The 23 patients treated for BA trunk aneurysms during the specified period comprised the study group (Table 1). The average age was 51.2±10.7 years, ranging from 31 to 73 years old. The cohort included 16 women (69.6%) and 7 men (30.4%).

Of these patients, 6 (26.1%) had multiple aneurysms. In cases of multiple aneurysms, the bleeding in 2 patients was caused by another aneurysm. Except for one, all patients with multiple aneurysms underwent one-stage surgery—microsurgically addressing all aneurysms in a single operation. All surgeries were primary, with none of the patients having undergone any previous endovascular or microsurgical intervention for their aneurysms.

In 19 patients (82.6%), the operation was performed in the acute phase of hemorrhage (Fig. 1), while 2 patients (8.7%) with unruptured aneurysms presented with oculomotor nerve dysfunction prior to surgery, and 2 patients (8.7%) had an asymptomatic disease course.

In total, 9 patients (39.1%) experienced oculomotor nerve dysfunction prior to surgery due to the mass effect from the distal BA trunk aneurysm. Of these, 7 had ruptured SCA aneurysms, and 2 were unruptured (Fig. 2).

In the majority of patients (87.0%), aneurysms were located in the distal third of the BA trunk, specifically at the origin of the SCA. In 3 patients (13.0%), the aneurysms were in the middle third of the BA trunk. The average size of the aneurysms was 7.5±3.5 mm (range 3-15 mm). All aneurysms were saccular. Perforating branches adhering to the aneurysm, emerging from its neck, were identified intraoperatively in 12 cases (52.2%).

In the vast majority of cases involving aneurysms at the SCA origin, a unilateral orbito-pterional approach was used (90.0%), with a pterional approach used in only two patients (10.0%). For aneurysms in the middle third of the BA trunk, a retrosigmoid approach was used in two instances (Fig. 3), and an orbito-pterional approach in one case. Endoscopic assistance was used in one case (Fig. 4). The choice of approach side was based on the aneurysm’s location, with access from the non-dominant hemisphere for aneurysms situated medially and proximally.

In 9 patients (39.1%), either intra- or extradural anterior clinoidectomy was performed to increase the surgical corridor and facilitate greater mobilization of internal carotid artery (ICA). Posterior clinoidectomy was more common, performed in 12 cases (52.2%). It also served to enlarge the surgical corridor, allow proximal control of the BA, enhance visualization of perforating branches, and increase aneurysm mobility. In several cases, posterior clinoidectomy was combined with sectioning of the tentorial edge (43.5%). In one patient with a middle third BA trunk aneurysm, partial resection of the dorsum sellae was performed alongside the posterior clinoid process (Fig. 5).

Temporal polectomy to enlarge the retro-carotid space was performed in 5 patients (21.7%), including one with a middle third BA trunk aneurysm (Fig. 6). In all cases, the resection was performed on the non-dominant (right) side, with its posterior boundary located 1.5-2.0 cm from the temporal pole.

Additionally, in 4 patients (17.4%), the posterior communicating artery was sectioned in an area free of perforating branches to further mobilize the ICA.

Intraoperative rupture of the aneurysm was observed in 4 patients (26.7%). In all cases of intraoperative rupture, bleeding was controlled with temporary clipping of the BA proximal to the SCA origin.

In all cases, standard techniques for clipping the aneurysm neck were used. Deconstruction of the arteries and revascularization methods were not employed in this series.

The median follow-up duration was 5.5 months (range: 3–18 months).

According to CT angiography data from the first day post-operation, all aneurysms were completely occluded. The patency of the arteries (BA, SCA, posterior cerebral artery (PCA)) was also preserved according to the angiographic findings.

Post-surgery, new oculomotor nerve dysfunction was noted in 5 patients (35.7%). In one of these patients, partial dysfunction persisted at the 6-month follow-up visit (4.3%). In all patients who had oculomotor nerve dysfunction before surgery due to mass effect, complete restoration of function was observed at follow-up. In one patient (4.3%), significant subcutaneous accumulation of cerebrospinal fluid was noted, which resolved after the placement of a lumbar drain. Another patient (4.3%) experienced worsening condition due to pronounced cerebral vasospasm, resistant to intra-arterial administration of nimodipine. Thus, the rate of persistent neurological deficit directly related to the surgery was 4.3% (partial oculomotor nerve dysfunction). There were no other surgical complications that led to neurological deterioration–such as brain contusion, intracranial hematoma, venous infarction, or surgical site infection.

Favorable clinical outcomes (mRS 0-2) were observed in 20 patients (87.0%). In this series, there were 2 fatal outcomes: one patient experienced a deterioration in condition after surgery due to progressive vasospasm, and the second patient initially presented in a severe condition due to massive subarachnoid hemorrhage (Hunt-Hess V).

Dynamic angiographic monitoring was available for 19 patients (82.6%). In these patients, the complete occlusion of the aneurysms was maintained without signs of recanalization.

The main drawbacks of endovascular treatment include lower occlusion rate, higher recanalization risk, particularly relevant in the acute phase of SAH, especially when using stent-assistance or flow-diverters that require dual antiplatelet therapy (DAT).

Microsurgical occlusion of saccular BA trunk aneurysms is reported in seven publications (Table 2). Reported occlusion rates for microsurgery range from 81.8% to 100% in published studies [2,6,9].

Rodríguez-Hernández et al. in their study described the results for 62 patients with SCA origin aneurysms, the largest series reported [9]. Reasons for opting for microsurgery, according to the authors, included factors like large aneurysm size, intraluminal thrombi, calcification in the aneurysm wall, unsuccessful embolization attempts, perforating branches emanating from the aneurysm, and fusiform or blister-like aneurysm shapes. The most common reason for incomplete occlusion is reported to be the emergence of perforating branches from the aneurysm neck or directly from the BA trunk. In Iizuka et al.’s work, one patient had a subtotally occluded aneurysm that showed an increase in the residual part under dynamic control, necessitating a second open intervention to re-clip the aneurysm [2]. In Jin et al.’s series, two patients had incomplete occlusion detected on follow-up angiography: one subcomplete and one partial. During dynamic follow-up, functioning remnants were stable, leading to continued observation [3]. Also, in 5 patients (8.3%) in the largest series presented by Rodríguez-Hernández and colleagues, subcomplete occlusion of the aneurysm was observed: in one case, the neck of the aneurysm was intentionally left unoccluded to preserve blood flow in the SCA, while in four patients, neck remnants were only identified on follow-up angiography [9].

Endovascular techniques used for SCA origin aneurysms and aneurysms of the BA trunk itself differ. In proximal BA trunk aneurysms, coil embolization without stent-assistance, which avoids the necessity for DAT, is the least commonly used according to literature—1.8-14.3%. Due to the peculiarities of vascular anatomy and aneurysm shape, stent-assisted embolization is more frequently employed—76.6-85.7%, with flow-diverting stents being used less frequently—15.0-21.6%.11-13) Immediate postoperative occlusion rates were 52.3-82.1% [12,13].

Conversely, in aneurysms at the origin of the SCA, embolization without the use of stents was much more commonly employed – in the largest series, it was used in 56.5-90.7% of cases [1,4]. In the series by Kim and colleagues, complete occlusion of aneurysms was achieved in 37.7% of cases, and subcomplete in 47.2% [4]. In the study by Acik and colleagues, complete and subcomplete occlusion of the aneurysm was observed in 89.4% of cases at long-term angiographic follow-up [1].

In our cohort, no ischemic injuries due to surgical manipulation were observed, and oculomotor nerve dysfunction—while common transiently—persisted in only one patient. Ischemic complications associated with clipping, in microsurgical series with ten or more patients, occurred relatively infrequently (0.0-16.7%). Such events included small ischemic areas in the territory of the perforating branches (small foci in the brainstem) and the main trunk of the SCA (large foci in the cerebellum and partially in the brainstem). In the series by Iizuka and colleagues of ten patients, one patient (10.0%) experienced cerebellar ischemia resulting from occlusion of the SCA trunk, which led to persistent neurological deficit and an unfavorable clinical outcome (mRS 3). Ischemia in the SCA territory was also noted in one patient in Nakagomi’s series after clipping a large, partially embolized aneurysm: the authors suggest that the cause might have been the slippage of the clip toward the neck, causing stenosis of the BA and the origin of the SCA, as the removal of coils was not performed during the operation [7]. Damage to the PCA trunk with subsequent development of an ischemia in the occipital lobe was noted in one patient (8.3%) in the series by Jin and colleagues [3]. Ischemia in the caudal sections of the brainstem leading to hemiparesis post-operation due to occlusion of a perforating branch was noted in one patient (1.6%) in the series by Rodríguez-Hernández and colleagues [9]. In the work of Nair and colleagues, one of 14 patients (7.1%) experienced a complication related to access to the SCA aneurysm: the patient developed a widespread venous infarct due to significant traction of the temporal lobe using a subtemporal approach, ultimately resulting in the patient’s death [6].

Complication rates after endovascular treatments of saccular aneurysms of the BA trunk itself ranged from 12.5-35.7%, most commonly involving ischemia in the territory of the perforating branches, less frequently severe ischemia due to BA thrombosis. In some cases, dysfunction of the abducens nerve was noted due to increasing mass effect associated with the aneurysm. Additionally, some cases reported rebleeding due to incompletely occluded aneurysms, recanalization, or under DAT [12,13]. In Zhong et al.’s work, delayed ischemic complications were also reported due to premature discontinuation of DAT [13].

At the same time, ischemic complications occur less frequently with SCA aneurysms. In the studies by Acik and Kim, isolated cases of ischemia in the territory of the PCA resulting from thromboembolism or due to premature discontinuation of DAT are described, which in one case led to a visual field defect, but nonetheless did not significantly reduce the functional status of the patients [1,4].

Bypass techniques were seldom applied in the reported series, typically characterized by individual clinical observations, thus making it challenging to conclusively evaluate the effectiveness and risks of these interventions. Only in study of Rodríguez-Hernández et al. are two cases described: one patient had an STA-SCA bypass, and another had a reimplantation of the anterior temporal artery into the SCA [9]. Both cases involved bypasses as a precautionary measure. There were no complications after these operations, and all anastomoses were patent according to the authors.

The main limitations of this study are its retrospective nature and the relatively small number of patients with proximal BA trunk aneurysms. This study is also a subject to selection bias, as patients were selected for microsurgical treatment based on aneurysm morphology, size, clinical condition, and multidisciplinary judgment, rather than random allocation. As a result, the cohort may not represent the full spectrum of basilar artery trunk aneurysms, particularly those more amenable to endovascular therapy. These factors limit the generalizability of our findings and preclude direct comparison with endovascular outcomes. Nevertheless, we present a large sample of patients with this rare vascular pathology of the brain, emphasizing the surgical nuances within illustrative clinical observations. This study showcases the high efficacy of microsurgical treatment of aneurysms, which are traditionally treated endovascularly. Nevertheless, certain morphological factors make clipping a more advantageous method in specific cases, such as a broad aneurysm neck, very small aneurysm size (less than 3 mm), acute hemorrhagic period combined with the necessity for DAT in endovascular treatment, and the presence of multiple aneurysms that can be occluded in one microsurgical session.