Introduction

Distal anterior cerebral artery (DACA) aneurysms are relatively uncommon, comprising only 2.1% to 9.2% of all intracranial aneurysms.6)10)11)15) Treatment of these aneurysms is challenging due to their low incidence and the associated lack of experience of medical professionals. Moreover, the

first structure to be exposed during dissection is the dome of the aneurysm, and difficulty in controlling the proximal artery results in a steep learning curve. As DACA aneurysms

result in high mortality and morbidity, we report our surgical results, experience, and limitations with these types of aneurysms.

The most common location for DACA aneurysms is near the genu of the corpus callosum at the point of division of the anterior cerebral artery into the pericallosal and callosomarginal arteries.2)5) There are several difficulties on surgery of distal anterior cerebral artery aneurysms such as a

narrow operative field, unfamiliar anatomical structures, and a high risk of rupture during the operation. Furthermore, small DACA aneurysms tend to rupture more frequently than do other small intracranial aneurysms, and they are known to have a poor prognosis.11) Endovascular treatment is complicated because the DACA aneurysm itself is located in far distal areas, making wire navigation difficult. Furthermore, coiling is difficult to perform due to the small

size of DACA aneurysms. For these reasons, DACA aneurysms have a high operative morbidity rate.

Materials and Methods

Between January 1998 and July 2008, a total of 504 patients with intracranial aneurysms were operated on at our institute. Twenty-six of these patients (5.2%) had unruptured DACA aneurysms or ruptured aneurysms in the acute stage of subarachnoid hemorrhage. The clinical and radiological features of all 26 cases were carefully reviewed through angiograms, medical records, and intraoperative findings.

We classified DACA aneurysm patients into two groups : one consisting of patients with a ruptured DACA aneurysm (ruptured group) and another consisting of patients with an unruptured DACA aneurysm (unruptured group). In the ruptured group at the time of admission, there were 11 patients with a Hunt-Hess grade between I to III, and six patients with a Hunt-Hess grade of IV or V. In terms of Fisher group, no patients in the ruptured aneurysm were group I, three patients were group II, seven patients were group III, and seven patients were group IV (Table 1).

Twenty-four of the patients with DACA aneurysms were operated on using a bifrontal anterior interhemispheric approach, while the remaining two patients were operated on via a pterional approach. Aneurysms were divided by diameter into smaller than 6 mm (n=18); medium, 7~10 mm

(n=6); and larger than 10 mm (n=2) (Table 2). Surgical outcomes were evaluated at discharge according to the Glasgow Outcome Scale (GOS) and were classified as good recovery (IV, V), moderate disability (III), severe disability (II), or death (I).

Results

The total incidence of DACA aneurysms was 5.2% of 504 patients with intracranial aneurysms, and 3.4% and 1.8% of DACA cases were assigned to the ruptured and unruptured groups, respectively. The gender ratio in the ruptured group was seven males to ten females, whereas it was two males to seven females in the unruptured group. The mean age of the ruptured group was 55.7 years old versus 56.5 years in the unruptured group.

The most common location for DACA aneurysms was the junction of the pericallosal and callosomarginal arteries. Among 26 patients with a DACA aneurysm, 23 patients had a DACA aneurysm originating in the pericallosalcallosomarginal (PC-CM) region, and three patients had

aneurysms originating in the pericallosal-frontopolar (PCFP) region.

In the ruptured group, aneurysms arose from the PC-CM area in 16 patients, and from the PC-FP area in one patient. In the unruptured group, seven patients had aneurysms that

arose from the PC-CM and two had aneurysms that arose from the PC-FP.

Fifteen patients had multiple aneurysms. In the ruptured group, multiple aneurysms were found in the middle cerebral arteries (MCA) in four patients, in the posterior communicating arteries (PCoA) in four patients, in the internal carotid arteries (ICA) in two patients, and in the

vertebral artery (VA) in one patient. In the unruptured group, one patient had multiple aneurysms on the MCA, two patients had multiple aneurysms in the PCoA, and one patient had two aneurysms, one on the MCA and the other on the anterior communicating artery (ACoA).

Associated vascular anomalies were found only in three ruptured aneurysm patients. These anomalies were azygos A2 involvement in two patients and bihemispheric ACA involvement in one patient. Twenty-four of the patients with DACA aneurysms were operated on via a bifrontal anterior interhemispheric approach and two patients were operated on via a pterional approach. All aneurysms had a saccular shape. Eleven (64.7%) of the 17 ruptured aneurysms and seven (77.8%) of the nine unruptured aneurysms were smaller than 6 mm. Medium-sized aneurysms of 7~10 mm were found in six of 26 aneurysms. Two patients, one in the ruptured group and one in the unruptured group, had aneurysms greater than 10 mm in diameter. No aneurysms

were>15 mm. Seven aneurysms (26.9%) had minor premature rupture during intraoperative proximal artery control (Table 2).

Good outcomes were obtained for 40% of patients with a Hunt-Hess grade >III in our study. Overall good outcome, that is, GOS V and IV, was achieved in 20 patients (77%); of

the remaining six patients, three (14.3%) had a poor outcome and three (28.6%) died. Among the three patients who died, one (33.3%) had aspiration pneumonia and two had delayed

ischemic neurological deficit (DIND). Two patients had transient complications; one had voiding difficulty, and the other had contralateral lower motor weakness (Table 3).

Discussion

Distal anterior cerebral artery aneurysms are thought to comprise 2.1~9.2% of all intracranial aneurysms. Most studies have reported a higher incidence of these aneurysms in women than in men.13)15)19) Distal anterior cerebral artery aneurysms present many special problems. Premature rupture during surgery is a lifethreatening concern. The propensity of DACA aneurysms to

rupture intraoperatively may be because the dome of the aneurysm faces the surgeon, resulting in exposure before identification of the aneurysm neck. If intracerebral hemorrhage (ICH) with a mass effect is present, the hematoma should be removed early in the dissection to provide additional working space. If hydrocephalus is also present, spinal drainage or intraventricular CSF

catheterization following evacuation of the hematoma may be useful to gain additional space.12)

Multiple aneurysms are present in about 20% of all patients with an aneurysm. However, patients with a DACA aneurysm more frequently have multiple aneurysms.11)12) For example, the incidence of multiple aneurysms ranged between 18.2 to 57.9% in patients with DACA aneurysms.8)10)11)16)18)20) We found multiple aneurysms in 15 cases (58%) in our series. Although the mechanisms underlying this multiplicity remain unclear, studies should focus on identifying the presence of another aneurysm in DACA aneurysm patients. The high frequency of congenital vascular anomalies may be a contributing factor in patients with DACA aneurysms. An azygos ACA has been reported in 0.26 to 2.6% of autopsy cases16)17) and in 0.2 to 3.7% of angiographically studied cases4). Recently, a high incidence of azygos ACAs, ranging from 7.1 to 15.8%, was reported in

patients with DACA aneurysms.11)14)16) Associated vascular anomalies were noted in three cases (12%) in our series.

Distal anterior cerebral artery aneurysms present unique challenges at surgery, including the narrownesses of the interhemispheric fissure and callosal cistern and the possibility of dense adherence between the cingulate gyri.1)9)12)20) The high frequency of broad neck aneurysms and

adhesion between the dome of the aneurysm and the pial layer result in retraction of the frontal lobe, which could cause premature rupture of the aneurysm before identification of the vascular anatomy. Major premature rupture during surgery is one of the most challenging surgical issues when treating DACA aneurysms. In our series, no major premature ruptures occurred. Seven

aneurysms (26.9%) had minor premature rupture during aneurysm neck dissection with intraoperative proximal artery control.

We prefer a bilateral interhemispheric approach (24/26 cases) to a unilateral one because we feel that less brain retraction occurs when a bilateral approach is used; furthermore, there is minimal anatomical distortion of the aneurysm and the parent vessels because of retraction compared to the unilateral approach. Ohno et al.11) reported finding small aneurysms of less than 5 mm in diameter in 36 of 49 cases. In other reports, most aneurysms were<10 mm, whereas no aneurysms were>15 mm.5)12) In our series, aneurysms smaller than 6 mm were found in 18 of 26 cases. There were only two aneurysms (7.7%) larger than 10 mm, whereas no aneurysms were

larger than 15 mm. The low incidence of large aneurysms is likely due to the tendency of DACA aneurysms to rupture before reaching a large size. Therefore, DACA aneurysms should not be overlooked and require aggressive treatment even if discovered incidentally in an unruptured state. Distal anterior cerebral artery aneurysms are known to have poor clinical course and prognosis compared to other supratentorial aneurysms, as well as relatively high morbidity and mortality rates.10)18) Operative mortality in reported series ranges from 0% to 23% with an average of 9.2%.2)3)5)7)9)20) Mortality has been found to be directly related to poor grade at presentation and also to severe bleeding associated with an intracerebral hematoma. Patients with

multiple aneurysms also may contribute to the poor outcome. In our series, seven patients had conjoined intracerebral hematoma, and seven patients were Fisher group III.

In terms of operative treatment of unruptured aneurysms, we believe that aneurysm size should not be the main factor used to decide whether to operate; even if the aneurysm is small, it may require operative treatment. We propose that ruptured distal ACA aneurysms should be operated upon depending on the size of the aneurysm, whereas unruptured distal ACA aneurysms require active management, regardless of their small size. However, because we only investigated a small number of ruptured cases and did not evaluate a control group, it is not plausible to confidently

conclude that DACA aneurysms tend to rupture more than intracranial aneurysms, even when they are small.

Evaluation of additional cases and logical comparisons of ruptured aneurysms according to size in other anatomical locations should allow further delineation of treatment guidelines for unruptured aneurysms in the future. Successful surgical treatment of DACA aneurysms depends on a precise understanding of their unique microsurgical anatomy, security of proximal control, and the surgeon’s experience.

Conclusions

Despite the small patient group, our data suggest that in general, DACA aneurysms tend to rupture at a smaller size than do intracranial aneurysms. We emphasize that unruptured DACA aneurysms need aggressive treatment even when they are small. Careful dissection and good

proximal control are mandatory to ensure the success of DACA aneurysm surgery.

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