Neurology India Medknow Publications on behalf of the Neurological Society of India ISSN: 0028-3886 EISSN: 1998-4022 Vol. 58, Num. 2, 2010, pp. 242-247

Neurology India, Vol. 58, No. 2, March-April, 2010, pp. 242-247

Original Article

Microsurgical subtemporal approach to aneurysms on the P ₂ segment of the posterior cerebral artery

Jing Zhitao, Wang Yibao, Wu Anhua, Ou Shaowu, Ban Yunchao, Zhou Renyi, Wang Yunjie

Department of Neurosurgery, The First Affiliated Hospital of China Medical University, Shenyang 110001, China

Correspondence Address: Wang Yunjie, Department of Neurosurgery, The First Affiliated Hospital of China Medical University, Shenyang-110 001, China, wangyunjie8@hotmail.com

Date of Acceptance: 29-Oct-2009

Code Number: ni10063

PMID: 20508343

DOI: 10.4103/0028-3886.63806

Abstract

Background: Aneurysms arising from the P ₂ segment of the posterior cerebral artery (PCA) are rare, accounting for less than 1% of all intracranial aneurysms. To date, few studies concerning the management of P ₂ segment aneurysms have been reported.
Objective: To review the microsurgical techniques and clinical outcomes of microsurgical treatment by different approaches in patients with aneurysms on the P ₂ segment of the PCA.
Materials and Methods: Forty-two patients with P2 segment aneurysms had microsurgical treatment by subtemporal approach. All the patients had drainage of cerebrospinal fluid for decompression, and indocyanine green (ICG) angiography was used in 20 patients to assess the effect of clipping.
Results: Of the 42 patients, 16 were operated by combined pterional-subtemporal approach. In 40 patients aneurysms were successfully treated by clipping the P ₂ aneurysmal neck while preserving the parent artery. Two patients with giant aneurysms were treated using surgical trapping. Postoperatively, 41 patients had a good recovery. One patient after aneurysm trapping had ischemic infarction in the PCA tertiary and presented with hemiparesis and homonymous hemianopia. However, this patient recovered after three weeks of treatment.
Conclusion: Subtemporal approach is the most appropriate approach to clip the aneurysms of the P ₂ segment. It allows the neurosurgeon to operate on the aneurysms while preserving the patency of the parent artery. Gaint P ₂ segment aneurysms can safely be treated by rapping of the aneurysm by combined subtemporal or pterional-subtemporal approach in experienced hands.ICG angiography will be an important tool in monitoring for the presence of residual aneurysm or perforating artery occlusion during aneurysm clipping. Preoperative lumbar drainage of cerebrospinal fluid may help to avoid temporal lobe damage.

Keywords: Indocyanine green, posterior cerebral artery, P ₂ segment aneurysm, subtemporal approach

Introduction

Aneurysms of the posterior cerebral artery (PCA) are rare and account for 0.7-2.3% of all intracranial aneurysms. Of the PCA aneurysms, P ₂ segment aneurysms are even rarer, with an incidence of about 0.3%. ^{[1],[2],[3],[4]} Aneurysms in the P ₂ segment arise between the junction of the posterior communicating artery (PCoA) with the PCA and the posterior part of the midbrain. The pterional, subtemporal, temporopolar, transpetrous and transcortical transchoroidal fissure are the surgical approaches which have been used to gain access to P ₂ segment aneurysms. ^{[1],[2],[3],[4]} Endovascular coil occlusion has rapidly evolved as a competing therapeutic alternative to surgical clipping in the treatment of P ₂ segment aneurysms. ^[5] However, surgery is still a well-established option for P ₂ segment aneurysms and complete closure of the aneurysm can be achieved by surgical clipping. ^[6] We studied the operative techniques and outcomes in 42 patients with P ₂ segment aneurysms treated with microsurgical clipping or trapping.

Materials and Methods

Clinical characteristics of patients

Forty-two patients (mean age 41.5 years (range 21-68 years); male : female: 17:25) . with P ₂ segment aneurysms were treated by microsurgical clipping or trapping between 1993 and 2008. The mean diameter of the aneurysms was 25±2.1 mm (range 10-45 mm) and 16 aneurysms had the diameter more than 25 mm (32±1.6 mm). Of the 42 patients, 25 patients presented with subarachnoid hemorrhage (SAH), one to three episodes. Severity of SAH at the time of presentation was Hunt and Hess Grade-III in four, ten with Grade-II in ten and eleven of Grade-I.in eleven. Other clinical presentations included oculomotor nerve palsy (n=8), headache and dizziness (n=13), contralateral hemiparesis (n=8), blurred vision (n=7) and epileptic seizure (n=1). Only one patient had incidental mass lesions in the ambient cistern during cranial computed tomography (CT) angiography (CTA) for head trauma [Table - 1].

Imaging

Cranial CTA revealed SAH in 25 patients and mass lesions in the ambient cistern in 16 patients. Magnetic resonance imaging (MRI) in 29 patients demonstrated uneven signals due to flow void or thrombosis in the aneurysms and also revealed the relationship between the aneurysm and adjacent brain tissues, especially the brainstem, thalamencephalon, major arteries and cranial nerves [Figure - 1]a. All 42 patients underwent digital subtraction angiography (DSA), and in 41 patients the aneurysms could clearly visualized [Figure - 1]b. In one patient aneurysm could not be visualized properly due to the complete thrombus of the aneurysm.

Surgical procedures

All 42 patients underwent microsurgery by the subtemporal approach, and 16 of them had a combined subtemporal and pterional approach. After general anesthesia, lumbar puncture with an indwelling catheter was performed for cerebrospinal fluid drainage. Forty patients underwent surgical clipping of the P ₂ aneurysmal neck without occlusion of the parent artery [Figure - 2] and [Figure - 3]a, and the remaining two patients with giant aneurysms underwent surgical trapping to relieve the mass effect.

Intra-operative indocyanine green (ICG) angiography with Leica (OH3) and related software was performed once or twice in 20 cases. The neck remnants of the aneurysm were found in two patients, inadvertent clipping of the branching vessels in one patient, and inadvertent clipping of the perforating vessels in two patients. The ICG angiography done after clip replacement showed complete residual elimination with patency of the branching and perforating arteries [Figure - 3]b.

Results

One of the patients who had trapping of the aneurysm had an ischemic infarct in the PCA tertiary, hemiparesis and hemianopia. However, this patient recovered following three weeks of hyperbaric oxygen treatment. The mean period of follow-up was 5.8 years (range 1.1-13.2 years). Of the 42 patients, 40 patients at the end of follow-up had good recovery and returned to work and eight of them with pre-operative hemiparesis had full recovery. Eight patients who had pre-operative oculomotor palsy, recovered within three postoperative months, and the one patient who developed hemiparesis recovered completely after six months. Three-dimensional CTA and DSA done at one week and three to six months after surgery [Figure - 4] revealed complete closure of the aneurysm in 41 patients.

Discussion

The P ₂ segment begins at the PCoA-PCA junction and courses through the distal peduncular and ambient cisterns to the posterior part of the midbrain, ^[1] where it gets further subdivided into a P _2A which courses through the crural cistern and a P _2P which courses through the ambient cistern. Generally, occlusion of the P ₂ posterior segment is safe, while occlusion of the anterior segment should be more carefully considered as the P ₁ -P ₂ junction and P ₂ anterior segment are mostly present in the perforating artery originating from the brainstem and thalamus ^[7] [Figure - 5]. Aneurysms of the PCA account for 0.7-2.3% of all the intracranial aneurysms, ^[2],[3],[4] and most occur in the P ₂ segment. In the series by Sakata et al. P ₁ segment aneurysms accounted for 27.3%, P1-P2 junction and P2 segment aneurysms for 54.5%, and P2 segment and beyond 18.2%. ^[2] Giant aneurysms account for only 6% of all intracranial aneurysms. However, among the aneurysms of PCA, the incidence of giant aneurysms is up to 20%, ^[5],[8],[9] a much higher figure than the aneurysms in the anterior circulation. Giant aneurysms exert a mass effect on the surrounding tissues and may present with oculomotor nerve palsy, visual field defects, memory impairment or epileptic seizures. ^[3],[4],[8] Drake reported 31 cases of PCA aneurysm, of which 13 were giant. ^[10] In the study by Yasargil and colleagues, seven of 14 cases were giant aneurysms. ^[11] There were 16 patients with aneurysms measuring 25 mm or more in diameter in the present study. Giant aneurysms increase the difficulty of surgical management. In addition, aneurysms of the PCA more often occur in association with other vascular abnormalities or disease, such as moyamoya disease, arteriovenous malformation and arterial occlusion, Marfan′s syndrome, and systemic lupus erythematosus. ^[4]

Selection of the optimal surgical approach is based on the type of the aneurysm, the relationship between the aneurysm and its parent artery and the exact location of the aneurysm. For saccular aneurysms, including some giant ones, it is safe and feasible to clip the neck completely. The subtemporal approach is a classic approach advocated by Drake to treat aneurysms of the PCA. ^[10] Surgical clipping via the subtemporal approach is simple for neurosurgeons to directly operate on aneurysms with the patent arteries well exposed. It is an ideal choice for cases where there is a need for reconstructing patent arteries. However, the extent of traction on the temporal lobe, particular when the aneurysm location is higher in position, may cause postoperative complications such as brain swelling and language disturbances. It may also cause compression of the Labbe vein and disruption of surface veins, possibly leading to brain edema. The damage due to these complications can be minimal with the use of mannitol administration, lumbar puncture or ventricular drainage during surgery in addition to avoiding excessive traction. ^[4],[8],[9] In our study, all patients had a lumbar puncture with indwelling catheter drainage prior to the operation to avoid temporal lobe damage. In our opinion, the subtemporal approach is a simple, rapid and safe approach for experienced surgeon to treat aneurysms on the P ₂ segment of the PCA. However, in case of larger aneurysms with outward growth, with subtemporal approaches one may have to handle the top of the aneurysm first which often may have severe adhesion after hemorrhage and which are also at a higher risk to rupture. Once the rupture occurs during the operation, it is difficult to control because the proximal parent artery is blocked, especially when the aneurysm is giant. Therefore, for this kind of aneurysm, a combined subtemporal and pterional approach is preferred. Such an approach enables proper brain relaxation by drainage of cerebrospinal fluid through the suprasellar cisterns and sometimes the lamina terminalis. This combined approach also enables proximal control of the PCA.

In the study by Onada ^[6] et al., successful neck clipping of the aneurysm was performed via the transcortical transchoroidal fissure approach by opening the cortex on the midtemporal gyrus. The possible risk with this approach will be development of visual field defects. This approach requires less retraction of the temporal lobe, and is very useful approach for the treatment of aneurysms located adjacent to the choroid fissure and at the temporal horn of lateral ventricle, more so in a setting of acute subarachnoid hemorrhage associated with brain edema. Transpetrous approach has been suggested by Ng ^[9] as an alternate approach to treat P ₂ aneurysms. P ₁ and P ₁ -P- ₂ junction aneurysms were approached by the pterion approach by Sakata et al. ^[2] and aneurysms of P ₂ and P ₃ segments by the subtemporal approach with good (67%) outcomes. Orbitozygomatic approach is the other approach suggested by Gerber ^[12] to treat P ₂ aneurysms, which minimizes retraction of the temporal lobe through cutting the zygomatic arch.

ICG angiography is a recent tool for intraoperative assessment of the clipping, and is easy and simple to operate. ^[13] Compared to other intraoperative angiographic techniques, it is low-cost, simple to operate, highly accurate, and requires less surgical time. It allows surgeons real-time observation of the aneurysm and its neck and perforating arteries to the brainstem during the operation, providing information for safe clipping. Postoperative DSA and CTA results corresponded to intraoperative ICG angiography findings, and there were no unexpected results, such as occlusion of parent or branching arteries.

As aneurysms of the P ₂ segment have a tendency to become giant aneurysms with which good preservation of the parent artery may not be possible. Because of this, trapping is the frequently done procedure with these aneurysms.. Because of good compensatory circulation in this segment of the artery, permanent ischemic brain injury is rarely seen. The P ₂ segment has three types of branches: central branches to the brainstem, ventricular branches, and inferior temporal branches that supply the cortical territory. ^[13] Yasargil reported five patients with unclipped PCA aneurysms who were successfully treated by trapping. ^[11] Of the nine patients with P ₁ and P ₂ aneurysms reported by Drake, ^[10] only one presented with visual field defects after trapping. However, occlusion of PCA still may be associated with risk of brain ischemia. ^[8],[14] In our study, two patients underwent trapping and PCA occlusion. One of these patients with the ligation of the anterior P ₂ had ischemic event with good recovery,. To avoid such a catastrophic event, the alternate preferred approach will be reconstruction of the P ₂ segment. The superficial temporal artery (STA) and the occipital artery (OA) are the preferred supply vessels in reconstruction. In addition, the superior cerebellar artery (SCA) and posterior inferior cerebellar artery (PICA) are considered alternative vessels when the STA and OA are poorly developed. ^[15]

In light of the advantages discussed above, we conclude that a basal lateral subtemporal approach appears to be useful for exposure of the P ₂ segment. When the aneurysm is giant, combination with a pterional approach is recommended. Lumbar puncture with indwelling catheter can be performed for cerebrospinal fluid drainage when the dura mater is exposed. ICG videoangiography is a recent tool for intraoperative clip assessment, and it is easy and simple to perform during surgery on posterior circulation aneurysms. P ₂ segment aneurysms can safely be treated exclusively by clipping or trapping through the subtemporal or combined subtemporal-pterional approach in experienced hands.

References

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