Introduction
According to the different locations and characteristics of intracranial aneurysms, surgical approaches are determined with the aim of

maximizing exposure and safety. The pterional approach has been used as a first option for various neurosurgical disorders.1)3)4)7) This

approach, which provides the option of accessing lesions in the anterior, middle and upper posterior fossae, cavernous sinus and sellar area

was introduced and popularized in contemporary neurosurgery for the treatment of cerebral aneurysms by Ya?argil et al.7)8)22)
Many traditional approaches have been widely used for the treatment of intracranial aneurysms; a common feature of these techniques is the

relatively extensive skin, bone, and brain exposure, possibly causing an increase in iatrogenic injury not related to the aneurysms itself. To

obviate these problems, many modifications of the pterional approach have been created and keyhole approaches have been developed for

use in modern microneurosurgery.9)17)21) They are designed to reduce iatrogenic traumatization by minimizing exposure and handling of

tissues, in order to achieve maximum efficacy in the treatment of neurosurgical diseases.5)6)9)18-20)
We adopted a previously reported pterional approach in our surgical practice and developed the neuronavigation-guided keyhole approach to

demonstrate the efficacy of this minimally invasive procedure.11) Herein, we report our experience of using this approach in the treatment of

32 patients with unruptured cerebral aneurysms.
Materials and Methods
Patient population
Between December 2008 and December 2010, 32 patients with unruptured intracranial aneurysms underwent direct surgical clipping through

the neuronavigation-guided keyhole approach in our hospital. Patients were selected according to the size and position of the aneurysm in

the circle of Willis. We used Vector vision compact? neuronavigation system (BrainLAB, Feldkirchen, Germany) and Star PACS? (Infinitt

healthcare, Seoul, Korea) for image registration. Because brain computed tomography (CT) is easily available, preoperative thin-slice (slice

thickness, 1 mm) brain CT with contrast enhancement was obtained for neuronavigation. The targeted aneurysm was outlined and

reconstructed in the axial, coronal and sagittal CT planes (Fig. 1B).

Operative methods
The patients were placed in a supine position, with their head elevated 10° to 15° such that the head position was above the heart and was

extended to allow the frontal lobe to fall away by gravity. The head was rotated from 15° to 45° toward the opposite side of the lesion, and

was fixed by a 3-point device such as the Mayfield head holder. Anatomical landmarks, including the sylvian line and the frontozygomatic

point, were identified on the scalp before the application of the surgical drapes, for incision planning (Fig. 1A). These points were estimated

via the neuronavigation system (Fig. 1B). A 4- to 5- cm skin incision was made along the frontotemporal area 1 cm behind each patient’s

hair line, and centered at 2/3 of the estimated location of the sylvian line, in order to expose the frontal lobe more than the temporal lobe

(Fig. 1A). After the temporal muscle was incised along the skin incision, the bone was exposed following subperiosteal dissection. The

sphenoid ridge, which separates the frontal and temporal lobes, was identified. A single burr hole was made at the most distal part of the

exposed bone and centered over the sphenoid ridge (Fig. 1C). Next, a Freer elevator was used to separate the dura from the bone to

prevent dural tearing, particularly in elderly patients. The craniotomy was performed using a high-speed craniotome (Fig. 1C). An oval-

shaped craniotomy, approximately 2.5×4cm in size, was completed as described above (Fig. 1D). The lateral part of the sphenoid ridge was

drilled off using a high-speed drill. After the dura had opened in a semilunar manner and reflected (Fig. 1E), we used the neuronavigation

system to verify the actual location of aneurysm, adjust the trajectory of the sylvian dissection. The remainder of the aneurysm surgery was

performed using conventional microsurgical techniques (Fig. 1F). Generally, no drainage was left in place. The skin was closed with skin

staples.
Results
The 32 enrolled patients comprised 15 men and 17 women, with a mean age of 63.06 years (range, 47 to 79 years). All patients had a single

aneurysm. Of these, 21 aneurysms were in the middle cerebral artery (MCA) bifurcation; 3, in the M1; 5, in the posterior communicating artery

(PComA) segment; 2, in the anterior communicating artery and 1, in the anterior choroidal artery segment (Table 1). The size of the

aneurysms ranged from 3.5 to 4.8mm. Mean operation time and hospitalization stay were 131 minutes (range, 100 to 150 minutes) and

5.4days (range, 4 to 8), respectively. Because many patients wanted to stay in hospital, mean hospitalization stay was longer than

expected. Mean operation time and hospitalization stay in conventional pterional approach were 192minute (range, 165 to 210minutes) and

8.3days (range, 6 to 10), respectively. All aneurysms were clipped successfully. There were no severe complications related to the

neuronavigation-guided keyhole approach, including infection, hematoma, cerebrospinal fluid leakage and nerve injury. Overall, the patients

were satisfied with the cosmetic results. 
Discussion
Various surgical approaches are available for the clipping of cerebral aneurysms. The pterional approach has been widely used, but has

several disadvantages including a large bone flap, damage to the facial nerve and trauma to the soft tissues. Since the implementation of the

minimally invasive concept in neurosurgery, several investigators have developed new techniques to decrease the size of the bone flap,

reduce brain retraction and enhance the exposure of the cranial base. Among them, the keyhole approach has shown convincing

advantages in the treatment of cerebral aneurysms.6)9)10)12-15)19) Currently, one of the most used keyhole approaches for treating

intracranial aneurysms is the subfrontal transciliary (front- olaterobasal, eyebrow) approach,5)6)10)13)16)20) which provides access to a

majority of the anterior circulation aneurysms. However, there are two main limitations to this approach.14) First, if the proximal MCA (M1)

segment is too long and the direction of the dome is lateral, clipping of some MCA bifurcation aneurysms might be difficult because the

surgical view and work are in a very deep plane and extensive dissection would be required. Furthermore, a thick blood clot around the

aneurysm would make the dissection more difficult in ruptured cases. Second, in case of PComA segment aneurysms, the angle of vision

with the eyebrow approach is more rostral than that obtained with the pterional approach and visualization of the aneurysmal neck is not

good in cases where the dome has a caudal direction.
Previously, Nathal et al.14) described the sphenoid ridge keyhole approach, which is centered over the sylvian fissure, for all anterior

circulation aneurysms except for the distal anterior cerebral artery. The current procedure allows sufficient brain exposure to ensure

dissection of the sylvian fissure.
We believe that the neuronavigation-guided keyhole approach is greatly useful in the treatment of MCA aneurysms. In cases of MCA

aneurysms, we were able to dissect the sylvian fissure minimally, due to exact identification of the aneurysm site using neuronavigation.

Therefore, we can reduce brain retraction, damage and operation time. Although our study restricted this approach to treat unruptured cerebral

aneurysms, less operative times and hospitalization stays as compared with standard pterional craniotomy were observed. Due to the small

size of the incision and the craniotomy, the integrity of the normal tissue was preserved. So, postoperative scalp and muscle swelling was

diminished and the patients were much more satisfied with the cosmetic results.
The main disadvantage of a minimal approach is the neurosurgeon’s limited ability to manipulate instrument, especially in the case of proximal

artery aneurysm. However, the superficially located, unruptured, and small aneurysms were not difficult to manipulate and clip.
We suggest that neuronavigation-guided keyhole craniotomy might be a good option for treating cerebral aneurysms. However, sufficient

surgical planning is required and patients should be carefully selected according to the position of the aneurysm in the circle of Willis,

neurological grade and the possible existence of associated pathological conditions such as cerebral hematoma, brain edema and

vasospasm.
Conclusion
With new concepts based on minimally invasive neurosurgery, the keyhole approach for treating cerebral aneurysms is gaining popularity.

Despite the small number of cases studied, we suggest that the neuronavigation-guided keyhole approach is safe and effective for the

treatment of anterior circulation aneurysms in selected cases.
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