26 Surgical Management of Posterior Circulation Aneurysms
Abstract
Posterior circulation aneurysms account for less than 10 to 16% of all intracranial aneurysms. Open microneurosurgical treatment of these lesions has always been challenging because of their close relationship to sensitive neuroanatomical structures. In this chapter, we review microsurgical approaches and techniques for complex aneurysms in the posterior circulation according to their location in the vertebrobasilar arterial system. Although individual surgical experience with posterior circulation aneurysms is declining as a result of improvements in endovascular techniques, the senior author (J.H.) of this chapter has treated more than 1,650 posterior circulation aneurysms, and the contents of this chapter are based on his personal experience.
Surgical Management of Posterior Circulation Aneurysms
Posterior circulation aneurysms account for 10 to 16% of all intracranial aneurysms. Open microsurgical treatment of these lesions has always been challenging because of their close relationship to sensitive neuroanatomical structures. 1 , 2 , 3 , 4 During the last two decades, microsurgical treatment of these lesions has shifted mostly to endovascular strategies because of improvements in modern endovascular techniques. This shift has led to a decline in the surgical treatment of posterior circulation aneurysms. Despite endovascular advances, select complex lesions continue to require microsurgical treatment. Additionally, in the developing world, the prohibitive cost of endovascular technologies has resulted in the continued need for microsurgical expertise.
Posterior circulation aneurysms often require complex skull base approaches because of the shape of the cranium, the narrowed basal cisterns, the proximity to cranial nerves (CNs), and the often tortuous course of the parent vessel. In general, these operations are performed in deep and narrow surgical corridors. Therefore, the microsurgical treatment of posterior circulation aneurysms cannot be uniformly described.
In this chapter, we will present special anatomical features of aneurysms and structural anatomical relationships in the subsections on each aneurysm position. We will also present classical microsurgical approaches and procedures as well as treatment strategies for complex aneurysms according to their location in the vertebrobasilar arterial system ( Fig. 26.1 ).
Surgical Approaches to Posterior Circulation Aneurysms
Upper Basilar Segment Aneurysms
Aneurysms of the upper basilar artery consist of aneurysms arising from the basilar bifurcation, posterior cerebral artery (PCA), superior cerebellar artery (SCA) junction, and proximal P1 segment of the PCA. 5 Some of the major surgical routes to treat these aneurysms include the subtemporal approach; the pterional transsylvian approach, with all its surgical variations; and the temporopolar or “half-and-half” approach. 6 , 7 , 8
An important landmark when choosing a surgical approach for upper basilar artery aneurysms is the relationship between the aneurysm neck and the posterior clinoid process. Aneurysms located 5 to 6 mm above the posterior clinoid process can be treated through a pterional transsylvian approach. However, when the aneurysm neck is not higher than 6 mm and not lower than 8 mm from the posterior clinoid process, a subtemporal approach is an appropriate surgical option. In the following sections, we will describe these surgical approaches and their modifications for upper basilar artery aneurysms. 9 , 10
Subtemporal Approach
In 1954 and 1959, respectively, Olivecrona and Drake described the subtemporal approach to access basilar artery aneuryms. 9 , 11 Subsequently, this approach was also adopted for PCA aneurysms. Since the first description of the subtemporal craniotomy, this approach has undergone several modifications. 12 , 13 The subtemporal approach offers a good exposure and visualization of the middle fossa floor and the interpeduncular space. Mainly used for the treatment of upper basilar artery aneurysms, this approach allows additional exposure of the proximal P2 segment. 7 , 14
Positioning
The patient is placed in the lateral park bench position, with the head fixed to the Sugita or Mayfield frame in a neutral position. The head is slightly elevated above the cardiac level, and the upper shoulder is retracted backward and caudally. During the subtemporal approach, cerebrospinal fluid (CSF) should be released via a lumbar drain, which is placed after positioning, with approximately 50 to 100 mL of CSF drained before the dura is opened. This maneuver allows for the brain relaxation that is necessary to avoid excessive retraction of the temporal lobe.
Skin incision
A horseshoe-shaped skin incision is made, starting 1 cm in front of the tragus, just above the zygomatic arch ( Fig. 26.2a ). The skin incision runs cranially, approximately 6 to 8 cm, and then curves posteriorly around the earlobe, reaching a line between the porion and the asterion. The skin is opened in a myocutaneous fashion and retracted caudally, using the Sugita frame spring hooks, which provide a strong retraction force. The monopolar cautery is used to detach the temporalis muscle, exposing the zygomatic arch and the suprameatal spine. During dissection, it is important to preserve and leave the external auditory canal intact because the skin around this area is very thin ( Fig. 26.2b ).
Craniotomy
A single bur hole is performed at the most cranial and superior end of the skin flap; an additional and optional bur hole can be placed just above the zygoma ( Fig. 26.2c ). This particular bur hole is used to detach the dura mater and place a bypass when necessary. Once the dura is detached from the bone, a craniotomy of approximately 4 to 5 cm in diameter is obtained.
The first cut with the craniotome starts from the first bur hole and is directed anteriorly and caudally to the base of the middle fossa. The second cut is made posteriorly toward the floor of the middle fossa. Finally, the bone is thinned down with the craniotome along the end of the previous two cuts, and then the bone flap is lifted and cracked ( Fig. 26.2d ). Multiple tack-up holes are drilled around the craniotomy to suspend the dura and prevent the formation of epidural hematoma. 15 The craniotomy can be widened using a diamond drill in the temporobasal direction, exposing the middle fossa floor. 7 , 13
Intracranial dissection
The dura is opened in a curvilinear manner, with the base directed caudally, and the dura edges are elevated over the craniotomy with multiple tack-up sutures. The main goal of the subtemporal approach is to reach the tentorial edge quickly, without causing damage or excessive compression to the temporal lobe. The mobilization of the temporal lobe starts anteriorly on the temporal pole and proceeds posteriorly and across the caudal surface, avoiding abrupt retraction of the middle portion of the temporal lobe because of the risk of tearing the vein of Labbé, which can lead to temporal lobe swelling and venous infarction. The spinal drain can be closed at this time. Once the temporal lobe is mobilized and the tentorial edge is visible, a wide retractor is placed to elevate the uncus, exposing the interpeduncular cistern and the oculomotor nerve (CN III). CN III can be mobilized by cutting the surrounding arachnoid membrane adhesions, although a higher risk of CN III palsy exists with even minimal manipulation of the nerve. In some circumstances, even with the retraction of the uncus and mobilization of CN III, the interpeduncular cistern space remains narrow. This problem can be resolved by placing a small, straight miniclip on the tentorial edge, just at the insertion and intradural course of the trochlear nerve (CN IV), allowing upward retraction of the tentorial edge ( Fig. 26.3 ). 13 If a wider exposure is required, the tentorium can be divided by performing a perpendicular cut posterior to the insertion of CN IV; this cut should not be more than 10 mm long. The tentorial flap is then fixed with straight aneurysm clips, increasing the surgical exposure of the upper basilar artery. 1 Venous bleeding from the tentorial cut can be stopped by injecting fibrin glue into this small opening. The dissection then continues, depending on the vascular segment to be treated. For low-lying basilar artery aneurysms, it is mandatory to split the tentorium. Careful preoperative planning is the key when deciding whether to split the tentorium to increase the surgical exposure ( Fig. 26.4 ).
Frontotemporal Approach
The approaches to the upper basilar artery have been grouped together to include the subtemporal approach, the pterional transsylvian approach, and the temporopolar or half-and-half approach. We have combined the pterional transsylvian and temporopolar approaches into the frontolateral or frontotemporal approaches, since both approaches follow a similar surgical route. However, slight intradural differences in these surgical approaches will be explained in more detail in the following sections.
The frontolateral or frontotemporal approaches include the pterional, the lateral supraorbital, the extended lateral supra-orbital, the anterior temporal, the temporopolar, and the orbitozygomatic approaches. 6 , 10 , 16 , 17 , 18 , 19 These approaches have been widely used to access aneurysms of the basilar bifurcation, proximal P1 segment, and SCA. The main objectives of the frontolateral approaches are to reduce retraction of the temporal lobe, to reduce damage to CN III and CN IV, to provide better exposure of the interpeduncular cistern anatomy, and to provide exposure necessary to treat other concomitant anterior circulation aneurysms.
Positioning, skin incision, and craniotomy
For the frontolateral approaches, the patient is placed in the supine position with the head rotated 15° to 30° toward the contralateral side, with the degree of positioning different for each approach ( Fig. 26.5 ). Frontolateral approaches are performed using a curvilinear frontotemporal skin incision. The length and extension of the skin incision vary, depending on the amount of exposure required. Table 26.1 summarizes the positioning, skin incision, and further craniotomy details of these approaches.
Intracranial Dissection (Pterional Transsylvian)
The dura is opened in a semicircular fashion, and multiple tack-up sutures are placed from the dura toward the craniotomy edges to prevent further epidural bleeding. The sylvian fissure is opened by sharp dissection, starting at the level of the pars opercularis of the frontal gyrus and following the middle cerebral artery (MCA), until complete exposure of the internal carotid artery (ICA) bifurcation is obtained. The superficial sylvian veins should be carefully detached from the frontal region toward the temporal cortex to increase surgical exposure. The arachnoid adhesions covering the opticocarotid (chiasmatic) cistern, carotid cistern, and Liliequist’s membrane are cut to obtain a wide exposure and to identify the posterior communicating artery (PCoA). The PCoA is followed posteriorly toward its junction with the ipsilateral PCA (P1–P2 segment). The basilar artery and its bifurcation are approached from an anterolateral direction between the ICA and the MCA. The dissection may continue laterally to the PCoA or medially to its perforators. Additionally, an alternative route, medially through the ipsilateral opticocarotid triangle, can be used as originally described by Yaşargil et al. 20 However, this route is not frequently required. In special circumstances, such as in the case of a low-lying basilar artery aneurysm, a wider exposure can be obtained through a pterional transcavernous route or through a pretemporal transcavernous transzygomatic route. 18 , 19 The transcavernous approach, originally described by Dolenc et al, 21 represents an expansion of the pterional approach. After a pterional approach is performed, the sphenoid wing is drilled off, from lateral to medial, until reaching the anterior clinoid process. Next, the superior orbital fissure is unroofed, exposing the meningo-orbital fold and the meningo-orbital artery. The meningo-orbital artery is subsequently coagulated and cut, allowing stripping of the temporal dura propria from the lateral wall of the cavernous sinus. Additionally, the superior and lateral walls of the orbit are drilled off to increase surgical exposure, while preserving the periorbita. An extradural anterior clinoidectomy is then performed, and the dura is opened in a T-shaped fashion, with the vertical arm of the T following the sylvian fissure and the indentation of the sphenoid wing. 6 , 7 , 10 , 14 The dural cut extends all the way down to the entrance of CN III and into the oculomotor triangle. This maneuver helps to further mobilize CN III and exposes the interpeduncular fossa and the posterior clinoid process. Then, if drilling of the posterior clinoid process is necessary, it can be performed. For high-lying upper basilar artery aneurysms, an orbitozygomatic or an anterior temporal approach with zygomatic arch trans-location enhances the subtemporal exposure of the middle fossa, providing a shallower depth of field to the temporal region and the upward trajectory for aneurysm exposure and dissection.
Intracranial Dissection (Anterior Temporal Approach)
Similar to the pterional transsylvian approach, the anterior temporal approach requires, in its initial stages, a wide sylvian fissure dissection, allowing complete exposure of the M2 segments of the MCA and the supraclinoid portion of the ICA. As previously mentioned, the superficial sylvian veins should be detached and mobilized from the frontal cortex toward the temporal cortex, following the dissection until the entrance of the veins into the sphenoparietal sinus. The mobilization of the superficial sylvian veins represents the key point of the anterior temporal approach, allowing safe posterior and medial retraction of the temporal pole. Posterior retraction of the anterior temporal pole allows for visualization of the PCoA, anterior choroidal artery, and PCA (P1) ( Fig. 26.6 ). Additionally, a lateral surgical trajectory can be obtained through the anterior temporal approach by retracting medially and elevating the anterior temporal pole from the middle fossa. This step requires the opening and sharp dissection of the arachnoid bands surrounding CN III and the ambient cistern until the PCA is visible.
Vertebrobasilar Aneurysms
Presigmoid Approach (Posterior Petrosal Approach or Combined Supratentorial Infratentorial Approach)
This approach is considered one of the most difficult approaches in neurosurgery. It was originally described for ventral brain-stem lesions and clival tumors, but its indications expanded to vascular lesions inaccessible through the traditional subtemporal, pterional, or retrosigmoid routes. The presigmoid approach offers a combined exposure of the middle and posterior fossa, as well as good visualization of the midbasilar segment. In our practice, this approach has been used to gain access to low-lying basilar tip aneurysms and basilar trunk (BT) aneurysms. 10 , 14 As described and refined by Hakuba et al 22 and Al-Mefty et al, 23 the modified presigmoid approach requires a partial labyrinthectomy to reduce hearing loss, and it requires complete mobilization and skeletonization of the sigmoid sinus after division of the tentorium and superior petrosal sinus. 10 , 14
Positioning
The patient is placed in the lateral park bench position, similar to the position for the subtemporal approach. As previously mentioned for the subtemporal approach, lumbar drainage or a ventriculostomy is necessary to obtain proper relaxation of the brain before proceeding with the approach ( Fig. 26.7 ).
Skin incision
A horseshoe-shaped skin incision, similar to the incision used in the subtemporal approach, is marked down, starting 1 cm anterior and superior to the root of the zygoma, directed upward and curving posteriorly 2 to 3 cm over the ear, before stopping 2 cm behind the mastoid line. A myocutaneous one-layer flap is performed and retracted caudally, using multiple spring hooks. The temporal and occipital muscles are detached caudally, completely exposing the temporal bone, the zygomatic arch, and the mastoid process.
Craniotomy
The craniotomy is made using three or four bur holes. The first one is at the most cranial part of the planned skin incision, the second one is just above the zygomatic arch, and the third one is at the posterior border of the skin incision, inferior to the transverse sinus projection. The fourth and optional bur hole is placed just superior to the expected course of the trans-verse sinus. This bur hole is helpful to detach the dura from the inner table and to reduce the risk of injury to the venous sinuses.
A first cut is performed with a craniotome, starting at the most cranial bur hole and directed toward the zygomatic bur hole. The second cut begins at the posterior fossa bur hole and is directed anteriorly and superiorly toward the first bur hole ( Fig. 26.7 ). The third cut starts at the zygomatic bur hole and is directed posteriorly toward the anterior aspect of the petrous bone. The remaining bone ridge is drilled off using a diamond drill, and the bone flap is cracked and lifted around this drilling line. Once the craniotomy is performed, an adequate exposure will demonstrate the transverse sinus, the dura of the posterior and middle fossa, and the sigmoid sinus.
Temporal bone drilling
Under the operating microscope, the squamous temporal bone is drilled off with a diamond drill to obtain an adequate supratentorial surgical corridor with minimal retraction of the temporal lobe. The drilling continues to the superior and posterior segment of the mastoid region of the temporal bone, increasing the sinodural angle exposure. The dura anterior to the sigmoid sinus is exposed, as necessary, and the drilling stops at the level of the antrum, without compromising the elements of the inner or middle ear. A posterior petrosectomy, including skeletonization of the semicircular canals, can be performed to reduce the risk of hearing loss. If a semicircular canal is inadvertently opened, it must be sealed off with bone wax, fibrin glue, fat, or muscle graft.
Dural opening
After the partial posterior petrosectomy is completed, the sigmoid sinus, the superior petrosal sinus, the presigmoid dura, and the temporal dura should be visible. The posterior fossa dura is opened under the microscope, just a few millimeters anterior to the sigmoid sinus. The opening is directed toward the superior petrosal sinus, which at the initial phase is left intact. The middle fossa dura is cut in a curvilinear fashion and directed toward the superior petrosal sinus. The superior petrosal sinus is then ligated using two sutures, and both previous cuts are connected, dividing the sinus and allowing for lifting of the dura by traction of the sutures.
Cutting the tentorium
The tentorium is cut from lateral to medial, anterior to the vein of Labbé and posterior to the tentorial insertion of CN IV ( Fig. 26.8 ). The retraction of the tentorium is performed subfrontally, and the splitting of the tentorium is conducted in a stepwise manner, starting with a small lateral cut on the tentorium and followed by bipolar coagulation to reduce the risk of bleeding. These two steps are repeated constantly, verifying supratentorially and infratentorially the course of CN IV, until the tentorium is completely split. The free flaps of the tentorium can be fixed to the dura of the temporal fossa using small aneurysm clips. 13
Vertebral Artery and Posterior Inferior Cerebellar Artery Aneurysms
The most frequent approaches to access vertebral artery (VA) or VA and posterior inferior cerebellar artery (VA-PICA) aneurysms are the far lateral and the lateral suboccipital or retrosigmoid approaches. 11 , 24 , 25 , 26 There are two main parameters to consider when selecting an approach to aneurysms in these segments. The first is the relationship of the aneurysm with the foramen magnum; those at least 10 mm above the foramen magnum can be approached through a retrosigmoid craniotomy. The second is the size and projection of the aneurysm. 24 , 25 , 26 Aneurysms located on the cortical PICA branches close to the mid-line require a median or paramedian suboccipital approach.
Far Lateral Approach
Most surgeons have favored and widely used the far lateral approach for VA-PICA aneurysms. Originally described by Heros, 27 this approach has undergone different modifications, including supracondylar, transcondylar, or paracondylar variants. Compared with the classic far lateral approach that requires removal of the posterior arch of C1 and almost complete drilling of the occipital condyle, this “enough lateral approach” is a fast and simpler modification, where the amount of condyle drilling is minimal and a hemilaminectomy of C1 is performed only when necessary. 24 , 25 , 26 , 28
Positioning
The patient is placed in the lateral park bench position with the head elevated approximately 20 cm above the cardiac level. The head is fixed to the frame, slightly flexed forward and laterally tilted toward the floor. This positioning increases the viewing angle toward the foramen magnum in a caudal trajectory ( Fig. 26.9 ).
Skin incision
A linear skin incision is marked 2 cm behind the mastoid process, starting just below the zygomatic line and extending 4 to 5 cm caudally to the mastoid tip. The subcutaneous fat and muscles are divided in a linear fashion using monopolar cautery. A self-retaining retractor is placed cranially, and then a second one is placed caudally. The muscle dissection continues until complete exposure of the occipital bone is obtained. The posterior arch of C1 and the foramen magnum are identified by finger palpation. At this point, the approach is performed under the operating microscope. The main objective is to identify the extradural course of the VA close to the transverse process of C1. This can be done using a micro-Doppler ultrasound to localize the artery. The idea is to expose the extradural segment of the VA above the posterior arch of C1 and its intradural entrance at the foramen magnum. Once the VA and C1 are completely identified, the occipital bone can be safely cleaned from attached muscles, all the way down to the foramen magnum.
Craniotomy
One bur hole is placed at the superior and posterior aspect of the exposed bone. Then a first cut with the craniotome is performed, directing slightly superior from the bur hole toward the mastoid as far as possible. A second cut directs slightly posterior and caudally toward the foramen magnum and as posterior as to where the VA makes its intradural entrance. Then, without footplate protection or a diamond drill, the craniotome is used to thin down, lift, crack, and remove the bony ridge at the anterior and lateral aspect of the planned craniotomy. Venous bleeding from the paravertebral venous plexus can follow elevation of the bone flap. Elevating the head and packing with hemostatic agents and fibrin glue will readily control the bleeding.
After the craniotomy is performed, the operating table is elevated to increase the surgical view toward the condyle. The bony window is then extended in an anterior direction with a diamond drill. Removal of the occipital condyle and skeletonization of the sigmoid sinus are rarely necessary. The drilling of the condyle is kept as minimal as possible, and the hypoglossal canal is left intact.
When a more inferior exposure is needed, a C1 hemilaminectomy can be added to the approach. This can be done using a diamond drill or a rongeur, starting medially and then proceeding toward the transverse foramen. 28
Intradural dissection
The dura is opened in a linear fashion, starting posterior to the intradural origin of the VA and curving anterolaterally toward the most superior segment of the craniotomy. Multiple tack-up dural stitches are placed over the craniotomy edges. The lateral flap of the dura is tensed tightly to the muscles to increase the lateral angle of exposure. After the dura is opened, the arachnoid adhesions are cut sharply. Additional CSF can be drained from the cisterna magna medially to increase cerebellar relaxation. In cases of proximal VAPICA aneurysms, the VA is followed for a short distance toward the PICA, and the aneurysm can easily be seen at its origin.
Larger aneurysms, or those located more distally along the PICA, require an approach between the lower CNs. Vertebrobasilar junction aneurysms are approached from a more inferolateral direction. The surgical corridor to these aneurysms still proceeds through the complex of lower CNs. 24 , 25 , 26
Retrosigmoid Approach (Lateral Suboccipital Approach)
For the treatment of VA-PICA aneurysms, Drake et al 11 favored the retrosigmoid approach, which requires a simple and smaller craniotomy than that for the far lateral approach. However, this smaller craniotomy requires a meticulous opening, the placement of a spinal drain to obtain maximal brain relaxation, and wide arachnoid dissection to provide the space necessary for aneurysm dissection. 28
Positioning and Skin Incision
The patient is placed in the lateral park bench position; the head is fixed to the Sugita or Mayfield frame and flexed slightly and tilted laterally ( Fig. 26.10 ). After the patient is positioned, a spinal drain is placed to obtain approximately 50 to 100 mL of CSF before the dura is opened.
A linear skin incision is marked 2 cm posterior to the mastoid process, 2 to 3 cm above the zygomatic line, and 4 to 6 cm caudal to this line. The skin incision has to extend several centimeters below the planned craniotomy to improve the passage of the craniotome during the craniotomy. The subcutaneous fat and muscles are split along the linear skin incision. The muscles are detached until the digastric groove is identified. The foramen magnum is identified by palpation. For the retrosigmoid approach, further dissection and exposure of the foramen magnum are not required.
Craniotomy
One bur hole is placed at the most superior and posterior aspect of the planned skin incision. The dura is detached from the inner table with a curved dissector without damaging the sigmoid or transverse sinus. Two cuts are then performed with the craniotome. The first one is made caudally toward the mastoid, and the second one is made superiorly and anteriorly toward the mastoid process. The bone ridge between these previous cuts is thinned down using the craniotome, without footplate protection or a diamond drill. The bone flap is then lifted and removed. A diamond drill is used to extend the craniotomy laterally until the sigmoid sinus is exposed. If the mastoid air cells are opened during the approach, they must be packed with fat, muscle, or fibrin glue to prevent a postoperative CSF leak.
Intradural Dissection
The dura is opened in a curvilinear fashion, with the base directed toward the mastoid. Multiple tack-up sutures are then placed over the craniotomy edges. Alternatively, a three-leaf or Y-shaped dural opening can be performed for cases requiring exposure of the transverse and sigmoid sinuses.
If the brain remains tight even after spinal drain placement, more CSF can be drained from the cisterna magna and the cerebellopontine cistern. After CSF is released and proper brain relaxation is achieved, the cerebellar hemisphere is gradually retracted and compressed. Arachnoid adhesions are sharply cut to enter the cerebellopontine cistern. Next, the lower CNs are identified.
Special attention should be taken to preserve bridging veins, including the petrosal vein complex. Since the retrosigmoid approach is a tailored craniotomy, its optimal location and extension depend on the aneurysm relationship with the foramen magnum.
Posterior Cerebral Artery Aneurysms
Table 26.2 11 , 14 , 15 , 19 , 24 , 26 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 summarizes the outcomes of the microsurgical treatment of posterior circulation aneurysms from some of the largest series in the surgical literature.
Epidemiology and Characteristics
Aneurysms of the PCA are rare, with an overall incidence of less than 1%, representing roughly 5 to 7% of all the aneurysms of the posterior circulation. The most frequently used classification is that of Zeal and Rhoton, 77 who divided the artery into four main segments: P1, between the basilar artery bifurcation and the PCoA; P2, between the PCoA and the posterior edge of the lateral surface of the midbrain; P3, between the posterior edge of the lateral midbrain and the origins of the parieto-occipital and calcarine arteries; and P4, terminal branches. We consider aneurysms of the P1 segment and the P1–P2 junction as proximal PCA aneurysms belonging to the circle of Willis and aneurysms of the P2, P3, and P4 segments as distal PCA aneurysms. Most PCA aneurysms are smaller than 10 mm, even when ruptured. Distal PCA aneurysms are ruptured more often than proximal PCA aneurysms. The incidence of fusiform PCA aneurysms is about 25%, and the P2 segment is the segment most often affected by fusiform PCA aneurysms ( Fig. 26.11 ). Saccular PCA aneurysms typically have a dome orientation in relation to the originating PCA segment: P1 segment, upward; P1–P2 junction, anterior or upward; P2 segment, lateral; and P3 segment, posterior. 6 , 7 , 11 , 12 , 78
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