Intracranial aneurysm rupture is the most common cause of subarachnoid hemorrhage (SAH). It has been a hot topic of neurosurgical research because of its high disability and mortality rate. In recent years, with the development of neuroimaging techniques, the number of incidental intracranial aneurysms has increased significantly [1]. Some scholars have reported that 3.6% to 6.0% of adults have occult unruptured cerebral aneurysms [2]. Although the International Study of Unruptured Intracranial Aneurysms (ISU IA) published two reports in 1998 and 2003, respectively, and achieved a certain degree of consensus, there is no consensus on U IA, especially in asymptomatic small U IA without a history of SAH. The management of asymptomatic small U I A, especially those without a history of SAH, is still divided. Is observation or surgical intervention preferred for small asymptomatic unruptured aneurysms? Should intervention or craniotomy be chosen? 1. Definition of unruptured asymptomatic small aneurysm An unruptured asymptomatic intracranial aneurysm is an aneurysm without a history of subarachnoid hemorrhage or pathologically confirmed incomplete breach of the aneurysm wall. Small aneurysms are generally defined as aneurysms less than 5 mm in diameter, a value derived from cerebral angiography, but with a 2 m error due to cerebral angiography. The International Study Group on Unruptured Intracranial Aneurysms, on the other hand, grouped them with a cut-off of 7mm [4]. , this paper is unified with the classification of the International Unruptured Intracranial Aneurysm Study Group. It includes 1) small asymptomatic unruptured aneurysms of occasional origin and 2) small unruptured aneurysms in multiple aneurysms detected by DSA due to subarachnoid hemorrhage. 2. natural history and risk factors for rupture 2.1 Natural history When deciding whether to treat patients with small unruptured aneurysms surgically, their natural history and surgical risk need to be weighed. in a 1998 [3] study by the International Study Group on Unruptured Aneurysms, the authors analyzed 1449 patients with a total of 1937 aneurysms with a mean follow-up of 8.3 months. The rate of rupture of aneurysms smaller than 10 mm was about 0.05% in patients without a history of subarachnoid hemorrhage (about 32.7% for aneurysms of 2-5 mm) and about 0.5% in 722 patients with a history of subarachnoid hemorrhage in various locations. (approximately 61.2% for aneurysms of 2-5 mm.) In 2003 [4] this organization prospectively studied the natural history of 1692 cases with a total of 2686 unruptured aneurysms with a mean follow-up of 4.1 years. Aneurysms smaller than 7 mm, except those located in the cavernous sinus segment of the internal carotid artery, had a rupture rate of approximately 0.15% per year in patients without a history of subarachnoid hemorrhage and 0.4% per year in patients with a history of subarachnoid hemorrhage. Makoto [5] et al. followed 446 patients with a total of 540 aneurysms for an average of 41 months. The study concluded that the rupture rate was about 0.54% per year for small unruptured aneurysms (less than 5 mm), 0.34% for single ones and 0.95% for multiple ones. 2.2 Risk factors for rupture The diameter of the aneurysm is considered to be the main risk factor predicting rupture, and Wiebers et al [4] showed that large aneurysms and those located in the posterior circulation and posterior communicating arteries increase the risk of rupture, and concluded that 7 mm posterior circulation aneurysms are more likely to rupture. In contrast, aneurysms located in the anterior circulation and less than 7 mm in diameter have a lower probability of rupture 5 years after diagnosis, but this is contrary to clinical practice, as most ruptured aneurysms are less than 10 mm in diameter [6]. In contrast, the study by Sonobe et al [7] was more precise and concluded that aneurysms with a diameter of 4-5 mm are at high risk, while Juvela et al [8] suggested that UI A rupture without SAH is more likely to be seen in aneurysms with a diameter increase of more than 1 mm than in large unbleeding aneurysms. Therefore, together with the above studies, we conclude that the annual growth rate of aneurysm diameter, rather than just its diameter size, plays an important role in ruptured aneurysms. Inagawa [9] concluded that anterior circulation aneurysms are prone to rupture, accounting for 90%, anterior cerebral artery (including anterior communicating artery and distal anterior cerebral artery) for 40%, middle cerebral artery for 25%, and internal carotid-posterior communicating artery for 25%, and concluded that anterior communicating artery aneurysms rupture mostly in males and internal carotid-posterior communicating artery aneurysms rupture mostly in females. Huttunen et al [10] concluded that 9% of ruptured aneurysms were located in the anterior communicating artery and 29% in the middle cerebral artery bifurcation, while Lindner et al [11] suggested that the risk of rupture of aneurysms in different locations should be related to their intrinsic properties. The natural history of unruptured aneurysms should be analyzed on a case-by-case basis, as many factors can influence aneurysm rupture. For example, family history of aneurysm, smoking, excessive alcohol consumption, female (especially postmenopausal), history of subarachnoid hemorrhage, size, location, and local symptoms of the aneurysm may affect the natural history. [12] In addition the life expectancy of the patient has to be considered. 3. Treatment strategy The current treatment strategy for small unruptured asymptomatic aneurysms is highly controversial. In 2000, the American Heart Association Stroke Committee made the following recommendations for the management of unruptured aneurysms: small aneurysms (less than 10 mm) found accidentally have a low risk of rupture and are generally selected for observation. Unruptured asymptomatic small aneurysms smaller than 5 mm should be conserved. Younger patients with unruptured aneurysms with severe psychological disturbances may be considered for treatment. Patients younger than 60 years of age with aneurysms larger than 5 mm need to be treated unless there are clear contraindications. In elderly patients, the location of the aneurysm plays a key role. Small aneurysms in the posterior circulation, anterior communicating artery and posterior communicating artery; cerebral aneurysms with morphological features such as large, irregular, and small vesicles in the apex/neck direction ratio are treated aggressively if there are no obvious contraindications. The patient’s wishes, life expectancy, concomitant diseases, number of aneurysms, location, morphology, arterial anatomical features, dynamic observation of aneurysm changes, symptomatic (acute or progressive) small aneurysms, family history of aneurysm or SAH, smoking history and physician’s experience should also be considered. 3.1 Waiting for observation When the patient is older than 70 years, accidentally discovered unruptured small aneurysms should be observed [14, 15]. They should be advised to avoid smoking, heavy alcohol consumption, treatment of hypertension, and diabetes mellitus. Meanwhile, imaging examinations (MRA,DSA if available, but whether frequent DSA examinations promote aneurysm growth or rupture is still not clearly reported,) are done every six months to one year for follow-up observation. If aneurysm enlargement or deformation occurs during the course of observation, or if the patient’s symptoms change, treatment should be re-evaluated. Patients with comorbidities should be treated aggressively with surgical treatment, and those with severe comorbidities should be treated aggressively with a balance of benefits and disadvantages. It should be especially noted that if, according to the characteristics of the aneurysm, it should be treated but the patient and the family request observation, when the patient and the physician lack trust in communication, they should be recommended to other physicians and institutions to listen to other physicians, and the family needs to be fully informed of its dangers, to preserve imaging data, and to make contingency plans. Because the diagnosis of an unruptured cerebral aneurysm can cause depressive symptoms and agitation in patients, care must be taken when explaining this. Psychological interventions are recommended for those with severe depression and agitation. 3.2 Endovascular treatment Endovascular treatment is the main measure for the treatment of intracranial aneurysms because of its minimally invasive nature, and in some countries there has even been a trend to gradually replace craniotomy clamping. The indications: 1. endovascular intervention should be chosen for elderly patients (>70 years old) or young patients with significant comorbidities (e.g. hematologic disease, not suitable for craniotomy), 2. aneurysm pointing, posterior pointing basilar bifurcation aneurysm and posterior superior pointing anterior communicating aneurysm with high surgical risk, 3. wide neck ( > 5 mm), atherosclerosis, neck and apical calcification, these aneurysms need neck reconstruction for embolization The location of the aneurysm, posterior circulation aneurysms have a poorer prognosis than anterior circulation aneurysms, and anterior communicating aneurysms in the posterosuperior direction, and internal carotid artery aneurysms in the mastoid segment and cavernous sinus segment have a higher risk. 5. However, the recurrence rate after endovascular embolization is relatively high. Murayama et al [16] reported that the recurrence rate of aneurysms is related to the apex and neck of the aneurysm. The recurrence rate of small aneurysms (4-10 mm) with aneurysmal necks (less than 4 mm) was only 5.1%. In contrast, the recurrence rate for small aneurysms with a wide neck (>4 mm) is approximately 20%. Although endovascular interventions are increasingly used to treat aneurysms, traditional surgical clamping is still the most mature and reliable method, which has long been the classic method for treating intracranial aneurysms, with a one-time complete clamping rate of more than 90%. At the same time, craniotomy also has advantages that cannot be replaced by endovascular embolization: 1. For aneurysms with wide neck or large neck/apical aneurysms that cannot be easily embolized, surgical clamping is effective, 2. Endovascular treatment is not suitable for aneurysms less than 3 mm in diameter, 5. Endovascular embolization may lead to embolization or thrombosis of the aneurysm-carrying artery, and 7. is relatively less expensive than endovascular embolization. Moroi et al. showed [17] that the disability and mortality rates for aneurysms less than 10 mm were 0.6% and 0. The authors noted that the risk of surgery for aneurysms of the anterior and middle cerebral arteries was 0. The disability and mortality rates for aneurysms of the internal carotid system less than 5 mm were 1% and 0. The disability and mortality rates for aneurysms of the vertebrobasilar system less than 5 mm were 0, and for aneurysms greater than The disability and mortality rates for aneurysms of less than 5 mm in the basilar system were 0, and 11.1% for those larger than 5 mm. It is undeniable that the level of the hospital and the experience of the surgeon have a greater impact on the success of the procedure. Hospitals with more aneurysms have a much lower mortality and disability rate than hospitals with fewer aneurysms, and experienced surgeons have better surgical results. Currently, there is a trend toward aneurysm clamping with a locked-hole approach. In conclusion, the management of small unruptured aneurysms should be evaluated in the context of their natural history and treatment risks. Family history of aneurysm, smoking, excessive alcohol consumption, women (especially postmenopausal), history of subarachnoid hemorrhage, size, location, and local symptoms of the aneurysm all influence its natural history. The current management of small unruptured aneurysms is mainly: observation, endovascular embolization, and surgical clamping. The treatment strategy is determined by considering the patient’s wishes, life expectancy, co-morbidities, number, location, and morphology of the aneurysm, anatomic features of the aneurysm-carrying artery, dynamic observation of the aneurysm, symptomatic (acute or progressive) small aneurysms, family history of aneurysm or SAH, smoking history, and the physician’s experience. The natural history of unruptured asymptomatic small aneurysms remains unclear because they are not easily detected clinically and there are no multicenter prospective studies with large samples. Currently, there is no clear treatment protocol for unruptured asymptomatic small aneurysms, which may be related to physicians’ preferences. The risk of treatment varies depending on the experience and treatment approach of different hospitals and different physicians. It would be beneficial to establish an aneurysm treatment center composed of experienced physicians from neurosurgery and interventional medicine, so that a comprehensive assessment of the patient can be made and an individualized and appropriate treatment plan can be proposed.