Nomenclature

The anterior cerebral artery (ACA) is divided into five segments (Fig. 74-1). The A1 segment, also known as the precommunicating or horizontal segment of the ACA, originates with the ACA at the internal carotid artery (ICA) bifurcation and extends to the junction with the ACoA. The A2 segment, also known as the postcommunicating segment, originates at the ACoA and extends to the genu of the corpus callosum, following the contour of the rostrum. The A3 segment curves around the genu and extends to the body of the corpus callosum, where it assumes a posterior course. The A4 and A5 segments continue over the body of the corpus callosum, the division between them being located at the plane of the coronal suture. Contrary to popular opinion, this nomenclature is not defined by the bifurcation into pericallosal and callosomarginal arteries. This bifurcation is usually located along the A3 segment (approximately 60% of patients), but it can be more proximal on the A2 segment (10%), it can be more distal on the A4 segment (12%), or the pericallosal artery can be absent (18%).⁵

FIGURE 74-1 Microsurgical anatomy of the ACA. Anterior (A) and oblique (B) views, showing the five ACA segments and 12 arteries that can enter the surgical field around an ACoA aneurysm. The five ACA segments are: A1, precommunicating or horizontal segment; A2, postcommunicating or infracallosal segment; A3, precallosal segment; A4, supracallosal segment; and A5, postcallosal segment. CmaA, callosomarginal artery; FpA, frontopolar artery; LSA, lenticulostriate artery; OfA, orbitofrontal artery; PcaA, pericallosal artery; RAH, recurrent artery of Heubner.

(From Lawton, MT. Seven Aneurysms: Tenets and Techniques for Clipping. New York: Thieme, 2010.)

Normal Anatomy

The A1 segment courses medially and anteriorly over the optic tract and chiasm to the ACoA complex. Numerous penetrating branches (average 8, range 2-15)⁵ originate from this segment and course superiorly to supply the anterior perforated substance, subfrontal area, dorsal surface of the optic apparatus, hypothalamus, anterior commissure, septum pellucidum, and paraolfactory structures. Collectively, these perforators are also known as the medial lenticulostriate arteries, as opposed to the lateral lenticulostriate arteries that originate from the M1 segment of the middle cerebral artery (MCA). The most important of these perforators is the recurrent artery of Heubner, which originates from the proximal A2 segment on its lateral wall, just distal to the ACoA. This artery can arise from the distal A1 segment, just proximal to the ACoA, in 14% of patients or at the level of the ACoA in 8% of patients but is within 4 mm of the ACoA in 95% of patients.⁵ The artery is almost always present (98%) and can be duplicated (2%).⁵ The artery follows a course parallel to the A1 segment, either superior (60%) or anterior (40%) to it.⁵ The recurrent artery is typically seen before the A1 segment when the frontal lobe is retracted, making it a useful landmark to identify the A1 segment and the ACoA. The recurrent artery of Heubner supplies the head of the caudate nucleus, putamen, outer segment of the globus pallidus, and anterior limb of the internal capsule. Therefore, arterial injury can produce weakness involving the contralateral face and arm and expressive aphasia in the dominant hemisphere.⁶

The ACoA joins the two ACAs as they arrive in the interhemispheric fissure, completing the anterior part of the circle of Willis. Normally, the ACoA diameter is about half that of the A1 segments, but there is a direct correlation between asymmetry in the A1 segments and ACoA diameter. In other words, the ACoA diameter increases as the caliber of a hypoplastic A1 segment decreases, thereby compensating for the asymmetry in the A1 inflow.⁷ The ACoA can be duplicated in one third of patients, and triplicated in 10% of patients, but is always present.⁵ An ACoA that is not visualized angiographically is usually explained by an absence of cross-filling rather than an absence of the ACoA itself, and it can be visualized by a carotid cross-compression maneuver during angiography. The ACoA gives rise to important perforating arteries, which originate from its superior (54%) and posterior (36%) surfaces.⁵ These perforators course to the hypothalamus, median paraolfactory nuclei, genu, columns of the fornix, septum pellucidum, and anterior perforated substance. There can be perforators originating from the anterior and inferior aspects of the ACoA, supplying the dorsal optic chiasm, but these are few in number.

After making a right-angle turn from the horizontal A1 ACA, the A2 ACA runs superiorly in the interhemispheric fissure, coursing in front of the lamina terminalis and tracing the curvature of the genu. The branches arising from this segment are important more for correctly deciphering the anatomy of the ACoA complex than for their vascular supply. The orbitofrontal artery is the first cortical ACA branch, arising from the A2 segment approximately 5 mm distal to the ACoA from the anterolateral surface and coursing perpendicularly over the gyrus rectus and olfactory tract.⁶ This artery supplies the gyrus rectus, orbital gyri (anterior, posterior, medial, and lateral), and olfactory bulb/tract. It is important to avoid mistaking the recurrent artery of Heubner for this orbitofrontal artery. Both can be seen on the medial aspect of the gyrus rectus, but their origins from the A2 segment are separated and their courses are different, with the recurrent artery of Heubner eventually rejoining the A1 segment, even if it meanders under the gyrus rectus along its more proximal course. Correctly identifying the recurrent artery is especially important when resecting gyrus rectus. The recurrent artery is also typically small in caliber, with a diameter of approximately 1 mm.⁵ The orbitofrontal artery is often encountered draped over or adherent to the dome of a superiorly projecting aneurysm, which can tether the dome and make it more difficult to mobilize. The course of the orbitofrontal artery can also lead it across the neck of an ACoA aneurysm, requiring additional dissection before clipping.

The last relevant branch artery is the frontopolar artery, which originates from the A2 segment 14 mm, on average, from the ACoA near the genu.⁵ This artery courses anteriorly in the interhemispheric fissure and supplies the ventromedial frontal lobes. Rarely, it can originate from a common trunk with the orbitofrontal artery. Like the orbitofrontal artery, the frontopolar artery can be draped over the dome of a superiorly projecting aneurysm and should not be misinterpreted.

The terminal branches of the ACA are the pericallosal and callosomarginal arteries, bifurcating at a variable point along the genu of the corpus callosum. The callosomarginal artery is usually the smaller of the two, and it runs across the cingulate gyrus to the cingulate sulcus, where it continues posteriorly. It gives rise to the anterior, middle, and posterior internal frontal arteries, which supply the medial frontal lobes back to the precentral gyrus. The pericallosal artery courses over the corpus callosum, giving rise to the paracentral, superior internal parietal (precuneal), and inferior internal parietal arteries. These cortical branches extend over the convexity to supply the superomedial surfaces of the hemisphere, anastomosing laterally with middle cerebral arteries and posteriorly with posterior cerebral arteries in these watershed areas.

Variant Anatomy

Variations from the normal ACoA anatomy are common and probably contribute to the abnormal hemodynamic that forms aneurysms. While the effects of variant anatomy on aneurysm pathogenesis are poorly understood, the anatomy itself must be thoroughly understood by the neurosurgeon to correctly interpret the anatomy intraoperatively and safely clip an aneurysm. Variations can be categorized as involving the afferent arteries (A1 segments), efferent arteries (A2 segments), or the ACoA.

Asymmetry in the caliber of the A1 segments is seen in approximately 10% of patients, with a hypoplastic segment defined arbitrarily as having a diameter of 1.5 mm.⁵ Smaller diameters are observed less frequently, with approximately 2% of patients having an A1 segment diameter of 1 mm.⁵ Aplastic A1 segments are rare, despite their suggestion on angiography with a contralateral A1 ACA that fills both distal ACAs. At surgery, a small A1 segment is typically found. The dominance of an A1 segment is relevant to aneurysm formation, projection of the dome, site of hematoma, and choice of side. Dominant A1 segments are frequently seen in association with ACoA aneurysms, which typically project in the direction of blood flow in that segment.⁷ As discussed later, it is usually advantageous to surgically approach these aneurysms from the side of the dominant A1 ACA, both for easier proximal control of the aneurysm and for better clipping of the neck. One final variant, the duplicated A1 segment, is rare (2%) and almost always unilateral.⁵

Variations in ACoA anatomy include duplications, triplications, and fenestrations.^8,9 Accessory ACoAs are typically small and without significant perforators, making it important to recognize the primary ACoA and preserve both it and its perforators. Fenestrations can be the site of aneurysm formation and require special attention to differentiate the normal artery from the pathology. More important than these structural variations in the ACoA is its orientation. In only 18% of patients is the ACoA oriented in the transverse plane, as illustrated in anatomic textbooks.⁵ Instead, the ACoA complex is usually rotated or tilted, causing the A2 ACAs to course obliquely to one another in the interhemispheric fissure. This variation in orientation can direct an A2 segment more posteriorly, making its visualization more difficult. Rotation of the ACoA can shift the location of perforators from posterior to lateral, again making visualization more difficult, particularly when the aneurysm lies between the neurosurgeon and the perforators. These subtle changes in angles can be misleading, particularly when visualization is compromised by factors such as a swollen brain, a large aneurysm, and thick hematoma.

Variant anatomy in the efferent arteries can be the most difficult to deal with and is best understood in terms of the classification by Baptista.¹⁰ After analyzing 381 brain specimens, he defined three types of efferent artery anatomy. The type I anomaly, referred to as the azygos or “unpaired” ACA, is a single midline vessel arising from the confluence of the A1 segments (Fig. 74-2). Distally, the azygos ACA divides into pericallosal and callosomarginal arteries, with bifurcations, trifurcations, and quadrifurcations having been reported. This variant occurs in 0.3% to 2% of patients. The type II anomaly, referred to as the bihemispheric ACA, is an A2 ACA that transmits branches across the midline to supply both hemispheres, usually in the presence of a contralateral A2 segment that is hypoplastic or terminates early in its course toward the genu. This anomaly can be seen in as many as 12% of patients. The type III anomaly, referred to as an accessory ACA, is defined as a third artery originating from ACoA, in addition to the paired A2 segments and usually between them (Fig. 74-3). This accessory ACA variant is the most difficult one surgically because it results in some of the most unusual anatomic puzzles. If this variation is not appreciated, then a critical A2 ACA could be missed or, worse, sacrificed inadvertently during the aneurysm clipping (Fig. 74-4). The accessory ACA varies in caliber from a small remnant of the median artery of the corpus callosum (MACC) to a hyperplastic ACA that can resemble an azygos ACA when the two A2 segments are small in caliber and terminate early. A careful analysis of the angiogram preoperatively can alert the neurosurgeon to this challenging variant.

FIGURE 74-2 A 67-year-old woman, with chronic headaches, underwent left carotid artery angiography that demonstrated a 1.5-cm-diameter azygos ACA aneurysm and a 7-mm ophthalmic artery aneurysm in oblique (A) and anterior–posterior (AP) (B) views. The ophthalmic artery aneurysm was coiled, and the azygos ACA aneurysm was clipped through a bifrontal craniotomy and anterior interhemispheric approach. No residual aneurysms were seen on postoperative lateral (C) and AP (D) projection angiograms.

FIGURE 74-3 A 65-year-old male presented with severe headache and diffuse SAH. Two cerebral angiograms failed to reveal any aneurysms. His angiogram demonstrates a small accessory A2 ACA.

FIGURE 74-4 A, This small, ruptured anteriorly projecting ACoA aneurysm appeared to have classic anatomy at first glance. B, Additional dissection revealed an accessory A2 ACA. C, The neck was clipped with one straight clip that preserved flow through all three efferent A2 ACAs.

(From Lawton, MT. Seven Aneurysms: Tenets and Techniques for Clipping. New York: Thieme, 2010.)

The MACC just mentioned originates during embryogenesis when elongating ACAs coalesce in the midline to form plexiform anastomoses at 44 days.¹¹ Normally, the MACC regresses and disappears as the A2 segments mature, but vestigial remnants can account for the accessory ACA.

Aneurysm Anatomy

ACoA aneurysms, as a group, arise from the complex of arteries around the ACoA, but their precise location can be subdivided into true ACoA aneurysms, A1–A2 junction aneurysms, A1 ACA aneurysms, and variant aneurysms associated with the anatomic variants described earlier. In addition, the distal ACA aneurysms, notably the pericallosal artery aneurysms, are often included in this group.

The true ACoA aneurysm arises from ACoA and is defined further by the projection of its dome in an anterior, superior, posterior, or inferior direction. In Yasargil’s experience with ACoA aneurysms, the superiorly and anteriorly projecting aneurysms were the most common (34% and 23%, respectively), while posteriorly and inferiorly projecting aneurysms were the least common (14% and 13%, respectively).¹² Some aneurysms (approximately 16%) have mixed projection or multiple lobes.

The anteriorly projecting aneurysm is a favorable one from the neurosurgeon’s perspective because the parent arteries are separated from the aneurysm. The two A2 ACAs course at right angles to the aneurysm’s axis, making their identification in the interhemispheric fissure straightforward. The neurosurgeon has a good view across the neck of the aneurysm, with the dome and its likely rupture site removed from where this critical dissection takes place. Posteriorly projecting perforators are well away from the aneurysm and are easily preserved. The anterior projection of the dome typically leaves room under the neck to view across to the contralateral A1 segment to complete the exposure for proximal control of the aneurysm. This orientation often adheres the dome to the frontal lobes, which can limit the mobility of the aneurysm, but the dome can be freed as a final step in the dissection if greater mobility is needed to see contralateral anatomy or to increase the aneurysm’s maneuverability for clip application. In addition to their anterior projection, these aneurysms typically tilt to one side, most commonly away from the dominant A1 segment.

The superiorly projecting aneurysm is less favorable than the anterior projecting aneurysm, mainly because of the contralateral A2 ACA and the perforators (Fig. 74-5). Proximal control is straightforward, as the aneurysm is away from the A1 segments and the view to the contralateral side is unobstructed. However, the aneurysm is interposed between the neurosurgeon and the contralateral A2 ACA, requiring some manipulation of the aneurysm to locate this artery. Furthermore, larger aneurysms can displace perforators laterally or posteriorly, and their adherence to the aneurysm necessitates some delicate dissection. In addition, the A2 segments can be adherent, requiring aggressive dissection along the plane between this efferent artery and the fundus to fully expose the neck. Unlike the anteriorly projecting aneurysms, where the view behind the aneurysm is panoramic, the view behind a superiorly projecting aneurysm requires more extensive dissection. The ipsilateral A2 ACA must be mobilized anteriorly and can require dissection of its branches (i.e., the orbitofrontal and frontopolar arteries). The clip application is often more complex with these aneurysms, sometimes requiring fenestrated clips that encircle the ipsilateral A2 ACA.

FIGURE 74-5 Cerebral angiograms demonstrate the different projections of ACoA aneurysms: inferiorly (A) and anteriorly (B) projecting aneurysms. C, A CTA demonstrates a superiorly projecting right A1–A2 junction aneurysm at the ACoA complex.

Posteriorly projecting ACoA aneurysms are arguably the most challenging to clip. With these lesions, the A1 and A2 segments can be identified readily, but the perforators are markedly more difficult to visualize and preserve. They are often displaced laterally, where they become obstacles to the clip blades during clip application, and/or they are displaced posteriorly, where they can easily elude detection. In addition, the parent arteries of the ACoA complex are interposed between the surgeon and the aneurysm neck, making it more difficult to dissect and apply the clips to the neck. Fortunately, these aneurysms are uncommon.¹²

The inferiorly projecting aneurysm is another favorable aneurysm, with the one caveat: its dome often adheres to the optic apparatus, making it susceptible to avulsion and rupture early in the dissection with frontal lobe retraction. From that standpoint, it can be treacherous, because at that point in the dissection, proximal control is inadequate and the aneurysm anatomy has not been analyzed or even exposed. However, care in retracting the frontal lobe can avert this complication, and the advantages of an inferior projection make it a relatively straightforward aneurysm. Like the anteriorly projecting aneurysm, the ipsilateral A1 and bilateral A2 segments are easily visualized. The contralateral A1 segment can be obscured, which could compromise proximal control of the aneurysm, but the other critical anatomy is accessible. Perforators are rarely a problem, and the necks of these aneurysms are easily closed. These aneurysms can be adherent to the optic nerves and/or chiasm, and often it is preferable to leave the dome undissected or amputate it after clipping the neck, rather than manipulating the optic nerves with unnecessary dissection.

A1–A2 junction aneurysms arise at the bifurcation of the A1 segment into the ACoA and A2 ACA, with a distinctly separate ACoA. These aneurysms have the same variability in their projection (anterior, superior, posterior, and inferior) but also tend to have a lateral projection leftward or rightward. This lateral deviation can result in rupture into frontal lobe parenchyma opposite from the dominant A1 ACA. The perforators tend to be more manageable with these aneurysms than with the ACoA aneurysms.

A1 segment aneurysms are uncommon and tend to be located more proximally toward the carotid bifurcation than the ACoA complex. They are associated with the perforators from this segment, or with curves and bends in the artery as it courses to the ACoA complex.

Variant aneurysms include aneurysms that arise from a fenestration, duplicated or accessory ACoA, accessory A2 ACA, or azygos ACA. Accessory anatomy can pose additional risks, because it can mislead the surgeon and result in inadvertent arterial occlusions if not carefully protected. For example, the neurosurgeon might erroneously clip an important accessory A2 segment if an ipsilateral and contralateral A2 segment have already been identified. Therefore, thorough preoperative review of the anatomy and intraoperative dissection is critical before the final clipping is performed. Infundibuli arising from the ACoA have been observed, particularly with other vascular lesions such as arteriovenous malformations. These infundibuli, like those at the posterior communicating artery and elsewhere, can appear on angiography like aneurysms but transmit normal arteries and must be preserved. Dissection along the course of these arteries distinguishes them from aneurysms. ACoA aneurysms can be giant, atherosclerotic, calcified, or thrombotic. Aneurysms at this location can also be nonsaccular and due to other causes, including infection, trauma, and dissection.

Exposure

The first decision in the surgical plan for an ACoA aneurysm is the choice of approach. For most aneurysms, a standard pterional craniotomy is sufficient. The alternative approach is the orbitozygomatic approach, which increases the exposure for large, giant, or complex ACoA aneurysms. The orbitozygomatic approach is, in essence, an application of skull base surgery principles, removing the orbital rim, roof, and inferior frontal skull to maximize the operating space under the brain and minimize retraction. The difference in exposure between the standard pterional and the orbitozygomatic approaches is well documented in cadaveric and clinical studies,^22,23 but the decision to remove the orbitozygomatic unit should be made judiciously. In the senior author’s experience with 485 ACoA aneurysms, the orbitozygomatic approach was used in just 16% of patients. As with most maneuvers to expose ACoA aneurysms, the orbitozygomatic approach should be used only when necessary, since additional surgery adds to the patient’s cumulative risk and impact from surgery.

The second decision in the surgical plan is the side of the craniotomy. In general, ACoA aneurysms are approached from the side of the dominant A1 segment, because this gives the neurosurgeon the best proximal control, avoids the dome of the aneurysm, and presents a more favorable vantage point for dissecting the aneurysm and applying the clips. When the A1 segments are symmetrical, the aneurysm is approached from the side of the nondominant hemisphere. With as many as 85% of ACoA aneurysms having asymmetrical or dominant A1 segments, this approach to choosing the craniotomy side results in a significant number of left-sided craniotomies. In the senior author’s ACoA experience, exactly half of the craniotomies were left sided. While there is some theoretical concern about operating on the dominant hemisphere, in our experience there has been no observable difference in neurologic or cognitive outcomes based solely on the side of the craniotomy. Therefore, the real anatomic advantages of a craniotomy ipsilateral to the dominant A1 segment outweigh the theoretical disadvantages of a craniotomy on the patient’s dominant hemisphere. In cases where there is intraparenchymal hematoma in the frontal lobe contralateral to the dominant A1 segment, there may be some inclination to approach the aneurysm from the side of the hematoma. However, in our experience, the hematoma connects to the aneurysm dome and can be easily evacuated from the contralateral side. In cases where there are multiple aneurysms, the choice of side may be influenced by the side of these other aneurysms to maximize the number of aneurysms that can be treated with a single craniotomy.

The technique of the pterional approach is well described and thus is not discussed in detail here, except for the critical aspects. The patient is positioned in the supine position and secured to the table with bolsters to enable extreme table tilting. The head is positioned in the Mayfield headrest with the midline rotated 30 degrees away from the operative side and the head extended to allow gravity to retract the frontal lobes. In this position, the malar eminence becomes the highest point in the surgical field. The skin incision extends from the zygomatic arch to the hair line in the midline, coursing in a semicircular arc. The CT scan and angiogram are reviewed preoperatively to determine the likelihood of encountering the frontal sinuses at the anterior–inferior edge of the craniotomy. In patients with large sinuses, the pericranium is harvested as a separate layer after just the scalp is reflected forward, because it is easiest at this point in the procedure to prepare a large and competent graft to cover the sinus during the closure. When the pericranium is dissected down from an elevated scalp flap at the end of the procedure, it is often more difficult to handle, smaller, and susceptible to perforations. The temporalis muscle is mobilized anteriorly after fashioning a cuff along the temporal line that is used to reattach the muscle during the closure. Fishhooks under tension retract and flatten this muscle to keep it from obscuring the operative field.

The pterional craniotomy is cut from a single bur hole in the temporal bone superior to the zygomatic arch. Extensive drilling of the greater and lesser wings of the sphenoid bone is essential to maximize the exposure. The inner table of the inferior frontal bone is drilled until flush with the anterior cranial fossa floor, and the ridges of the orbital roof are flattened. The temporal bone is drilled until flush with the middle fossa floor, and the anterior temporal bone is drilled until the exposure is contiguous with the lateral orbital wall. All intervening pterional bone is drilled away until the superior orbital fissure is identified and opened. Drilling is continued down the medial sphenoidal ridge to the base of the anterior clinoid process, which can leave a prominence in the surgical corridor if it is not flattened. Bleeding from the base of the anterior clinoid process can well up and run into the surgical field during microdissection, making absolute hemostasis mandatory. The dura is opened in a semicircular flap based on this hinge point at the anterior clinoid process. When reflected and tacked with suture, this dura flap should form a flat surface and afford an unobstructed view to the carotid cistern.

The orbitozygomatic approach differs only in the dissection of the temporalis muscle and in the osteotomies of the orbit and zygoma.²⁴ We prefer the subfascial dissection to expose the zygoma and lateral orbit, but an interfascial dissection can also be used.²⁵ The osteotomies are performed after the craniotomy, yielding a separate bone flap and orbitozygomatic unit that reassemble with excellent cosmetics postoperatively. The plates that replace the orbitozygomatic unit are applied and screw holes are predrilled before the unit is detached to ensure proper realignment. In addition, we prefer a reciprocating saw over a high-speed drill to make the osteotomies, and we prefer stepped cuts rather than linear cuts because they interlock the orbitozygomatic unit to the skull, enhancing the reconstruction.

A critical decision in the microdissection is whether to dissect the sylvian fissure. There is no need to expose the MCA or its branches, but the sylvian dissection separates the frontal and temporal lobes to open the corridor to the ACoA complex. The important maneuver is the gentle elevation of the frontal lobe to expose the recurrent artery of Heubner and the A1 ACA, and without splitting the sylvian fissure, the temporal lobe often is pulled into that corridor. Therefore, some degree of sylvian fissure dissection is required, sometimes just the proximal portion extending several centimeters from the carotid cistern but other times a full fissure split down to the limen insula. In older patients, this extensive dissection can be straightforward, but in younger patients with edematous brains after SAH, this dissection can be difficult. The neurosurgeon must weigh the facility of opening the fissure with its necessity in determining the extent of dissection.

With the proximal sylvian fissure dissected and the carotid bifurcation exposed, a retractor can be placed with its tip on the posterior portion of the medial orbital gyrus. This retractor gently elevates the frontal lobe and exposes the A1 segment. A site should be prepared along this segment early on to place a temporary clip for proximal control. The A1 segment can then be followed to the ACoA complex. The recurrent artery of Heubner can also be used as a guide to the ACoA complex and is often seen before the A1 ACA. The exposure of an ACoA aneurysm requires progressively shifting the frontal retraction forward, from the posterior medial orbital gyrus to the gyrus rectus. In anticipation of greater retraction and to optimize brain relaxation, the lamina terminalis is opened early in the dissection to release CSF. The arachnoid around the optic nerves is incised, extending the dissection into the chiasmatic cistern and across the chiasm to the contralateral optic nerve. The lamina terminalis is then identified posterior to the chiasm and beneath the ipsilateral A1 ACA and ACoA. Inferiorly projecting aneurysms and some anteriorly projecting aneurysms may impede access to the lamina terminalis, in which case this maneuver is best deferred until after the aneurysm is clipped.

The frontal retractor can then be advanced forward on the medial orbital gyrus, with the tip just lateral to the olfactory tract. The recurrent artery of Heubner is dissected away from the frontal lobe so that it lies with the A1 ACA. It is important to identify and mobilize this delicate artery to avoid inadvertent occlusion by the retractor blade or inadvertent injury with gyrus rectus resection. When the retractor is lateral to the olfactory tract, the gyrus rectus overlies the ACoA complex and the neurosurgeon can evaluate whether it should be resected or whether the interhemispheric fissure can be opened to elevate the gyrus rectus. Patients with unruptured aneurysms or atrophy often have interhemispheric fissures that can be opened widely to expose the ACoA complex without gyrus rectus resection. In others, gyrus rectus resection is the simplest, safest maneuver to expose the ipsilateral A2 ACA and proximal neck of the aneurysm.

The pia over the gyrus rectus is coagulated and incised, and brain is removed until the pia on the medial frontal lobe is reached. The orbitofrontal artery often courses across the gyrus rectus and should be avoided. When sufficient gyrus rectus has been removed, the retractor can be advanced again, with the tip of the blade within the resection cavity. The tip of the retractor should lie over the shoulder of the recurrent artery, as it originates from the ipsilateral A2 ACA, to protect this artery. The medial arachnoid is then incised to enter the interhemispheric fissure where the A2 ACA and its distal branches can be visualized.

Clipping

Successful clipping depends on thorough dissection and interpretation of the anatomy, which can be challenging at the ACoA complex (Fig. 74-6). In all, 11 arteries need to be identified: bilateral A1 ACAs, bilateral A2 ACAs, bilateral recurrent arteries of Heubner, ACoA, bilateral orbitofrontal arteries, and bilateral frontopolar arteries. The last two arteries are often outside the surgical field and are relevant only in rare cases where they course inferiorly into the surgical field. The first two arteries are the most important, because they provide the proximal control. Identification of the contralateral A1 ACA requires dissection across the region of the aneurysm, so its projection must be appreciated and the dissection should steer clear of the dome. Gyrus rectus resection or interhemispheric fissure dissection identifies the ipsilateral A2 ACA. The contralateral A2 ACA is the most difficult of the arteries to identify because it is the deepest artery in the surgical field and its visualization is usually obscured to some degree by the aneurysm. Therefore, the contralateral A2 ACA is typically the last of the five major arteries (two A1 ACAs, two A2 ACAs, and ACoA) to be found, often requiring some manipulation of the aneurysm. Alternatively, it can sometimes be seen in the interhemispheric fissure distal to the aneurysm and traced back to the ACoA complex. The orbitofrontal arteries are identified at their origin just distal to the recurrent artery and often course over the aneurysm or region of the dissection. Anatomic variations at the ACoA complex may add to or subtract from the 11 arteries.

FIGURE 74-6 ACoA aneurysm dissection steps: (1) following the A1 segment and recurrent artery of Heubner, (2) identifying the A2 segment, (3) crossing the midline via the ACoA, (4) controlling the contralateral A1 segment, (5) entering the distal interhemispheric fissure, (6) tracing the contralateral A2 segment proximally, and (7) separating the perforators from the aneurysm neck.

(From Lawton, MT. Seven Aneurysms: Tenets and Techniques for Clipping. New York: Thieme, 2010.)

After visualizing the parent arteries, attention is directed to the aneurysm neck and perforator dissection. ACoA aneurysms are notoriously adherent to surrounding structures. Aneurysms projecting superiorly typically adhere to the A2 ACAs, while those projecting inferiorly adhere to the optic apparatus. Posteriorly projecting aneurysms typically adhere to perforators, while anteriorly projecting aneurysms adhere to the frontal lobes. Therefore, the final steps in the dissection are often completed with the aneurysm softened with temporary clips on the A1 segments, mobilizing it from its surrounding adhesions and preparing it for clip application. For this, cerebroprotection with barbiturates is used, in conjunction with neurophysiologic monitoring.²⁶ Electroencephalography and somatosensory evoked potentials are monitored during administration of barbiturates, with the anesthetic dose titrated to achieve electroencephalogram burst suppression. Mild hypothermia is also used routinely for cerebroprotection. Temporary clips can then be applied, with neurophysiologic monitoring indicating the patient’s tolerance to parent artery occlusion during the dissection. If changes in neurophysiologic monitoring are observed, temporary clips can be removed to reperfuse the ischemic territory, or the blood pressure can be elevated with vasopressor agents. Typically, only a few minutes are needed to complete the dissection and apply the clips. Minimizing temporary clip times requires maximizing the dissection that can be done beforehand, preselecting permanent clips, and having an experienced scrub nurse. Surgeon speed is of utmost importance during temporary clipping to reduce risk of ischemic injury. Temporary clipping relaxes the aneurysm to give the neurosurgeon the opportunity to perform the final, risky maneuvers, whether mobilizing the aneurysm to identify the remaining anatomy, clearing the perforators from the path of the clip blades, and/or establishing the cleavage planes between branch arteries and the aneurysm neck.

Permanent clipping depends on the aneurysm projection, size, and anatomy of the A1–A2 junction (Fig. 74-7). Aneurysms that project anteriorly and inferiorly can usually be clipped with a straight clip, since the aneurysm and the afferent/efferent arteries are on opposite sides of the ACoA. With these aneurysms, the viewing angle is along the length of the neck, parallel to the ACoA, with good visualization of posterior perforators and parent arteries. Aneurysms that project superiorly can be difficult because of the intimate relationship between the neck and the A2 segments. A clip that parallels the ACoA must be positioned so that its tips do not compromise the contralateral A2 ACA, with the tips either stopping just short of this segment or passing anterior or posterior to it. On the ipsilateral side, a fenestrated clip is often needed to close the proximal neck without compromising the ipsilateral A2 ACA. These requirements demand precise clip application, but fortunately the perforators are not as difficult to dissect as with posteriorly projecting aneurysms. Alternatively, a more posteriorly directed clip can sometimes be applied perpendicular to the ACoA. This clip application can produce a dog-ear remnant below the clip, which might require additional clips below the original one. Aneurysms that project posteriorly can be difficult because not only are the A2 segments intimately associated with the neck on its anterior aspect but so are the perforators on its posterior aspect. These aneurysms often require fenestrated clips such as the superiorly projecting aneurysms, because the clip blades pass through or around the ACA vessels. Most importantly, the back blade must course safely through a complex array of treacherous perforators.

FIGURE 74-7 Clipping techniques for ACoA aneurysms projecting inferiorly (simple clipping, straight clip) (A), anteriorly (simple clipping, straight clip) (B), superiorly (tandem clipping) (C), and posteriorly (simple clipping, angled fenestrated clip) (D).

(From Lawton, MT. Seven Aneurysms: Tenets and Techniques for Clipping. New York: Thieme, 2010.)

The large and giant ACoA aneurysms have challenges associated with their size, as they do at other locations. Wide necks, fusiform morphology, or an atherosclerotic artery contribute to the complexity for endovascular and microsurgical treatment. These aneurysms often necessitate closure with multiple clips. Single straight clips on these aneurysms tend to fail, because tissue in the proximal blades prevents the distal tip from closing. Fenestrated clips address this problem, with the fenestration encircling only the proximal neck of the aneurysm rather than an A2 segment. Then, a second clip can close the proximal neck encircled by the first clip, thereby completing the repair and contouring the reconstruction. Some of the most challenging aneurysms are those with A2 ACAs originating from the base of the aneurysm (Fig. 74-8), with acute angles between the A1 and the A2 segments or, even worse, with A2 segments that leave the aneurysm parallel to the A1 segments. Clip reconstruction for this unusual anatomy is associated with a significant risk of efferent artery compromise and often requires intraoperative angiography.

FIGURE 74-8 A 44-year-old woman with a family history of intracranial aneurysms underwent screening neuroimaging that revealed aneurysms. Lateral (A) and anterior–posterior (B) views of a left carotid artery angiogram demonstrated a large ACoA aneurysm measuring approximately 14 × 7 mm with an ill-define neck, as well as a small right superior hypophyseal artery (SHA) aneurysm (not shown). Note the incidental persistent left trigeminal artery. Endovascular coiling of the aneurysm was attempted, but the anatomy was not favorable for this modality. The patient then underwent a left orbitozygomatic–pterional craniotomy for clipping of the ACoA and right SHA aneurysms. The aneurysm anatomy is demonstrated before clipping (C) and after clipping, with views below the clips (D) and above them (E). The aneurysms are indicated by the asterisk. Postoperative cerebral angiography showed exclusion of both ACoA and SHA aneurysms and flow in bilateral ACA arteries (F, right carotid artery angiogram, anteroposterior view; G, left carotid artery angiogram, anterior oblique view).

Once the aneurysm is clipped, it is critical to carefully inspect the anatomy to be certain the aneurysm no longer fills, there is no neck remnant, and the parent arteries are patent. Gross inspection and palpation of the dome with a Rhoton No. 6 instrument can often indicate whether an aneurysm still fills, with persistent pulsations observed in those that are incompletely occluded. Incompletely occluded aneurysms typically have some neck remaining distal to the tips that may not have been appreciated during clip application, or the neck may have elongated as the blades closed. This situation requires advancement of the clip across this remnant. If the clips are across the neck completely but the aneurysm still fills, it may require an additional stacked clip to reinforce the closure. When the aneurysm appears completely closed, it is punctured and deflated to be sure. At this point, the dome can be transected or aggressively mobilized to visualize anatomy that might have been poorly appreciated prior to clipping. Sometimes additional dissection after an aneurysm is clipped can confirm the interpretation of anatomy or proper clip application or can reveal technical errors. Therefore, it is critical to dissect beyond the point of aneurysm clipping. The neck beneath the clips along the ACoA is inspected next to ensure that no dog-ear remnants remain. These remnants are easily closed with additional microclips. Lastly, the 11 parent arteries are inspected for patency, often with a micro-Doppler probe and/or intraoperative angiography.

Alternative Techniques

The ACoA complex is home to complex aneurysms that cannot be treated with conventional clipping, including giant aneurysms, thrombotic aneurysms, calcified/atherosclerotic aneurysms, fusiform aneurysms, and mycotic/infectious aneurysms. For these aneurysms, other techniques may be needed.

Giant aneurysms require techniques not unlike the ones already discussed. The additional room provided by the orbitozygomatic approach makes it routine for these aneurysms.²² Temporary clipping is particularly valuable, because a giant aneurysm that no longer has antegrade filling is deflated and mobile, thereby opening the surgical field and facilitating the dissection and clipping.

With thrombotic aneurysms, the aneurysm does not respond to temporary clipping because thrombus transforms the aneurysm into a solid mass. Therefore, thrombectomy is needed to convert this mass into a deformable sac (Fig. 74-9). After trapping the aneurysm with temporary clips on both A1 and both A2 ACAs, it is entered by incising the wall. With some giant thrombotic aneurysms, complete control is not possible at the outset because the bulk of the aneurysm prevents visualization of contralateral anatomy. In these cases, bleeding can be controlled from inside the aneurysm with a Cottonoid over the orifice of the artery until the involved artery is isolated and temporarily clipped. A Cavitron ultrasonic surgical aspirator (CUSA) is typically used to break up thrombus and clean out the aneurysm lumen, with attention focused at the neck where the clip needs to be placed. Transecting the aneurysm can expedite thrombectomy by freeing the essential tissue needed to reconstruct a neck, rather than gutting the entire lumen and mobilizing the sac. This transaction maneuver can be important, because it minimizes ischemia time and risk of neurologic complication.

FIGURE 74-9 A 50-year-old man presented with a grand mal seizure and was diagnosed with an unruptured giant ACoA aneurysm. A, An axial T2-weighted magnetic resonance image revealed a giant, partially thrombosed aneurysm in the anterior interhemispheric fissure, with surrounding cerebral edema. Lateral (B) and anterior–posterior (C) views of the left carotid artery angiogram demonstrated an inferiorly and anteriorly projecting, bilobed aneurysm. D, The patient underwent a left orbitozygomatic craniotomy to widely expose the aneurysm. Before the aneurysm neck could be clipped, the aneurysm was incised (E) and the intraluminal thrombus was removed with a CUSA (F). Postoperative left carotid artery angiography revealed good flow in the A1 and A2 branches bilaterally and no residual aneurysm filling (G, lateral view; H, anteroposterior view).

Previously coiled aneurysms that have residual filling or that recur are often managed like thrombotic aneurysms (Fig. 74-10). They too behave like a solid mass, and their intraluminal content is largely thrombus. Ideally, coils that have compacted create a neck beneath the coils that will accept a clip, thereby simplifying the management. In other cases, thrombectomy is required to mobilize the neck for clip reconstruction. The CUSA can be used as with purely thrombotic aneurysms, leaving the coil mass in place and clipping around the coils. Only rarely is it advisable to attempt to extract coils from the aneurysm.

FIGURE 74-10 A, A 40-year-old woman presented to an outside hospital with SAH on an axial CT scan. B, She was found to have right MCA and ACoA aneurysms on a right carotid artery angiogram. She underwent a right pterional craniotomy to clip the right MCA aneurysm and was then referred to this institution for coiling of the ACoA aneurysm. C, Three years later, the ACoA aneurysm recurred and the patient was referred for microsurgical clipping. D, She underwent a right orbitozygomatic craniotomy, and the coiled anterior portion of the aneurysm could be visualized. E, The recurrent posterior portion of the aneurysm was located behind this coil mass and was occluded with a fenestrated clip that encircled the coils. F, Postoperative angiography revealed no residual filling of the aneurysm.

Bypass techniques allow complex aneurysms to be proximally occluded or trapped with minimal risk of ischemic complications. However, revascularization of the distal ACA territory for ACoA aneurysms is more difficult than with other vascular territories and aneurysms. The superficial temporal artery (STA) and occipital artery, the two most commonly used donor arteries for other extracranial-to-intracranial bypasses, are not long enough for a distal ACA bypass. Interposition grafts, such as the radial artery or saphenous vein, bridge from the carotid artery or STA to the ACA. They are long, require extensive exposure, and are prone to late occlusions. In situ bypasses that connect the A3 or A4 ACAs in the interhemispheric fissure with a side-to-side anastomosis can be used when blood flow in one of the efferent A2 arteries can be preserved during the aneurysm occlusion (Fig. 74-11). Most of these bypass procedures require an interhemispheric approach, separate from the pterional approach used for the ACoA aneurysm. A multimodality approach that creates a surgical bypass first and then occludes the aneurysm endovascularly in a separate stage has appeal. It minimizes the extent of surgery, limiting the craniotomy to just an interhemispheric approach rather than a combined pterional–bifrontal approach. It also confirms bypass patency before arterial sacrifice and allows the patient’s medical status and hemodynamics to be optimized for the second-stage procedure. This multimodality strategy has proved effective with atherosclerotic, fusiform, mycotic/infectious, and traumatic aneurysms involving the ACoA complex.²⁷

FIGURE 74-11 A, A 78-year-old woman presented with aphasia and headaches. Her head CT scan demonstrated obstructive hydrocephalus due to a giant, partially thrombosed ACoA aneurysm. B, Left carotid artery angiogram (anteroposterior view) revealed a superiorly projecting aneurysm without a discrete, clippable neck. The left A1 and A2 ACAs were separated by the base of the aneurysm, indicating a fusiform morphology. A multimodality treatment was planned, with a bypass to the distal ACA as a first stage and endovascular coil occlusion of the aneurysm as a second stage. She underwent bifrontal craniotomy, interhemispheric approach, and A3–A3 ACA side-to-side anastomosis to revascularize the ACA territory distal to the aneurysm. The back walls of the anastomosis are sutured from within the arterial lumen first (C), followed by the front walls (D). E, The depth of this surgical corridor is shown. F, Postoperative left carotid artery angiography demonstrated a patent bypass with good distal flow in bilateral ACA territories. G and H, The patient then underwent coil occlusion of the aneurysm and the proximal left A1 ACA, with right carotid artery angiography demonstrating obliteration of the aneurysm and blood flow in bilateral ACA territories that originates from the right A2 ACA and crosses to the left hemisphere through the bypass. H, The anastomosis site is indicated by the large black arrow.