The Microsurgical Relationships between Internal Carotid-Posterior Communicating Artery Aneurysms and the Skull Base

Abstract

Objective  This study aimed to review the anatomical and clinical characteristics of internal carotid-posterior communicating artery (IC-PC) aneurysms, especially those located close to the skull base.

Methods  The microsurgical anatomy around the posterior communicating artery (PComA) was examined in a dry skull and five formalin-fixed human cadaveric heads. The clinical characteristics of 37 patients with 39 IC-PC aneurysms, who were treated microsurgically between April 2008 and July 2016, were retrospectively reviewed.

Results  The anterior clinoid process (ACP), as well as the anterior petroclinoidal dural fold (APF), which forms part of the oculomotor triangle, are closely related to the origin of the PComA. Among the 39 IC-PC aneurysms, anterior clinoidectomy was performed on 4 (10.3%) and a partial resection of the APF was performed on 2 (5.1%). Both of these aneurysms projected inferior to the tentorium, or at least part of the aneurysm's dome was inferior to the tentorium.

Conclusion  Proximally located IC-PC aneurysms have an especially close relationship with the ACP and APF. We should be familiar with the anatomical relationship between IC-PC aneurysms and the structures of the skull base to avoid hazardous complications.

Introduction

Internal carotid-posterior communicating artery (IC-PC) aneurysms, which arise from the internal carotid artery (ICA) at the branching of the posterior communicating artery (PComA), are common aneurysms. ¹ Most of them are easily visualized and clipped through a standard pterional craniotomy with an opening just proximal or sphenoidal segment of the sylvian fissure. ²

Endovascular therapy has changed the management of IC-PC aneurysms. Angiographical views of the neck of the aneurysm and parent artery are more easily obtained for IC-PC aneurysms than for anterior communicating artery and middle cerebral artery aneurysms. ³ In addition, IC-PC aneurysms often have features that are favorable for coiling, including easier catheter accessibility, a smaller size, a narrower neck, and the ability to use balloons or stents to facilitate aneurysm packing. ⁴ In the current endovascular era, IC-PC aneurysms that may require microsurgical clipping have a complex morphology, wider necks, larger sizes, and abnormal PComA branching or fetal posterior cerebral arteries (PCAs). ^{4 5}

Occasionally, IC-PC aneurysms have a complex anatomical relationship with the structures of the skull base, including the anterior clinoid process (ACP) and anterior petroclinoidal dural fold (APF). In such cases, a resection of the ACP or APF is required for IC-PC aneurysm clipping, though microsurgical clipping can become hazardous. Only a few studies have been published in the past several decades that focus on the relationships between IC-PC aneurysms and the structures of the skull base. ^{5 6} This serves to refocus our attention on the anatomy of IC-PC aneurysms and their relationship with the structures of the base of the skull.

In this study, we reviewed the anatomical relationships between IC-PC aneurysms and the structures of the skull base, including the ACP and APF. In addition, we also reviewed the clinical features of IC-PC aneurysms that required resection of them for clipping.

Materials and Methods

Cadaveric Dissections

A dry skull and five adult cadaveric heads whose arteries and veins were injected with red and blue colored silicone were examined under an operating microscope at 3 to 40 ×magnifications. The bones were dissected with a high-speed drill.

Clinical Study

We retrospectively examined the preoperative clinical and radiological, as well as operative, findings of IC-PC aneurysms that were treated microsurgically at the Department of Neurosurgery, Kyushu Central Hospital between April 2008 and July 2016. Thirty-seven patients with 39 IC-PC aneurysms were included in this study. This study was approved by the Institutional Review Board at the Kyushu Central Hospital.

Surgical Management

All patients in this study underwent surgery via a pterional approach. The patient's head was turned approximately 30 degrees contralaterally in the supine position, and a frontotemporal craniotomy was performed. The sphenoid ridge was drilled extradurally. When the IC-PC aneurysm located close to the skull base ( Fig. 1A, B ), the ACP or APF obscured exposure of the aneurysm. After opening the dura, the sylvian fissure was opened from lateral to medial to expose the optic nerve, ICA, A1, and M1 ( Fig. 1C, D ). The details of the anterior intradural clinoidectomy procedure are well described elsewhere. ^{7 8} When the neck of the aneurysm was obscured by the APF, resection was performed from the apex of the ACP toward the roof of the cavernous sinus, as previously described ( Fig. 1E, F ). ⁶ The decision to perform the resection was made by more than two trained neurosurgeons (S.M., Y.K., K.M., S.H., and T.I.). A partial resection of the APF provided sufficient space around the proximal neck for clipping ( Fig. 1G–J ).

Fig. 1.

Preoperative CTA, intraoperative photo, and schema showing neck clipping of a right IC-PC aneurysm that projects inferior to the APF. ( A ) CTA surgical view of the right IC-PC aneurysm. The aneurysm located close to the skull base. ( B ) Slightly lateral view from ( A ). The anterior clinoid process has been removed (indicated by blue dotted line). The multilobular aneurysm can be seen. ( C ) Right pterional exposure of the parasellar region. The sylvian fissure has been opened to expose the optic nerve and supraclinoid portion of the carotid, anterior choroidal, and middle cerebral arteries. The APF obscured the dome of the IC-PC aneurysm. ( D ) Schematic drawing of ( C ). ( E ) The APF has been partially resected. Gentle retraction of the ICA provides visualization of the proximal neck of the aneurysm. ( F ) Schematic drawing of ( E ). ( G ) The clip blade is safely passed through which are provided by the partial resection of the APF. ( H ) Schematic drawing of ( G ). ( I ) The aneurysm is clipped with a straight titanium clip. The course of the anterior choroidal artery is seen. ( J ) Schematic drawing of ( I ). A, artery; Ant, anterior; APF, anterior petroclinoidal dural fold; C, clip; Car, carotid; Chor, choroidal; Clin, clinoid; CTA, computed tomography angiography; ICA, internal carotid artery; IC-PC, internal carotid-posterior communicating artery; M.C.A., middle cerebral artery; Petroclin, petroclinoid; PComA, posterior communicating artery; S, suction probe; *, dome of the aneurysm.

Results

Cadaver Dissections

Osseous and Dural Relationships

The osseous and dural structures that are closely involved in IC-PC aneurysms are the ACP and the ligaments that are attached to the process ( Figs. 2 and 3 ). The ACP is located at the medial end of the lesser wing of the sphenoid bone ( Fig. 2A ). The process is a bony projection and is directed posteriorly from the medial side of the lesser sphenoid wing. The border between the process and the lesser wing is undefined. The process is attached to the body of the sphenoid bone at two sites: the anterior and posterior roots of the ACP. ⁹ The anterior root extends medially from the base of the ACP over the optic canal to the body of the sphenoid bone, and forms the roof of the optic canal ( Fig. 2A ). The posterior root, referred to as the optic strut, extends medially below the optic nerve to the body of the sphenoid bone and forms the floor of the optic canal ( Fig. 2A ). The anterior bend of the ICA, which is a part of the intracavernous ICA, courses upward along the posterior surface of the optic strut and becomes the clinoid segment ( Fig. 2A–D ). ¹⁰ The segment lies medial to the ACP and courses upward to become the supraclinoid segment ( Fig. 2B ). A small prominence, which is called the middle clinoid process, is situated medial to the ACP and inferolateral to the tuberculum sellae. Occasionally, an osseous bridge connects the anterior and middle clinoid processes to form a bony canal, called the caroticoclinoid foramen, through which the ICA passes. ^{9 10} The posterior clinoid process is an osseous prominence that is located at the superolateral tip of the dorsum sellae. An osseous bridge, which is called the interclinoidal osseous bridge, may connect the anterior and posterior clinoid processes. ¹¹ These bridges among the anterior, middle, and posterior clinoid processes may make the removal of the ACP difficult. ¹²

Fig. 2.

Superior view of the right parasellar region. ( A ) The parasellar region of a dry skull. The cavernous segment of the ICA courses forward along the carotid sulcus of the sphenoid bone to reach inferomedial to the ACP and posterior to the optic canal, where it becomes the clinoid segment. The middle clinoid process is located at the medial side of the terminal of the carotid sulcus and projects upward. In this case, an osseous bridge extends from the middle clinoid process to the ACP, thus forming an osseous ring around the artery that is called the caroticoclinoid foramen. ( B ) The parasellar region of an injected brain. The right frontal and parietal lobes and part of the temporal bone have been removed to show the relationships between the PComA and surrounding structures. The oculomotor nerve arises from the mid brain and travels below the posterior cerebral artery, medial to the uncus as it courses forward. ( C ) The relationship between the oculomotor nerve and structures of the skull base. The right hemisphere has been removed to show the course of the oculomotor nerve. The nerve passes through the oculomotor triangle, which is formed by the APF and posterior petroclinoidal dural fold and interclinoidal dural fold, and travels inferior to the ACP. ( D ) An enlarged view of the oculomotor triangle. The ACP has been removed to expose the course of the oculomotor nerve. The fetal type of PComA originates from the posterolateral wall of the ICA, where it enters almost directly above the oculomotor nerve into the oculomotor triangle. ( E ) A course of the adult type of the PComA. The PComA courses backward and medially and slightly above and medial to the oculomotor nerve, and joins the PComA. A, artery; Ant, anterior; A.C.A., anterior cerebral artery; ACP, anterior clinoid process; APF, petroclinoidal dural fold; Car, carotid; Chor, choroidal; Clin, clinoid; CN, cranial nerve; Fiss, fissure; For, foramen; ICA, internal carotid artery; Interclin, interclinoidal; Less, lesser; Mid, middle; M.C.A., middle cerebral artery; Oculom, oculomotor; Orb, orbital; Ped, peduncle; Pet, petrous; Petroclin, petroclinoid; Post, posterior; P.C.A., posterior cerebral artery; PComA, posterior communicating artery; S.C.A., superior cerebral artery; Seg, segment; Sulc, sulcus; Sup, superior; Temp, temporal; Tent, tentorium; Triang, triangle.

Fig. 3.

Lateral view of the stepwise dissections of the paraclinoid region and cavernous sinus. ( A ) Lateral view of the right paraclinoid region and cavernous sinus. The hemispheres have been removed to expose the relationship between the PComA and its surrounding structures. As the oculomotor nerve passes through the peduncular cistern, it courses downwardly and inferior to the PComA to enter the cavernous sinus. ( B ) Lateral view of the relationship among the PComA, oculomotor nerve, and APF. The dura lining covering the ACP and middle fossa, and the outer layer of the dura in the lateral wall of the cavernous sinus have been removed to expose the ACP and oculomotor and trochlear nerves. ( C ) Lateral view of the relationship between the PComA and the structures of the skull base. The inner layer of the dura in the cavernous sinus has been removed. Removing the ACP exposes the clinoid segment of the ICA. The oculomotor nerve enters into the cavernous sinus below the origin of the PComA. A, artery; Ant, anterior; ACP, anterior clinoid process; Car, carotid; Cav, cavernous; Clin, clinoid; CN, cranial nerves; Gr, greater; ICA, internal carotid artery; N, nerve; Pet, petrosal; P.C.A., posterior cerebral artery; PComA, posterior communicating artery; Seg, segment.

The anterior end of the tentorium is attached to the petrous apex and anterior and posterior clinoid processes ( Fig. 2C, D ). ¹³ This attachment to the petrous apex and the clinoid processes forms three dural folds: the APF and posterior petroclinoidal dural fold and the interclinoidal dural fold ( Fig. 2C, D ). The APF and posterior petroclinoidal dural fold extend from the petrous apex to the anterior and posterior clinoid processes, respectively. The interclinoidal dural fold extends from the ACP to the posterior clinoid process. A triangle formed by these dural folds is referred to as the oculomotor triangle, through which the oculomotor nerve enters the cavernous sinus ( Fig. 2D ).

Neural Relationships

The neural structure that is closely involved in IC-PC aneurysms is the oculomotor nerve ( Fig. 2B ). The oculomotor nerve arises from the midbrain on the medial aspect of the cerebral peduncle and travels between the PCA and superior cerebellar artery ( Fig. 2B–E ). As it passes forward, it travels slightly downward and anterolaterally to reach the area below the tip of the ACP ( Fig. 3A, B ). Then it penetrates the center of the oculomotor triangle, which is formed by the APF and posterior petroclinoidal dural fold and the interclinoidal dural fold to enter the cavernous sinus ( Figs. 2C, D , and 3 ). The oculomotor cistern, which is an arachnoid sleeve, accompanies the oculomotor nerve through the cavernous sinus roof to the area below the lower edge of the ACP. ¹⁴

Vascular Relationships

The supraclinoid ICA begins where the artery penetrates the dura mater inferomedial to the optic nerve and lies medial to the ACP ( Fig. 2B ). The supraclinoidal segment travels posteriorly, superiorly, and slightly laterally to reach the lateral side of the optic chiasm, and bifurcates at the medial end of the sylvian fissure ( Fig. 2B ). Usually, the PComA branches off from the posteromedial surface of the ICA and travels backward and medial to the oculomotor nerve to join the PCA ( Fig. 2D ). The fetal type of PComA courses further laterally and above or lateral to the oculomotor nerve ( Fig. 2E ).

Clinical Study

Overall, 37 patients (27 women and 10 men) with 39 IC-PC aneurysms were included in this study. The mean age of the patients was 64.1 ± 13.1 years (range: 40–87 years). Most of the aneurysms were ruptured (29 cases, 74.3%). At admission, the patients' Hunt and Hess grades were as follows: grade 1 in 5 cases (12.8%), grade 2 in 15 cases (38.5%), grade 3 in 5 cases (12.8%), grade 4 in 4 cases (10.3%), and grade 5 in 0 case (0%). An oculomotor nerve deficit was present at the time of surgery in five patients (12.8%) and developed during the postoperative period in four (10.3%). The orientation of the fundus of the aneurysm was identified in the coronal plane of preoperative computed tomography (CT) angiography imaging studies. ¹⁵ The most frequent directions of the aneurysms were lateral (16 cases, 41.0%) and inferolateral (16 cases, 41.0%). The least frequent directions of the aneurysms that were seen on the coronal plane were superior and inferior (one case, 2.7%) and inferomedial directions (one case, 2.7%). None of the fundi of the aneurysms was directed medially or superomedially. The fundus of the aneurysm projected above and below the tentorium in 21 (53.8%) and 14 patients (35.9%), respectively. In four patients (10.3%), the fundus of the aneurysm projected against the tentorium, or around the tentorium with lobules, both supra- and infratentorially, as previous reported ( Table 1 ). ²

Table 1. Characteristics of the patients with the IC-PC aneurysm.

Characteristics		All aneurysms ( n  = 39)	No. of aneurysms
			ACP group ( n  = 4)	APF group ( n  = 2)
Age (y)	Mean ± SD	64.1 ± 13.1	57.0 ± 6.7	73.5 ± 13.4
	Range	40–87	58–76	64–83
Female ( n , %)		27 (69.2%)	4 (10.3%)	2 (5.1%)
Dome direction ( n )	Inferolateral	16	2	0
	Lateral	16	1	2
	Inferior	5	0	0
	Inferomedial	1	0	0
	Superior and inferior	1	1	0
Relation to the tentorium ( n )	Inferior	14	4	1
	Superior	21	0	0
	Superior and inferior	4	0	1
Aneurysm size (mm)	Neck × dome (average)	4.4 × 6.5	5.0 × 9.2	3.3 × 4.4
CN III palsy ( n )	Preoperative	5	0	1
	Postoperative	3	2	0
	Permanent	0	0	0
Intraoperative rupture ( n )		3	1	0

Abbreviations: ACP, anterior clinoid process; APF, anterior petroclinoidal dural fold; ACP group, IC-PC, internal carotid-posterior communicating artery; IC-PC aneurysms that required anterior clinoidectomy for clipping; APF group, IC-PC aneurysms that required resection of the APF for clipping; CN, cranial nerve.

Based on the intraoperative findings, the ACP or APF was resected intradurally to obtain sufficient room around the proximal neck for clipping of the aneurysms or proximal control of the ICA. An anterior clinoidectomy was performed in four aneurysms (10.3%, ACP group) because of the very proximal location of the aneurysms (Y.K.: two cases, K.M.: two cases). In two aneurysms (5.1%, APF group), the APF was partially resected to obtain sufficient room around the proximal neck to clip the aneurysm (Y.K.: one case and S.M.: one case). No aneurysms required combined resection of both the ACP and APF. The most frequent direction of the aneurysms in the ACP group was inferolateral (two patients, 50%), while that of the APF group was lateral (two patients, 100%). Both of the aneurysms in the ACP and APF groups projected inferior to the tentorium, or at least part of the aneurysm's dome was located inferior to the tentorium. The average size of aneurysms that required anterior clinoidectomy was larger than that of aneurysms that required resection of the APF. Half of the cases in the ACP group showed transient oculomotor nerve palsy, but none of the patients suffered from permanent oculomotor nerve palsy. No patient presented trochlear nerve palsy before or after clipping of the aneurysms. We experienced three intraoperative aneurysm ruptures in our series, one in ACP group (1/4, 25%) ( Table 1 ).

Discussion

IC-PC aneurysms, both unruptured and ruptured, are one of the most common types of aneurysms, ^{2 16 17 18} and most of them are easily visualized and clipped through a standard pterional craniotomy. ² However, if the aneurysm is located close to the skull base, it has a complex anatomical relationship with the structures of the skull base, such as the ACP and APF. In this study, we briefly reviewed the anatomical relationships between IC-PC aneurysms and the structures of the skull base. Our clinical study demonstrated that infratentorial IC-PC aneurysms and IC-PC aneurysms with at least part of the dome located inferior to the tentorium can require a resection of the ACP and APF.

Anterior clinoidectomy is an inevitable technique for neck clipping of paraclinoid and basilar apex aneurysms. ^{10 12 19 20 21} This technique exposes the canalicular segment of the optic nerve and clinoid segment of the ICA, and facilitates mobilization of the supraclinoid and clinoid segments of the ICA if the upper ring is opened. ^{22 23} Partial intradural anterior clinoidectomy is usually enough to treat IC-PC aneurysms, unlike paraclinoid aneurysms. ^{4 15} In previous reports, 4.3 to 11.0% of IC-PC aneurysms required an anterior clinoidectomy due to proximal vascular control, an unmovable ICA, and limited space for clipping of the aneurysm. ^{4 24 25} Ochiai et al and Park et al examined preoperative angiography scans and reported that a short distance between the ACP and the neck of the aneurysm, as well as a tortuous intracranial ICA path, predicts the need for an anterior clinoidectomy. ^{24 25} The supraclinoid portion of the ICA travels tortuously so that it is hard to measure the real distance and angles of the supraclinoid ICA and IC-PC aneurysms. In addition, the movability of the ICA, which is difficult to predict preoperatively, depends on the severity of atherosclerosis and calcification. Therefore, we should always be prepared to resect the ACP during any surgery to treat IC-PC aneurysms.

The aneurysm size can be used as the preoperative predictor for the need of a resection of the ACP for IC-PC aneurysms clipping. Large IC-PC aneurysms tend to require anterior clinoidectomy. ^{26 27} The average size of aneurysms that required an anterior clinoidectomy was 5.0 × 9.2 mm in this study. According to previous reports, relatively large aneurysms, or those that were larger than 1.0 cm, required anterior clinoidectomy. ^{6 27} Dolenc commented that extradural anterior clinoidectomy is recommended when the largest diameter of the aneurysm is close to 1 cm. ²⁵ We should prepare for resection of the ACP when the size of the aneurysm is larger than 1 cm.

Cerebrospinal fluid leakage, intraoperative aneurysm rupture, and damaging the cranial nerves (CN II and CN III) are considered as major complications related to the anterior clinoidectomy. ²⁸ The intradural technique offers early visualization of the CN II, ICA, and ACP, and this provides an evaluation of the anatomy of the lesion and the amount of bony removal before drilling. Although one of the main advantages of the extradural technique is that the dura mater serves as a protective shield preventing injury of the neurovascular structures including ICA, CN II, and CN III if the drill slips off the bone. Though the risks and benefits of each technique have been well investigated, ^{7 8 29 30} it is impossible to decide which technique is less harmful due to the heterogeneity of the reported patient populations. We think the intradural anterior clinoidectomy may be preferred for IC-PC aneurysm clipping. Intradural techniques allow us tailored ACP removal and can be done within direct sight of the aneurysm. Though either intradural or extradural techniques have much higher risks of intraoperative rupture are controversial, we believe this helps avoid intraoperative rupture. ²⁸ In addition, in our case series, the decision to resect the ACP or APF was done always intraoperatively based on the microsurgical findings, so that it was done intradurally.

Two anatomic variants of bony structures, caroticoclinoid foramen and interclinoid osseous bridge, make an anterior clinoidectomy difficult. ¹² The incidence of caroticoclinoid foramen has been reported from 6.0 to 35.67%. ³¹ There are considerable racial differences in the incidence. ³¹ The interclinoid osseous bridge is less frequent than caroticoclinoid foramen. The incidence of it has been reported from 1.01 to 8.68%. ³¹ We should carefully interpret preoperative CT images to find these anatomic variants because they make the complete removal of the ACP difficult. ^{12 32} In addition, retraction of the ICA without preoperative recognition can cause it to tear. ³² Ota et al reported that intradural anterior clinoidectomy following extradural anterioclinoidectomy provides complete removal of the ACP with the caroticoclinoid foramen and interclinoid osseous bridge. ¹² A hybrid method can make the removal safe. ²⁸ We should be familiar with both intradural and extradural anterior clinoidectomies to avoid complications related to the anterior clinoidectomy.

Similar to the ACP, the APF also obscures the proximal neck of IC-PC aneurysms; however, only a few studies have focused on resection for IC-PC aneurysm clipping. ^{5 6 15} Kim et al reported that resection of the ligament was performed in 6.8% of 117 IC-PC aneurysms. Nossek et al described an APF fenestration technique that was performed for 10.5% of 86 IC-PC aneurysms. ⁵ González-Darder et al performed resection for 15.0% of 84 IC-PC aneurysms. ¹⁵ We performed resection in 5.1% of patients with IC-PC aneurysms. Accordingly, the frequency of performing resection of these aneurysms seems almost equal to that of anterior clinoidectomy.

The characteristics of aneurysms requiring resection of each structure seem to be different. Kim et al reported that aneurysms requiring resection of the ACP or APF mainly projected posteroinferiorly. ⁶ Nossek et al speculated that patients with small- to medium-sized aneurysms with wide necks that project inferior to the tentorium or have a more lateral orientation and lie in the direction of the tentorium may need to undergo resection of the APF. ⁵ The relationships between IC-PC aneurysms and the tentorium, as well as the direction of the projection of the dome, predict the need for resection of the structures of the skull base. Based on our small case series, no supratentorial IC-PC aneurysm required anterior clinoidectomy or resection of the APF. Though the number of cases in our study was small, we can speculate that infratentorial aneurysms and at least part of the aneurysm's dome were located below the tentorium, and aneurysms that project in a direction other than medial may require resection of the AC or APF. Regarding the size of the aneurysm, relatively small aneurysms, even those that are 3 mm, ³³ required resection of the APF.

The preoperative predictors for need of the resection of the APF for IC-PC aneurysm clipping have not been described very well. It is apparently difficult to predict the need for resection of the APF. As the ligamentous structures rarely ossify, evaluating the relationship between IC-PC aneurysms and these structures on preoperative CT images can be difficult. ³⁴ Heavily weighted T2 magnetic resonance images (MRI) are potentially valuable for identifying the relationships between IC-PC aneurysms and the surrounding structures. ³⁵ However, subarachnoid hemorrhage may weaken the contrast between the cerebrospinal fluid and the nerves coursing through the cistern, and the condition of patients with subarachnoid hemorrhage may not allow performing MRI scan.

The procedure for resection of the APF for IC-PC aneurysm clipping has not been focused on in previous studies. Ozawa et al performed the resection from immediately below the tip of the ACP and exposed the oculomotor nerve that coursed through the oculomotor cistern. ³³ Then they opened the oculomotor cistern backward to the oculomotor porus, along the lateral side of the nerve. ³³ Kim et al reported using a similar technique, proposing that this technique can substitute for anterior clinoidectomy in selected cases. ⁶ Nossek et al presented a fenestration of the APF technique. ⁵ They resected only part of the APF that was adjacent to the proximal neck of the aneurysm, and reported that this procedure can minimize the risk of intraoperative rupture, oculomotor nerve damage, and bleeding from the cavernous sinus. ⁵ We believe that partial resection of the APF provides enough space around the proximal neck of the IC-PC aneurysm. The oculomotor cistern is cone shaped; its greatest diameter is at the oculomotor porus and tapers toward its anterior end. ¹⁴ The oculomotor cistern has a maximum diameter of 5.5 ± 1.1 mm and maximum length of 6.5 ± 1.5 mm. ¹⁴ Opening the cistern from the oculomotor porus and dividing the APF along the lateral surface of the oculomotor nerve until it enters the cavernous sinus, as reported by Takami et al, can minimize the risk of damage to the oculomotor nerve. ³⁶ Therefore, we can open the cistern ∼5 mm from the entry point without opening the cavernous sinus. However, patients with subarachnoid hemorrhage usually experience brain swelling; therefore, resecting the APF following the cisternal part of the oculomotor nerve can be difficult. Occasionally, we have no choice but to begin resection of the APF from near the tip of the ACP. The oculomotor nerve travels close to the tip of the ACP, and care should be taken to avoid damaging the nerve. ⁹

Resection of the ACP and APF can cause neurovascular complications. As shown, cadaver dissection, the ICA and optic, oculomotor, and trochlear nerves located close to the ACP and APF, and have a potential risk to be damaged during resection. Based on our result, resection of the ACP has a higher risk of damage to these neurovascular structures. Kim et al speculated that resecting APF can be a partial alternative to anterior clinoidectomy and can decrease the injuring of the ICA and optic nerve, which were expected during anterior clinoidectomy. ⁶ We advocate, as Nossek et al reported, partial resection of APF is more useful technique for clipping the IC-PC aneurysm. However, as mentioned earlier, aneurysms requiring the resection of the ACP and APF have different characters so that we should be familiar with both techniques.

Conclusion

Proximally located IC-PC aneurysms have close relationships with the ACP and APF, which occasionally need to be resected to obtain proximal control of the ICA and/or improve visualization of the proximal neck of the aneurysm. Although resection of these structures is infrequently needed, all neurosurgeons who treat IC-PC aneurysms microsurgically should be familiar with the resection of them because these procedures have potential to cause hazardous complications due to damaging the ICA, premature rupture, or damaging the oculomotor nerve. A precise understanding of the anatomical relationship between IC-PC aneurysms and the structures of the skull base will make the surgery more accurate and safe.