Introduction
Posterior circulation aneurysms remain a challenge to vascular neurosurgeons.[1]
[2] The complex anatomy of the posterior fossa, with close relation to the brainstem
and cranial nerves, in addition to the depth of the surgical field have encouraged
neurosurgeons to develop refined microsurgical techniques to access aneurysms of the
posterior circulation.[1]
[3]
[4]
Approximately 10–15% of aneurysms are located in the posterior circulation. The most
common site is the basilar bifurcation (63%), followed by the superior cerebellar
artery (SCA) and the posterior inferior cerebellar artery (PICA).[1]
[2] Aneurysms of the posterior circulation usually present secondary to subarachnoid
hemorrhage (SAH). The International Study of Unruptured Intracranial Aneurysms (ISUIA)
concluded that a posterior circulation aneurysm represents an independent risk factor
for rupture in its natural history.[5] The PHASES score estimates a 5-year aneurysm rupture risk based on age, aneurysm
location, size, history of subarachnoid hemorrhage (SAH), and presence of hypertension.
Patients younger than 70 years old and who have posterior circulation aneurysms without
history of SAH or hypertension, presented with 1%, 2%, 5%, and > 15% predicted risk
of rupture in 5 years for aneurysms with less than 7 mm; between 7.0–9.9 mm, 10–19.9 mm,
and ≥ 20 mm in size, respectively, compared with rates of 0%, 1%, 1%, and 5%, respectively,
for the same size categories involving the anterior circulation.[6] Therefore, the management of unruptured posterior circulation aneurysms depends
on those variables mentioned above. It is, of course, important to mention that the
risk of treatment must not to be higher than risk of rupture.
Despite the technical challenges, the timing of treatment for ruptured aneurysms may
also pose a hard decision for neurosurgeons.[1]
[4] The high incidence of rebleeding, risk of vasospasm and the clinical conditions
should be considered for surgical treatment of ruptured posterior circulation aneurysms.
At 2 months follow-up, untreated ruptured vertebrobasilar aneurysms had 63% mortality
compared with 10% of patients surgically treated.[4] Hernesniemi et al (1994)[4] reported good outcomes in the first postoperative month in 80% of the patients operated
within 7 days after SAH; adding to that, the early-surgery group showed lower mortality
rate at the 1-year follow-up. Several authors through different research designs also
concluded the early treatment as best treatment option for ruptured posterior circulation
aneurysms, except in the case of high grade patients (Hunt-Hess IV and V), who have
a mortality rate higher than 80% in the first 48 hours and would certainly not tolerate
high complexity-brain surgery.[4]
[7]
[8]
Aneurysms of the posterior circulation have unique anatomical features depending on
their location, morphology and aneurysm dome projection. Careful individualized preoperative
planning based on angiographic findings and bone anatomy of the posterior fossa is
mandatory for each patient. Therefore, we addressed the main aneurysms of the posterior
fossa in different sections discussing their anatomical landmarks and relationships,
best surgical approach, and outcomes.
Discussion
Vertebral Aneurysms
Saccular vertebral aneurysms are extremely rare conditions accounting for 0.5–3% of
all intracranial aneurysms[1]
[4] ([Fig. 1]). The vast majority of vertebral aneurysms are presented as fusiform aneurysms,
due to dissections with high risk of bleeding and rebleeding.[9] However, those aneurysms are usually not eligible for clipping techniques, and the
surgical treatments include: proximal occlusion of the parent vertebral artery, trapping
procedures, clip reconstruction, and occipital-PICA or PICA-PICA bypass.[1]
[2]
[10]
[11] Aneurysms of the vertebral artery arise from the PICA onset or at the vertebrobasilar
junction (VBJ); therefore, this issue will be discussed in the following section.
Fig. 1 Vertebral aneurysm case treated by microsurgery. (A, B) Digital subtraction angiography
(DSA) showing saccular aneurysms on the right vertebral artery. (C) A far lateral
approach was used, and microdissection was performed. (D) Microdissection of the aneurysm
dome with adequate exposure of the neck. (E) Aneurysm successfully clipped.
Posterior Inferior Cerebellar Artery Aneurysms
Anatomical Features
The PICA is the second and largest branch of the vertebral artery. It has five segments
defined by its relationship with the lower cranial nerves and trajectory on the medulla
and cerebellum surface: anterior medullary (PI1), lateral medullary (PI2), tonsillomedullary
(PI3), telovelotonsillar (PI4), and cortical segments (PI5). The PI1 beings at the
PICA's origin and its trajectory follows through the hypoglossal rootlets and the
medial edge of the inferior olive. The PI2 extends from the olive to the rootlets
of the ninth, tenth and eleventh cranial nerves, and the lateral edge of the inferior
olive. The tonsilomedullary segment (PI3) courses down on the inferior pole of the
cerebellar tonsil where it forms an infratonsillar loop to reverse the course and
ascend along the medial tonsil. The telovelotonsilar segment (PI4) beings at the level
of the midpoint of the medial tonsil, and it ascends until the roof of the fourth
ventricle, where it changes direction to form a cranial loop and courses to the tonsillobiventral
fissure. The beginning of PI5 is considered to be the point where PI4 emerges from
the fissure and it is immediately bifurcated in the medial trunk, which supplies the
vermis and adjacent hemispheres, and the lateral trunk, which supplies the cortical
surface of the tonsil and the hemispheres.[2]
[12]
[13]
Surgical Strategy
There is still debate regarding the ideal surgical approach for such complex aneurysms.
With regards to the PICA-vertebral junction (PICA-VJ), the surgical approach depends
on its precise anatomic location. Lesions in the anterior surface of the brain stem
might be clipped through far lateral transcondylar approach. Aneurysms located in
the anterolateral and lateral surface can be operated through lateral suboccipital
approach. Peripheral PICA aneurysms are mostly treated through midline or paramedian
suboccipital approach.[12]
[13]
During the surgical procedure, a major cause of morbidity is inadvertent injury of
the lower cranial nerves. The tortuous trajectory of the PICA, passing through the
cranial nerves' rootlets, facilitates stretch lesions of the cranial nerves. Delicate
retraction and refined microsurgical technique are critical for preserving the integrity
of the nerves. Another issue is the risk of PICA occlusion, resulting in a lateral
medullary syndrome (Wallenberg). This type of lesion would be prevented with a detailed
angiographic preoperative study of the complex neurovascular anatomy associated with
sharp dissection of structures.[1]
Outcomes from the Literature
The high complexity of the approach to the posterior fossa has fueled neurosurgeons
to develop innovative techniques for accessing the region. In the 1980s, advances
made in the microneurosurgery field allowed the first promising case series about
surgical treatment of posterior circulation aneurysms.[14] Drake (1969)[15] published the first series of clipping vertebrobasilar aneurysms. He obtained 75%
of success in the treatment of PICA-VJ using the suboccipital approach.
In 1999, Ogilvy et al[16] achieved good outcomes in more than 90% of the PICA aneurysms sample (n = 26) using combined surgical and endovascular treatment. In the surgical treatment,
more refined techniques were adopted, such as far lateral suboccipital and transfacial-transoral
approach. More recently, the same group from Massachusetts General Hospital published
an updated series with 197 posterior circulation aneurysms. Nineteen patients with
PICA aneurysms underwent surgical clipping, with a 94% rate of excellent clinical
outcome and 90% of complete occlusions in the postoperative angiography; both clinical
and radiographic outcomes showed higher efficacy with the surgical treatment. So,
they recommend that, whenever possible, posterior circulation aneurysms should be
treated by surgery; the endovascular treatment modality should be considered for worse
clinical conditions, unfavorable anatomy, and high-grade patients.[7] Although the endovascular treatment has become more sophisticated overtime, with
surprising outcomes, it still has not equaled the data from surgical treatment for
anterior circulation aneurysms.
Basilar Artery Trunk Aneurysms
Basilar artery (BA) trunk aneurysms are defined as lesions located on the VBJ and
the branching point of the superior cerebellar point, and they represent 8% of posterior
circulation aneurysms.[1]
[2] The eloquence of surrounding structures and the complex neurovascular anatomy increase
the potential for surgical morbidity. Neurosurgeons must have accurate anatomical
background and affinity with surgical approaches to safely access this region.
Anatomical Features
The BA originates at the VBJ, at the level of the pontomedullary sulcus, and ascends
anterior to the brainstem until its bifurcation in the interpeduncular cistern.
Additional care should be taken regarding the perforating arteries to the brainstem
with origin in the posterior or lateral surfaces of the upper portion. Two main branches
originate from the basilar artery trunk: the anterior inferior cerebellar artery (AICA)
and the SCA. The AICA originates right above the vertebral basilar junction, near
the abducent nerve, and courses backward through the cerebellopontine angle adjacent
to the foramen of Luschka to finally supply the petrosal surface of the cerebellum,
pons, and eighth cranial nerve. The SCA originates within 2.5 mm of the basilar apex
and courses underneath the third cranial nerve (CN), medial to the tentorial edge.
It dips caudally to the trigeminal root and enters in the cerebellomesencephalic fissure,
following the fourth CN and the superior cerebellar peduncle. Moreover, multiple perforating
vessels typically arise from the midbasilar trunk, at the level of pons. These perforators
are organized in caudal, middle, and rostral groups; they also include the superolateral
and inferolateral pontine arteries, which supply the paramedian and lateral pontine
surfaces.[1]
[12]
The anatomical landmark of basilar trunk aneurysms (BTAs) is the origin of the AICA,
site of aneurysms formation. Therefore, upper BTAs are the ones located between the
AICA and SCA, while lower BTAs are those located between the AICA and VBJ.[17]
[18]
Surgical Strategy
Based on the damage to the internal elastic lamina and the state of the intima, non-atherosclerotic
BTAs can be classified into four causal subtypes (Mizutani classification):[19] (1) Acute dissecting aneurysms; (2) Segmental ectasia; (3) Mural bleeding ectasia
(chronic dissecting); and (4) Saccular aneurysm. It is suggested that saccular aneurysms,
are related to genetic cerebrovascular vulnerability, usually related to multiple
aneurysms with elevated risk of rupture ([Fig. 2]). The surgical treatment is more feasible for this type of aneurysm, while acute
dissecting, segmental ectasia (chronic dissecting), and mural bleeding ectasia are
better treated through endovascular techniques.
Fig. 2 Basilar trunk aneurysm case. (A, B) Digital subtraction angiography showing thrombosed
basilar trunk aneurysm; (C, D) A petrosectomy was performed with facial nerve skeletonization;
(E) the anterior inferior cerebellar artery was dissected from the basilar artery
and the aneurysm; (F) Aneurysm exposure; (G) Temporary clipping of the basilar artery,
proximal and distal to the neck of the aneurysm. Thrombus was evacuated, and definitive
clipping was performed
Traditionally several routes of approach have been described for BTAs: orbitozygomatic,
subtemporal, retrosigmoid, transpetrosal, anterior petrosectomy, transoral, and lateral
suboccipital.[20] The pterional and orbitozygomatic approaches provide satisfactory exposure of the
upper basilar complex with visualization of both sides of the patent artery; however,
it has a narrow and deep surgical field, requiring anterior and posterior clinoid
removal for enhanced view and a limited aneurysm clipping direction of an anteroposterior
trajectory.[1]
[2]
[20] Sugita et al[20] reported good outcomes with the middle subtemporal approach, except for aneurysms
located on the distal basilar artery. Retraction of the trigeminal nerve, pons and
the aneurysm itself for dissecting were pointed as critical steps for safe and precise
aneurysm clipping. Kawaseet et al[21] devised the subtemporal transtentorial approach with an extradural component along
the floor of the middle fossa to the petrous ridge. A lateral middle basilar view
is reached through tentorial incision between the fifth cranial nerve and the seventh/eighth
nerves. This extradural approach protects the venous system of the temporal lobe;
but, it provides limited view of the clipping route and, sometimes, clip placement
parallel to the BA is not reliable for broad neck aneurysms.
The pretemporal transcavernous approach for low-lying basilar artery aneurysms provides
an enlarging surgical field through the interpeduncular and prepontine cisterns. It
includes removal of the anterior clinoid, cutting distal and proximal dural rings,
opening the cavernous sinus, and drilling the dorsum sellae and the clivus. Although
more demanding, this approach overcomes the constraint of the limited deep operative
area inherent to previous techniques.[22]
Certain types of aneurysm might be considered unclippable due to their complex morphologies.
Alternative treatment modalities must be considered, such as trapping or occlusion
of the patent vessel. Preoperative tests for BA occlusion tolerance and detailed angiographic
study are mandatory for these procedures.
Outcomes from the Literature
Basilar trunk aneurysms represent less than 1% of intracranial aneurysms. Due to their
rarity, experience acquisition is limited to small case series with individual morphological
features, which raises difficulties regarding the treatment decision.[17]
[18]
Sugita et al[20] presented surgical clipping of 10 basilar trunk aneurysms, with a success rate of
80% using the middle subtemporal approach. Higa et al[18] reported series of 22 saccular aneurysms of the BA trunk treated by surgery or endovascular
techniques. Eleven patients underwent direct clipping. There was no procedure-related
mortality; however, one patient died due to clinical complications. All cases were
admitted with SAH; the pretreatment neurological status was considered as a predictor
for the clinical outcome. The most common surgical approach employed was the subtemporal
transtentorial (5/11); three patients needed a second operation, one because of partial
clipping, the second due to inadequate surgical approach (anterior transpetrosal)
and last one due to regrowth. Good clinical outcome was achieved in 6 out of 11 patients,
and this result was not affected by choice of treatment (endovascular x surgery).
Saliouet et al[17] included 52 patients with aneurysms of the BTA with different morphologies. Eighteen
patients underwent treatment, nine by surgery and nine through endovascular procedures.
Ten patients had SAH as first clinical presentation. five out of nine saccular aneurysms
were treated by surgery; four patients were submitted to clipping, while the trapping
was chosen for the remaining ones. Besides those saccular aneurysms, four dissecting
aneurysms were also surgically treated, through bypass (¾) and one case with patent
vessel occlusion.
Basilar Artery Bifurcation Aneurysms
The basilar bifurcation is the most common site of aneurysms of the posterior circulation.
Between 5–8% of brain aneurysms are located at the basilar bifurcation and account
for almost half of all posterior circulation aneurysms.[1]
[2] As with other aneurysms presented above, the surgical management of basilar apex
aneurysms remains challenging, even with significant advances in microsurgical techniques.
Understanding of the anatomy of the surrounding area as well as of the surgical approaches
are necessary to achieve satisfactory outcomes.[23]
Anatomical Features
Apex basilar aneurysms are usually located at the bifurcation of the basilar artery
into the posterior cerebral arteries (PCAs). They are usually located inside the subarachnoid
space of the interpeduncular cistern, which has the posterior clinoid process and
clivus as an anterior limit, the cerebral peduncles posteriorly, the temporal lobe
and tentorial edges laterally, and the mammillary bodies and posterior perforated
substance superiorly. The preservation of perforator vessels is a determining factor
for good recovery postoperatively. The thalamoperforating arteries might arise from
the posterior aspect of the basilar trunk, P1 segment of PCA, or from the posterior
communicating arteries (PComAs). It is noteworthy that the projection of the dome
of these aneurysms indicates their relationship with the thalamoperforating arteries.
Aneurysms with domes directed posteriorly, toward the interpeduncular cistern, have
close thalamoperforating arteries, posterior to the neck or dome. Therefore, careful
dissection around the neck is mandatory prior to definitive clipping. When the aneurysm's
dome is directed forward, the perforators come from the posterosuperior aspect of
the basilar bifurcation, toward the posterior perforator substance.[1]
[12]
[13]
Another important anatomic issue in the region is the third CN that passes between
the PCA and the SCA right after its emergence from the brainstem. The membrane of
Liliequist is the anterior limiting of the interpeduncular cistern; some authors define
it as the interpeduncular “curtain.” It consists of a thick arachnoid layer that anchors
from the mammillary bodies superiorly and takes anterior and inferior directions before
folding posteriorly to form the roof of the prepontine cistern.[1]
[2]
[12]
Surgical Strategy
To access the interpeduncular fossa, three classic surgical approaches were described:
Pterional-transsylvian, orbitozygomatic and subtemporal approach.[1] All of the approaches present advantages and disadvantages. The pterional-transsylvian
approach provides good access to the basal cisterns and the proximal control is straightforward;
however, it offers an antero-lateral view of the aneurysm, while the perforators coming
off the basilar tip are usually coming from behind. This relation does not allow clear
visualization of perforators; in addition, inspection of the distal part of the clip
is difficult. Furthermore, the pterional-transsylvian approach to the interpeduncular
fissure includes the great depth of the surgical field, limiting the aneurysm's dissection
and clipping[24] ([Fig. 3]).
Fig. 3 Basilar bifurcation case. (A) Digital subtraction angiography image showing an aneurysm
on the top of the basilar artery. (B) Pterional approach was performed showing internal
carotid artery (1), optic nerve (2) and posterior cerebral artery (3). (C) Microdissection
of the aneurysm and surrounding structures showing the aneurysm sac (1), basilar artery
(2), posterior cerebral artery (3), superior cerebellar artery (4) and oculomotor
nerve (5). (D) Final disposition of the aneurysm clipping
The subtemporal approach offers a good lateral view of the basilar tip with proper
visualization of the perforators. This approach, however, does not provide good access
to the basal cisterns and also presents high risk of injuring the vein of Labbé or
other veins on the basal surface of the temporal, toward the tentorium, during retraction
of the temporal lobe[1] ([Fig. 4]).
Fig. 4 Basilar bifurcation aneurysm case. (A) Digital Subtraction angiography image showing
basilar bifurcation artery aneurysms associated with a brain arteriovenous malformation.
(B) Digital subtraction angiography image showing a hyperplasic posterior communicating
artery. (C) Right subtemporal approach was performed. (D) The aneurysm was dissected
from posterior cerebral and thalamogeniculate arteries
The pretemporal approach, described in the early 90s, offers the possibility of enhancing
the angles of the view to the interpeduncular region, ranging from the anterolateral
view of the pterional-transsylvian approach to the lateral view of the subtemporal.
Retracting the temporal lobe posteriorly, this approach provides multiple surgical
routes and the interpeduncular region can be reached through the transsylvian, temporopolar,
and subtemporal approaches. The pretemporal approach might be achieved through pterional
or orbitozygomatic craniotomies, according to surgical needs. This last one is helpful
for high basilar bifurcation aneurysms that require a more superiorly angled surgical
field.[25]
The basic principles of surgical suitability are related to the relationship of the
basilar apex to the posterior clinoid, and the orientation of the dome and the PCAs.
It is mandatory to preserve the perforating arteries, especially when the artery of
Percheron is responsible for the thalamus vascularization. Moreover, the lack of proximal
control in low-lying aneurysms suggests that endovascular treatment should be considered
as the first treatment option.
Outcomes from the Literature
The deep location and the eloquence of the surrounding region brings several challenges
to the surgical treatment of basilar apex aneurysms. Even with microsurgical expertise,
the morbidity and mortality rates were defined at 30.6%, for ruptured, and 17.5% for
unruptured aneurysms.[26] The endovascular treatment came out as a less invasive alternative with most favorable
outcomes at immediate postoperative time. However, in the long-term follow-up, a 41%
recurrence rate was described, with 26% needing retreatment.[27]
Hauck et al (2010)[28] performed a retrospective analysis of 21 patients with small unruptured basilar
tip aneurysms (< 7 mm in size). The aneurysms size ranged from 2–6 mm, with a median
diameter of 4.4 mm. All patients underwent the pterional-transsylvian approach. In
the 1-year follow-up, 81% (17) of the patients referred excellent recovery, while
the 4 remaining ones presented minor disabling only. These authors advocate that non-favorable
outcomes related to the surgical treatment of those aneurysms must not be considered
in the case the small ones (< 7 mm). The surgical clipping could be safely performed
through the pterional approach with 100% success rate in their series. If argued whether
to treat or not those small aneurysms, their natural history supports that progressive
growth of aneurysms on this topography may increase the risk of rupture per year.
Therefore, the early surgical treatment of small unruptured basilar bifurcation aneurysms
might be taken within the “therapeutic window,” avoiding SAH complication.
Bohnstedt et al (2017)[29] reported data on 208 basilar bifurcation aneurysms, with 47 cases surgically treated.
The pterional approach was employed in 24 patients, while the subtemporal was adopted
for 23 patients. The pterional approach showed less neurologic deficits (third CN
palsy; p = 0.001) plus better performance status (Glasgow outcome scale) at discharge. The
mortality rate was 2.1%. In this study, the endovascular treatment was compared to
open surgery. There was no significant result regarding to morbidity or mortality
rates. However, remnants and recurrences were significantly more frequent in the endovascular
group.
Superior Cerebellar Artery Aneurysms
Anatomical Features
The SCA is divided into four segments: (1) anterior pontomesencephalic, (2) lateral
pontomesencephalic, (3) cerebellomesencephalic, and (4) cortical. The anterior pontomesencephalic
segment begins at the SCA origin, courses underneath the third CN and ends at the
anterolateral brainstem, sided by the tentorial edge medially. The lateral pontomesencephalic
segment takes caudal trajectory passing through the upper limit of the trigeminal
root and ends at the entrance to the cerebellomesencephalic fissure. The cerebellomesencephalic
segment courses posteriorly following the fourth CN and superior cerebellar peduncle.
The cortical segment is represented by small branches coming from the cerebellomesencephalic
fissure responsible for blood supply of the cerebellum's tentorial surface.[12]
[13]
Surgical Strategy
Even though the endovascular management of SCA aneurysms can be effective, the relatively
shallow location with straightforward surgical approach in addition to the lack of
perforating arteries in the surrounding region favor the surgical treatment. The conventional
pterional approach provides good surgical field for dissection of the aneurysm's neck,
which begins from the axilla of the P1 PCA or the shoulder of the SCA, not intimate
with the thalamoperforating arteries. The PCA and SCA can be identified in both sides
of the third CN and dissected distally to find the aneurysms neck ([Fig. 5]).
Fig. 5 Superior cerebellar artery aneurysm case. (A, B) Angiography with 3D reconstruction
showing type A SCA aneurysm. (C, D) Fronto-orbitozygomatic approach was performed
with adequate exposure of the basilar artery and aneurysm neck. (E, F) Aneurysm clipping
with single curved definitive clip. (G) Seventh day postoperative angiography showing
complete occlusion of the aneurysm sac
Furthermore, the SCA aneurysms usually lie off the midline, below the basilar bifurcation,
right in the center of the carotid-oculomotor triangle, facilitating visualization
and dissection compared with basilar bifurcation ones.
Outcomes from the Literature
Jin et al (2012)[30] compared endovascular vs surgical treatment of 33 SCA aneurysms. Twelve patients underwent surgery; 66% of
the patients presented with SAH. Good clinical and radiological outcomes were achieved
in 66.6% and 75%, respectively. Procedure-related third nerve palsy occurred in five
patients (5/12), with gradual and spontaneous recovery. The authors explained that
the relatively low rate of success was due to bad clinical conditions at admission.
However, there was no significant difference comparing the microsurgical outcomes
with the endovascular treatment results.
Patra et al (2016)[3]
[31] studied the role of microsurgery for SCA aneurysms even in the endovascular era.
As previously demonstrated, the microsurgery approach showed satisfactory clinical
outcome in 66% of the patients (vs endovascular; p = 0.54), with complete occlusion in 88.8% (vs endovascular; p = 0.45). However, in a pooled analysis of 12 studies, they found better clinical
outcomes with the endovascular treatment; while microsurgery showed better radiological
outcomes of sustained complete occlusion.
Posterior Cerebral Artery
Anatomical Features
The PCAs originate from the BA right after emergence of the SCAs. The PCA is divided
into four segments. The first segment (P1) begins in the basilar bifurcation and ends
where the PComA arises. It is also denominated as precommunicating segment, which
is usually larger in caliber compared with the PComA; however, some patients have
fetal anatomy and the posterior circulation is fed by the internal carotid artery
(ICA) through augmented PComA. The P2 segment (postcommunicating segment) has its
onset at the PCA-PComA junction and ends at the posterolateral margin of the midbrain.
The P2 might be divided into two segments: anterior (P2A) and posterior (P2P). The
P2A, or crural segment, walks around the cerebral peduncle through the crural cistern
and the P2P, or ambient segment, courses around the lateral midbrain within the ambient
cistern. The P3 segment (quadrigeminal segment) beings at the posterolateral margin
of the midbrain, walks over the free edge of the tentorium, courses through the quadrigeminal
cistern, and ends at the anterior limit of the calcarine sulcus. The calcarine segment
(P4) courses through calcarine fissure until the cortical surface of the occipital
lobe.[1]
[2]
[12]
[13]
An important anatomical landmark of the PCA surface is the origin of the posterior
thalamoperforators, which arise from the posterior side of the middle third of the
P1segment. When those arteries are represented by only one vessel with bilateral territory,
it is called Percheron artery. Another important branch of P1 is the posterior choroidal
artery, which takes trajectory through the basal cisterns (crural, ambient and quadrigeminal),
coursing over the superior colliculus and pineal gland to reach the choroid plexus
in the roof of the third ventricle and the floor of the lateral ventricles. The peduncular
and thalamogeniculate perforators emerge from the P2 segment and ascend superiorly.
The long and short circumflex perforators also originate from the P1 and P2 segments,
but they course parallel to the PCA rather than ascending.[1]
[2]
[12]
[13]
Surgical Strategy
Zeal & Rhoton[32] suggested a new classification of PCA segments, with direct application in the context
of best surgical approach for those aneurysms. The PCA was divided into three segments:
S1, S2, and S3. The S1 segment is located inside the interpeduncular, crural, or ambient
cistern, lateral to the cerebral peduncle. On angiograms, it is located from the basilar
bifurcation to the most lateral aspect of the PCA trunk. The S2 is located inside
the ambient and quadrigeminal cisterns and extends from the most lateral aspect of
the artery within the quadrigeminal cistern, the collicular point. The last segment,
S3, is located in the quadrigeminal cistern and its extension is from the collicular
point to the distal branches of the PCA inside the calcarine sulcus and parietooccipital
sulci. Based on this classification, the best surgical approach for aneurysms along
the PCA is determined, such as: aneurysms located in the S1 segment are usually best
accessed via pterional, pretemporal or a subtemporal approach; the S2 segment is best
managed through subtemporal approach with some additional resection of the parahippocampal
gyrus; and aneurysms located on the S3 segment are best approached through an occipital
interhemispheric route ([Fig. 6]).
Fig. 6 Posterior cerebral artery aneurysm case. (A) Angiography with 3D reconstruction showing
posterior cerebral artery aneurysm. (B) Orbitozygomatic approach was performed. (C,
D) The aneurysm neck was exposed and clipped.
Outcomes from Literature
The PCA aneurysm is a relatively rare condition, representing ∼ 1% of all intracranial
aneurysms. The lack of large published series impairs the correct judgment about treatment
modalities and outcomes. Seoane et al[33] published a series with 15 aneurysms surgically treated, 9 by clipping occlusion,
3 treated by trapping, and the 3 remaining aneurysms were treated through bypass,
proximal PCA occlusion (giant aneurysm) and direct coagulation (small size). Out of
these, 80% presented as ruptured aneurysms, even the ones that were small. The authors
achieved excellent outcome in 66.6% of the patients, 4 out of 15 cases presented reversible
neurological deficits, and 1 mortality. Currently, endovascular treatment of PCA aneurysms
is becoming popular; however, it offers high risks and it is challenging to completely
obliterate the aneurysm while preserving the flow of the parent artery. Large and
giant PCA aneurysms are a unique condition that should be discussed in a separate
section. For the small PCA aneurysms, on the other hand, surgical treatment shows
encouraging results.
