Microsurgical treatment of distal anterior cerebral artery aneurysms: A 25 year institutional experience
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.193827
Source of Support: None, Conflict of Interest: None
Introduction: We present a retrospective study of distal anterior cerebral artery (DACA) aneurysms operated at our institute.
Keywords: Aneurysm; clipping; distal anterior cerebral artery; microneurosurgery
Intracranial aneurysms are typically located at the arterial branching points near the skull base. Origin of an aneurysm distal to the circle of Willis is uncommon. One of the infrequent aneurysm locations is the portion of the anterior cerebral artery (ACA) distal to the anterior communicating artery (ACoA), also termed as the distal anterior cerebral artery (DACA). The DACA aneurysms represent approximately 6% (2–9%) of all intracranial aneurysms. These aneurysms have special features such as a small size, in concordance with the relatively small caliber of the DACA itself, and a broad base with originating branches. In addition, they are associated with vascular anomalies of the ACA, such as azygos, bihemispheric, and triplicated pericallosal arteries, arteriovenous malformations (AVMs), and multiple aneurysms. Though endovascular treatment has been used recently, microsurgery has been the treatment of choice for DACA aneurysms for several decades. There are few large series of DACA aneurysms available in literature. We reviewed cases of DACA aneurysms treated microsurgically at our institute to determine the clinical profile and distribution of DACA aneurysms along the different segments of distal anterior cerebral artery and to evaluate the vascular anomalies of this region, the surgical approach adopted in treating these aneurysms, and the outcome of surgery.
This study is a retrospective review of the data of patients who were treated surgically for DACA aneurysm between 1988 and 2013 at our institute. The patients' clinical case files and radiological data were reviewed for the relevant information. The aneurysms were identified by conventional catheter angiography or recently by computerized tomographic angiography (CTA). The site of origin of the aneurysm on DACA, size of aneurysm, number of aneurysms, and presence of vasospasm were noted. The anomaly of DACA was categorized according to the classification of Baptista; type I: the normal common pattern, type II: an accessory artery (or third A2 segment also called as median callosal artery), type III: the ayzgos trunk, and type IV: Bihemispheric distribution of cortical branches. The surgical approach and operative findings were recorded. The outcome at discharge was categorized as following: Good – no neurological deficits, mild disability – lower limb paresis with motor power ≥3/5, moderate disability – lower limb paresis with motor power <3/5, severe disability – complete paraplegia/hemiplegia/altered sensorium, and death. The outcome at follow-up was determined using the Glasgow outcome scale.
During the study period, a total of 2137 patients with an intracranial aneurysm underwent surgery, of which DACA aneurysms comprised 132 (6.2%) cases. The clinical details are presented in [Table 1]. All patients presented with subarachnoid hemorrhage. The most common clinical presentation was headache. Most patients presented in World Federation of Neurosurgical Societies (WFNS) grade 2, and CT scan Fisher grade 3. The most common location of the aneurysm was at the A3 segment in 115 (87.3%) cases. The most common projection was either anterior or superior [Figure 1] and [Figure 2]. Thirty-three (25%) patients had a very small (≤2.5 mm) aneurysm, 66 (50%) had a small (2.5–6 mm) aneurysm, and 8 (6.1%) had a large (>6 mm) aneurysm. The size of the aneurysm was not available in 25 (18.9%) cases. Angiographic vasospasm was seen in 36 (27.3%) cases, and multiple aneurysms were found in 16 (12.1%) cases. An azygous A2 was seen in 13 (9.8%) cases [Figure 3]. One patient (0.7%) had a single A2 artery; however, during surgery, a very thin A2 was also seen, confirming the diagnosis of a bihemispheric A2.
The aneurysms were approached through the frontotemporal/pterional craniotomy in 7 (5.3%) cases, bifrontal craniotomy interhemispheric and subfrontal approach in 64 (48.5%) cases, and unifrontal parasagittal craniotomy and interhemispheric approach in 61 (46.2%) cases. The choice of approach was dependent on the site of origin and direction of projection of the aneurysm as well as on the surgeon's preference, particularly in the case of A2 and A3 segment aneurysms. The A2 segment aneurysms were approached through the frontotemporal/pterional craniotomy or bifrontal craniotomy with an interhemispheric and subfrontal approach. The A3 segment inferiorly or anteriorly directed aneurysms were approached through the bifrontal craniotomy and interhemispheric approach. The A3, A4, and A5 segment aneurysms were approached through the unifrontal parasagittal craniotomy and interhemispheric approach. Intraoperative neuromonitoring was not performed in any of the cases. Intraoperative rupture of the aneurysm occurred in 13 (9.8%) cases. However, the stage of the surgery at which intraoperative rupture of the aneurysm occurred could not be retrieved from the available data. Temporary clipping was used as and when required either during dissection of the neck of the aneurysm or at the time of permanent clipping. Successful clipping of the aneurysm was possible in 123 (93.2%) cases. Wrapping of the aneurysm was done in 2 (1.5%) cases. The surgery was abandoned in 7 (5.3%) cases due to severe brain swelling or due to broad neck of the aneurysm. A routine postoperative check angiogram was not done in all patients. A check digital subtraction angiography (DSA) was, however, done if intra-arterial nimodipine therapy was required. [Figure 4] and [Figure 5] show an illustration of two cases.
The outcome was favorable in 74.2% cases at the time of discharge, and the in-hospital mortality was 6 (4.5%). The follow-up was available for 83 (62.9%) patients. There was no significant difference in the baseline characteristics of patients with and without follow up [Table 2]. The outcome was favorable in most of the patients at the time of follow up [Table 3].
The DACA aneurysms are relatively rare. They also manifest as akinetic mutism, bilateral leg weakness, behavioral changes, and cognitive deficits due to involvement of bilateral cingulate gyri and the supplementary motor areas either because of intracerebral hemorrhage (ICH) or infarction. In our series, lower limb weakness was present in 12.9% cases. We also had a relatively high incidence (18.9%) of seizures at initial presentation.
Computerized tomography scan findings
The incidence of ICHs is higher following rupture of DACA aneurysms due to the narrow pericallosal cistern and adherence of the aneurysmal fundus to the adjacent brain surface. The incidence of ICH following rupture of DACA aneurysms in the reported literature is 17–73%. The incidence in our series was 10.6%. Intraventricular hemorrhage (IVH) is seen in 25–30% of the patients with ruptured DACA aneurysms. The incidence of IVH in our series was 17.4%.
The DACA can be classified into the following segments. The A2 segment originates at the anterior communicating artery and extends till the rostrum of corpus callosum. The aneurysms arising from this segment can take origin from the frontobasal branches, proximal pericallosal arteries, or from the main arerial trunk. The reported incidence of A2 aneurysms is 5–22% of all DACA aneurysms.,, The incidence of A2 aneurysms in our series was 9.1%. The A3 segment spans around the genu of corpus callosum till its body. The aneurysms of the A3 segment are the classical pericallosal aneurysms. These aneurysms can be further divided depending on their origin. They can arise from the inferior, anterior, or superior portion of A3. This further classification of A3 (pericallosal) aneurysms is important because the inferior A3 (subcallosal) aneurysms are more difficult to access as these aneurysms are hidden below the corpus callosum and sometimes embedded in it. A more basal approach may be required to visualize the subcallosal aneurysms. The A3 segment is the most common site for the origin of aneurysms. The incidence of A3 aneurysms is 69–82% of all DACA aneurysms., The incidence of A3 (pericallosal) aneurysms in our series was 87.1%. The A4 segment continues till the vertical plane of coronal suture, and A5 segment beyond that. The aneurysms from A4 and A5 segments arise from the distal cortical branches or distal pericallosal branches. The incidence of A3 and A4 segment aneurysms is 5–20% of all DACA aneurysms., The incidence of A3 and A4 aneurysms was 2.3 and 1.5%, respectively, in our series.
The DACA aneurysms are small at the time of rupture. The mean size varies from 5 to 8 mm, with more than half of them being smaller than 7 mm.,, In our series, half of the aneurysms were small, and one-fourth were very small. Only 6.1% cases had large aneurysms. In addition to a small size, DACA aneurysms often have a broad base and a branch originating at the base., The small size and configuration makes endovascular treatment less suitable for most DACA aneurysms.
In addition to classification of segments of DACA, one has to look for associated vascular anomalies with DACA aneurysms. Different anomalies of the ACA have been observed in 7–35% of the patients with DACA aneurysms. The azygos (single distal) ACA is the most common anomaly associated with DACA aneurysms. In a series of 3572 cases who underwent three-dimensional time-of-flight magnetic resonance angiography (3D-TOF MRA), 14 (0.39%) patients were identified as having an azygous ACA. The incidence of azygous ACA aneurysms is 3–22% of all DACA aneurysms. The incidence of azygous ACA aneurysms was 9.8% in our series. Another anomaly, a “bihemispheric ACA,” is very difficult to distinguish from an azygous ACA in routine DSAs. The incidence of bihemispheric ACA is 0.2–12%, and the triplication of ACA occurs in 3–13% of patients with DACA aneurysms. In one of our cases, an azygous ACA was visible on DSA; however, during surgery, a very thin A2 was also seen, confirming the diagnosis of a bihemispheric ACA. We had only one case with a bihemispheric ACA. Thus, its incidence in our series was 0.7% of all DACA aneurysms. We did not encounter triplication of ACA.
Multiple associated aneurysms have been reported in 25–55% of patients with DACA aneurysms, a figure which is higher than the usual 28–35% reported for other aneurysmal locations.,, In our series, the incidence of multiple aneurysms was 12.1%. Angiographic vasospasm was seen in 27.3% cases.
Microsurgical clipping of DACA aneurysms is relatively difficult as compared to other aneurysms located on the circle of Willis. The difficulties are related to the approach deep inside the narrow interhemispheric space, the small pericallosal cistern, the lack of anatomic landmarks, the dense attachments, and embedding of the aneurysm in the surrounding brain tissue. Despite these challenging features, microsurgical clipping still offers better treatment results as compared to coiling. Though the number of patients undergoing coiling for DACA aneurysms has been increasing globally, it is still not in proportion to aneurysms in other locations. Endovascular occlusion of a DACA aneurysm is difficult because of its small size, relatively wide neck, branches originating close to its base, the small caliber of the parent artery, and the distal location of the aneurysm. The complication rate related with coiling is 7–18%. Rupture during the procedure is seen in 3–18% cases. The complete occlusion rate is 75–92% with coiling.,,,,
In our series, all patients underwent microsurgical treatment, which is the first choice of treatment for most of the aneurysms treated at our institute. The choice of surgical approach varied with the surgeon in our series. Although all aneurysms at and distal to the anterior communicating artery (AComA) can be treated using the interhemispheric approach, it is a necessity for those aneurysms that were 2 cm or more distal to the AComA. For the interhemispheric approach, the patient was positioned supine with the head in the neutral position. When the aneurysm occurred below or proximal to the genu of the corpus callosum (A2 or inferior A3 segment), the same positioning was employed but the head was extended. For more proximal aneurysms (A2 segment) lying just above the ACoA, the patient is positioned with the head rotated 15–30° away and maximally extended, an approach similar to any AComA aneurysm approached through the pterional route. In our series, 5.3% patients underwent a pterional craniotomy, 48.5% a bifrontal craniotomy, and 46.2% a unilateral parasagittal craniotomy. The bifrontal craniotomy was preferred for more proximal aneurysms (A2 or A3 inferior), and the unilateral approach for distal aneurysms. Unlike for other aneurysms, proximal arterial control could always be achieved later for DACA aneurysms. Some surgeons routinely employ the bifrontal basal approach for DACA aneurysms due to the achievement of an early and better proximal control associated with these aneurysms. Some surgeons also use neuronavigation. We did not use navigation for any of our cases. Navigation may be useful in more distal aneurysms such as those present in the A4 and A5 segments, or for aneurysms at the peripheral branches along the medial surface of the frontal lobe. The aneurysms can be successfully clipped in most cases. A complete occlusion rate of more than 90% with microsurgical clipping has been reported. In our series, successful clipping could be done in 93.2% cases. In 2 (1.5%) cases, wrapping of the aneurysm was done with muscle because of its broad neck and due to major vessel arising from the aneurysm. The surgery was abandoned in 7 cases (5.3%) because of severe cerebral edema. The complications of surgery for DACA aneurysms include an intraoperative rupture and venous infarction. Intraoperative rupture has been reported in 15–40% cases. In our series, the rate of intraoperative rupture was 9.8%. We did not encounter a case of venous infarction. Venous infarction is more likely during surgery for a more distal aneurysm, as the major draining veins at or posterior to the coronal suture are at risk of rupture.
Microsurgical series of patients with ruptured DACA aneurysms have reported favorable outcomes in 58–83% and a mortality in 7–21% cases. The in-hospital mortality in our series was 4.5%. At the time of discharge, 11.4% patients had a severe disability due to major neurological deficits, 54% had a mild to moderate disability, and 29.5% did not have any neurological deficits. The follow-up duration of 6 months or more was available for 83 (62.9%) patients. At follow-up, 85.6% patients had a favorable outcome.
The main limitations of our study was its retrospective nature and a study duration of many years. As the incidence of DACA aneurysms is low, it is not possible for a single center to do a prospective study. It will require a multicenter study to collect a large data base related to these aneurysms over a short period of time. The other limitation of our study was the lack of follow-up of all our patients. We compared the three major baseline predictors of outcome: age, clinical grade, and CT scan grade for patients with and without follow up. There was no statistically significant difference between the two groups. Hence, our data of patients in whom follow-up was available can be extrapolated to the entire group.
DACA aneurysms are uncommonly present. In spite of the technical difficulty encountered during their surgery, microsurgical treatment is preferred and gives good results.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]