We are IntechOpen, the world’s leading publisher of Open Access books Built by scientists, for scientists 6,300 170,000 185M Open access books available International authors and editors Downloads Our authors are among the 154 TOP 1% 12.2% Countries delivered to most cited scientists Contributors from top 500 universities Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (BKCI) Interested in publishing with us? Contact [email protected] Numbers displayed above are based on latest data collected. For more information visit www.intechopen.com 11 Surgical Management of Posterior Circulation Aneurysms: Defining the Role of Microsurgery in Contemporary Endovascular Era Leon Lai* and Michael Kerin Morgan The Australian School of Advanced Medicine, Macquarie University, Sydney, Australia 1. Introduction In the period between 1930 and 1960, surgical treatment of posterior circulation aneurysms were only possible by indirect trapping or parent vessel ligation. Olivecrona was said to have performed the first unplanned trapping of a posterior inferior cerebellar artery (PICA) aneurysm in 1932. In 1937, Tonnis inadvertently opened a cerebellopontine angle aneurysm having assumed that it was a tumour preoperatively1. Dandy performed the first vertebral artery ligation beneath the atlas to treat a vertebral aneurysm in 1944 2. In 1948, Schwartz3 reported his experience with direct surgical approach to a large basilar artery aneurysm and successfully trapped it using silver clips. Logue4 and Mount5 formally described the techniques of vertebral artery and basilar artery ligation in 1958 and 1962, respectively. By the 1960s, neurosurgeons were attempting direct surgical clipping of vertebrobasilar aneurysms. Early attempts, however, were not met with great success. Dr Charles Drake from Canada published his initial experience with direct surgical clipping of four ruptured basilar bifurcation aneurysms in 19616. Although two of his patients died postoperatively, the two survivors made dramatic functional recoveries. He concluded that ‘direct surgical attack was feasible and worthwhile under exceptional circumstances, when life was threatened by repeated haemorrhages’. Dr Ken Jamieson from Australia reported 19 surgical cases in 1964, 10 of whom had died and 5 were left with severe morbidity7. He commented ‘it is clear that the basilar bifurcation is no place for the faint of heart. Only time and greater experience will indicate whether it is a place for neurosurgeons at all.’ The introduction of the surgical microscope to neurosurgery in the late 1960s and their propagation in the 1970s, 1980s and through to the 1990s greatly influenced the results of aneurysm surgery. Perhaps the greatest impact of the operating microscope was not just in enhancing the results of experienced aneurysm surgeons, but in accelerating the learning curve of young neurosurgeons that enabled them to master microsurgical skills and achieved competitive results within a shorter period of time. Worldwide reported surgical mortality rates for posterior circulation aneurysms have dropped from 34.4% in the 1960s to * Correspondig Author www.intechopen.com 236 Explicative Cases of Controversial Issues in Neurosurgery 7.5% (1970s), 5.6% (1980s), 6.0% (1990s) and 5.0% in the new millennium. Surgical morbidity averaged between 9.8% and 12.8% throughout this time period. The invention by Guglielmi8 in the early 1990s to treat intracranial aneurysms by detachable platinum coils once again revolutionised the practice of cerebrovascular neurosurgery. This change was most dramatic and rapid for aneurysms located in the posterior circulation where surgical approaches continued to impose significant morbidities to the patients. By the end of 1990s, endovascular treatments of posterior circulation aneurysms were already well established in many centres across the United States9101112 and Europe1314. The International Subarachnoid Aneurysm Trial (ISAT)15, which compared endovascular coiling to microsurgical clipping, included only 58 patients (2.7%) with posterior circulation aneurysms from their cohort of 2143 because most authors by that stage did not perceived clinical equipoise between the two treatment modalities for aneurysms in this location. Endovascular procedures were regarded as the new promises for treatment of posterior circulation aneurysms. Two decades on, we learnt that the effectiveness of endovascular treatment, measured by its durability, is a major technical limitation. Complete obliteration is frequently not achieved. Overall, recurrent filling is seen in 15% of aneurysms on angiograms obtained at 6 months after treatment. Longer term follow up in most series suggest complete obliteration rate is possible in just over 50% of coiled posterior circulation aneurysms. This carries significant implications on retreatments, monitoring and risk of rebleeding for many patients. Until endovascular techniques evolve to a point where recurrence and rebleeding rates are within acceptable limits, surgery remains a viable and competitive treatment option for aneurysms of the vertebrobasilar system. The challenge for contemporary vascular neurosurgeons is to understand the differing but complementary role each treatment modality currently has to offer, and to maintain the proficiency and technical skills to deal with an emergence of complex and recurrence of previously coiled aneurysms. 2. The role of endovascular coiling for posterior circulation aneurysms Endovascular therapy has changed the way we practice cerebrovascular neurosurgery. In the past, endovascular techniques were used to occlude aneurysms when there is ‘anticipated surgical difficulty’, ‘failed clipping’, ‘patient or physician preference’, and ‘poor medical condition’. Today, the reverse appears to be true. Endovascular therapy has largely replaced microsurgery as the firstline treatment modality for aneurysms located in the posterior circulation. In many neurosurgical centres in recent years, this trend is even more evident for unruptured posterior circulation aneurysms. The essential characteristic and therapeutic goal of the endovascular procedure is to induce thrombosis within the aneurysm by the deployment of platinum microcoils. From a neurointerventional perspective, the key determinant for success is aneurysmal morphology, not so much location. Small aneurysms with small necks and those at a right angle to blood flow are considered appropriate for endovascular procedures. Because of the complexity or the infrequency (therefore negative impact upon confidence of competence) of surgical access to the posterior circulation, endovascular repair has gained dominant mode of treatment in this location. Other important factors to consider whether to treat or www.intechopen.com Surgical Management of Posterior Circulation Aneurysms: Defining the Role of Microsurgery in Contemporary Endovascular Era 237 not and which mode of treatment to employ include the patient’s clinical status and the available institutional expertise. Elderly patients, or those in poor clinical grade post subarachnoid haemorrhages, may be better treated using endovascular techniques irrespective of aneurysm morphology. Although good data is available regarding endovascular repair by means of coiling, many aneurysms are now repaired with more complex techniques including additional stents, bioactive coils, balloon re-modelling, and the addition of ethylene-vinyl alcohol copolymer and flow diversion stents. Each of these techniques offers promise to deal with problems that the simple coiling procedure was found wanting. However, with complexity comes complications, and their risks and expectations of treatment await further experience and analysis. Not all aneurysms are amenable to endovascular treatment. For large or wide-necked aneurysms, or where the dome-to-neck ratio is less than 2, coiling is less effective. In this situation, aneurysm neck and parent vessel may be best reconstructed by microsurgical techniques. Other factors that may limit successful endovascular aneurysm occlusion include inadequate endovascular access or the presence of unstable intraluminal thrombus. When an arterial branch is incorporated in the neck of an aneurysm, as in the case of many basilar bifurcation lesions, effective endovascular treatment can be difficult. Observational studies suggest that endovascular occlusion of ruptured aneurysms is comparable to that of conventional microsurgery in the short term and can prevent early rebleeding. These studies suggest that endovascular techniques provide protection against rebleeding in the first few months, when rebleeding occurs most frequently. A review of the literature on endovascular treatment outcomes for both ruptured and unruptured posterior circulation aneurysms is demonstrated in Table 1. In summary, around 70 to 91% of patients with posterior circulation aneurysms achieve independence (mean 85%) if treated by endovascular techniques. The overall morbidity is 4.4% (range 0 to 9.6%) and mortality is 9.1% (range 0 to 18.2). The risk of post coiling haemorrhage is 1.5% out of the 961 reported cases between 1990 to 2005 (Table 1). The rate of complete occlusions is 52.1% (compared to >90% in most surgical series)16 17. The degree of initial occlusion has important ramifications on retreatments, monitoring, and risks of rehemorrhages. Long-term results of the ISAT suggested that rebleeding rate is 3 times more likely in patients who have recurrent aneurysms from incomplete coiling than patients with completely treated aneurysms. Of those patients that experienced rebleeds, mortality rate was up to 70%18. It is therefore prudent that younger patients with unruptured posterior circulation aneurysms be recommended for surgical management where long-term durability by this technique is an advantage. 3. The role of microsurgery for posterior circulation aneurysms According to the International Study of Unruptured Intracranial Aneurysms (ISUIA), unruptured posterior circulation aneurysms, particularly at the basilar bifurcation, carry a more aggressive risk of rupture than that of similarly sized lesion located in the anterior circulation19. Over a five-year period, aneurysms over 6mm diameter bear a cumulative risk of rupture of at least 15%. This compares to 2.6% for those in the anterior circulation. www.intechopen.com 238 Explicative Cases of Controversial Issues in Neurosurgery Therefore, Younger patients (age <50 years) with unruptured posterior circulation aneurysms should be treated, given the accumulated risk of rupture during a period of many years. Although endovascular treatment options must be considered in all cases, higher partial obliteration rates and recurrence rates make microsurgical obliteration more favourable in relatively young patients without extenuating medical circumstances. Mean % Study No. of % follow Post GDC Independent Morbidity Mortality Author/Year Complete Period patients SAH up haemorrhage (%) (%) (%) occlusion (months) Guglielmi 1990-1991 43 56 2 40 1/43 83 4.8 7 199220 McDougall 1991-1995 33 70 15 21 1/33 NA 3 12.1 199621 Pierot 1993-1994 35 91 4.8 73 0/35 91 0 8.8 199622 Klein 1993-1996 21 76 9.8 67 0/21 91 4.8 4.8 199723 Nichols 1992-1995 28 100 6 61 0/28 80 0 15.4 199724 Raymond 1992-1995 31 74 15.5 42 0/31 87 3.2 6.5 199725 Vinuela 1990-1995 403 100 NA NA NA 84.3 9.6 6.1 199726 Eskridge 1991-1995 150 49 12 75* 4/150 78 6.7 18.2 199827 Bavinzski 1992-1998 45 75 27.4 54 1/45 73 4.4 15.5 199928 Gruber 1993-1996 21 52 26 14 0/21 90 9.5 0 199929 Steiger 199930 1990-1998 16 69 6 69 0/16 88 6.3 6.3 Lempart 1991-1998 112 100 13.1 54 1/112 83 2.8 15 200031 Tateshima 1990-1999 75 58 31.3 45 1/75 86 4.1 8.4 200032 Birchall 1992-1998 35 46 42.7 46 1/35 86 3.4 8.6 200133 Uda 200134 1990-1999 41 69 21 32 1/41 90 2.6 7.7 Pandey 1995-2005 275 61.5 31.8 87.8** 3/275 87.4 5.1 4.9 200735 Summary 1364 71.7 17.6 52.1 1.5 (14/961) 85.2 4.4 9.1 *% complete occlusion defined as >90% by source author; **% complete occlusion defined as >95% by source author Table 1. Endovascular treatment outcomes of posterior circulation aneurysms: analysis of published series. www.intechopen.com Surgical Management of Posterior Circulation Aneurysms: Defining the Role of Microsurgery in Contemporary Endovascular Era 239 Unlike endovascular treatment that depends on aneurysmal morphology, microsurgical success relies critically on 1. The specific aneurysmal location along the vertebrobasilar system. 2. Aneurysm size and patient’s age Location Location determines surgical approaches, which largely affects the outcomes. In general, surgical approaches to posterior circulation aneurysms are difficult because: 1. Surgical exposure is deep. This translates into long surgical corridor with narrow confines, thus limiting manoeuvrability and the proficiency to which a clip can be optimally placed on the aneurysm. The ability to attain good proximal and distal control may be restricted, further increasing the operative risk in the presence of subarachnoid haemorrhage. 2. The margin of error is small. The close proximity of posterior circulation aneurysms to the brainstem with interposing cranial nerves and perforator arteries makes the anatomy around this region complex and unforgiving. 3. The infrequency of these lesions. Posterior circulation aneurysms account for approximately 10 to 15% of all intracranial aneurysms, thereby giving few surgeons the opportunity to gain the necessary experience to manage them well. The emergence of endovascular therapy in the last 20 years further reduces the number of posterior circulation aneurysms available for surgical repair. Size and patient’s age Raaymakers et al36, in a meta-analysis of case series published between 1966 and 1996 found that the morbidity and mortality of surgery for non-giant unruptured posterior circulation aneurysms was 12.9% and 3.0% respectively. They found that age; aneurysm size and location of the aneurysms (anterior versus posterior) were factors that predicted a greater chance of a favourable outcome. In ISUIA II, patients’ age was an important factor in overall surgical outcome. Other predictors of poor outcome included large aneurysmal size, history of ischaemic cerebrovascular disease, and presence of aneurysmal symptoms other than rupture. In Ogilvy and Carter’s logistic regression model37, posterior circulation, size of aneurysm and age of the patient were associated with poor outcome. Eftekhar et al38 reminded us of the overall low risk associated with surgical clipping at dedicated cerebrovascular centres, when treating patients with small unruptured posterior circulation aneurysms. In their surgical treatment of 136 unruptured vertebrobasilar aneurysms in 120 patients, the combined surgical mortality and morbidity for aneurysms <9mm in size was 3.2%. They emphasized that younger age patients and smaller sized aneurysms were favourable surgical predictive factors. This view is well supported by the works at other dedicated cerebrovascular centres39. In general, aneurysms that are most suitable to surgical clipping are: 1. Superior cerebellar artery (SCA) aneurysms 2. P1 Posterior cerebral artery (PCA) aneurysms 3. Distal anterior inferior cerebellar artery (AICA) aneurysms 4. PICA aneurysms www.intechopen.com 240 Explicative Cases of Controversial Issues in Neurosurgery Aneurysms that are difficult to approach microsurgically are: 1. P2 Posterior cerebral artery aneurysms 2. Basilar trunk 3. Proximal AICA 4. Vertebral-basilar junctions 4. Preoperative consideration A wide variety of operative approaches exist and the surgeon must select the most appropriate for the aneurysm location, size and projection. A number of critical factors must be considered prior to making the decision to operate. 1. Imaging: An angiogram combined with bone imaging reveals important anatomical features, of value not just in determining the optimal approach but also in indicating the operative risks. Note the a. Height of the aneurysm neck in relation to the posterior clinoids or clivus b. Size and direction of the aneurysm fundus. c. Any associated crucial perforator anatomy d. Any co-existent anterior circulation aneurysms that may alter the side of intended approach 2. Neuro-anesthesia and cerebral protection: Mild hypothermia and barbiturate-induced electroencephalographic burst suppression are necessary for complex basilar bifurcation and trunk aneurysms. Both techniques are essential when considering using temporary clipping as an adjunct to final aneurysm dissection and permanent clipping. It is important that these are thoroughly communicated with the anaesthetists and the rest of the neurosurgical team throughout the case. 3. Side of approach: In general, access to the parent artery immediately prior to the aneurysm dictates the side. For midline locations, a right-sided approach is preferable if either side provides equal access to the parent artery. Other factors may be taken into consideration but only if access to the parent artery is ensured. These factors include: a. Coexistent left-sided anterior circulation aneurysm. b. Hearing loss where a medial petrosectomy is required. 4. Types of approach: In considering the approach, it is important to keep in mind the principles underlying most cranial base surgical strategies including a. Shortest trajectory to the lesion b. Bone removal rather than brain retraction c. Maximization of extradural exposure d. Skeletonization/decompression of cranial nerves and vascular structures e. Reconstitution of all dural openings. From an anatomical perspective, it is useful to subdivide the vertebrobasilar arterial system into 3 compartments (Table 2). 1. Upper vertebrobasilar: incorporating basilar bifurcation, posterior cerebral artery (PCA) and superior cerebellar artery (SCA). 2. Middle vertebrobasilar: incorporating low-lying basilar bifurcation, basilar trunk, proximal Anterior Inferior Cerebellar Artery (AICA), and vertebra-basilar junction (VBJ). www.intechopen.com Surgical Management of Posterior Circulation Aneurysms: Defining the Role of Microsurgery in Contemporary Endovascular Era 241 3. Lower vertebrobasilar: incorporating vertebral and PICA arteries aneurysms. The selection of a particular approach depends on a number of important factors 1. Location of aneurysm along the vertebrobasilar system 2. Size and projection of fundus of aneurysm 3. Surgeon’s familiarity with specific approaches Compartments Aneurysms Surgical corridorApproach Options Upper Basilar bifurcation Anterolateral Pterional approach vertebrobasilar Posterior cerebral artery Orbitozygomatic approach Superior cerebellar Subtemporal approach artery Upper basilar trunk Middle Midbasilar trunk Lateral Transpetrosal approach vertebrobasilar Anterior inferior Combined supra- and cerebellar artery infratentorial approach Extended middle fossa approach Transoral approach Lower Vertebrobasilar junctionPosterolaterally Far-lateral approach vertebrobasilar Vertebral artery Extended far-lateral approach Posterior inferior Midline suboccipital approach cerebellar artery Table 2. Surgical approaches to posterior circulation aneurysms 5. Skull base approaches for aneurysm occlusion In vascular neurosurgery, exposure is extremely important. Only with adequate exposure can neurosurgeons directly visualize vascular anatomy, obtain proximal and distal control, apply meticulous microsurgical technique, and manoeuvre a clip to occlude an aneurysm successfully with a good outcome. In the last 3 decades, skull base neurosurgeons have disassembled and reassembled the skull in every possible way with the intention to maximise exposure and minimise neurological injury. These techniques have been designed to reduce the distance between the surgeon and the aneurysm, increase surgical manoeuvrability, and reduce retraction on neighbouring neurovascular structures to improve safe aneurysm clipping. In this section, we described only a selected few approaches that are practised by the senior author (MKM) in approaches to aneurysms of the posterior circulation. Orbitozygomatic approach The orbitozygomatic (OBZ) approach dramatically enhances the standard pterional craniotomy. It allows exposure of, and access to, the medial end of the sphenoid wing and www.intechopen.com 242 Explicative Cases of Controversial Issues in Neurosurgery middle fossa floor, providing a much greater scope for manoeuvre in the vertical dimension than through conventional anterolateral techniques. The “orbito” aspect involves removing the superior and lateral orbit, which opens up the roof of the operative corridor when the patient’s head is rotated away from the aneurysm and extended. In addition, extending the zygomatic removal by removing the zygomatic arch is utilised when there is an advantage in creating a flat trajectory with the middle cranial fossa floor (e.g. for medial petrosectomy). A widened operative corridor improves illumination, eliminates the need for brain retraction, and optimises manoeuvrability. A good OBZ approach gives the neurosurgeon a wide sweep of surgical trajectories ranging from supraorbital to transsylvian to pretemporal to subtemporal. Surgical trajectory can then be tailored to the pathology at hand. There are a number of important limitations to this technique, although the risks are low: 1. Cosmetic concerns including a. Temporalis atrophy b. Subtle orbital asymmetries that bother some patients c. Frontalis nerve injury, d. Pulsatile enophthalmos, 2. Orbital problems such as a. Orbital entrapment, b. Diplopia from extraocular muscle or nerve injury, c. Blindness 3. Infection: communication with the frontal or ethmoidal sinus may increase the risk of infection or cerebrospinal fluid leakage. Key steps in an orbitozygomatic approach 1. Patient’s head is placed in a 3-point fixation head frame in slight extension such that the malar process is upper most. An imaginary line, starting from the lateral canthus of the ipsilateral eye to the external occipital protuberance, should be positioned perpendicular to the floor. This will ensure the Sylvian fissure remains vertical, such that after wide splitting of the fissure, the frontal and temporal lobe fall away from the operative field. 2. A curvilinear incision is planned from just anterior to the ipsilateral tragus up to the superior temporal line. The incision then gently curves to terminate at the hairline superior to the contralateral midpupillary line. A small strip of hair is shaved with clippers along the course of the planned incision. 3. The skin is incised and haemostasis is obtained with Raney clips. The inferior limb of the incision is complete after the scalp is dissected from the temporalis fascia with a periosteal elevator. 4. The scalp flap is mobilised anteriorly and the temporalis fascia is exposed. The fascia is sharply incised and elevated separately in a subfascial dissection to protect the frontalis branch of the facial nerve running along the superficial surface of this fascial plane. 5. Dissection continues anteriorly to expose the orbital rim, malar eminence, and the zygomatic arch. 6. The temporalis muscle is raised separately, exposing the zygomatic root and pterion. The muscle flap is left attached to the cranium at its vascular pedicle in the infratemporal fossa. www.intechopen.com Surgical Management of Posterior Circulation Aneurysms: Defining the Role of Microsurgery in Contemporary Endovascular Era 243 7. The scalp flaps and temporalis muscle are retracted anteriorly and inferiorly using surgical hooks. 8. The periorbital is a delicate lining and can be carefully stripped from the undersurface of the orbit with a Mitchell dissector. Periorbita can be preserved by beginning the dissection where it is thickest inferolaterally near the inferior orbital fissure, by using side-to-side sweeps with a round-tipped dissector, and by advancing circumferentially along the orbital roof and the lateral wall. This dissection gradually deepens towards the orbital apex. 9. Two burrholes are placed over the temporal bone near the root of the zygoma and a pterional craniotomy is performed. 10. The OBZ unit consists of the orbital rim, orbital roof, lateral orbital wall, and zygomatic arch. Removal of the zygoma is optional. The OBZ unit can be removed with the cranial flap as one integrated piece, which provides a better cosmetic result than a two-piece technique, although more difficult. 11. Additional bone is removed around the orbital apex, resecting what remains of the orbital roof, lateral orbital wall, and medial sphenoid wing, back to superior orbital fissure. 12. The dura is then opened in a semicircular incision, and reflected anteriorly and inferiorly over the periorbita and temporalis and tacked to the over lying scalp. This way, the profile of the periorbital contents is flattened to enhance the exposure. Arachnoid dissection The Sylvian fissure is the gateway to aneurysms along the circle of Willis. Separating the frontal and temporal lobes with the fissure split is one of the most important skills a vascular neurosurgeon needs to master. It is important, when performing the Sylvian fissure split, to keep the following principles in mind: 1. No retraction should be used. 2. Superficial Sylvian veins are the guardians of the Sylvian fissure. Knowing which way to dissect beyond the veins is an important skill. In general, superficial Sylvian veins course inferiorly and bridge to the sphenoparietal sinus under the sphenoid ridge. Dissection, therefore, should be along the frontal side to preserve these connections. However, it is important to maintain the venous connections of the larger veins in the region of the frontal operculum to minimise the risk of venous infarction in this region. 3. Cortical and deep arachnoid dissection must always be sharp, precise and controlled, using only the inverted tip of a No. 11 scalpel blade. Blunt dissection places stress on arachnoid-bound structures and increases the risk of complications from bleeding and neural injuries. 4. The Sylvian fissure is entered from distal to medial, and from deep to superficial along the direction of the middle cerebral artery branches. 5. All cisterns must be opened maximally to allow CSF egression and optimise brain relaxation. This manoeuvre eliminates the need for fix brain retraction. The Sylvian cistern is entered first, followed by the opticocarotid and the chiasmal cisterns. Fenestration of the lamina terminalis is encouraged to allow more CSF drainage from the third ventricle in cases where CSF outflow obstruction due to haemorrhage may be present. This further enhances brain relaxation. 6. The deep Sylvian cistern is opened and the carotico-oculomotor triangle is dissected. www.intechopen.com