Introduction

Posterior circulation aneurysms account for about 10~15% of all aneurysms.21) Due to the recent advances in cranial base surgical techniques, surgery for aneurysms in the posterior fossa is now possible. However, as their location is relatively deep in comparison to aneurysms in the anterior

circulation and because they are located adjacent to the brainstem and cranial nerves, posterior circulation aneurysms require the use of sophisticated surgical techniques. In addition, due to a low incidence density, the opportunity for surgical maneuvers is rare, and this leads to poor surgical results in comparison to anterior circulation aneurysms.12) It has been known that factors related to the

prognosis of patients with posterior circulation aneurysms are Hunt and Hess grade, patient age, and location of the aneurysm.9)25)26) Since the introduction of endovascular treatment, cases in which surgical approaches or clipping would be difficult, those in which surgical risks were high due to advanced age or severe health problems, those in which clipping failed and those in which surgery was refused by the patient were successfully managed without craniotomy. Endovascular treatment has been proven to be a

suitable treatment method, especially for posterior circulation aneurysms, due to the anatomical properties and surgical difficulties associated with these aneurysms.4)5)7)8)17)

Recent studies have reported that endovascular treatment results in favorable outcomes when compared to surgical clipping in certain aspects of primary treatment.19)31) This study was performed to analyze the factors that influence the prognosis of patients with posterior circulation aneurysms and to evaluate the results of endovascular treatment in comparison to those of surgical clipping.

Materials and Methods

Between January 1994 and May 2007, 1,333 patients were diagnosed with cerebral aneurysms by angiography in our institution, and 101 (8%) patients with posterior circulation aneurysms were selected(Fig. 1). Treatment outcome was evaluated using the Glasgow outcome scale (GOS) 6 months

after the initial insult. GOS scores of 1~2 were considered indicative of a poor outcome, a GOS score of 3 was considered indicative of an intermediate outcome and GOS scores of 4~5 were considered indicative of a favorable outcome. The rates of mortality and morbidity were investigated, and various causes of death were analyzed. Initial Hunt and Hess (H-H) grade, aneurysm location, size of the aneurysm and therapeutic modalities were considered to be factors that could affect the clinical outcome. These factors were subjected to a comparison analysis. Subjects over the age of 50 were classified as ‘'aged’'. Patient status was considered good in patients with an H-H grade of 1~2, moderate in those with an H-H grade of 3, and poor in those with an H-H grade of 4~5. When it comes to the location of

the aneurysm, standard being in the direction of the heart to arterial blood, distal aneurysm location was determined from the vertebro-basilar junction and thereafter, while a proximal aneurysm was determined to be at the forepart of that junction. Aneurysms less than 10mm in diameter were classified as small, and aneurysms over 10mm in size were classified as large. The subjects were divided into 2 treatment groups: an endovascular treatment group and a surgical clipping group. Comparison analysis was performed for all of these factors. Statistical analysis was performed by Chi-square test and

Fisher’'s exact test(p<0.05).

Results

The patients ranged in age from 28 ~ 81, and the mean age was 52.8. Thirty-four male patients and 67 female patients participated in the study. There were more female patients, and this tendency was stronger in the older patients (Fig. 1). The frequencies of various locations of posterior circulation

aneurysms were determined. The aneurysm originated from the basilar artery (BA) bifurcation in 55 (54%) patients, the origin of the superior cerebellar artery (SCA) in 12%, the posterior cerebral artery (PCA) in 4%, the BA trunk in 4%, the vertebral artery (VA) at the origin of the posterior

inferior cerebellar artery (PICA) in 13% and the PICA in 1%. Aneurysms in the distal area were most common (79%), and the basilar artery (BA) bifurcation was the most common location (54%).

We performed 61 operations (60.4%), and 32 cases were treated using endovascular therapy. All craniotomies were carried out 2 weeks after the initial insult. Among the 29 cases of craniotomy, ligation was performed in 21 cases, aneurysm wrapping was performed in 6, and aneurysm trapping was performed in 2. The surgical approach was determined based on the location of the aneurysm. A

pterional approach was used in the treatment of distal aneurysms in 18 cases, showing the highest frequency. The lateral suboccipital approach was used in 3 cases and a midline suboccipital approach was used in 2 cases. Forty patients had undergone only conservative treatment due to aneurysms that were classified as inoperable (8 cases), poor neurological status including fatal rebleeding (17 cases),

refusal against surgery (14 cases), and one aneurysm that spontaneously disappeared.

The overall mortality rate at 6 months was 17.8%, as 18 of 101 patients died. The morbidity rate was 13.9%, as 14 of 101 patients had Glasgow Outcome Scale scores of 2~3 grade. Rebleeding before surgery was the most frequent cause of death(44.4%)(Table 1). The association between patient age and outcome was analyzed, but the result was insignificant (Fisher’'s exact test, p 0.05). A relatively good status, GOS grade of 4 or more, was achieved in 60.3% of patients over the age of 50, and

79% of patients below age 50 had GOS scores of 4 or higher. Twenty-five of the 30 patients under the age of 50 (83%) and 21 of the 31 patients over the age of 50 (68%)

showed good prognoses after craniotomy or endovascular therapy. The patients receiving treatment showed similar

results. There were significant differences in prognosis according to initial H-H grade (Chi-square test, p 0.05). Forty-two of the 49 patients with H-H grades of I~II (85.7%), and only 6 (30%) of the 20 patients with H-H grades of IV~V showed good prognoses(Fig. 2). Prognosis regarding the location of the aneurysm was better in proximal aneurysms, but this was found to be insignificant (Fisher’'s exact test, p=0.072). Nineteen of the 22 patients with proximal aneurysms showed good prognoses (86.3%),

and 50 of the 79 patients with distal aneurysms had good prognoses (63.2%). In addition, endovascular therapy was preferred in patients with distal aneurysms. There was no significant difference in prognosis according to the size of the aneurysm when an aneurysm size of 10mm was considered to be small (Fisher’'s exact test, p 0.05). The occurrence of large aneurysms was 24.7%, and 15 out of 25

cases showed good prognoses (60%). A good prognosis was observed in 67.4% of patients with small-sized aneurysms. There were also 5 cases of fusiform aneurysms and 4 cases of dissecting aneurysms. Due to the low rate of occurrence, we were not able to obtain significant results, but the

prognoses were generally shown to be poor. The prognosis was significantly better in patients treated with endovascular therapy(Fisher exact test, p=0.032)(Fig. 3). Eighteen of the 22 patients who underwent craniotomy showed a good prognosis (58.6%), while 29 of the 32 cases treated with endovascular therapy showed a good prognosis (90.6%). To prevent bias in the selection of subjects according to H-H

grade, the H-H grades were compared in every group, but there was no significant difference. Since the introduction of endovascular therapy to our hospital in June 1996, craniotomy has been performed for the treatment of posterior circulation aneurysms in 7 cases, and endovascular therapy was performed in 32 cases. Endovascular therapy was frequently performed after the initial introduction, with a ratio of 1:4.57 (craniotomy vs. endovascular therapy). The short-term (6 months) prognosis of patients treated with

endovascular therapy was shown to be better than craniotomy, but this finding was not statistically significant (Fisher exact Test, p=0.074). A good prognosis before the introduction of endovascular therapy was found in 60.0% (18/30) of patients, while 71.8% (51/71) of patients showed good prognoses after the introduction of endovascular therapy. Treatment refusal by patients decreased from 15.2%

to 13.2% after the introduction of endovascular therapy. The percentage of inoperable cases decreased from 12.1% to 8.8% in our hospital. The prognoses of patients treated with endovascular therapy improved, and the frequency of complications decreased in comparison to the initial-stage application. This may be due to the accumulation of experience, improvement of skills and advancement of equipment.

Discussion

Early operation is generally more efficacious in the treatment of brain aneurysms than delayed surgery.2)3)27)28)

However, when it comes to the posterior circulation, a lack of surgical experience with posterior circulation aneurysms due to surgical difficulty and low frequency of occurrence has led to delayed operation, while early operations were frequently performed for anterior circulation aneurysms.9)12)13)22) However, Peerless et al. carried out a study of 206 patients with posterior circulation aneurysms.23)

They reported that aneurysm rupture during early operation was not frequent and early operation may be beneficial for patients with a relatively good neurological status. Hillman et al. performed a clinical analysis of patients with ruptured posterior circulation aneurysms.10) They reported that there

was no significant difference between early operation and delayed operation, but the general prognosis was better in the group of patients who underwent early operation. In a study of 25 patients who underwent early operation for ruptured posterior circulation aneurysms, including 2 cases treated using endovascular therapy, Sagoh et al. reported that there was no significant difference in treatment results

between early and delayed operation.25) They also suggested that age is an important factor, and early operation should be performed selectively. This suggests the necessity of early operation in patients with posterior circulation aneurysms, depending on the properties of various factors. Zhao et al.

recently studied 153 patients with posterior circulation aneurysms, including 102 cases in which surgery was performed and 51 cases treated conservatively, and the frequency of rebleeding was found to be 9.2% per year.31)  Thus, they suggested that early operation should be performed actively for the treatment of posterior circulation aneurysms. A direct comparison was not possible, as early surgery was not performed in our study. However, rebleeding before operation was the most frequent cause of

death. This indirectly suggests that it is better to perform early operation or endovascular therapy than to delay treatment.

Cerebral aneurysm surgery by craniotomy is now feasible for all aneurysms in the posterior fossa due to advances in various techniques and surgical approaches, especially those of the skull base. Since the introduction of the deep hypothermia circulatory arrest technique, surgery for giant aneurysms, which were previously thought to be inoperable, is now feasible, and several studies reported that the

prognosis was relatively good.1)14) Clinical experiences with 42 cases of posterior circulation aneurysms treated with endovascular therapy were initially reported by Guglielmi et al in 1992.8) In 1997, Redekop et al. analyzed the results of aneurysm clipping and endovascular therapy in 49 patients with distal basilar artery aneurysms (52 aneurysmss).24) Distal basilar artery aneurysms are known to have poor

prognoses when compared to proximal posterior circulation aneurysms. They reported that most of the distal basilar artery aneurysms had satisfactory results from surgical clipping, suggesting a comparable alternative treatment for posterior circulation aneurysms. Since the introduction of endovascular therapy, applications of this treatment have been extended. Furthermore, it was reported to be safe and

effective in a 3-year follow-up study.15) Posterior circulation aneurysms are especially suitable for endovascular therapy due to surgical difficulties and anatomical properties.

Indications for endovascular therapy include inoperable cases due to the size, shape and location of the aneurysm or cases in which clipping is unfeasible. Patients who are over the age of 70 or have poor health conditions should be treated by endovascular therapy as well. This may also be applied when the patient refuses surgery due to an insufficient understanding of the risks of surgery and in cases in which clipping is unsuccessful.4)6) Wiebers et al. reported that 20~65% of patients with cerebral aneurysms

only received conservative treatments due to the difficulty of craniotomy.30) There are many advantages of endovascular therapy. Brain damages from craniotomy can be prevented, and the therapy is feasible, even in patients with poor health conditions and in cases in which clipping is unsuccessful.11)29)

Disadvantages are that complications such as aneurysm rupture may occur during procedure, and the therapy itself may induce a cerebrovascular spasm. In addition, the results of long-term follow-up studies remain unclear.16)18)20) Ogilvy et al. recently reported the results of 352 cases of posterior circulation aneurysms treated using endovascular therapy, which were publicized in the literature until 2002.21)

According to the results, the total mortality rate was 13.9%, the treatment-related mortality rate was 2.6% and the morbidity rate was 7.9%. The prognosis was somewhat better when compared to craniotomy. Mordasini et al. performed endovascular therapy on 47 patients with posterior circulation aneurysms for 10 years.19) Permanent morbidity was 4.3%, and the mortality rate was 0%. This

suggested that endovascular therapy is safe and effective for use in the treatment of posterior circulation aneurysms. In our study, short-term prognosis (6 months) and results of endovascular therapy proved to be better than craniotomy. It was suggested that endovascular therapy may be more effective than craniotomy in preventing short-term rebleeding after the rupture of aneurysm, leading to improvement of prognosis. Endovascular therapy may be used prior to other treatment modalities. Mordasini et al.

indicated that the mortality and morbidity rates decreased statistically after the use of 3 dimensional GDC and that the benefits of endovascular therapy would be enhanced through the continuous advancement of technique and equipment.19)

Statistical analysis was not performed in our study, but improvement in prognosis and results due to accumulation of experience and newest technologies were observed in comparison to the period of initial introduction. Hillman et al. reported that the initial Hunt-Hess grade is the most important factor in the prognosis of patients with posterior circulation aneurysms.10) Sagoh et al. reported that older patients and patients with distal aneurysms may have poor prognoses.25) Therefore, the age of the patient and

location of the aneurysm are critical factors in determining the prognosis. In our study, there was no significant difference in the results of patients above and below the age of 50. Patients with good H-H grades had favorable prognoses, and this was proven to be statistically significant.

Proximal aneurysms seemed to have a better prognosis, but this was not statistically significant due to the small sample size. However, it was suggested that the location of the aneurysm might play an important role in the prognosis of aneurysms.

Conclusions

The results obtained from this study are as follows.

1) Non-surgical endovascular therapy was suggested to be beneficial for improving the short-term prognosis of patients with posterior circulation aneurysms. Long-term follow-up studies are necessary, but endovascular therapy should be performed prior to any other treatment for posterior circulation aneurysms.

2) As rebleeding before operation was the most frequent cause of death, it is suggested that early treatment should be actively performed in order to decrease the mortality of rebleeding and improve the overall prognosis.

3) Initial Hunt-Hess grade may be the most important factor in determining the prognosis of patients with posterior circulation aneurysms. This was not statistically significant due to the small sample size but the location of the aneurysm may be an important factor affecting the prognosis of patients with aneurysms.

REFERENCES

11) Day JD, Fukushima T, Giannotta SL. Cranial base approaches to posterior circulation aneurysms. J Neurosurg 87:544-54,

1997

12) Disney L, Weir B, Petruk K. Effect on management mortality of a deliberate policy of early operation on supratentorial

aneurysms. Neurosurgery 20:695-701, 1987

13) Drake CG. Progress in cerebrovascular disease:management of cerebral aneurysm. Stroke 12:273-83, 1981

14) Eskridge JM, Song JK. Endovascular embolization of 150 basilar tip aneurysms with Guglielmi detachable coils:results

of the Food and Drug Administration multicenter clinical trial. J Neurosurg 89:81-6, 1998

15) Fernandez Zubillaga A, Guglielmi G, Vinuela F, Duckwiler GR. Endovascular occlusion of intracranial aneurysms with

electrically detachable coils:correlation of aneurysm neck size and treatment results. AJNR Am J Neuroradiol 15:815-20, 1994

16) Gobin YP, Vinuela F, Gurian JH, Guglielmi G, Duckwiler GR, Massoud TF, et al. Treatment of large and giant fusiform

intracranial aneurysms with Guglielmi detachable coils. J Neurosurg 84:55-62, 1996

17) Guglielmi G, Vinuela F, Dion J, Duckwiler G. Electrothrombosis of saccular aneurysms via endovascular approach:part 2. preliminary clinical experience. J Neurosurg 75:8-14, 1991

18) Guglielmi G, Vinuela F, Duckwiler G, Dion J, Lylyk P, Berenstein A, et al. Endovascular treatment of posterior circulation aneurysms by electrothrombosis using electrically detachable coils. J Neurosurg 77:515-24, 1992

19) Hernesniemi J, Vapalahti M, Niskanen M, Kari A. Management outcome for vertebrobasilar artery aneurysms by early surgery. Neurosurgery 31:857-861, discussion 861-2, 1992

10) Hillman J, Saveland H, Jakobsson KE, Edner G, Zygmunt S, Fridriksson S, et al. Overall management outcome of ruptured

posterior fossa aneurysms. J Neurosurg 85:33-8, 1996

11) Hodes JE, Aymard A, Gobin YP, Rufenacht D, Bien S, Reizine D, et al. Endovascular occlusion of intracranial vessels for curative treatment of unclippable aneurysms:report of 16 cases. J Neurosurg 75:694-701, 1991

12) Kassell NF, Torner JC, Haley EC Jr, Jane JA, Adams HP, Kongable GL. The International Cooperative Study on the

Timing of Aneurysm Surgery:part 1. overall management results. J Neurosurg 73:18-36, 1990

13) Kassell NF, Torner JC, Jane JA, Haley EC Jr, Adams HP. The International Cooperative Study on the Timing of Aneurysm

Surgery:part 2. surgical results. J Neurosurg 73:37-47, 1990

14) Lawton MT, Daspit CP, Spetzler RF. Technical aspects and recent trends in the management of large and giant midbasilar

artery aneurysms. Neurosurgery 41:513-20, discussion 520-1, 1997

15) Malisch TW, Guglielmi G, Vinuela F, Duckwiler G, Gobin YP, Martin NA, et al. Intracranial aneurysms treated with the

Guglielmi detachable coil:midterm clinical results in a consecutive series of 100 patients. J Neurosurg 87:176-83, 1997

16) McDougall CG, Halbach VV, Dowd CF, Higashida RT, Larsen DW, Hieshima GB. Causes and management of

aneurysmal hemorrhage occurring during embolization with Guglielmi detachable coils. J Neurosurg 89:87-92, 1998

17) McDougall CG, Halbach VV, Dowd CF, Higashida RT, Larsen DW, Hieshima GB. Endovascular treatment of basilar

tip aneurysms using electrolytically detachable coils. J Neurosurg 84:393-9, 1996

18) Mericle RA, Wakhloo AK, Lopes DK, Lanzino G, Guterman LR, Hopkins LN. Delayed aneurysm regrowth and

recanalization after Guglielmi detachable coil treatment:case report. J Neurosurg 89:142-5, 1998

19) Mordasini P, Schroth G, Guzman R, Barth A, Seiler RW, Remonda L. Endovascular treatment of posterior circulation

cerebral aneurysms by using Guglielmi detachable coils:a 10-year single-center experience with special regard to technical

development. AJNR Am J Neuroradiol 26:1732-8, 2005

20) Murayama Y, Malisch T, Guglielmi G, Mawad ME, Vinuela F, Duckwiler GR, et al. Incidence of cerebral vasospasm after

endovascular treatment of acutely ruptured aneurysms:report on 69 cases. J Neurosurg 87:830-5, 1997

21) Ogilvy CS, Hoh BL, Singer RJ, Putman CM. Clinical and radiographic outcome in the management of posterior circulation aneurysms by use of direct surgical or endovascular techniques. Neurosurgery 51:14-21, discussion 21-2, 2002

22) Ohman J, Heiskanen O. Timing of operation for ruptured supratentorial aneurysms:a prospective randomized study. J

Neurosurg 70:55-60, 1989

23) Peerless SJ, Hernesniemi JA, Gutman FB, Drake CG. Early surgery for ruptured vertebrobasilar aneurysms. J Neurosurg