Anterior communicating artery (ACoA) is the most common site for intracranial aneurysms as revealed by landmark clinical studies.4) Despite the high prevalence of ACoA aneurysms, surgical clipping of ACoA aneurysm can still be challenge to even experienced neurosurgeons because of variable anatomy of the ACoA complex and its surrounding structures. The selection of the best surgical approach should include minimum brain retraction, minimum unnecessary operative manipulation, familiarity of surgical approach for operator, and easy access to the aneurysm, which depends on location, direction and size. The selection of surgical approach for high-positioned ACoA aneurysm can be even more difficult because surgical obliteration of high-positioned ACoA aneurysm requires higher technical skill and the use of adjunct procedures, such as, extended craniotomy, resection of gyrus rectus, and revascularization techniques. In 1953, Norlen and Barnum first described a transfrontal approach for ACoA aneurysm with partial resection of frontal lobe.5) Kempe and VanderArk published a modified transfrontal approach with limited rectus gyrus resection.3) Nevertheless, a conventional pterional approach with resection of rectus gyrus is still used for the surgical obliteration of ACoA aneurysm. In our institution, we modified procedures from the conventional pterional approach. We dissect the olfactory tract and detach from frontal lobe. In this study, the purpose is to evaluate surgical outcome of our pterional-subolfactory approach for the high positioned ACoA aneurysm through comparison with conventional pterional approaches.

Between February 2005 and December 2012, 463 ACoA aneurysms were surgically treated in our institution. Among them, 48 high positioned anterior communicating aneurysms were treated with pterional-subolfactory or conventional pterional approaches. The pterional-subolfactory approach includes sharp dissection and detachment of olfactory tract to free from brain parenchyme and resection of the gyrus rectus immediately medial and inferior to the olfactory tract. Thirty-four of the 48 cases were treated with pterional-subolfactory approach and 14 were treated conventionally. High positioned anterior communicating artery aneurysms were defined as aneurysms located higher than 10 mm above the anterior clinoid process.

For the 34 cases treated with pterional-subolfactory approach, the mean location of ACoA aneurysm was 12.62 mm (range from 10.22 to 16.13 mm) from the anterior clinoid process. For the 14 cases treated with conventional approaches, the mean location of ACoA aneurysm was 12.74 mm (range from 10.11 to 17.26 mm). Sizes of aneurysms treated with subolfactory approach were varied from 3 mm in diameter to 16 mm (average 5.52 mm in diameter). The average diameter of conventionally treated aneurysms was 6.36 mm. The operation duration for conventional pterional and pterional-subolfactory approaches were measured, and conventional pterional approach took 58 minutes more than the pterional-subolfactory approach (p = 0.001). Patient characteristics are summarized in Table 1.

A rectus gyrus approach for anterior communicating aneurysm was first described by Kempe et al.3) in 1971. Since then, resection of the rectus gyrus has become the world-wide method for proper exposure of an anterior communicating aneurysm. In the standard method, the retractor is repositioned over the medial orbital gyrus just lateral to the olfactory nerve aiming toward the estimated level of the ACoA junction in the interhemispheric fissure. The pia of the gyrus rectus, immediately medial and parallel to the olfactory nerve, is cauterized and resected. This standard method provides a proper view of the anterior communicating aneurysm. In 1992, Horikoshi et al.2) reported clinical outcomes after rectus gyrus resection during conventional pterional approach. He concluded that resection of rectus gyrus helps to achieve safer aneurysm clipping without causing definite neurological sequelae affecting the postoperative outcome. However, with a highly positioned aneurysm, to retain the proper view of ACoA aneurysm, more retraction and elevation of frontal lobe is required and direct retraction injuries from the retractor can develop. Our pterional-subolfactory approach is different from the conventional pterional approach with removal of rectus gyrus in that we cauterize the pia of the rectus gyrus, immediately medial and inferior of olfactory tract after proper dissection of olfactory tract. In this way, we can preserve the olfactory tract most importantly, and achieve proper view and operation field for highly positioned ACoA aneurysm with no direct retraction needed (Fig. 1, 2). Our pterional-subolfactory approach requires no retractor to elevate frontal lobe.

In our study, we achieved good clinical outcome with no surgical related permanent complications in both conventional pterional approach and pterional-subolfactory approach group. The pterional-subolfactory approach resulted in shorter operation duration, and that difference was statistically significant. The olfactory dissection procedure might take additional operative time, but makes identification of aneurysmal neck easier for clipping.

In 1972, Pool demonstrated bifrontal craniotomy for ACoA aneurysms. Pool operated upon 60 ruptured ACoA aneurysms using low bifrontal craniotomy.7) He noted good clinical outcomes among good and fair risk group patients with 2.8% mortality and no surgical related complication. However, low bifrontal craniotomy can increase the possibility of postoperative infection due to frontal sinus opening and retraction injuries can develop during an interhemispheric approach due to bilateral brain retraction. Also, most neurovascular surgeons are more familiar with pterional craniotomy than interhemispheric approach with bifrontal craniotomy.

Anosmia is a well-known surgical related complication after ACoA aneurysm surgery. Hiroaki et al.1) performed a comparison study of anosmia after ACoA aneurysm surgery between the anterior interhemispheric and basal interhemispheric approaches in 1995. They reported 31% anosmia after anterior interhemispheric approach and 1.9% for basal interhemispheric approach. They concluded that frontal lobe depression and elevation during surgery may injure the olfactory nerve. Park et al.6) concluded olfactory dysfunction followed by pterional approach for cerebral aneurysms in 2009. Their study revealed high risk of olfactory dysfunction after pterional approach for ACoA aneurysm with 14.3% (10 of 70 patients). They concluded that direct pressure damage to the olfactory tract by spatula can be the possible reason for olfactory dysfunction. Olfactory dysfunction can pose a higher risk for older patients because of less elastic and more vulnerable olfactory nerves. In our study, we assessed olfactory nerve injury for unruptured aneurysm patients and found 1 case (9.1%, 1 of 11 patients) with pterional-subolfactory approach, very low compared to the literature.

The primary limitations of the present study are the retrospective design and the relatively small number of patients. In addition, olfactory dysfunction outcome was checked only for the unruptured aneurysm group due to preoperative mental status. Subarachnoid hemorrhage might have affected patient's preoperative mental status, and proper preoperative olfactory function evaluation was difficult. Also olfactory function of the conventional approach group was excluded in this study due to lack of old medical history. In the future, olfactory dysfunction outcome for both groups should be studied for comparison of both approaches.

In this study, we achieved excellent clinical outcome for both unruptured and ruptured ACoA aneurysms with no surgical related permanent complications except 1 case of olfactory nerve dysfunction. We concluded that pterional-subolfactory approach can be highly preferable for cases of high positioned ACoA aneurysm located higher than 10 mm above anterior clinoid process. Pterional-subolfactory approach can prevent olfactory bulb tearing, resulting in preservation of olfactory tract without any surgical complication. Also the subolfactory approach can provide wider and effective operation view with minimal retraction of brain. In conclusion, the pterional-subolfactory approach can be an alternative surgical approach for high positioned ACoA aneurysms.