Relevant Anatomy

The DACA is defined as the extension of the ACA beyond the ACoA (Fig. 369-1). The DACA ascends anteriorly and superiorly from the juncture of the ACoA to the genu of the corpus callosum in the interhemispheric fissure, running between the medial aspects of the frontal lobes. Superior to the lamina terminalis, the DACA then curves around the genu of the corpus callosum and divides into the inferior pericallosal and superior callosomarginal arteries. These arteries pass over the body of the corpus callosum, where they form anastomotic connections with splenial branches of the posterior cerebral artery (PCA). The DACA provides the vascular supply for the medial surface of both hemispheres and a large segment of the anterior corpus callosum.

FIGURE 369-1 Anatomic schematic of the distal anterior cerebral artery (DACA) including sites of aneurysm formation: 1, orbitofrontal artery; 2, frontopolar artery; 3, anterior internal frontal artery; 4, middle internal frontal artery and callosomarginal artery; 5, posterior internal frontal artery; 6, paracentral artery; 7, superior parietal artery; 8, inferior parietal artery. The asterisk (*) denotes typical sites for traumatic aneurysm of the DACA. CMA, callosomarginal artery.

There have been several proposed naming schemes for the various elements of the DACA, the most accepted being the nomenclature proposed by Perlmutter and Rhoton.²⁵ The DACA is subdivided into four segments, A2 to A5, with A1 being the segment of the ACA proximal to ACoA.²⁵ The A2 segment is defined as the section of the artery from the ACoA to the junction of the rostrum and genu of corpus callosum. The orbitofrontal and frontopolar arteries arise from this segment. The A3 segment extends from the genu to a point where the artery makes a posterior turn above the genu. The callosomarginal artery arises from A3. The A4 segment is the posterior extension of the artery from A3 and extends to a point bisected by the coronal suture. The A5 segment then extends distally to include the anastomoses with the splenial arteries.

The eight cortical branches of the distal ACA include the orbitofrontal and frontopolar from A2; anterior middle, posterior internal frontal, and callosomarginal from A3; paracentral from A4; and superior and inferior parietal arteries from A5. The largest branch of the DACA, the callosomarginal artery (CMA) is the most common site of DACA aneurysms.²⁵ The CMA gives rise to a variable number of cortical branches; most commonly, the middle internal frontal artery, followed by posterior and anterior internal frontal arteries and the paracentral artery; and less commonly, the superior parietal artery. The frontopolar artery is the second most common site of DACA aneurysms.

Vascular anomalies are commonly described in association with DACA aneurysms, and the most commonly observed variants in distal ACA anatomy include azygous ACA (type I variation), bihemispheric ACA (type II), and triplicate or accessory ACA (type III) (Fig. 369-2).^25,26 It is thought that an association between these vascular variants and DACA aneurysms may be related to a flow disturbance that then leads to aneurysm formation. For example, up to 10% of DACA aneurysms are associated with an azygous ACA, in which the distal segments of both anterior cerebral arteries are represented by a single common vessel. The presence of these variants has significant implications, both in considering the surgical approach and in clinical outcome, because damage to the single trunk may result in bihemispheric deficits.

FIGURE 369-2 The usual pattern and common variations of anatomy of the distal anterior cerebral artery.

(Adapted from Royand F, Carter P, Guthkelch N. Distal Anterior Cerebral Artery Aneurysms. New York: McGraw-Hill; 1995.)

Clinical Presentation

The average age at presentation of patients with DACA aneurysm is about 50 years, and there appears to be a slight female preponderance.⁷ Symptomatic aneurysms may present with subarachnoid hemorrhage (SAH), which is often prominent in the interhemispheric fissure across the top of the corpus callosum (Fig. 369-3), a pattern that may be confused with that from superiorly directed ACoA aneurysms. These aneurysms may also bleed into the adjacent frontal lobe and occasionally into the ventricles. Intraventricular hemorrhage generally results from extension of a frontal lobe hematoma into the ipsilateral frontal horn, rather than downward extension into the third ventricle.

FIGURE 369-3 Characteristic head computed tomography scan of patient with a ruptured distal anterior cerebral artery arising at the origin of the right callosomarginal artery, demonstrating thick clot in the interhemispheric fissure as well as diffuse subarachnoid hemorrhage.

Generally, patients with a ruptured DACA aneurysm present with classic findings of SAH. Additionally, intracerebral hemorrhage (ICH) is a common complication of ruptured DACA aneurysm and may be found in as many as 50% of cases.^3,27 In large part resulting from this high incidence of ICH, some authors have reported an unusually high incidence of poor clinical grades in patients presenting with ruptured DACA aneurysms relative to aneurysms at other sites.^2,27 One of the difficulties associated with aneurysms in this area is a lack of reliable localizing neurological findings. Nevertheless, the presence of a focal hemorrhage leading to vascular compromise and ischemia may result in memory loss, mutism, or rarely, a monoparesis in the case of a distally located clot. A hemispheric disconnection syndrome may also occur following a significant intracallosal hemorrhage.²⁸

All patients with suspected ruptured DACA aneurysms should undergo four-vessel angiography in an attempt to identify the ruptured aneurysm as well as to uncover additional aneurysms (Fig. 369-4). However, it is often difficult to determine the side of origin of DACA aneurysms even with a high-quality angiogram. Magnetic resonance imaging (MRI), magnetic resonance angiography, and venography are useful in planning the skin incision and bone flap. These images also can also be used to determine the proximity of the aneurysm dome to the cingulate gyrus.

FIGURE 369-4 Anteroposterior (A) and lateral (B), as well as three-dimensional reconstruction (C), of a large, bilobed, broad-necked aneurysm involving the origins of both the right pericallosal and callosomarginal arteries of the A2 segment. This aneurysm was successfully clipped.

Surgical Considerations

Although endovascular coiling is considered the first-line therapy for many cerebral aneurysms, distal ACA aneurysms are often treated microsurgically because of their peripheral location, small size, unfavorable neck-to-parent artery ratio, and tendency to “blow out” the bifurcation at which they occur. Moreover, the amount of brain retraction and muscle dissection needed to clip these lesions is comparatively minimal relative to other aneurysms. Factors that favor endovascular treatment include extreme age, significant medical comorbidities, poor neurological condition, and irreversible coagulopathy.

For ruptured lesions, surgical timing and medical management are identical to those for other cerebral aneurysms. We recommend operating on ruptured aneurysms acutely and delay surgery only in patients who are so devastated neurologically that a functional recovery is unlikely. If there is an unruptured DACA aneurysm and a ruptured aneurysm at another location, some authors advocate repairing all aneurysms during one procedure to facilitate treatment of vasospasm with hemodynamic therapy.²⁹ Our practice is to repair only the ruptured aneurysm and leave the unruptured DACA aneurysm because it requires both extending the craniotomy and further brain manipulation and operative time. We do not believe this additional risk is warranted because we have never seen an unruptured aneurysm bleed during hemodynamic therapy, intra-arterial vasodilator therapy, or angioplasty.

Preoperative Preparation

All patients are managed with normovolemia and normotension, so long as severe vasospasm is not an issue. Hydrocephalus resulting in neurological decline is treated with external ventricular drainage. However, if communicating hydrocephalus is being tolerated, intraoperative spinal drainage is preferred. Dexamethasone, gastrointestinal prophylaxis, perioperative antibiotics, and intraoperative phenytoin are administered. In good-grade disease patients with SAH and unruptured aneurysms, a loading dose of phenytoin is administered intraoperatively but not continued postoperatively. Phenytoin is continued both in poor-grade disease patients and those with risk factors for seizures such as intraparenchymal clots or other parenchymal pathology. Antifibrinolytic therapy decreases early aneurysmal rebleeding but increases ischemic deficits from vasospasm.³⁰ Therefore, we employ this in all patients in whom surgery is delayed for any reason.

Operative Procedure

Skin Incision and Craniotomy

The scalp incision and bone flap take into account any associated aneurysms as well as the precise location of the DACA aneurysm. For extremely proximal A2 lesions, a bicoronal, modified bicoronal, or pterional incision may be used in preparation for a pterional craniotomy with or without orbital or orbitozygomatic extension. For the usual DACA aneurysm arising from the callosomarginal artery origin in the region of the genu, we use a modified bicoronal scalp incision extending from the ipsilateral zygoma across the midline to the contralateral superior temporal line, at the hairline (Fig. 369-5). We make a parasagittal craniotomy beginning at the margin of the frontal air sinus and extending 4 to 6 cm superiorly toward the coronal suture, laterally to the superior temporal line and across the superior sagittal sinus. Some surgeons place the bur holes directly over the sagittal sinus. We prefer to place slots across the sinus so that the dura of the contralateral frontal lobe is visible. This allows one to raise the bone flap with the footplate attachment never contacting the sinus and allows one to rotate the sinus slightly to the contralateral side after opening the dura, resulting in increased exposure. Care must be taken to not occlude the sinus with this maneuver. More distal pericallosal artery aneurysms are also best approached interhemispherically (see Fig. 369-5). The exact site of the scalp incision and craniotomy is determined by the specific location of the aneurysm as well as the underlying venous anatomy. Frameless stereotactic coregistered to an MRI, magnetic resonance angiogram, or venogram can be helpful in planning the exact location of both. Attempts should be made to avoid retraction on the motor cortex, and broader-based scalp flaps that cross the midline allow flexibility to operate in front of and behind critical venous anatomy.

FIGURE 369-5 Surgical approaches for distal anterior cerebral artery aneurysm. 1, Approach for supracallosal aneurysms; 2, approach for proximal aneurysms.