Intracranial basilar apical aneurysms account for about 50% of posterior circulation aneurysms, with deep location and close relationship with important thalamic perforating arteries, direct clamping of the aneurysm necks is very difficult, and it is a very challenging procedure in neurosurgery. Although most cases have been treated by endovascular embolization in recent years, there is still a fatal risk of aneurysm recurrence after embolization and rebleeding in the subperitoneal space. In addition, some aneurysms with wide necks and giant aneurysms are not suitable for endovascular embolization (1-3), so neurosurgeons also need to master the microsurgical treatment of aneurysms of the apical region of the basilar artery. I. Microanatomy of the apical region of the basilar artery. The apical part of the basilar artery is located in the interpeduncular pool, with the slope and posterior bedspread anteriorly, the cerebral peduncle posteriorly, the papillary body and posterior perforating mass superiorly, and the medial aspect of the temporal lobe and the rim of the canopy laterally. The basilar artery is located approximately 15 mm posterior to the internal carotid artery and gives off a superior cerebellar artery approximately 1 mm in diameter bilaterally as it approaches its tip in the interpeduncular pool; this artery may also mutate into unilateral or bilateral double branches. The basilar artery is apical in the vicinity of the dorsum of the saddle and branches into bilateral posterior cerebral arteries with a diameter of about 2 to 3 mm. The diameter of the first segment of the posterior cerebral artery (segment P1, i.e., the posterior cerebral artery from the bifurcation of the apical part of the basilar artery to the point of confluence with the posterior communicating arteries) is dependent on how much the posterior communicating arteries supply the distal posterior cerebral artery. The posterior aspect of the basilar artery, the proximal part of segment P1, and the posterior communicating artery give rise to the thalamic perforating artery, which supplies the thalamus. The cranial nerve most closely related to the apex of the basilar artery in this region is the motor nerve, which passes between the posterior cerebral artery and the superior cerebellar artery in the interpeduncular pool, and forward into the roof of the cavernous sinus.The Liliequist’s membrane, a thickened bead of reticulum, covers the interpeduncular pool prior to the papillary body, which attaches upward, extends anteriorly and inferiorly, and then folds posteriorly to form the apex of the pontine anterior pool (4). II General principles of surgery for aneurysms of the apex of the basilar artery. When selecting cases, middle-aged and young patients with low Hunt and Hess levels should be treated more aggressively with microsurgery, taking into account the perennial cumulative chances of rupture of the aneurysm, the risk of incomplete embolization and recurrence, etc. Interventional therapy should be the mainstay of treatment for elderly patients and those with high Hunt and Hess levels after subpial hemorrhage, and in addition, size, location, and pointing of the apex of the aneurysm, as well as the patient’s own intention to choose are also important factors to consider when choosing a treatment modality (5). The basic principles of surgical operation for aneurysms in other parts of the skull are applicable to aneurysms of the apex of the basilar artery, which include protection of the aneurysm-carrying artery and control of the proximal end, the method of sharp dissection, appropriate and adequate visualization, and preservation of the perforating artery. Before separating the apex of the basilar artery from the aneurysm, a temporary block should be prepared in an area without a perforating artery below the superior cerebellar artery of the basilar artery trunk. Gentle retraction of the basilar artery trunk can better expose the dorsal side of the aneurysm to adequately visualize and free the thalamic perforating artery attached to the neck of the aneurysm, and appropriate aneurysm clips should be chosen to clip the neck of the aneurysm, and if the vessel and the aneurysm have a close or complex relationship, the aneurysm should be prepared with various angles, forms with skeletons, and other forms of clips. If the relationship between the vessel and the aneurysm is close or complex, aneurysm clips should be prepared with various angles, skeletonized holes, etc. (6). When aneurysm clips are placed, the basilar artery, which is the perforating artery supplying the brainstem and thalamus, must be preserved, and care should be taken not to cause stenosis of the posterior cerebral artery or entrapment. After clamping the aneurysm, intraoperative angiography and ultrasound Doppler testing can be used to verify the patency of the carrier vessel and the completeness of the neck clamping to decide whether to end the procedure or to reposition the aneurysm clip. C. Common surgical approaches for apical basilar artery aneurysms. 1. Wing-point translateral approach: Yasargil (7) was the first to use a translateral approach to clip an aneurysm of the apex of the basilar artery. Opening the Liliequist’s membrane in the wing-point approach can fully expose the pedunculopontine pool and facilitate the observation of the ipsilateral posterior communicating artery and P1 segment, but this approach is difficult to observe the thalamus penetrating artery behind the aneurysm, and there is a dead space in the field of view when the neck of the aneurysm is clamped. The classical lateral fissure approach has good anterior visual field for aneurysms with necks located between the level of the middle pterygoid and 1 cm above the posterior bed process, but it is not good for the visualization of aneurysms with necks located below the level of the middle pterygoid or those with very high necks. It is necessary to use the infratemporal or orbital zygomatic approach, or open the cavernous sinus and abrade the posterior bed process to solve the anatomical and visual field difficulties encountered due to the low necks of aneurysms in the operation. 2.Infertemporal approach: Drake (8) firstly used the infertemporal approach to clip the aneurysm of the apical basilar artery, which has the advantages of easy observation of the perforating artery on the dorsal side of the aneurysm, easy blockage of the basilar artery trunk, and the use of a skeletonized aneurysm clip to facilitate the protection of the ipsilateral P1, and the intraoperative incision of the edge of the canopy can be as much as 1/3 of the slope, but it is hard to observe the contralateral P1 and its emanation of the perforating artery, and the field of the operation is narrower, particularly However, it is difficult to observe the contralateral P1 and its perforating artery, and the operative field is narrower, especially in the case of temporal lobe edema after subretinal hemorrhage, and it is also difficult to control the intraoperative bleeding. Translateral fissure and inferotemporal combined approach: first proposed by Drake, also known as “half-and-half” approach (halfapproach), using frontotemporal combined craniotomy, the operation can be both translateral fissure pool, but also from the inferotemporal to reach the top of the basilar artery, with the common advantages of the pterygoid point approach and inferotemporal approach, Sano (9) found the advantages of wing point approach and inferotemporal approach. Sano (9) found that this approach was blocked by the anterior part of the temporal lobe, and it was necessary to change the field of view in the lateral fissure pool and the inferotemporal area, which caused inconvenience in surgical operation, and further improved this approach into the temporal pole approach, which fully exposed the temporal pole during craniotomy, so as to retract the temporal pole posteriorly to increase the exposure space in the operation. 4. Orbital-zygomatic approach: In recent years, a group led by Spetzler has conducted anatomical and clinical studies on the orbital-zygomatic approach for clamping aneurysms of the apex of the basilar artery, which has made this approach another new approach in addition to the two classic approaches, namely, the pterygoidal point and the inferotemporal approach (10). The advantages of this approach are increased view angle, reduced strain on the brain, shorter operative depth, and occasionally, after confirming that the ipsilateral posterior cerebral artery is supplied by the basilar artery, the posterior communicating artery can be dissected to further increase the exposure space of the internal carotid artery-motor nerve triangle. Bilateral posterior cerebral arteries and superior cerebellar arteries can be observed in the operative field. If the aneurysm is located below the posterior peduncle, the posterior peduncle and the upper part of the slope can be resected, and if the aneurysm is located above the posterior peduncle and hidden in the pedunculopontine fossa, the view provided by this approach can adequately show the aneurysm and its surrounding structures. 5. Anterior temporal transzygomatic transcavernous sinus approach: Krisht (11) reported that the anterior temporal transzygomatic transcavernous sinus approach was used to clip the aneurysm at the apex of the basilar artery, which allowed for the removal of the posterior pedicle and the adequate freeing of the internal carotid artery and the arterio-ocular nerve. Intraoperatively, the upper part of the zygomatic arch was removed and the jagged part of the temporal bone was completely removed, and further removal of the pterygoid ridge and the posterior one-third of the extra-orbital and superior orbital wall preserved the orbital wall periosteum to reach the anterior bed process, which was able to expose the meningo-orbital artery, serve as a starting point for the separation of the lateral temporal lobe dura mater and the lateral wall of the cavernous sinus outside of the dura mater, and expose the epidural segment of the motoneye, and then remove the anterior bed process by the superior part of the optic canal to open the dura mater. If there is insufficient space for placing a temporary blocking clip on the proximal end of the basilar artery, we can continue to open the cavernous sinus for 5-10 mm at the posterior medial base of the triangular part of the motor nerve and at the posterior medial part of the cavernous sinus segment of the internal carotid artery, and then abrade the bone of the rocky oblique intersection area as well as the posterior bed protrusion on the medial and lateral side of the motor nerve, until the main trunk of the BA is exposed and there is sufficient space for placing a temporary blocking clip. After there is sufficient space for placing a temporary blocking clip, a temporary blocking clip is placed and the aneurysm is clamped. 6. Kawase approach: This approach is performed anteriorly in the petrous bone, removing the bone of Kawase’s triangle and Glasscock’s triangle, opening the suprasellar sinus, and viewing the basilar artery through the gap between the III-IV, IV-V, and V-VII nerves, and is suitable for aneurysms of the apical basilar artery that are located 5 mm below the posterior bed eminence and on the superior slope (12). Other accesses include trans-triventricular approach and trans-labyrinthine approach (4), but fewer cases have been reported in the literature. IV. Surgical outcomes of apical basilar artery aneurysms. The incidence of apical aneurysms of the basilar artery is low, and the surgical difficulty is high, so there are few reports of large cases. In 2004, Lozier et al. (13) from Columbia University in New York reported that 98 cases of apical basilar artery aneurysms were treated by microsurgery at the Department of Neurosurgery of the Institute of Neuroscience of the same university during 1987-1999, of which 42 cases were small aneurysms, 36 cases were large aneurysms, 19 cases were giant aneurysms, and 1 case recurred after GDC tethering. 48 cases were unruptured aneurysms, and 50 cases were ruptured aneurysms. 48 cases were unruptured aneurysms, 50 cases were ruptured aneurysms with preoperative subpial hemorrhage, of which Hunt and Hess graded, 22 cases were graded 1 and 2, 23 cases were graded 3, 4 cases were graded 4, and 1 case was graded 5. 21 cases were operated within 72 hours of rupture of the aneurysm, 17 cases were operated within 4-10 days, 11 cases were operated only in the late stage, and 1 case was operated again at 501 days after SAH due to recurrence after GDC embolization. All surgeries were performed by the renowned Prof. Solomon. The transfemoral point lateral fissure approach was used in 64% of the cases, the temporal pole approach in 36%, and the infratemporal approach was not used, and the orbital-zygomatic craniotomy was combined in 9 out of the 16 cases of basilar artery apex deviation. Results Clamping of the tumor neck and keeping the carrier artery patent was performed in 84 cases (85.7%), wrapping in 9 cases, isolation in 1 case, intraoperative abandonment in 4 cases, and postoperative reintervention in 6 cases. Intraoperative aneurysm neck clamping could not be performed in 5 cases due to the inability to avoid injury to the carrier artery, in 2 cases the carrier artery could not be observed because of the high position of the top of the basilar artery, and in 3 cases the aneurysm involved bilateral posterior cerebral arteries and superior cerebellar arteries. 36 cases (36.7%) presented without complications. The most common surgical complication was injury to the motor nerve (21 cases), but the vast majority recovered after 3 months. The most common long-term complications were injury to the perforating artery (14 cases), as well as cerebral ischemia (8 cases) and hydrocephalus (9 cases) due to subpial retinal hemorrhage. Other disabling complications included large-vessel infarction (3 cases), cerebral traction injury (2 cases), and medically induced arterial entrapment formation by hypothermic circulation (1 case). The percentages of postoperative discharge and self-care at 3 months were 67% and 79%, respectively, and the long-term prognosis was 70% for mild disability and good, and the prognosis of patients with unruptured aneurysms and non-giant aneurysms was better than that of other cases. The annual postoperative subpial hemorrhage rate was 0.18% in all cases with clamping and 0% in cases with complete clamping. In 2005, Krisht (11) reported on a group of 21 cases of surgical treatment of an aneurysm of the apex of the basilar artery; 95% (20/21) of BA aneurysms were successfully clipped, and 1 case could only be wrapped because of the extreme base of the aneurysm, with a perforating artery traveling through the aneurysm wall. 90.5% of the cases had a GOS of 4 or 5 at discharge, 95% had a GOS of 4 or 5 at 1 year after surgery, and one case had a GOS of 4 or 5 at 3 years after surgery. At 1 year after surgery, 95% of cases had a GOS of 4 or 5, and 1 case died of other diseases at 3 months after surgery. Postoperative angiography showed no residual aneurysm in any of the clamped cases. one case had a small thalamic infarct, causing short-term memory impairment, which recovered four months after surgery. All cases had a transient actinic palsy, but all recovered within three to six months. One case of cerebrospinal fluid leak was cured after repair by transnasal butterfly approach. one case developed transient hyponatremia and secondary epilepsy. In China, Shi Xiang’en et al. (14) reported 5 cases of aneurysms of the parietal bifurcation of the basilar artery with the onset of subarachnoid hemorrhage, 4 cases were treated with frontotemporal zygomatic (cutting off the zygomatic arch and enlarging the base of the middle cranial fossa) surgical approach, and 1 case of aneurysm was clipped by a pterygoid point of approach. 4 cases resumed their normal work and life after the operation, and 1 case took care of itself half a year later. Chen Jianliang et al(15) reported 163 cases of intracranial aneurysms clamped by microsurgery,including 3 cases of apical basilar artery aneurysms.When choosing the access route,the site of the aneurysm must be combined.For example,when DSA diagnoses an aneurysm of the basilar artery,the cranium should be left unclipped,and then contrast photographs should be taken,in order to understand the relationship between the aneurysm body and the posterior bed protuberance.If the aneurysm is lower than the tip of the posterior bed protuberance,an inferotemporal access route should be chosen,and if the position of the aneurysm is If the aneurysm is higher than the tip of the posterior bed eminence, then the pterygoid approach can be chosen. The orbital-zygomatico-temporal approach can show the whole cerebral basilar artery ring, optic nerve, arterial nerve, and buccal nerve on the diseased side, and it can deal with all the aneurysms of the basilar artery trunk. In addition, because the basilar artery has more tiny perforating branches into the brainstem, the neck of the aneurysm should be carefully separated before the aneurysm is clipped on the upper aneurysm, and the emphasis on the total freeing of the whole aneurysm tumor is not. In conclusion, with the continuous improvement and innovation of neurointerventional techniques, more and more cases of apical basilar artery aneurysms have been treated by interventional methods, but before the interventional methods completely replace surgical clamping of aneurysms, neurosurgeons should also be fully equipped with the knowledge and skills maintained by surgical treatment of apical basilar artery aneurysms.