well as natural history. In Group II, the number of UIA <5 mm was observed to increase from 37% to 54% of all sizes of UIA. There was no difference in aneurysm location between the two groups. The surgery rate in Group II declined remarkably from 41% to 20% in all aneurysm sites, especially in middle cerebral artery aneurysms (from 67% to 23%). The mean aneurysm diameter in Group II (2.9 mm) was slightly smaller than in Group I (3.2 mm). The mean duration of follow-up was 3.8 years. Among 97 patients in the follow-up group, one had SAH, resulting in an annual rupture rate of UIA <5 mm of 0.2%. In the surgical group, no mortality or morbidity was noted. The present study indicated that the development of a “brain check-up” system for the healthy population in Japan may be increasing the prevalence of small UIA <5 mm.
Influenced by several recent reports regarding the natural history and surgical outcome in patients with small UIA, both providers and receivers of such preventive medical care might be getting more careful about prospective repair of small UIA, resulting in a decreased surgery rate.

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