Interactive Article

CLINICAL STUDIES

Embolization of Cerebral Arteriovenous Malformations: Part I­­Technique, Morphology, and Complications

Departments of Neurology (CL) and Interventional Radiology (GW, PS), Sahlgrenska University Hospital, Göteborg, Sweden

OBJECTIVE: The aim of this two-part study is to give a full account of all patients referred for embolization of arteriovenous malformations (AVMs) from 1987 to 1993. This article (Part I of II) presents the patient study, including angiographic features and their relation to the immediate outcome of embolization.
METHODS: Of the 192 patients referred, 150 were subsequently treated. Most patients were referred by neurosurgeons, and 85% of the AVMs were Spetzler-Martin Grade greater than or equal to 3. We have accounted for the 42 patients who did not undergo embolization.
RESULTS: Occlusion from embolization alone (total embolization) was obtained in 13% of patients. Full treatment (total embolization or embolization and then stereotactic radiation or surgery) was achieved in two-thirds of all patients (n = 100, 66%), and combined treatment with stereotactic gamma radiation was the most important part of the treatment strategy. The procedural mortality was 1.3%. The total incidence of complications after embolization was high (40%), but only 6.7% of cases were labeled severe. Of all angiographic features that were considered, large size and the presence of deep feeders were predictors of failure to achieve full treatment. Thirty-four patients with AVMs <8 cc were included in the study. These could have been irradiated as the sole treatment. In this group of small AVMs, the results of embolization were far better than in the whole group. Fourteen of the AVMs had volumes of <4 cc, and 10 of these (71%) were totally embolized. One patient had a hemianopsia. Among AVMs 4 to 8 cc in volume (n = 20), the total embolization rate was 15%, the full treatment rate in combination with gamma treatment was 75%, and 10% of the patients were operated on after embolization. Severe complications occurred in 15% of patients, but no complications occurred after November 1990.
CONCLUSION: In a series of AVMs, most of which were regarded as unsuitable for surgical excision, two-thirds were reduced to a size suitable for gamma knife treatment or totally occluded by embolization alone. The total complication rate was high, but the combined rate of death and complications affecting lifestyle was 8.0%, equal to ~3.2 years of natural history. (Neurosurgery 39:448­459, 1996)

Key words: Arteriovenous malformations, cerebral; Arteriovenous malformations, embolization; Complications; Treatment results

In 1960, the first modern report on embolization of cerebral arteriovenous malformations (AVMs) appeared in the literature (23). There has been continuing development since then. The progress has concerned both materials and techniques as well as increased understanding of the lesions. The agents used for embolization have changed from materials like muscle, dura mater, silicon beads, polylene threads, and concentrated alcohol to the now-prevailing polyvinyl alcohol (PVA, Contour; Nycomed Ingenor, Paris, France), mainly for preoperative embolization, and the tissue adhesive N-butyl-2-cyanoacrylate (NBCA, Histacryl Bleu; B. Braun Melsungen Ag, Melsungen, Germany). Improved delivery devices have led to a change from surgical exposure, via balloons with calibrated leaks, to guidewire-guided microcatheters like Tracker (22) (Target Therapeutics, Fremont, CA) and flow-guided catheters like Minitorquer (Nycomed-Ingenor, Paris, France) and Magic (Balt, Montmorency, France) (8). The improved devices have decreased the risk of vascular damage and increased access to the lesion.

Increasingly better results with surgery and stereotactic radiation have been published (6, 16, 17, 28, 30, 34). However, the figures for frequency and level of complications and outcome are often difficult to evaluate. The purpose of this report is to present the experience from one center where embolization is the primary treatment of patients with AVMs coupled with an endeavor to use stricter criteria for evaluation of results.

PATIENTS AND METHODS

Radiological criteria

An AV fistula is defined as a direct communication between artery and vein, without an intervening nidus network; it is seen at angiography or fluoroscopy or is considered to be present when a microcatheter (2.2 Fr ~ 0.7 mm) is able to pass through the nidus into a draining vein. An AV fistula can exist alone or within an AVM nidus. For our study, vascular steal was labeled severe when almost no vessels except those involved in the AVMs were seen on an angiogram. Moderate steal meant less than normal contrast filling of nonfeeders.

The size of the AVMs was measured on the films. Feeding arteries and draining veins were not included. Magnification factors were always taken into account; until 1991, they were calculated by measurement of cranial size by computed tomographic (CT) scanning and after 1991 by means of 10-mm washers taped on both sides of the head. The AVMs have been approximated to ellipsoids and their volumes calculated using the formula:

where d_1­3 are the three diameters of the AVMs in centimeters and V is the volume in cubic centimeters (28).

Eloquent area location according to Spetzler-Martin classification was judged from anatomic data obtained mainly from angiography (32). CT and magnetic resonance imaging scans were also used when available. Cortical brain mapping and functional imaging have not been used for classification.

Embolization techniques

2) A microcatheter (Tracker) used over a guidewire or a purely flow-guided microcatheter (Magic) is used for rapid injection of cyanoacrylate without flow control. The Tracker catheter is also used for injection of particles (PVA).

3) A purely flow-guided microcatheter (Magic) in a wedged position (the catheter totally occluding the feeder) makes it possible to inject cyanoacrylate with flow control. In this situation, cyanoacrylate can be injected slowly in large amounts spreading throughout the AVMs (Fig. 1, A­D).

A predecessor to the Magic microcatheter was used from 1987 through 1989. It was a very floppy flow-guided catheter used for embolization in the same manner as the Magic microcatheter but without flow control (29). This catheter was injected by means of a propelling chamber. The Magic microcatheter became available in 1990 and has been widely used since 1991 for most procedures, in the wedged position when possible.

During the first years of the study, isobutyl-2-cyanoacrylate (Bucrylate; Ethicon Inc., Sommerville, NJ) was used as the main embolic material. This agent was withdrawn from the market; we have used NBCA since 1990. The cyanoacrylate concentration was varied from 30 to 100% by mixing with Lipiodol (Guerbet, Aulney-Sous-Bois, France). Tantalum powder (Nycomed Ingenor, Paris, France) was used as contrast agent in the mixture until the end of 1990. PVA has been used less frequently over time and, for the last 3 years, only in the external carotid territory. During the first 2 years, polylene threads or Viñuela cocktail (a mixture of contrast medium, ethanol, Avitene [bovine collagen; Medchem Products Inc., Woburn, MA], and PVA) (34) was used occasionally.

Pharmacological treatment

Strategy of embolization

When an aneurysm was present in the nidus, we tried to close it early in the treatment process. Only during the last year of the study were detachable coils available for the treatment of feeder aneurysms; we did not previously treat aneurysms endovascularly. When a hemorrhage was the presenting manifestation of AVMs with a feeder aneurysm, the symptomatic lesion was treated first (aneurysm clipping). Currently, when a feeder aneurysm is coiled, the shunts are embolized shortly after to decrease hemodynamic stress. We have only occasionally performed dedicated preoperative embolizations.

Definitions of outcome from embolization

Patients undergoing partial treatment had AVMs that after embolization still had AV shunting, but no plans were made for further treatment with embolization, surgery, or radiation because of inaccessibility or size.

Our grading of complications is based on the National Institutes of Health (NIH) stroke scale (3). A severe complication indicates decreased function that has an influence on quality and conduct of life (>5 points on the NIH scale). A moderate complication corresponds to a deterioration of functional status but usually no influence on lifestyle (2­5 points). A slight complication (0­1 point) corresponds to a deficit that has a negligible influence on functional status.

If extravasation was observed in conjunction with a complication during angiography or was seen on a CT scan after a procedural complication, the event was classified as a hemorrhage. Other events were classified as ischemic. Occasionally, small hemorrhages may have been undetected on CT scans because of artifacts from injected glue-contrast mixture.

Radiological and clinical follow-up

Statistical methods

RESULTS

The AVMs were located on the left side in 54.0% and on the right side in 42.0%, and they were crossing the midline in 4.0% of patients. The lobar distribution is presented in Table 3. The largest diameters of AVMs are presented in Figure 2 and volumes in Figure 3. Statistical measures for diameters are 10 to 120 mm (42.3 ± 17.6, mean ± SD) and for volumes are 0.5 to 280 cc (25.8 ± 31.1, mean ± SD).

Eloquent areas according to the Spetzler-Martin classification (32) were involved in 72.7%, possibly involved in 6.7%, and not involved in 20.7% of patients. The distribution of Spetzler-Martin gradings is shown in Figure 4. Spetzler-Martin gradings were 1 through 5 (3.3 ± 0.9, mean ± SD).

Angioarchitecture

Perforating arteries and/or vessels to the basal ganglia were involved as feeders in 31.3% of all patients. Feeder aneurysms were found in 16% and nidus aneurysms in 12% of patients. One patient had both a feeder and a nidus aneurysm.

In 20.7% of patients, a direct fistula with a diameter >0.7 mm was seen on the scans or were evident when a microcatheter (2.2 Fr ~ 0.7 mm) went through the nidus into a draining vein. Only one patient had a large single-hole fistula without any surrounding microshunts. In all other patients, the fistulas were part of the nidus in an AVM.

Vascular steal was classified in three groups (see Patients and Methods). Severe steal was observed in 22.7%, moderate steal in 42.0%, and no steal in 35.3% of patients.

Venous drainage was purely central in 10.7%, purely cortical in 46.0%, and a combination of both in 43.3% of patients.

Venous pathological findings searched for included stenoses and ectasias. Stenoses were found in 9.3% and ectasias in 38.7% of patients. Diagnostic angiograms were never aimed at disclosing venous pathological abnormalities.

Embolization technique

Cyanoacrylate was used as the only embolic agent in 66.0% of patients (in 23.3% isobutyl-2-cyanoacrylate [before 1990] and in 42.7% NBCA [1990 and after]). It was used in combination with PVA in an additional 24.6% of patients. PVA only was used in 5.3% of patients. In the remaining 4.1%, cyanoacrylate or PVA was used in combination with polylene threads or Viñuela cocktail.

Complications of embolization

Seventeen additional patients had hemorrhages in connection with embolization. Five patients had severe complications, six had moderate complications, three had slight complications, and three had no complications. Complications were also related to ischemic events. In all, severe complications occurred in 10 (6.6%), moderate complications in 23 (15.3%), and slight complications in 26 (17.3%) patients, as judged from the outcome 4 weeks after completed embolizations. All complications occurred ¾4 days; most were directly related to the procedure.

Regarding the reasons for complications in individual patients, we particularly searched for and found hemorrhage (14 patients), vascular damage (dissection) (4 patients), vasospasm (2 patients), untoward embolization of normal brain feeder (7 patients), thromboembolic complication on the arterial side (5 patients), and venous occlusion from glue or thrombus (0 patient). In 21 patients, no particular event was noticed that could be linked to the complication. A small hemorrhage could easily be overlooked on a CT scan because of artifacts from glue after embolization.

In patients with hemorrhages, we failed to pinpoint a probable reason for the hemorrhage, such as venous occlusion. However, during the 1980s, hemorrhages in direct conjunction with treatment were mostly caused by guidewire perforation or by a ruptured vessel from balloon overinflation. Venous compromise after embolization has always been thoroughly sought out. In 12 patients, a microcatheter was glued in, and 6 of these patients experienced complications. It was impossible to verify whether the complication was caused by the glued-in microcatheter.

Predictors for complications

Of particular note is that location of eloquent areas was not accompanied by an increased incidence of complications after embolization. The presence or absence of an aneurysm on a feeder or in the nidus did not influence the incidence of complications, nor did the level of shunting as shown by grade of steal. The presence or absence of deep venous drainage did not affect the outcome, nor did the presence of venous varices.

Predictors for partial treatment of AVMs

Morphological outcome and total management

To be able to compare the results with those of patients who undergo only gamma knife treatment, certain groupings are made in Tables 10 and 11, showing the treatment results among the smallest of all lesions.

DISCUSSION

The decision of how to treat cerebral AVMs depends heavily on knowledge of the natural history and alternative treatment possibilities. Perusal of newer literature reveals an almost unanimous opinion that the natural history for a conservatively managed patient includes an annual cumulative bleeding rate of 2 to 4%, an annual mortality of ~1%, and a combined mortality and severe morbidity rate of 2.5% (5, 7, 27). Ondra et al. (27) found no difference in the risk of hemorrhage or death, regardless of presenting symptoms, in their well-controlled series. Brown et al. (5) made the same observation and could not detect any difference in bleeding rate that was dependent on size.

Surgery

Stereotactic radiation

Endovascular treatment

Our series of patients was selected from the AVM population by choice of the referring neurosurgeon (or, in a few instances, neurologists after consultations with local neurosurgeons), who regarded active treatment as the appropriate option and considered embolization an attractive alternative to surgery or gamma knife treatment. This is illustrated by the fact that very few patients with small AVMs were referred to us. Most of the patients had high Spetzler-Martin grading (84% are Grade greater than or equal to 3; see Fig. 4), indicating increased surgical risk. This surgical selection favored left side and eloquent region location in patients referred for embolization. The large number of feeders and the large percentage of patients with perforating arteries involved as feeders to the AVMs indicated the surgical complexity of the lesions as well as the problems facing the interventionalist. We had problems achieving full treatment for patients with perforating arteries involved as feeders. The distribution of different Spetzler-Martin grades (Fig. 4) reflects the level of surgical difficulty; it is not truly valid for endovascular treatment but for the points given for size. We did not find the other Spetzler-Martin scale variables, such as deep venous drainage and, in particular, eloquent area location, predictive of either subsequent complications or failing treatment. The Spetzler-Martin gradings are included to facilitate comparison with other studies.

We found many aneurysms. This may depend on the angiographic equipment, with improved image quality, and more selective catheterizations. Intranidal aneurysms are regarded as risk factors for subarachnoid hemorrhage but are mostly presented as indicators for previous hemorrhage (12, 25). The significance of this is that if the increased risk of early recurrent hemorrhage indicated by some studies (4, 9, 11, 18) is true, the finding of an intranidal aneurysm calls for early treatment (12). We treated a patient who had had eight subarachnoid hemorrhages and in whom we found an aneurysm in centrally located AVMs. We aimed selectively for the aneurysm and occluded it with glue, leaving the rest of the AVMs untouched. That patient had no further hemorrhages during 3 years of follow-up.

In our current technique of embolization, we use a flow-guided catheter (Magic) for injection of NBCA, as described above. This catheter makes it possible to reach very distally into the intracranial circulation (Fig. 1B). The results from this type of embolization are continuously improving with increased experience. The level of complications and treatment results are not evenly distributed over the period studied, but there is an obvious tendency to improvement (37).

Continuous development with the wedged injection technique (Fig. 1, A­D) seems to further improve results, with a decreased number of complications and increased percentage of fully treated and totally embolized AVMs (preliminary data).

The techniques developed earlier and used during the earlier part of the study are still occasionally used. The balloon with a calibrated leak is at times advantageous for embolizing very large fistulas. The flow control obtained with the balloon makes it possible to have the glue stick to the vessel wall even in instances with extreme flow. The balloon we use is a detachable latex balloon (Balt). The detachability makes it possible to retrieve the catheter even if the balloon is glued.

We always use guidewire-guided catheters in the external carotid circulation, because flow-guided catheters do not function well there. In the internal carotid circulation, this kind of catheter is rarely used. It could be advantageous if too large a difference in flow exists between a main artery and the targeted feeder to allow a flow-guided catheter to enter or if a feeder leaves at an extreme angle.

The complication rate with embolization (mortality, 1.3%; severe complications, 6.7%; moderate complications, 15.3%; slight complications, 17.3%; and no complications in 59.3%) is not worse than results after surgery (16, 17). The mortality rate resulting from embolization was 1.3%; in the case of the two patients who died, circumstances possibly explaining the outcomes were present. The first patient, who had a very large fistula, had a possible parallel to the neurosurgical phenomenon of normal perfusion pressure breakthrough (33). We have since closed several large fistulas but under a regimen with pharmacologically lowered blood pressure; no hemorrhagic complications resulted. The other patient, who had an enormous AVM, had two embolization procedures only a few days apart, with a substantial affect on hemodynamics. We scheduled sessions over a short period with only 3 or 4 days' intermission. Currently, we always allow at least 4 weeks between sessions. Several more hemorrhages occurred; during the 1980s, those in direct conjunction with treatment were mostly caused by guidewire perforation or vessel rupture from balloon inflation. Hemorrhages occurring days later were probably caused by hemodynamic changes with increased feeder pressure after closure of shunts or possibly vascular wall necrosis caused by the embolic agent. Several hemorrhages were not followed by neurological sequelae.

Neurological deficits as complications are more frequent in our series than in other series of embolized patients (2, 10, 26). Several factors account for this. The AVMs were mostly large and complicated, because patients with lesions suitable for radiation or surgery were rarely referred. We probably used a more aggressive approach, resulting in approximately two-thirds of the patients having a reasonable chance of becoming free from AVMs through embolization alone or in combination with gamma knife treatment or surgery. Another factor was the strict grading of complications and the thorough clinical evaluation by a neurologist who recorded even the smallest aberration from the normal. As shown in Tables 10 and 11, the complications occurred among small AVMs concentrated to the earlier years of the study; this tendency is confirmed for most AVMs in a separate study (see Addendum, below) (37).

One finding that raises concern was the number of patients (n = 50) without fully treated AVMs. As described earlier, those patients had large AVMs that were considered impossible to treat actively with other methods. Whether embolization was of any benefit regarding further hemorrhage and death for them must be determined by continuing follow-up.

The morphological outcome and complication rate were tested against location regarding lobes and eloquent areas, size and volume, number and type of feeders, presence of aneurysms, fistulas and different levels of steal, type of venous outflow and venous pathological abnormalities, and number of treatment sessions. The only reliable predictors found were size (Table 7) and the presence of perforators or ganglionic vessels in conjunction with morphological outcome. The rate of no or slight complications was approximately the same in all size groups (Table 5).

Because the occlusion rate after gamma knife treatment is heavily dependent on the volume irradiated (34) (occlusion rate after irradiation related to AVM volume, <1 cc [88%], 1­3 cc [78%], and 4­8 cc [50%] obliteration after 2 yr), and the result of embolization of small AVMs is good (Tables 10 and 11) and continues to improve (see Addendum, below), it seems reasonable to raise the question whether embolization could be an initial treatment even for small AVMs and whether the gamma knife should be used if areas of nonobliterated nidus remain. Any size reduction would be beneficial to gamma knife treatment results.

The combined risk of death and severe complications amounts to 8.0%. This is approximately equal to 3.2 years of natural history mortality/morbidity. Whether this concentration of risk is acceptable may be debated. If the same calculations are made from results from the last 2 years of the study, the figure is far more favorable, 0% mortality and severe morbidity and a higher percentage of fully treated patients (37).

ADDENDUM

CONCLUSION

ACKNOWLEDGMENT

Received, May 24, 1995.
Accepted, March 20, 1996.
Reprint requests: Gunnar Wikholm, M.D., Section of Interventional Neuroradiology, Department of Radiology, Sahlgrenska University Hospital, S-413 14 Göteborg, Sweden.

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COMMENTS

Wikholm et al. report their experience with embolization, stereotactic radiosurgery, and surgery in 150 consecutive patients with difficult AVMs, between 1987 and 1993. The article is well written and organized; it is also very thorough in the reporting of anatomic results and technical and clinical complications related to embolization techniques. The authors have used numerous delivery systems, including balloons with calibrated leaks, flow-guided microcatheters, and microcatheter / microguidewire combinations. Eighty-five percent of AVMs were Spetzler-Martin Grade 3 or higher. Occlusion for embolization alone was achieved in 13% of patients, and combined treatment with stereotactic radiosurgery was the most important part of the treatment strategy. The procedural mortality was 1.3%, and the total incidence of complications related to embolization was 40%, with 6.7% labeled as severe.

The authors did not relate their technical complications to the delivery systems that they used. It is possible that the majority of vascular perforations, dissections, and spasm occurred during the use of balloons with calibrated leaks and microcatheter/microguidewire combinations. They described a significant reduction in technical complications after 1990, and this positive outcome could be related to a change in technique and the more frequent use of flow-guided microcatheters. The authors describe an unusually high percentage (39.3%) of ischemic complications. They do not specify whether the ischemic complications also decreased with the use of flow-guided microcatheters. Were these ischemic complications related to untoward embolization of normal cerebral arteries? Or were they related to progressive retrograde thrombosis of an AVM feeder also supplying eloquent cortex? Or, were they related to the development of brain swelling caused by untoward occlusion of the AVM venous drainage with embolic material? The use of superselective amytal testing in AVM feeders helped us decrease postembolization ischemic complications resulting from delayed thrombosis of the arterial feeder also supplying proximal eloquent cortex. The motor-sensory and visual cortices can be identified by anatomic means, but the language cortex, spread in the perisylvian area, cannot be depicted by anatomic imaging.

It should be emphasized that the authors depicted numerous minor or moderate neurological ischemic complications after thorough neurological examinations performed by an unbiased neurologist. The majority of these immediate complications resolved in long-term clinical follow-up studies.

We agree with the authors that it is very difficult to clearly identify the source of postembolization hemorrhagic complications. The perforation of an arterial feeder used to be seen as the most common cause of periembolization hemorrhage, but the incidence has dramatically decreased with the use of flow-guided catheters. In our present experience, postembolization untoward occlusion of an AVM venous drainage in a partially embolized nidus is the most common cause of postembolization hemorrhagic complications. This is particularly true in large AVMs with multiple arterial feeders or in AVMs associated with high-flow arteriovenous fistulas. On occasion, we have identified rupture of a previously undetected intranidal aneurysm as the cause of postembolization hemorrhagic complication. Intranidal aneurysms in perforators such as thalamoperforators, or in lenticulostriate arteries and sometimes in anterior choroidal arteries, may be difficult to embolize because of the technical difficulties encountered in the catheterization of these arteries.

Seventy-five percent of embolized AVMs between 4 and 8 cc in volume (n = 20) were then treated with stereotactic radiosurgery, and 10% of patients were operated on after embolization. The authors highlight the point that they search not only for a decrease in angiographic density of the AVM nidus but also for a true diminution in diameter and volume of the AVM. They embolize from the periphery to the center of the AVM. Is it possible that this methodology increases the ischemic complications? The periphery of an AVM nidus is usually supplied by smaller arteries and arteries "en passage" that have "less sump effect" than centrally located feeders.

Fernando Viñuela
Interventional Neuroradiologist
Los Angeles, California

Wikholm et al. present a critical review of their experience with 150 patients with cerebral AVMs. This two-part study is an ambitious retrospective analysis of the radiological and clinical data in embolization of relatively high-grade (Grades 3­5) AVMs with liquid adhesive and polyvinyl alcohol. It reflects the world standard in regard to the technique of embolization and includes both surgical and radiosurgical treatment as well as AVMs treated with embolization alone. This careful analysis of the cases from a single center provides important information regarding the efficacy of embolization in preventing hemorrhage from AVMs and further defines complication rates for embolization of high-grade AVMs.

One hundred and fifty patients met the inclusion criteria for the study; 84% were Grade >3. Forty-four percent were referred for subsequent radiosurgery. The total procedural morbidity was close to 40%, although most of the morbidity was minor with a 1.3% overall procedural mortality. However, over the course of the study period, six patients died from AVM hemorrhage. One would expect five deaths for a cohort of 150 patients followed for 6 years, but if more than 5 patients died during the study period, it would imply that the natural history of this cohort is worsened by treatment or by the selection bias of inclusion criteria (i.e., high grade or lesion angioarchitecture).

The division of the analysis into "completely treated" and "partially treated" lesions provides important data regarding the effect of embolization on the hemorrhage rate of AVMs. Patients considered "fully treated" (100 patients) included patients with complete obliteration of the nidus by embolization alone (10%) as well as those lesions reduced to a size suitable for radiosurgery. Three postprocedural hemorrhages occurred in this group while the patients were awaiting further therapy, which implies that there was no protective effect of embolization on hemorrhage rate. Of greater concern is the group of patients with "partially treated" lesions in which nine hemorrhages after treatment occurred in the follow-up period of 4.2 years, which exceeds the expected natural history. This observation raises the serious question of whether these patients should be treated by embolization alone. Smaller AVMs are not more benign.

Scott C. Standard
L. Nelson Hopkins
Buffalo, New York

Wikholm, Lundqvist, and Svendsen are well-known individuals in the field of interventional neuroradiology and endovascular neurosurgery. I have followed their work for over 20 years. The senior author, Svendsen, has always been a highly reliable and honest individual who follows his patients carefully. Therefore, this information is of great assistance in understanding the role of embolization in the overall management of cerebral arteriovenous malformations. For the most part, the results, degree of occlusion, type of complications, and outcome are similar to others as reported. The authors' excellent ability to follow patients gives us some insight as to the efficacy of this treatment. The authors' persistence and honesty, as well as their ability to have the material reviewed by independent neurologists, helps us all in the treatment of AVMs. Important concepts are mentioned in this article that are compatible with those reported in our series (1). We also concur with the authors' analysis of the angioarchitecture and correlation with hemorrhage. Patients in whom they were able to close the intranidal, or pedicle, aneurysm were protected against hemorrhage. However, seven patients in whom they did not achieve this goal developed hemorrhages; two of the patients died. This again is compatible with our experience. Also compatible with our experience are those patients in whom hemorrhage occurred after embolization. The outcome in this group, which may be caused by a very select group of patients with very difficult malformations, seems to be worse than the outcome after spontaneous hemorrhage. We also concur with the authors that the location of the malformation is less critical from an endovascular point of view than it is from a surgical point of view. Endovascular treatment aims to close the abnormality and preserve all normal arteries in the vicinity, regardless of location in an "eloquent" or "noneloquent" area. Therefore, location as a risk factor seems to be less critical for endovascular embolization. We also concur with the authors that the preliminary angiogram, showing the size of the malformation and presence of perforators as part of the supply to the malformation, will predict success for endovascular treatment.

The authors seem to have a higher complication rate than the rates reported in other series, but it is probably related to their more honest and more thorough analysis and neurological examination. The complications of all interventional neuroradiological procedures in large series such as this one may get lost in view of the fact that these series are comprehensive and are conducted over a long period of time (7 yr). In the last several years, the use of nonballoon-tipped catheters and flow-guided catheters has markedly decreased the complication rate because vessel perforation and rupture are related to trauma from balloons or guidewires. On the other hand, microcatheter gluing has also decreased because of the decreased concentration of NBCA in the mixture. Therefore, longer injections are now possible and bonding of the glue to the microcatheter and vessel wall has also been markedly decreased. This group of investigators also follows several of our concepts; anatomic guidelines are the most important predictors of potential hazards and neurological complications, reaching the nidus and preserving all normal vessels decreases the risk, and sodium amytal was not used as a "provocative" test and the outcome was very good. Therefore, the use of sodium amytal as an excuse for poor technique, lack of superselectivity, or lack of knowledge of anatomy is clearly established in this group of patients. We also concur that the Spetzler-Martin classification is a surgical description of surgical skills and does not correlate with endovascular treatment. We do concur with the authors that we use this classification for the sake of comparison, because it is frequently requested from interventional neuroradiologists. Again, we do concur that this does not correlate with the difficulty of managing patients from an endovascular point of view. Furthermore, no matter how effective the embolization, we can only try to affect the "size" of the remaining malformations. However, no interventional procedure can change the location of the malformations, venous drainage, or, in reality, size. Therefore, the Spetzler-Martin classification is difficult to change with endovascular procedures. We also concur with the statement that in those patients in whom cure is not possible by either embolization or a combination of techniques, the aim of the embolization is a targeted approach in an attempt to close the "weakness" of the angioarchitecture of the malformation, such as intranidal aneurysms or aneurysms in the pedicle, or to reduce the pressure in the veins that have an outflow restriction (1). We agree with the technique of embolization and, for the most part, use the same approach. Based on the experience of the authors as well as ours, embolization may play a role even in small malformations that are suitable for radiosurgery to reduce the weakness of the angioarchitecture and hopefully afford protection during the latency period of the radiosurgery and to further reduce the volume (the smaller the volume, the more effective the radiosurgery).

Overall, this article is very important for the understanding of the long-term efficacy of this intervention. For the authors' benefit, the techniques available currently for intranidal embolization (Moret J, personal communication) are far superior to those available at the beginning of the study; the future outcome should be further improved because of these technical advancements. The authors also clarify the long-standing controversy concerning the use of liquid adhesive versus particles, confirming our own experience that particles are less effective and carry similar risks.

Alex Berenstein
Interventional Neuroradiologist
New York, New York

1. Berenstein A, Lasjaunias P: Classification of brain arteriovenous malformations: Angioarchitecture, in Surgical Neuroangiography. Heidelberg, Springer-Verlag, 1993, vol 4.

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