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Endovascular Treatment of Intracranial Aneurysms
Clinical Pearls
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Endovascular neurosurgery is quickly emerging as a first-line treatment for many intracranial aneurysms.
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Aneurysm coils (with or without stents) and flow-diverting stents are the primary devices used for the endovascular neurosurgical treatment of an intracranial aneurysm. Treatment considerations include aneurysm location, aneurysm morphology, and risk of rupture.
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If an endovascular approach is not preferable, surgical clipping of the aneurysm remains the primary alternative treatment.
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After aneurysm treatment with any kind of stent, patients are often placed on dual antiplatelet therapy for 3 to 6 months to mitigate device-related thrombosis. Stent-assisted treatment of ruptured aneurysms is limited by the risks of dual antiplatelet therapy.
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During aneurysm coiling, adjunctive devices such as removable balloons and permanent stents provide structural support to the coils, preventing coil herniation into the parent vessel.
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Flow-diverting stents are emerging as a common treatment for wide-neck aneurysms. Compared to traditional stents, flow-diverting devices utilize a greater metal surface area to direct blood flow away from the aneurysm.
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Emerging endovascular technologies are currently focused on vascular reconstruction devices for challenging aneurysm locations, as well as incorporation of bioactive materials.
Introduction
Intracranial aneurysms can be treated with a variety of endovascular techniques, each with a specific set of pros and cons. This chapter describes the current state of endovascular treatment options with a focus on their advantages and disadvantages in order to explore the practice of treating intracranial aneurysms. The discussion also briefly highlights the future directions of the endovascular treatment of intracranial aneurysms.
Clip or Coil?
Due largely in part to the published outcomes of the International Subarachnoid Aneurysm Trial (ISAT) and the Barrow Ruptured Aneurysm Trial (BRAT), endovascular therapy for intracranial aneurysms is considered first-line treatment for most aneurysms. However, each aneurysm, patient, and surgeon is unique. For example, an aneurysm easily treated with stent-assisted coiling presents a challenge if ruptured, due to the potential complications of placing a patient with a ruptured aneurysm on antiplatelet agents, which are required for any stenting technique. Additionally, ruptured aneurysms often need to be excluded from the circulation immediately, making some techniques, like flow diversion, less attractive. Lastly, subtle details of aneurysm architecture will influence the ultimate decision on how to best treat each individual aneurysm. Trying to apply absolute “dos and don'ts” is often fraught with difficulty, and thus we aim to present coherent guidelines in this chapter.
Aneurysm Morphology
Many geometric parameters have been evaluated in attempts to best determine if an aneurysm can be treated by endovascular therapy as opposed to surgical clipping. Furthermore, these same parameters have been used to best determine if an aneurysm can be treated with primary coiling or if balloon-assisted coil embolization (BACE) or stent-assisted coil embolization (SACE) will be needed. The four main parameters are dome-to-neck ratio, aspect ratio, dome size, and neck width ( Fig. 24.1 ).
Dome-to-Neck Ratio
A maximum dome width to maximum neck width ratio of ≥2 is favorable for primary coiling. A dome-to-neck ratio of ≥1.6 is occasionally amenable to primary coiling and nearly always amenable to BACE or SACE techniques. A dome-to-neck ratio of <1.2 makes standard endovascular techniques more challenging. Other solutions can be helpful in this situation, like flow-diverting stents or consideration of surgical clipping.
Aspect Ratio
A maximum dome height to maximum neck width ratio of ≥ 1.6 tends to be favorable for primary coiling. If the aspect ratio is <1.6, the aneurysm is typically shallow and BACE, SACE, or flow diversion techniques should be considered.
Neck Width
An aneurysm neck of ≤4 mm is usually favorable for primary coiling, unless the dome of the aneurysm is also very small. A neck >4 mm may make primary coiling challenging—regardless of other parameters—and BACE, SACE, or flow diversion techniques should be considered.
Dome Size
An aneurysm dome of <3 mm is challenging for standard endovascular techniques. Flow diversion can be considered, but as aneurysms <3 mm usually do not pose an imminent risk of rupture, observation is often considered. Surgical clipping, especially in the setting of aneurysmal rupture, is often favored for very small aneurysms.
Aneurysm Coils
Aneurysm coils are usually made of a platinum/tungsten alloy, as they are inert metals readily visible on fluoroscopy. Each coil comes in various dimensions. The first number is in millimeters and refers to the diameter of the coil loops (“D,” Fig. 24.2 ) formed during deployment. The first loop of coil is often ~75% of this width to aid in safe deployment. The second number is in centimeters and refers to the total unwound length of the coil.
The coil itself is attached to a pusher wire by an attachment site that can be released electrolytically, thermally, or mechanically. The coil and pusher wire come packaged together in a plastic sheath that is placed in the hub of the microcatheter for introduction into the catheterized aneurysm.
Coil softness is a difficult variable to quantify but is influenced largely by the primary structure diameter (see Fig. 24.2 , “D”) and pitch angle (see Fig. 24.2 , α). The softer the coil, the easier it is to place in the aneurysm without kickback of the microcatheter or rupture of the aneurysm, but it also potentially lends itself to worsening coil compaction over time. Observations of recurrence rates of 10% to 30% after treatment with bare platinum coils led to alternative coil surface modifications. Several “bioactive” coil systems are on the market containing materials meant to induce an inflammatory response triggering fibrosis within the aneurysm. Fibered coiled VortX coil (Stryker, Boston, Massachusetts), Nexus coil (ev3; Covidien, Irvine, California), and Axium MicroFX (Covidien) have been shown in several single-center studies to allow for occlusion rates >90% despite lower packing density. Polyglycolic acid (PGLA) coils (Matrix coil, Stryker) have shown mixed outcome results in the literature but do appear to be safe. Polyglycolic acid (PGA) coils (Cerecyte, Micrus Endovascular, San Jose, California) have a PGA filament loaded within the coil and hydrolyze in response to blood flow resulting in an inflammatory response. Similar to fibered coils, several observational studies have demonstrated increased durability of PGA coils compared to standard bare platinum ; however, two randomized trials failed to show a difference in radiographic outcomes or retreatment rates between PGA and bare platinum coils. Additionally, these trials actually suggested a worse clinical outcome at discharge for the ruptured aneurysm cohort when PGA coils were used. Lastly, hydrogel coils (HydroCoil embolic system; MicroVention, Tustin, California) are composed of a bare platinum coil coated in hydrogel. Hydrogel coils do not induce an inflammatory response but rather swell to fill dead space, increasing packing density in that respect. Although some single center studies have also reported improved packing density, a randomized trial showed no difference in radiographic outcome or clinical outcome between hydrocoils or bare platinum. Additionally there were five cases of unexplained hydrocephalus in the hydrocoil group.
To coil embolize an aneurysm, the concept of “frame, fill, finish” is used ( Fig. 24.3 ). For a given aneurysm ( Fig. 24.3A ), framing coils are first chosen ( Fig. 24.3B ). These coils are typically round or circular (sometimes termed 3D or 360 degree) and are designed to provide an outer spherical foundation for subsequent “filling coils” and “finishing coils.” Filling coils typically follow the framing coil and are deployed to occupy the space within the aneurysm ( Fig. 24.3C ). These coils have intermediate stiffness and usually have a 360-degree or 2D helical shape. Finally, the finishing coils, designed for final packing of the aneurysm, are deployed for complete obliteration ( Fig. 24.3D ). Finishing coils are usually the softest and thinnest coils. Several studies have demonstrated an inverse relationship between packing density and aneurysm recurrence with optimal outcomes at 20% to 25%.
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