Intracranial aneurysms constitute a significant public health problem in the United States.
Spontaneous rupture of cerebral aneurysms typically results in subarachnoid hemorrhage and affects 10-30/100,000 patients/year. As many as 10 % of patients die before reaching the hospital. Of those who do get to the hospital, the most significant risk to life is aneurysm re-bleeding, although cerebral vasospasm makes a substantial contribution to overall morbidity and mortality. Considering that as many as 50% of subarachnoid hemorrhage patients eventually die as a result of their hemorrhage and an additional 25% suffer permanent neurological injury, any cerebral aneurysm deserves a thorough evaluation for potential treatment.
The primary goal of treatment of cerebral aneurysms is to prevent future rupture. What can we tell patients about their risk of future rupture? The best available data suggest that previously unruptured aneurysms carry a risk of hemorrhage of about 1-2 %/year. If subdivided by size, the risk is probably slightly lower for small aneurysms (< 1cm) and slightly higher for large aneurysms (1-2.5cm). The presence of multiple aneurysms and a family history of subarachnoid hemorrhage probably also raise the risk of rupture. Once an aneurysm has ruptured, the chance of re-hemorrhage dramatically increases. Of those patients who survive an initial subarachnoid hemorrhage, approximately 20 % re-bleed in the first 2 weeks and 35% over the first month if the aneurysm is left untreated. After the first 30 days, the risk falls back to 1-2 % per year.
Surgical clipping is the mainstay of treatment of both ruptured and unruptured cerebral aneurysms. In this approach, the affected artery must be exposed, and the aneurysm must be visualized directly. The surgeon can then carefully apply a metal clip to the aneurysm's base, blocking blood flow. The risk of hemorrhage is eliminated once the aneurysm is eliminated from the blood flow. The advent of micro neurosurgical techniques and advancements in cerebrovascular surgery (temporary clipping, neuroprotection, etc.) have extended the applicability of aneurysm surgery and improved surgical outcomes. Despite these advancements, even in the best hands, aneurysms remain challenging to clip.
In 1991, Guglielmi detachable coil (GDC) embolization was introduced as an alternative method for treating selected aneurysm patients. The GDC system comprises a soft, flexible, microcoiled platinum wire with intrinsic helical memory attached to a delivery mandrel. The coil is connected to the mandrel by a unique soldered joint and can be "detached" once in the aneurysm by electrolysis. GDC embolization involves transarterial delivery of these platinum coils into the lumen of the aneurysm by way of a microcatheter placed into the aneurysm. The treatment aims to prevent blood flow into the aneurysm sack by filling the aneurysm with coils and thrombus. It has been shown in animals that this approach leads to the organization of the thrombus in the aneurysm and, with further healing, to the formation of a membrane across the neck of the aneurysm. This completes the exclusion of the aneurysm from the flow of blood. Using the analogy of the surgical treatment, the technique is, in effect, placing an endovascular "clip."
Theoretically, there are several advantages of GDC over surgery. These procedures are performed under general anesthesia in the neuroangiography suite utilizing the standard transfemoral approaches used in diagnostic angiography. Potentially, the treatment could be combined with the initial diagnostic cerebral angiogram, thereby reducing the risk of re-rupture. Because the approach is transarterial, multiple aneurysms can be treated during a single procedure. GDC treatment could be combined with co-morbid conditions such as vasospasm (intra-arterial papaverine or angioplasty). Also, since there is no scarring of the route of approach to the aneurysm as there is in the case of surgery, staged or multiple procedures can be performed without added difficulty. Finally, since the approach is endovascular, the issues that make any particular location relatively straightforward or more difficult are entirely different. Therefore, some places that are relatively complex for surgery (basilar tip) are straightforward for GDC. (Of course, the opposite is also true of other sites, e.g., the middle cerebral.)
Despite these potential advantages, surgery remains the primary treatment modality for most aneurysms. Surgical clipping has been successfully applied to intracranial aneurysms for many years, and the treatment's durability has been proven. This minimizes the need for post-procedure angiographic studies, which are required for any patient treated with GDC. Surgery also provides controlled access to areas of intricate anatomy and allows for arterial reconstruction for aneurysms with complex shapes and wide necks where GDC could not be applied.
Massachusetts General Hospital was one of the sites in the multicenter trial involving some 1200 patients that ultimately achieved FDA approval for the GDC technology. Since the beginning of the trial, we have developed an increasing practice in GDC therapy here at the MGH, and we are currently treating 20-30 aneurysms per year with this technique. The results indicate that over 80% of GDC-treated aneurysms have a narrow neck.
Since its inception, GDC embolization has evolved due to both clinical experience and the introduction of technological improvements. We are now much better at selecting aneurysms appropriate for treatment. Recently published work (Malisch 1997) confirms our experience that giant aneurysms are less likely to have a successful outcome after GDC embolization. Such aneurysms have a significant likelihood of coil compaction into the aneurysm, causing a recurrence of the aneurysm lumen and, more concerning, an alarmingly high hemorrhage rate after embolization (33% of such patients in a small series with an average follow-up interval of 3.5 years). We also now know that aneurysms that project along the direction of blood flow in the parent artery (e.g., superiorly directed basilar tip) are at increased risk for coil compaction into the aneurysm. Finally, there is the suggestion from autopsy data that thick-walled, calcified giant aneurysms are less likely to undergo clot organization and ultimate exclusion of the aneurysm from the circulation than is thought to be the case for small, thin-walled aneurysms. Indeed, we have the best results with small aneurysms, which also have small necks.
Technological advances in GDC technology have also improved this method of treatment. Over the last several years, the number of coil sizes has increased, multi-dimensional coils allowing safer initial coil placement have become available, and more recently, much softer coils have been introduced. The manufacturing process, which forms the attachment of the platinum coil with the stainless steel introducing wire, has also been improved such that this detachment zone dissolves much quicker during the electrolytic process than previously. This improvement has dramatically reduced the time needed to detach each coil from up to several hours to almost always less than 10 minutes. Another significant technical refinement has been in the area of microcatheters. We now have braided, hydrophilically coated microcatheters, which allow improved access to many aneurysms, thereby increasing our chances of obtaining complete aneurysm obliteration.
Our current approach is to consider GDC treatment as a complement to the surgical treatment of cerebral aneurysms. Using GDC in any patient undergoing surgery may carry an increased risk. These typically include patients with co-morbid medical conditions, with high Hunt and Hess Grades (³4), or with increased intracranial pressure following SAH. Aneurysms in which surgical exposure carries an unacceptable risk ( i.e., posteriorly directed basilar tip aneurysms or paraclinoid aneurysms which are partially within the cavernous sinus) or those which may not easily hold a surgical clip such as calcified atherosclerotic aneurysms GDC becomes the primary treatment option. GDC is also considered in patients who wish to have treatment but desire not to have a craniotomy.
Christopher M. Putman, M.D., Frank Huang-Hellinger, M.D. Ph.D., and Christopher Ogilvy, M.D.
The Interventional Neuroradiology service at Massachusetts General Hospital and the MGH Brain Aneurysm & AVM Center.
Guglielmi G, Vinuela F, Sepetka I, et al. Electrothrombosis of saccular aneurysms via endovascular approach. Part 1: Electrochemical basis, technique, and experimental results. J Neurosurg, 75:1-7, 1991.
Guglielmi G, Vinuela F, Dion J, et al. Electrothrombosis of saccular aneurysms via endovascular approach. Part 2: Preliminary clinical experience. J Neurosurg 75:8-14, 1990.
Malisch TW, Guglielmi G, Vinuela F, Duckwiler G, Gobin YP, Martin NA, Frazee JG. Intracranial aneurysms treated with the Guglielmi detachable coil: midterm clinical results in a consecutive series of 100 patients, J Neurosurg 87:176-183 (1997).