Abstract
BACKGROUND AND PURPOSE:
Whether hydrogel coils (HGCs) can reduce intracranial aneurysm recurrences when compared with bare platinum coils (BPCs) remains a disputed subject. Thus, we sought to perform a systematic review and meta-analysis to evaluate the efficacy of hydrogel coils in the context of intracranial aneurysm treatment.
MATERIALS AND METHODS:
Following PRISMA 2020 guidelines, we systematically reviewed PubMed, Scopus, Embase, and Web of Science for randomized controlled trials (RCTs) comparing HGC to BPC. Outcomes of interest were: end of procedure and last follow-up occlusion rates, including complete occlusion and major recurrence, complication rates, morbidity, and mortality. Risk ratios (RRs) and 95% CIs were calculated.
RESULTS:
The study selection: 5 RCTs, including 2126 patients (HGC n = 1064, BPC n = 1062), were analyzed. HGC showed comparable rates of immediate complete occlusion (RR = 0.89, 95% CI = 0.68–1.16, P = .29) and packing attenuation (MD = 27.17, 95% CI = –16.59–70.93, P = .12) compared with BPC. At an average 18-month follow-up, HGC significantly reduced major recurrence rates (RR = 0.75, 95% CI = 0.60–0.94, P = .03). Complete occlusion rates at the last follow-up were higher for HGC after outlier exclusion (RR = 1.29, 95% CI = 1.18–1.42, P < .001). All the outcomes related to complications, including hemorrhagic and thromboembolic complications, were similar between both groups (P > .1 for all). Finally, HGC resulted in similar rates of mRS 0–2 and mortality compared with BPC (RR = 0.98, 95% CI = 0.95–1.01, P = .15 and RR = 0.72, 95% CI = 0.31–1.65, P = .33, respectively). Only 5 RCTs were included in this meta-analysis, which may limit the generalizability of our findings. The absence of long-term follow-up also limits the assessment of treatment durability.
CONCLUSIONS:
Our meta-analysis of RCTs suggests that the use of HGC in the endovascular treatment of intracranial aneurysms results in significantly lower rates of recurrence compared with BPC, with both coil types showing similar initial occlusion rates and safety profiles.
Treatment of a ruptured intracranial aneurysm (IA) is a matter of urgency to avoid an early second rupture, which is most often fatal.1 Treatment of an unruptured aneurysm, on the other hand, is a preventive treatment whose indication is based on consideration of risk factors for rupture linked to the anatomy of the aneurysm and the patient.2 Endovascular treatment by coiling was developed in the 1990s and marked a important milestone for patient care. Since the results of the International Subarachnoid Aneurysm Trial, which showed better outcomes with endovascular coiling than with neurosurgical clipping for the treatment of intracranial aneurysms, endovascular coiling remains the preferred treatment for most patients.3-5
Minimally invasive aneurysm embolization with coils has been shown to be safe and effective. However, a clear limitation to this technique appeared with aneurysm recurrence occurring in approximately 20%–30% of aneurysms treated with coils.3,6,7 Aneurysm treatment aims to completely exclude the aneurysm; placement of coils within the aneurysm’s sac promotes thrombosis and mitigates the risk of rupture.8 Recurrence is the consequence of incomplete and unstable thrombus formation lacking myofibroblast infiltration and collagen deposition in the aneurysm dome and poor endothelialized neointimal layer at the neck of coiled aneurysms.8
Because of the high recanalization rates following endovascular coiling and the importance of attenuated packing of the aneurysmal sac in improving long-term occlusion rates, a number of second-generation coils have been developed. The hydrogel coil (HGC; MicroVention) was developed with an expandable polymer that allows improved aneurysm filling.9 The pressing question of whether HGC offers advantages over the established bare platinum coils (BPCs) in the context of treating intracranial aneurysms remains disputed, though repeatedly evaluated through randomized controlled studies (RCTs).6,10-12
A previous meta-analysis of RCTs found that HGC significantly reduced IA recurrences in the midterm follow-up but did not show any significant differences in other outcomes, including complete occlusion at midterm.13 The authors recommended further exploration of the potential benefits of HGC, particularly second-generation HGC, on complete occlusion and clinical outcomes. Since then, a new RCT with the largest population to date, using second-generation HGC, was published, providing further favorable results for HGC but only in terms of recurrence.14 In response, we conducted an updated systematic review of the literature and meta-analysis of RCTs to objectively evaluate the effectiveness of HGC compared with BPC in the treatment of IA. This research aims to provide a clear and direct assessment of the angiographic and clinical outcomes associated with these 2 coil types, contributing to a more informed understanding of their respective roles in neuroendovascular interventions.
MATERIALS AND METHODS
Search Strategy
On January 17, 2024, following PRISMA recommendations for systematic reviews,15 we conducted a comprehensive literature search within the Nested Knowledge Autolit software based on our prespecified protocol. The search covered 4 databases from their inception: PubMed, Embase, Web of Science, and Scopus. Only English-language studies were included.
For each database, we employed different combinations of keywords and/or MeSH terms. The terms included “Hydrogel,” “Hydrocoil,” “HydroSoft,” “HydroFrame,” “HydroFill,” “coil,” “aneurysm,” and “randomized.” Boolean operators (AND, OR) were used to ensure a comprehensive retrieval of studies related to hydrogel-coated coils in the treatment of aneurysms.
In addition, we conducted a manual search through the references of included articles to identify any potentially missed relevant studies. All search results were exported to a Nested Knowledge Autolit for deduplication and screening according to the inclusion criteria.
Screening Process
We included all original studies fulfilling our predetermined Population, Intervention, Comparison, and Outcome (PICO). The population was patients with IA, intervention treatment with HGC, and control group treatment with BPC; the outcomes of interest were immediate and last follow-up complete occlusion and adequate occlusion, major recurrence, retreatment rate, favorable outcome (mRS = 0–2), mortality rates, and complication rates (including ischemic and hemorrhagic complications). RCTs comparing treatment outcomes for IA by using BPC versus HGC were included without restrictions on publication date, country of origin, aneurysm location, or rupture status. All other study designs were excluded. Two authors (S.G. and O.M.A.-J.) did the title and abstract screening against the predefined criteria. This was followed by a full-text screening of any retained studies of the first screening step. In both stages, the senior author was consulted to resolve any conflicts in the decisions. Complete occlusion and adequate occlusion were defined according to specific criteria. For complete occlusion, Raymond-Roy Occlusion Classification (RROC) grade I was used as a benchmark. Additionally, for adequate occlusion, defined as complete aneurysm occlusion along with a neck remnant, RROC grades I and II were considered.16
Data Extraction
The extraction of selected articles was performed by 2 authors (J.C. and A.Z.). The extracted data included study characteristics, baseline data of the included patients, and the aforementioned outcomes of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.
Risk of Bias
The “Cochrane RoB 2: a revised tool for assessing risk of bias in randomized trials” was used to assess the risk of bias, with 2 independent reviewers evaluating all studies (A.Z. and O.V.).17 Two authors evaluated the quality of each study, and it was adjudicated by a third one when needed.
Statistical Analysis
In this study, we performed a random-effects meta-analysis by using R software Version 4.3.1 and the “meta” statistical package.18 We calculated risk ratios (RRs) along with their respective 95% CIs. The confidence intervals for the random-effects estimates were derived by using the Restricted Maximum Likelihood estimator, incorporating Jackson’s modification of the Hartung-Knapp/Sidik-Jonkman variance correction, and the “Hybrid method 2” in Jackson et al19 as an ad hoc correction. To handle studies with zero events in both treatment groups (double zeros), we applied the treatment arm continuity correction method not to discard the information in such cases.20,21
Heterogeneity was assessed by using the Cochran Q test and the I2 statistic. A P value < .05 for Cochran Q was considered indicative of statistically significant heterogeneity, while I2 values >50% were interpreted as substantial heterogeneity. To explore sources of heterogeneity, if substantial, we conducted sensitivity analysis by using the leave-one-out method, examining the impact of excluding each study on the overall effect size. Because of the small number of studies (fewer than 10) included in the meta-analysis, we were unable to perform Egger regression to assess publication bias or conduct meta-regression.22
RESULTS
Search and Screening Results
A total of 134 records were identified across 4 databases: PubMed (n = 21), Embase (n = 54), Scopus (n = 24), and Web of Science (n = 35). After 68 duplicate records were removed, 66 records remained for screening. During the screening of titles and abstracts, 45 records were excluded for various reasons, including a comparison between clipping versus coiling, secondary analyses, book chapters, letters, and studies unrelated to the topic.
From the remaining records, 21 full-text reports were sought for retrieval, all of which were successfully retrieved and assessed for eligibility. After further evaluation, 16 reports were excluded for reasons including conference abstracts (n = 2), duplicates (n = 12), prospective observational studies (n = 1), and comparisons of stent-assisted coiling versus coiling (n = 1). Ultimately, 5 studies were included in the final review (Fig 1).
FIG 1.
PRISMA flowchart of the included studies.
Study Characteristics and Risk of Bias
All 5 studies included in our analysis were RCTs. The size of the included studies ranged from 96 patients to 600 patients, with a total of 2126 patients enrolled and pooled for the analysis (hydrogel n = 1064, BPC n = 1062). Study and patient characteristics are detailed in Supplemental Data.
RCTs included in our analysis were deemed to have a “low” risk of bias for 3 of them, “some concerns” for 1 of them, and the last study was considered to have a “high” risk (Fig 2).
FIG 2.
Results of the methodologic quality assessment of the included studies based on Cochrane RoB 2. “Traffic light” plots of the domain-level judgments for each individual result.
End of Procedure Occlusion Rates
All 5 RCTs, with 2059 patients, reported the rate of immediate complete occlusion. HGC resulted in comparable rates of immediate complete occlusion compared with BPC (RR = 0.89, 95% CI = 0.68–1.16, P = .29). There was substantial heterogeneity among the included studies (I2 = 68%, P value = .01) (Fig 3A). The sensitivity analysis with the leave-one-out method resulted in similar results, regardless of what the study omitted (Supplementary Data).
FIG 3.
Forest plot of the random effects proportion meta-analysis of the reported rates of the end-of-procedure occlusion outcomes. A, Immediate complete occlusion. B, Immediate adequate occlusion. C, Immediate residual neck. D, Immediate residual aneurysm. E, Packing attenuation.
HGC also resulted in comparable rates of immediate adequate occlusion compared with BPC (RR = 0.99, 95% CI = 0.95–1.03, P value = .50), which was reported by 4 RCTs, with 1488 patients. There was no heterogeneity among the included studies (I2 = 0%, P = .59) (Fig 3B).
Furthermore, immediate residual neck and immediate residual aneurysm were comparable between both groups (Fig 3C, -D).
Finally, 3 RCTs, with 1291 patients, reported the packing attenuation at the end of the procedure. HGC resulted in comparable rates of packing attenuation compared with BPC (MD = 27.17, 95% CI = −16.59–70.93, P = .12). There was substantial heterogeneity among the included studies (I2 = 99%, P < .001) (Fig 3E).
Last Follow-Up Occlusion Rates
Average last follow-up in all studies was 18 months (range 12–24 months).
All 5 RCTs, with 1953 patients, reported the rate of major recurrence at the last follow-up. HGC resulted in significantly reduced rates of major recurrence at the last follow-up compared with BPC (RR = 0.75, 95% CI = 0.60–0.94; P = .03). There was no heterogeneity among the included studies (I2 = 0%, P value = .44) (Fig 4A). It should be noted that slightly different definitions of major recurrence were used. The Hydrogel Endovascular Aneurysm Treatment (HEAT) trial defined it as an increase on the RROC scale from 1 to 3, 2 to 3, or as a grade 3 that showed progression on the Meyer scale. For the GREAT trial and Poncyljusz et al,10 major recurrence was defined as any change from complete aneurysm occlusion or neck remnant at the end of the index procedure to residual aneurysm at angiographic follow-up. The HELP and Patients Prone to Recurrence After Endovascular Treatment (PRET) trials defined it as a recurrence sufficiently large enogh to technically allow placement of further coils.
FIG 4.
Forest plot of the random effects proportion meta-analysis of the reported rates of the last follow-up occlusion outcomes. A, Major recurrence. B, Complete occlusion. C, Adequate occlusion. D, Residual neck. E, Residual aneurysm. F, Retreatment.
Four RCTs, with 1409 patients, reported the rate of complete occlusion at the last follow-up. HGC resulted in similar rates of complete occlusion at the last follow-up compared with BPC (RR = 1.21, 95% CI = 0.99–1.48, P = .06). There was substantial heterogeneity among the included studies (I2 = 54%, P = .09) (Fig 4B). When conducting the sensitivity analysis, the omission of the outlier study led to a significantly higher rate of complete occlusion at the last follow-up for the HGC group compared with BPC (RR = 1.29, 95% CI = 1.18–1.42, P < .001) (Supplementary Data).
Adequate occlusion at the last follow-up was similar between both groups (RR = 1.08, 95% CI = 0.92–1.27, P = .31), which was reported in 3 RCTs, with 956 patients. There was no significant heterogeneity among the included studies (I2 = 15%, P = .310) (Fig 4C). Similarly, residual neck at the last follow-up was similar between both groups (RR = 0.74, 95% CI = 0.32–1.71, P = .26) (Fig 4D).
Three RCTs, with 956 patients, also reported the rate of residual aneurysm at the last follow-up. HGC resulted in significantly reduced residual aneurysm at the last follow-up rates compared with BPC (RR = 0.75, 95% CI =0.61–0.93, P = .01). There was no heterogeneity among the included studies (I2 = 0%, P = .95) (Fig 4E).
Finally, 5 RCTs, with 1986 patients, reported the rate of retreatment. HGC resulted in similar rates of retreatment compared with BPC (RR = 0.83, 95% CI = 0.52–1.33, P = .34). There was no heterogeneity among the included studies (I2 = 0%, P = .43) (Fig 4F).
Complications
All the outcomes related to complications—hemorrhagic complications, thromboembolic complications, all strokes, coils migration, hydrocephalus, procedural complications, total complications—were similar between both groups (Supplemental Data). There was no heterogeneity among the included studies for each outcome.
Clinical Outcomes
All 5 RCTs, with 2123 patients, reported the rate of mortality. HGC resulted in similar rates of mortality compared with BPC (RR = 0.72, 95% CI = 0.31–1.65, P = .33). There was no significant heterogeneity among the included studies (I2 = 25%, P = .26) (Fig 5A).
FIG 5.
Forest plot of the random effects proportion meta-analysis of the reported rates of mortality (A) and morbidity (mRS 0–2; B).
Four RCTs, with 2006 patients, reported the rate of mRS 0–2. HGC resulted in similar rates of mRS 0–2 compared with BPC (RR = 0.98, 95% CI = 0.95–1.01, P = .15). There was no heterogeneity among the included studies (I2 = 0%, P value = .72) (Fig 5B).
DISCUSSION
Our meta-analysis, including 5 RCTs comparing HGC to BPC, demonstrates that HGC provides a similar safety profile with superior long-term durability, evidenced by reduced recurrence rates. These results are clinically significant, particularly for unruptured aneurysms or those prone to recurrence, where retreatment rates can pose additional risks to patients.
Key Findings and Clinical Implications
The primary finding of our meta-analysis is the significantly lower rate of major recurrence at midterm follow-up with HGC compared with BPC. This observation is consistent with previous studies, such as the GREAT and HEAT trials, which reported reductions in recurrence rates and trends toward decreased retreatment for HGCs.11,14 Moreover, a systematic review of first- and second-generation HGCs compared with BPCs highlighted the reduced recurrence and favorable long-term outcomes, supporting their use in clinical practice.23 The reduction in recurrence with HGCs may be attributed to the unique hydrogel coating, which expands upon contact with blood, resulting in increased packing attenuation and a more stable thrombus formation within the aneurysm sac.8
Other systematic reviews and meta-analyses have reached similar conclusions regarding recurrence rates.13,24,25 Nonetheless, we conducted an up-to-date analysis to specifically address the comparison between HGC and BPC in RCTs only. This approach allowed us to aggregate the largest randomized population, yielding more reliable results on additional occlusion outcomes and clinical outcomes. We found a significantly higher rate of complete occlusion at midterm follow-up with HGC compared with BPC, a result not achieved in previous meta-analyses.13 Earlier studies included either retrospective analyses23 or varied types of bioactive coils.24,25
Our analysis also found no significant differences in initial complete occlusion rates between HGC and BPC, which suggests that HGCs do not compromise immediate procedural efficacy. While this similarity in occlusion rates might initially seem to negate the benefits of HGC, the reduced recurrence rates highlight the importance of long-term stability over immediate outcomes. This could be particularly valuable in aneurysms with a higher baseline risk of recanalization or in cases where complete initial occlusion is challenging to achieve.12
Safety Profile of Hydrogel Coils
The safety profile of HGCs was comparable to BPCs across various adverse events, including hemorrhagic complications (RR = 0.73), coil migration (RR = 1.53), hydrocephalus (RR = 1.30), and thromboembolic complications (RR = 0.73), with no significant heterogeneity among the studies for these outcomes (all I2 = 0%). This is consistent with findings from a meta-analysis of randomized controlled trials, which reported similar complication rates between hydrogel and bare platinum coils across a pooled sample of 1526 patients.13 Furthermore, the GREAT trial also reported comparable overall procedural complication rates and similar adverse composite outcomes between HGC and BPC groups, demonstrating that HGCs do not compromise safety even in high-risk populations.11 The PRET trial, which focused on large and recurrent aneurysms, found similar safety outcomes between the 2 coil types, highlighting that HGCs are a safe alternative even for challenging aneurysm profiles.12,26 These findings indicate that the benefits of HGCs in reducing recurrence are not achieved at the cost of increased procedural risks. The stability of these results, even in high-risk populations and across different centers, reinforces the safety of HGCs as an alternative to BPCs for intracranial aneurysm treatment.
Histologic Considerations and Mechanistic Insights
The reduced recurrence rates observed with HGCs can be attributed to the distinct biologic properties of the hydrogel coating. Upon contact with blood, the hydrogel expands, leading to better aneurysm sac filling and a denser, more stable thrombus that reduces porosity and minimizes the risk of coil compaction.8 This structural stability provides an optimal scaffold for cellular infiltration, particularly by myofibroblasts, which play a critical role in stabilizing the aneurysm through collagen synthesis and fibrous tissue deposition.27 The increased collagen deposition at the aneurysm neck strengthens the neointima layer, effectively sealing the aneurysm and reducing the likelihood of delayed recanalization. This suggests that HGCs not only offer mechanical stability but also promote a more organized healing process at the aneurysm neck. Experimental studies have confirmed these histologic changes, suggesting that the unique properties of HGCs may provide enhanced long-term stability compared with BPCs.8
Limitations and Future Directions
Several limitations should be noted. First, the inclusion of only 5 RCTs limits the generalizability of our findings. While certain outcomes, such as major recurrence and residual aneurysm rates, showed no heterogeneity (I2 = 0%), others, such as immediate and long-term complete occlusion, displayed moderate heterogeneity (I2 = 68% and 54%, respectively). High heterogeneity was observed for packing attenuation at the end of the procedure (I2 = 99%), indicating considerable variability across studies, likely due to differences in procedural techniques and aneurysm characteristics. This variability may limit the robustness of our conclusions for some end points. Additionally, incomplete reporting of long-term outcomes in several studies could limit the assessment of treatment durability. Future studies should focus on aneurysm subtypes in different clinical scenarios, such as ruptured, unruptured, and reanalyzed aneurysms, to provide more comprehensive insights.
CONCLUSIONS
Our meta-analysis supports the use of HGC as a safe and effective alternative to BPC for the treatment of intracranial aneurysms, particularly in reducing major recurrence rates without compromising safety. Given the comparable complication profiles and potential for improved midterm outcomes, HGCs should be considered in clinical scenarios where long-term aneurysm stability is critical, such as wide-neck aneurysms or cases with a high risk of recurrence. Further studies are warranted to confirm these findings and explore their role in different aneurysm types and clinical contexts.
Acknowledgments
J.C. received educational grants from the French Society of Neuroradiology (SFNR), the French Society of Radiology, the Philippe Foundation, and the Foundation Thérèse and René Planiol.
ABBREVIATIONS:
- BPC
bare platinum coil
- HGC
hydrogel coil
- IA
intracranial aneurysm
- MD
Mean Difference
- RCT
randomized controlled trial
- RR
risk ratio
- RROC
Raymond-Roy Occlusion Classification
Footnotes
Disclosure forms provided by the authors are available with the full text and PDF of this article at www.ajnr.org.
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