Endovascular Approaches to ACoA Aneurysms

Coiling remains the endovascular treatment of choice for ACoA aneurysms.^19,20 The acute angle of the A2 segments as they originate from the ACoA often renders balloon remodeling and stenting difficult. Several studies have validated the efficacy and safety of coiling these aneurysms. Nonetheless, they remain a challenging subgroup of lesions to treat²¹ because of a number of factors: difficulty catheterizing the A1 segment, tortuosity at the A1-2 junction, and suboptimal imaging caused by multiple arterial branches arising near the aneurysm site. Improvements in catheter and microwire technology and refinement of three-dimensional (3-D) angiography have reduced these limitations to a certain degree.²²

Accurate visualization of the ACoA aneurysm through 3-D angiography is essential to assess whether the lesion can be embolized. The location of the aneurysm, specifically whether it arises from the ACoA segment itself or from the A1-2 junction, is also critical in determining the endovascular approach. In patients with only one A1 segment, preservation of the ACoA is the only way to preserve flow into the contralateral A2. In such cases, care must be taken to prevent compromising the ACoA segment by coils protruding from the aneurysmal sac. Obviously, coil herniation and thrombosis of the ACoA artery would be better tolerated in patients with bilateral patent A1s. For these reasons, angiography of both ICAs often must be undertaken before the aneurysm is addressed.

For aneurysms arising from the ACoA segment itself, the direction that the aneurysm points is a good predictor of the success of treatment. Inferiorly and posteriorly projecting lesions are more difficult to coil because visualization is often suboptimal and catheter stability within the aneurysm can be tenuous. Steam shaping the catheter tip or using an appropriately angled catheter can reduce the tendency of catheter “kick back” while coiling. That ACoA aneurysms are typically small further complicates treatment.^23,24 This factor, coupled with the tortuosity of the A1 and ACoA segments, often renders catheter stability precarious within the aneurysm sac. The operator may be forced to reposition the catheter and to recatheterize the aneurysm several times before a stable coil mass is achieved within the lesion. These repeated efforts increase the likelihood of these aneurysms rupturing during the procedure.

The use of balloon remodeling can serve three important purposes in the treatment of ACoA aneurysms.²⁵ Specifically, this technique can improve the likelihood of coil retention within wide-necked lesions, increase the density of the coil pack, and provide a means of tamponade in the event of an intraprocedural rupture.²⁶ Navigating a balloon, however, into a small or tortuous A1 segment can be difficult and often impossible (Fig. 374-1). The smaller caliber of the balloon catheter microwire and the relative stiffness of the device itself can preclude navigation along this segment. A microcatheter exchange, in which a more supple and navigable microcatheter is first directed into the anterior cerebral circulation and then exchanged for the balloon catheter over a long microwire, can be used when balloon remodeling is required. Nevertheless, this exchange technique is technically challenging and poses a high risk of devastating complications, such as vessel perforation and thrombosis.

FIGURE 374-1 A, Right ICA injection demonstrates an ACoA aneurysm with a 3-mm neck. B, Coil embolization with balloon assistance. The arrow delineates the inflated balloon. C, Angiography demonstrates complete obliteration of the aneurysm.

(Used with permission from Barrow Neurological Institute.)

Another elegant, but technically difficult, means of balloon remodeling is to navigate a microcatheter into the aneurysm from one A1 and the balloon catheter from the contralateral A1. This technique underscores the need for a clear understanding of the bilateral anatomy of A1 and ACoA. One trajectory may be better for navigating into the aneurysm while the other may be better for navigating the balloon across the ACoA segment and into the contralateral A2. Finally, the so-called kissing balloon technique, in which balloons are navigated from both sides and inflated while a third catheter is used to coil the aneurysm is perhaps the most challenging catheter approach used to treat these lesions.²⁷ The high morbidity and mortality rates associated with these “transcirculation” techniques should first prompt the operator to consider clip ligation of these challenging aneurysms.

Stent-supported coil embolization can also be used to treat ACoA aneurysms. The recent evolution of easily navigable microstents permits catheterization of the distal anterior cerebral arteries (ACAs).²⁸ Nonetheless, because antiplatelet medical prophylaxis is needed, this strategy is typically reserved for unruptured aneurysms and used only as a “salvage” technique in cases of subarachnoid hemorrhage (SAH). Salvage situations include herniation of coils into the parent artery or vessel thrombosis, in which a stent can be deployed to reestablish vessel patency. For unruptured wide-necked aneurysms of the ACoA region, stenting offers a potential alternative to microsurgical clipping. The risks of stenting, including the technical factors associated with catheterization and the need for long-term single and occasionally dual antiplatelet medical therapy, must be weighed against the comparative risks of clip ligation. For older patients or those with other medical comorbidities, stent-supported embolization may be the treatment of choice for wide-necked aneurysms in the region of the ACoA.

A number of technical factors must be considered before embarking on stent-assisted coiling of ACoA aneurysms. The luminal diameter of the A1, ACoA, and A2 segments must be sufficient to accommodate stents with diameters ranging from 2 to 4.5 mm. Placement of an oversized stent in this region may encourage in-stent stenosis or thrombosis. An oversized stent may also preclude navigation of a second microcatheter across the side wall of the stent because the tines in a small artery may be too constrained to allow the microcatheter to pass. This complication can be prevented by “jailing” a catheter in the aneurysm before the stent is deployed. Catheterization of the A2 and ACoA segments can also be difficult, requiring a catheter exchange to navigate the stent to the target zone. As discussed, these technically challenging exchanges increase the risk and must be undertaken only in select cases.

Endovascular Approaches to Pericallosal Aneurysms

Pericallosal aneurysms are rare and frequently amenable to coil embolization. These lesions usually arise from the branch point of the pericallosal and callosomarginal arteries. As such, the most salient concern in their treatment is preservation of these parent arteries (Fig. 374-2). Wide-neck lesions of this region are best addressed through an interhemispheric craniotomy because both stent and balloon-assistance can be difficult to achieve in their distal location. However, in patients with large caliber ACAs and straightforward arterial anatomy, such adjuvant techniques can be employed. As with ACoA aneurysms, 3-D angiography is essential to delineate the aneurysm and its anatomy relative to the parent pericallosal and callosomarginal arteries. Bilateral carotid angiography is recommended to determine the easiest A1 route to the aneurysm. Navigation across the ACoA segment to the contralateral A2 may be required but is seldom difficult to accomplish.

FIGURE 374-2 A, Angiogram demonstrates a 4-mm pericallosal aneurysm. B, Angiogram demonstrates complete obliteration of the aneurysm.

(Used with permission from Barrow Neurological Institute.)

Endovascular Approaches to PCoA Aneurysms

This variable group of lesions accounts for the second most common type of aneurysm after those of the ACoA. Both ruptured and unruptured PCoA aneurysms can manifest with acute palsy of the third cranial nerve.²⁹ Which treatment improves function of the third cranial nerve in this setting is under debate.²⁹ Surgical clip ligation offers rapid relief of aneurysmal mass effect, while progressive thrombosis and retraction of the aneurysm after coiling may be a relatively prolonged process. Regardless, this subgroup of aneurysms is readily treated through a variety of endovascular techniques.

Once again the use of 3-D angiography is paramount. This modality facilitates visualization of both the PCoA and parent ICA and delineates their relationship to the aneurysm neck.³⁰ Another crucial point is determining whether the PCoA is fetal. Both computerized tomographic angiography (CTA) and magnetic resonance angiography (MRA) can adequately demonstrate the posterior circulation. In their absence, however, digital subtraction angiography (DSA) of the posterior circulation must be undertaken to determine the presence of the ipsilateral P1 artery. If a fetal PCoA is present, its sacrifice by a protruding coil mass within the aneurysm can produce an infarction of the distal territory of the posterior cerebral artery (PCA). Conversely, the operator can be more aggressive and allow coils to encroach on the PCoA when the vessel is not fetal.

Once the anatomic characteristics of the PCoA have been delineated and the optimal working angle for visualization of the neck achieved, coil embolization of these lesions is typically straightforward. Many operators prefer to use balloon remodeling for most PCoA aneurysms. In this setting, balloon remodeling may allow greater packing of the aneurysm and serve as a means of tamponade in the event of intraprocedural rupture.^31–33 The highly compliant balloons currently available for use also partially herniate into aneurysms, potentially improving the chances of preserving a PCoA arising from the proximal neck of the aneurysm. Finally, in patients with a patent ACoA segment, the balloon can be left inflated for prolonged periods while multiple coils are placed into the aneurysm without subjecting the patient to an inordinate risk of stroke. Electrophysiologic monitoring is also useful for guiding the length of time the balloon remains inflated. A diminution of the somatosensory evoked potentials or a depression in the electroencephalogram (EEG) should prompt rapid deflation of the device.

Every effort must be made to preserve the patency of a fetal PCoA. As described, herniation of a compliant balloon into the aneurysm, partially covering the orifice of the fetal artery, may provide adequate protection. When this configuration cannot be accomplished, navigation of the balloon directly into the PCoA can be considered. Often, however, the acute angulation of this arterial segment off the ipsilateral ICA precludes navigation of the balloon catheter. A more steerable microcatheter may take this arterial turn more easily and can then be exchanged for the balloon catheter over a 300-cm microwire. Transcirculation catheterization of the fetal PCoA via the contralateral ICA can also be undertaken but is more technically challenging and requires adequate patency of the ACoA segment (Fig. 374-3). These more demanding catheter techniques should first prompt the operator to consider the feasibility and safety of clip ligation for these challenging lesions.

FIGURE 374-3 A, Right ICA injection demonstrates a wide-necked PCoA aneurysm. The arrows demarcate a fetal PCA. B, Left vertebral artery angiographic injection confirms the fetal nature of the right PCA. C, Simultaneous bilateral ICA angiogram demonstrates the patency of the ACoA. D, Native image demonstrating the transcirculation balloon-assisted coiling of the right PCoA aneurysm. E, Road map imaging during coil embolization. F, After balloon-assisted coiling, there is near-complete obliteration of the aneurysm.

(Used with permission from Barrow Neurological Institute.)

Stent-assisted coiling is an effective means of treating wide-necked PCoA aneurysms. The proximal location of this arterial segment is easily navigable, facilitating delivery and accurate placement of the stent. Stenting can also be used after balloon remodeling to mitigate the risk of damaging or dislodging the stent if placed primarily. Similarly, stenting may be employed as a “bailout” strategy either to push herniated coils back into the aneurysm sac or to pin herniated coils against the parent ICA wall. The large caliber of the ICA along the PCoA segment allows adequate expansion of the stent, thereby facilitating passage of a microcatheter through the side wall of the device. A jailing technique also can be used and offers the added benefit of stabilizing the microcatheter within the aneurysm during coiling.

Finally, balloon-assisted Onyx HD 500 (Onyx, Irvine, CA) infusion can be used to treat large and wide-necked PCoA aneurysms.³⁴ This technique requires placement of a long compliant balloon across the neck of the aneurysm to achieve a “seal” and to prevent inadvertent propagation of the embolisate into the distal circulation. The durability of Onyx HD 500 has yet to be established.³⁴ However, several preliminary studies suggest that this agent may be more effective than coiling for the treatment of large and wide-necked aneurysms.³⁴ The adjunctive use of coils and stents with Onyx embolization has also been reported and may further decrease the likelihood of an aneurysmal recurrence.

Endovascular Approaches to Ophthalmic Aneurysms

Like PCoA aneurysms, this subgroup is easily managed through endovascular techniques. In contrast, surgical exposure of these aneurysms can be quite difficult. Often, removal or drilling down the anterior clinoid process is required for adequate delineation of the aneurysm neck. Proximity to or compression of the optic nerve by the aneurysm can further complicate clip ligation. Finally, exposure of the ICA proximal to the aneurysm for temporary clipping is difficult to achieve. For these reasons, embolization may be the treatment of choice for ophthalmic aneurysms.

The most important caveat for the endovascular treatment of ophthalmic aneurysms is to verify the location of the ophthalmic artery as it relates to the neck of the aneurysm. Frequently, the artery arises from the proximal portion of the neck rendering coiling a less than ideal means of treatment. Several studies have reported that proximal occlusion of the ophthalmic artery is well-tolerated by patients.^10,16 However, the gravity of monocular visual loss should prompt surgical exploration as the treatment of choice in these select cases. Nevertheless, when the ophthalmic artery is distinct from the aneurysm, endovascular treatment is straightforward and consists of many of the techniques described earlier.

Catheter stability, especially in this location where the aneurysm typically arises just after the siphon turn, is the most challenging aspect of embolization. Steam shaping or employing a preshaped catheter may afford greater purchase of the catheter within the aneurysm during coiling. Similarly, in cases of wide-necked aneurysms, balloon remodeling or the jailing/stenting technique may provide additional support for the catheter as it enters the neck of the aneurysm, thereby facilitating embolization (Fig. 374-4). These adjuvant techniques are frequently required for the treatment of these aneurysms. The proximal location of the lesion and the relatively large caliber of the ICA along this segment simplify the use of these devices.

FIGURE 374-4 A, Left ICA angiogram demonstrates a large periophthalmic aneurysm. B, The neck of the aneurysm measures approximately 10 mm. C, The arrow demonstrates the navigation of the microcatheter over the dome of the aneurysm in preparation for an exchange for the stent delivery device. D, After it was navigated to a distal middle cerebral artery branch, the catheter was pulled from the dome. E, Complete obliteration of the aneurysm after stent-assisted coiling. F, Lateral angiographic projection confirms the patency of the ophthalmic artery.

(Used with permission from Barrow Neurological Institute.)

Nonetheless, both the accurate inflation of the balloon across the neck of the aneurysm and precise placement of a stent are complicated by the siphon turn of the ICA. This turn often results in propagation of the balloon (“watermelon seeding”) toward the ICA terminus. This turn can also propel a stent delivery catheter inferiorly during placement. Careful and slow balloon inflation and stent deployment reduce the likelihood of device propagation. Once these devices are accurately deployed, coiling the aneurysm is straightforward.

If the catheter is pushed out of the aneurysm and into an acutely placed stent, renavigation through the sidewall of the device is not difficult. Movement of the stent during this process, however, should prompt the operator to abort treatment and allow the stent to scar or “endothelialize” in place before retreatment is pursued. Onyx HD 500 embolization with balloon assistance also can be used to treat this subgroup of aneurysms.

Approaches to Cavernous Aneurysms

This heterogeneous group of aneurysms has a variety of indications for treatment.³⁵ These lesions have an exceedingly low propensity for life-threatening sequelae even after hemorrhage. Nonetheless, there are several compelling reasons to treat cavernous aneurysms: the risk of rupture with the creation of a carotid-cavernous fistula (CCF), compressive cranial neuropathy, progressively worsening headache, and erosion of the sphenoid sinus by the aneurysm. CCFs obviously threaten visual integrity and optic mobility and therefore mandate urgent treatment. Headache, especially ipsilateral retro-orbital pain, can be debilitating and treatable by embolization of the aneurysm. Compressive cranial neuropathies causing ptosis and paralysis of eye movement can be addressed through endovascular flow diversion techniques, in which progressive thrombosis and shrinkage of the aneurysm relieve the compressive forces. Finally, erosion by the aneurysm of the sphenoid sinus potentially exposes the patient to life-threatening epistaxis. These lesions are unsecured and should be treated urgently.

Cavernous aneurysms requiring treatment tend to be large or giant.^36,37 Indeed, it is difficult to argue for the treatment of small cavernous aneurysms given their benign natural history. Treatment techniques have evolved considerably for this subgroup of aneurysms. Techniques of historical interest include hunterian parent artery ligation either surgically or through endovascular methods.^38–40 Parent artery ligation was typically preceded by balloon test occlusion (BTO).^41,42 In the event of BTO failure, sophisticated bypass techniques were used to augment cerebral blood flow before the parent artery was sacrificed.⁴³ This deconstructive strategy was largely supplanted by detachable balloon embolization of the aneurysm and later by the advent of aneurysm coiling. Adjuvant techniques, such as balloon remodeling and stent-supported coiling, were often used to address the typically wide-necked anatomy of these aneurysms. The evolution of flow diversion technology, however, will likely supplant all of these modalities as the treatment of choice for large and giant cavernous aneurysms. Nevertheless, the current lack of widespread availability of these products will prolong the use of these other techniques.

BTO, followed by parent artery ligation, with or without surgical bypass, remains a commonly employed treatment strategy for this group of aneurysms.^44,45 BTO is performed through bifemoral access in which the balloon catheter is inflated in the target artery while diagnostic angiography of the contralateral ICA and vertebral artery is performed. This technique allows assessment of the patient’s collateral circulation to the affected hemisphere. Nuclear medicine scans are also more readily obtained to improve quantification of the effect of balloon inflation on cerebral perfusion and to reduce the likelihood of false-negative results. Failure of BTO mandates revascularization, which usually involves a high-flow bypass with a radial artery or saphenous vein graft to anastomose the cervical ICA with the middle cerebral artery (MCA). The ICA can be ligated at the time of bypass or in a delayed fashioned through embolization. If the patient passes the BTO, the artery can be occluded with a coil anywhere from the cervical portion to the aneurysmal segment.

With the advent of flexible stents designed specifically for navigation in the intracranial vasculature, coil embolization of these aneurysms became substantially easier. Stent-supported coiling can be accomplished either through the jailing technique or with primary passage of the microcatheter through the sidewall tines of the stent. Again, movement of the stent during this process should prompt the operator to abort the procedure to allow the stent to scar in place. In cases of ultrawide-necked aneurysms, a balloon-in-stent technique improves assessment of the parent artery during coil embolization.⁴⁶ In this technique, a balloon is inflated within the stent during coiling and then deflated on blank roadmap imaging. Coil migration toward the “ghost” of the balloon confirms herniation into the parent artery and should prompt repositioning of the coil.

In cases of large, wide-necked cavernous aneurysms, navigation of a microcatheter beyond the lesion can be challenging for two reasons. It is often difficult to visualize the distal ICA beyond the aneurysm neck, and the catheter preferentially enters the aneurysm rather than the parent artery. The operator may be forced to drive the microcatheter over the dome of the aneurysm and then back into the distal ICA. Once this position is achieved, the microcatheter can usually be pulled from the aneurysm while distal purchase is maintained. An exchange for the stent delivery device must then be performed.

Balloon remodeling has also facilitated the coiling of cavernous aneurysms. Multiple or prolonged balloon inflations, however, may subject the patient to ischemic and thromboembolic complications. Electrophysiologic monitoring can mitigate the likelihood of this complication. The presence of a patent ACoA or PCoA ensures continued perfusion of the ipsilateral hemisphere during balloon inflation and lessens the chance of ischemic insult. Navigation of the balloon beyond the aneurysm can be challenging and may require a microcatheter exchange. Because both stent- and balloon-supported embolization of these aneurysms are challenging, the number of complications is higher than that associated with straightforward coiling. This risk reinforces the need for an accurate appraisal of the risk associated with the aneurysm if left untreated.

Flow diversion will likely supplant these technically difficult endovascular procedures in the treatment of symptomatic cavernous aneurysms (Fig. 374-5). These devices, which are stents constructed with a higher density of metallic strands, allow flow to be diverted through the central lumen of the device and away from the aneurysm neck. Flow diversion produces stagnation and eventual thrombosis within the aneurysm sac. Thrombosis shrinks the size of the aneurysm and can ameliorate cranial nerve compression and potentially reduce the severity of headache. Recent multicenter experience documents a nearly 100% rate of aneurysm obliteration with the use of these devices. Nonetheless, because of the novelty of this technique, the long-term patency of the stented vessel is not yet known nor is it well-understood how and for how long the patient should be treated with antiplatelet medications. Preliminary results, however, suggest persistent patency and excellent efficacy of aneurysm obliteration.

FIGURE 374-5 A, Left ICA angiogram demonstrates a giant left cavernous aneurysm. B, Lateral projection confirms the wide-necked nature of this lesion. C and D, Three-dimensional reconstructed images confirm the wide-necked nature of this aneurysm and a smaller aneurysm located more proximally along the cavernous ICA. E,

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