Natural History of Unruptured Intracranial Aneurysms

Robert D. Ecker, M.D.; L. Nelson Hopkins, M.D.


Neurosurg Focus. 2004;17(5) 

In This Article

Abstract and Introduction

Since the publication of the retrospective part of the International Study of Unruptured Intracranial Aneurysms (ISUIA) in 1998, there has been a significant focus in the neurosurgical literature on the natural history of these lesions. The prospective data from the second part of the ISUIA, which was published in 2003, provided further evidence that small, asymptomatic intracranial aneurysms may have a more benign course than previously believed. With the data from the ISUIA as a reference point, in this paper the authors strive to provide a source of practical clinical data to aid cerebrovascular physicians in the initial decision to treat or observe a patient with a small, asymptomatic intracranial aneurysm. The issues covered will include previous rupture, symptoms other than rupture, aneurysm size, site, and aspect ratio. It is the authors' goal to provide a useful practical framework on the relevant clinical issues as an aid to practitioners treating patients who present with intracranial aneurysms.

The publication of the retrospective arm of the ISUIA in 1998 indicated that the risk of rupture of an aneurysm smaller than 10 mm in a patient with no previous SAH was 0.05%.[10] Furthermore, the morbidity and mortality rates associated with the surgical management of these lesions were 17.5% at 30 days and 15.7% at 1 year. With a risk of rupture 10 to 12 times lower than previously estimated and the risk associated with treatment approximately double that for historical controls, the recommendation was to manage patients expectantly when their aneurysms were smaller than 10 mm. This recommendation led to a maelstrom of controversy, with prominent cerebrovascular surgeons noting that the ISUIA population was heavily selected by cerebrovascular specialists to include patients with low risk for rupture because those with particularly worrisome symptoms, aneurysm morphological configuration, and family history of SAH were preferentially treated and not included in the study.[3–5,13,21]

In 2003, the prospective arm of the ISUIA was published ( Table 1 ).[27] The investigators who conducted this new, powerful trial reported that the 5-year cumulative rupture rates for aneurysms in the anterior circulation in patients with no previous SAH was 0% in lesions smaller than 7 mm and 2.5% in lesions of the PCoA or posterior cerebral circulation ( Table 2 ). The risk associated with surgical treatment was 13.7% at 30 days and 12.6% at 1 year in patients with no previous SAH. Endovascular treatment resulted in overall morbidity and mortality rates of 9.3 and 9.8% at 30 days and 1 year, respectively. Aneurysm size and location were significant predictors of rupture, and patient age combined with aneurysm size and location was a significant predictor of treatment outcome. Although significantly different from the retrospective data, the ISUIA prospective data underscores the need for individual counseling with respect to lesion size, site, patient age, and comorbidities in each patient who presents with a cerebral aneurysm.

Exhaustive literature reviews on the natural history of unruptured intracranial aneurysms are available.[24] In this review of the natural history of unruptured intracranial aneurysms, we will focus on the relevant practical data that a clinician should consider in the initial decision to treat or observe a patient with an unruptured aneurysm. Although previously reported risk factors for SAH include female sex, smoking, excessive alcohol consumption, hypertension, family history, ischemic heart disease, autosomal-dominant polycystic kidney disease, and use of oral contraceptive drugs, the ISUIA investigators found that aneurysm size and posterior cerebral location outweighed all other factors.[17] Therefore, in this review we will focus on the risk factors of previous SAH, aneurysm size, aspect ratio (Fig. 1), aneurysm location, and symptoms other than rupture.

Schematic drawing showing the aspect ratio calculated by dividing the aneurysm's depth by its neck ratio.