Korean Journal of Cerebrovascular Surgery 2003;5(2):123-129.
Published online September 1, 2003.
Posterior Circulation Aneurysm Surgery.
Oh, Chang Wan , Han, Dae Hee
Department of Neurosurgery, Seoul National University College of Medicine, Seongnam, Korea.
Abstract
Surgical treatment of posterior circulation aneurysms are still challenging to the neurosurgeons, requiring highly skilled hands. Patients with intracranial aneurysms operated on from January 1984 to January 2003 have been reviewed retrospectively. During that period 104 patients with 110 posterior circulation aneurysms underwent operation in our institute. Among them 47 patients had 49 aneurysms at the basilar bifurcation. The posterior cerebral artery aneurysms (PCEAA) 11, the superior cerebellar artery aneurysms (SCAA) 18, the anterior inferior cerebellar aneurysms (AICAA) 6, the vertebral artery aneurysms (VAA) 8 and the posterior inferior cerebellar artery aneurysms (PICAA) 18. The surgical approaches for BBAA, SCAA and PCEAA (proximal to P4) were pterional route in 70 aneurysms and subtemporal in 4. Modified pterional approach was suitable for most of such aneurysms. For lower basilar trunk aneurysms (AICAA and VBJA), both far lateral suboccipital craniectomy and petrosal presigmoid approach had been tried and the presigmoid one seemed to be the choice of approach. The author achieved aneurysmal neck clipping in the 82 (73%) aneurysms, wrapping in other 15 and proximal clipping in the other 13. The operative mortality and morbidity were 6% and 17% each, which were comparable to the other series. Concerning surgical complications, transient oculomotor palsies were most frequent (38%), followed by transient hemiparesis, thalamic infarction, status epilepticus and peripheral infarction of the parent-arterial territory.
Key Words: Posterior circulation aneurysm, Modified pterional approach, Petrosal presigmoid approach

Introduction


  
Surgical treatment of vertebrobasilar aneurysms are still challenging to the neurosurgeons, requiring highly skilled hands. Especially, aneurysms at the basilar trunk and distal vertebral artery are vexing lesions because of their rarity and crucial location at the skull base. Their location is deep and we should traverse many important structures to reach these aneurysms. Accordingly the surgery of these aneurysms has a high risk of complications. Furthermore, the perforators around these aneurysms have a critical role, and all of these problems prevent us from surgery of these aneurysms. Several approaches have been utilized to reach the aneurysms of this location. However, most routes to this area share the problems of long operative distance, restricted operative field and risk of injuring vital structures such as brain stem and cranial nerves. In addition to usual harmful effects of the subarachnoid hemorrhage such as a direct injury, cerebral vasospasm, acute hydrocephalus and intrcerebral hemorrhage, the risk of injury to the vital structures during surgery could be very high in this surgery.4)13)27)33)35)

Surgical Experience of Posterior Circulation Aneurysms 

   Several approaches have been proposed to reach the aneurysm at the basilar bifurcation, and the senior author(DH Han) has utilized modified pterional route in most cases. In this section, the surgical tactics and the results of this approach will be presented with discussion.

Material and Method

1. Material
  
Patients with intracranial aneurysms operated on from January 1984 to January 2003 have been reviewed retrospectively. During that period 104 patients with 110 posterior circulation aneurysms underwent operation in our institute. Among them 47 patients had 49 aneurysms at the basilar bifurcation. Their medical records and radiological data were analyzed.

2. Method of modified pterional approach
  
In our institute, most of the aneurysms at the basilar bifurcation were operated through the modified pterional approach, which consists of the usual pterional and the anterior subtemporal routes. This approach could also be applied to the aneurysms at the junction of the basilar and the superior cerebellar arteries and the proximal posterior cerebral arteries.
   For the planning of surgery we examine the angiogram meticulously. The site, direction and size of the aneurysm and its anatomical relationship with the dorsum sellae, the posterior communicating artery and the posterior cerebral artery are very important factors to expect what we will do during the surgery. The degree of atherosclerosis of the intracisternal internal carotid artery(ICA) is another factor to determine the feasibility of this approach.
   The craniotomy must be wide enough. The usual pterional bone flap should be extended to the temporal side. In addition, we try to make the brain as slack as possible to access the deep location. This can be accomplished by such maneuvers as a head up position, hyperventilation, osmotic agent and lumbar or ventricular CSF drainage. After sylvian dissection we often cut the sylvian vein at the temporal tip for the mobilization of the temporal lobe which caused no harmful effect on the patient's outcome. After the dissection of the optic nerve and the internal carotid artery it is very important to select either optico-carotid trinangle or carotid-tentorial triangle. This is determined mainly by the anatomy of the ICA, especially the length of it or the stiffness due to atherosclerosis. If it is long enough either the opticocarotid avenue or the caroticotentorial spaces can be utilized. However in cases with short ICA the caroticotentorial triangle may be the choice of approach. We utilized the caroticotentorial route twice as often as the opticocarotid triangle. Generally speaking the caroticotentorial route provides with the better access to the highly located basilar tip aneurysm.
   For the retraction of the ICA we usually utilize tapered retractors with the tip curved to fit the outer curvature of the artery. This technique can preserve the blood flow in that artery during the retraction the ICA which is very important for the success of our approach. Usually three retractors are applied: one at the temporal lobe, the other one at the frontal lobe and the third one on the ICA.
   After the opening of the membrane of Liliequist the basilar artery and the proximal basilar artery near the posterior clinoid process are exposed. In this step if more space is required we remove the posterior clinoid process or incise the tentorium. We have cut the posterior communicating artery in one case to get more space but this should be done with a great care to preserve many important perforators arising from this artery.
   After the exposure of the basilar artery we usually apply temporary clips. One temporary clip on the basilar artery is usually enough but sometimes temporary clipping on both P1 may be necessary. Occaisionally for getting more space the removal of the temporary clips is necessary. During the dissection, perforators behind the aneurysmal neck and on the both P1 segments should be searched carefully. The important anatomical land-mark at this step is the 3rd nerve between the superior cerebellar and the posterior cerebral arteries. We dissect the 3rd nerve on the medial side only to decrease the postoperative third nerve palsy.
   During application of the aneurysmal clip the preservation of the perforators around the aneurysmal neck is critical. We are trying to confirm the perforators as much as possible. Packing with Surgicel to displace the perforators is a useful tactic to prevent them from being clipped.
   Sometimes the removal of retractor can facilitate the surgical procedure by allowing more space, especially at the time of aneurysmal clipping.

Results

1. Characteristics of the posterior circulation aneurysms
  
During the past 19 years we have operated on 110 aneurysms at the vertebrobasilar system in 104 patients. During this period 47 patients with 49 basilar tip aneurysms underwent the operative procedure. The incidence of multiple aneurysms was high in our cases with posterior circulation aneurysms with the overall incidence of 38 percent. In patients with basilar tip aneurysms the incidence was as high as 56 percent. This characteristics of multiple combined anterior circulation aneurysm makes the pterional approach more useful because we can treat these lesions in the one surgical procedure.
  
The choice of surgical approaches of posterior circulation aneurysms is varied by their location. In our series most aneurysms around the basilar tip were approached through the pterional route while in only 3 cases the subtemporal avenue was chosen. We prefer the presigmoid transpetrosal approach for the aneurysms located at the lower basilar trunk(Table 1), as described by Al-Mefty3) while some authors prefer the far lateral suboccital appraoch.18)

2. Characteristics of the basilar tip aneurysms
  
About half of the cases had large or giant basilar tip aneurysm. The direction of dome was superior or superoposterior in about two-thirds of the patients and only two cases showed posteriorly directed aneurysmal dome.
   Of the 47 aneurysms in 45 patients approached through the pterional route, the aneurysmal neck clipping was possible in 40 cases and the other 7 aneurysms were treated by wrapping. The preoperative Hunt and Hess grade was the most important factor for the overall outcome. Patients with good preoperative grade had the excellent outcome in 75 percent with mortality and morbidity of 5 percent and 20 percent, which are very similar to other report51. The overall mortality was 9 percent.
   The most frequent complication was transient third nerve palsy(49%), which disappeared in two and half months on the average. About 29 percent of the patients suffered from also transient hemiparesis. However, serious complications such as thalamic infarction or midbrain syndrome occurred in 5 patients.

Discussion

1. Microsurgical anatomy
  
Knowledge of the microsurgical anatomy around the basilar tip is crucial for surgery of aneurysm at this location. The basilar tip is located in the interpeduncular fossa. This fossa is bounded superiorly by the mammillary bodies and anterior half of the posterior perforated substance, posteriorly by posterior half of the posterior perforated substance(PPS) and anterior part of the mesencephalic tegmentum, inferiorly by the upper pons, lacteally by the cerebral peduncles and anteriorly by the basilar bifurcation and the proximal superior cerebellar artery.
   In this fossa, crucial brain stem structures are fed by perforators arising from basilar bifurcation. The anterior half of PPS is supplied by the paramedian thalamic and the posterior half by the superior paramedian mesencephalic arteries. The inferior paramedian mesencephalic arteries go to the posterior one third of this fossa. 
   Most of these perforators arise from P1 segment as a single or double trunks on each side. And then, they soon split into the paramedian thalamic and the superior paramedian mesencephalic arteries. On P1 segment, 97 percent of them arises from the posterior or the superior side, and only 3 percent comes from the anterior side. The most common patterns of origin of perforators to PPS are single trunk bilaterally or single and double trunk on each side. The inferior paramedian mesencephalic arteries originate from the posterior wall of P1, posterior and inferior side of the superior cerebellar artery or posterior wall of the basilar artery.31)42)

2. Features of basilar tip aneurysms
  
Of all intracranial aneurysms, those at the posterior circulation constitute 5-10 percent4)13)27)33)35) and 52-62 percent of them arise from the bifurcation of basilar artery.14)44)49) In our series, of the 110 posterior circulation aneurysms, 49 (45%) were located at the basilar bifurcation. 
   The incidence of multiple intracranial aneurysms has been reported as 14 to 19 percent in the clinical series and 23 to 28 percent at the autopsy.26)28)29)47) Compared with these report, the incidence of multiplicity in our series of posterior circulation aneurysms was a little higher(38%). Moreover, in our cases with aneurysm at the basilar bifurcation, the incidence(56%) was even higher than the usual one. Other authors also have reported high incidence of multiple aneurysms in patients with basilar tip aneurysms, ranging from 36 to 49.2 percent.48)51) The reason for this high incidence of multiplicity may be referral bias, but it may also indicate higher propensity of these patients to develop intracranial aneurysms.

3. Surgical approaches to the basilar tip aneurysms
  
Several approaches have been reported to reach aneurysms at basilar bifurcation, among which the subtemporal and the pterional routes have been used most widely. For each case with this aneurysm, appropriate approach is selected mainly according to the findings of the angiography, such as the size and direction of aneurysmal dome, relation to the posterior communicating and posterior cerebral arteries and distance from the dorsum sellae.13)36)44)49)51) In addition to these factors, the familiarity of the surgeon with each approach also plays an important role in selection of the route. The subtemporal approach, first described by Drake8)13) and good for identification of the perforators arising from the posterior aspect of the basilar aneurysm, is excellent for those aneurysms with posteriorly directed dome and low-lying neck.36) However this route has some drawbacks, such as a narrow operation field and the high risk of injury to the temporal lobe and the 3rd and 4th cranial nerves. The pterional approach, first described by Yasargil.50)52) eliminated these drawbacks of the above approach with better appreciation of the anatomy of the interpeduncular cistern and provided with opportunity to treat the concurrent aneurysms at the anterior circulation, although it also has disadvantages such as surgical difficulties in cases with obstruction by the internal carotid artery and the posterior communicating artery. 
   Recently, however, the modification and/or combination of these two approaches seem to be more popular than the classic ones. These modified approaches involve standard pterional craniotomy with more posteroinferior extension to expose the anterior aspect of the temporal lobe. Drake, in 1978, called this technique as the "half and half" method,10) while Sano described similar technique as a "temporopolar approach.37) The temporopolar approach consisted of posterior retraction of the temporal tip, after extension of pterional craniotomy to the anterior temporal base, allowing wider operating field and better access to the high basilar artery aneurysms. In the publication of Sundt,46) this combined method was named a "modified pterional(anterior temporal) approach". In his paper, Heros depicted the technique he used under the name of the "combined pterional/anterior temporal approach".19) His approach involved modifications, such as cutting the temporal muscle anteroinferiorly at the zygoma to allow it to be retracted posteriorly over the ear and resecting some of the uncus to achieve better exposure between the third nerve and the tentorial edge. In summary, most recently utilized modified approaches are combinations of the conventional pterional and the anterior temporal approaches with some modifications, to achieve wider operation field and less retraction to the temporal lobe. In our series of 38 basilar top aneurysms, we approached 37 aneurysms through modified pterional route, and only one case through subtemporal corridor. 
  
To enter the interpeduncular cistern, either the opticocarotid or the caroticotentorial(retrocarotid) space is selected. This selection is made according to the anatomical relationship of the internal carotid artery, the optic nerve, the oculomotor nerve, the tentorial edge and the posterior communicating artery.8)51)52) Yasargil51)52) utilized the opticocarotid route when at least 5 mm existed between the optic nerve and the internal carotid artery. Tanaka et al.48) reported that they chose this approach route under the following situations:1) laterally protruded and/or highly sclerotic internal carotid artery;2) long and redundant A1 segment;3) an associated aneurysm of the internal carotid artery obstructing the retrocarotid space;and 4) a short and/or large posterior communicating artery obstructing the retrocarotid space. This space usually provides with narrower space and more restricted access to the high basilar bifurcation than the caroticotentorial one.48) In this approach, the A1 segment should be dissected to make it redundant or may be sectioned when it is hypoplastic.48)51) If necessary, the optic canal may be unroofed to mobilize the optic nerve or the anterior clinoid process is drilled off to increae the motility of the internal carotid artery.48) The caroticotentorial route, which permits wider range in the direction of clip application and in the height of basilar bifurcation, is good for cases with closely approximated optic nerve and internal carotid artery.48)51) In our series, of the 45 cases, 33 were operated through this space. In this approach, the space either medial or lateral to the posterior communicating artery can be utilized to dissect and clip the aneurysmal neck, the latter one providing with more space and accordingly being adopted most often. More often than not, both of the opticocarotid and the caroticotentorial spaces are employed to adequately delineate the aneurysm and the basilar bifurcation area.48)51)
  
Sometimes, the intervening posterior communicating artery may be sectioned when it is hypoplastic.51) However, this procedure should be performed after every effort to preserve blood flow to the thalamoperforators in order to avoid catastrophic sequelae34) such as tuberothalamic infarction. Hypoplastic P1 segment may also be cut with the same precautions.51) To avoid injury to the perforators arising from the proximal P1 segment, this artery should be dissected along the inferior side.9)31)53) The third nerve should be dissected to mobilize it, and we usually do this only on the medial side of the nerve to lessen the postoperative dysfunction. For cases with low basilar bifurcation in relation to the dorsum sellae or for application of temporary clip, lower extension of the working space may be desired. In such situation, the posterior clinoid process can be drilled off or the tentorial edge may be incised with care taken not to injure cranial nerves III and IV.51) Resection of some part of the uncus may also be helpful for this purpose.19)51) The best site for application of temporary clip to the basilar artery is between the origins of the posterior cerebral and the superior cerebellar arteries, where the numbers of perforators are least, and the temporary clips with narrow blades are recommended.31) 
   Another, less frequently adopted route to enter the interpeduncular cistern is the suprabifurcation one.13) This approach is utilized in patients with short, immobile internal carotid artery, which hinders both of the above approaches. In this approach, care should be taken not to injure the lenticulostriate arteries and the optic tract.23)36)



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