Intracranial aneurysms are common in the general population. The overall prevalence of unruptured intracranial aneurysms (UIAs) is estimated of 2.3-3.2% in the population without specific risk factors for SAH. As noninvasive imaging modalities are more commonly used than before, UIAs are increasingly being detected. Most patients with small aneurysms (less than 5mm) are incidentally found in clinical practice. Some studies indicate that the majority of patients with UIAs, particularly with small aneurysms (<7mm), have a low risk of rupture, and others have found that small ruptured aneurysms have a high proportion in patients with SAH. Therefore, there is a lot of controversy regarding which small aneurysms can be left untreated, or which aneurysms are needed to be treated with clipping or coiling.

The prevalence varies widely among different detection methods, race/ethnicity or patients with other inherited diseases. Although a wealth of data is available for the natural history of UIAs, the true natural history remains unknown because case selection bias occur in almost all studies. However, data on Chinese UIA is unknown. Using the MR angiography (MRA) to detect aneurysms, the prevalence is 7% of selected adult population in China. Therefore, small UIAs are very common and are increasingly being detected in clinical practice. Conservative treatment, surgical clipping and endovascular coiling are the three treatment options for UIAs. The optimal treatment remains controversial, particularly for small aneurysms (less than 7mm). To date, no clinical trials have compared the safety and efficacy between conservative treatment and surgical clipping or endovascular coiling for UIAs. It may be impossible to conduct the randomized controlled study considering aneurysm ruptured as a devastating event. However, surgical clipping or endovascular treatment itself carries a risk of immediate morbidity or mortality. Therefore, a substantial variability widely exists in treatment decision-making for UIAs, and this may lead to a great variability in clinical recommendations.

Our study is a prospective observational study to identify the incidence of rupture of small aneurysms in the first year after the diagnosis of the aneurysm which is left untreated. Meanwhile, we determine the differences of outcomes, procedural complications, and rates of retreatment between surgical clipping and endovascular coiling for small UIAs in China.

1. Background: There is a lot of controversy regarding which aneurysms can be left untreated, or which aneurysms are needed to be treated with clipping or coiling. To date, no clinical trials have compared the safety and efficacy between conservative treatment and surgical clipping or endovascular coiling for small UIAs.
2. Study design: A multicenter prospective observation registry study. This study is undertaken to conform to the study protocol. Patients will be recruited between December 2016 and December 2018. Patients are eligible for the study if they meet the inclusion criteria and they are not eligible if any of the following exclusion criteria are met.
3. Procedures: All patients were interviewed by a multidisciplinary team that consisted of vascular neurosurgeons, interventional neuroradiologists and anesthetists. If patients meet all of the inclusion criteria: an unruptured aneurysms ≤5mm are enrolled and then followed up at 6 and12 months. Clinical observation, surgical clipping and endovascular coiling are the three treatment options for UIAs. In general, when an UIA is detected, it should be needed to quit smoking, to aggressively manage hypertension, and to control alcohol use. When an aggressive treatment is considered, treatment risks should be balanced against the risk of rupture.

Inclusion Criteria:

1. Intracranial unruptured aneurysms confirmed by digital subtraction angiography DSA, CT angiography or MR angiography (MRA);
2. The maximum size of unruptured aneurysm ≤5mm;
3. Informed consent

Exclusion Criteria:

1.Patients with severe systemic disorders and expected life span less than one year; 2.Pregnant or lactating women.

Five hundred patients with unruptured aneurysms will registered in this study during the 3 years.