Introduction There have been several case reports and small series of postprocedure events following the endovascular occlusion of unruptured aneurysms that include, but are not limited to, headaches, aseptic meningitis and delayed hydrocephalus. In some single center experiences, these events have been reported in 1–3% of coiled aneurysms treated with HydroCoils. However, these same occurrences have also been seen with platinum and other second generation coils. The Ventricular Enlargement Study was initiated to further investigate these events.

Method This study consisted of an international, multicenter, retrospective analysis of patients treated with Hydrocoils (Microvention, Tustin, California, USA) that evolved to edema, inflammation or ventricular enlargement postprocedurally. Predetermined data points included preoperative, intraoperative and postoperative imaging, patient demographics, details related to aneurysm size, location and morphology, procedural details such as coils used, medications administered and intraprocedural complications, and all postprocedure follow-up including clinical status of the patient and any complications. This cohort of patients was compared with aneurysms treated using Hydrocoil, in the same time period, from the same centers, but did not experience postprocedure events, looking for any predictors of inflammatory reaction and ventricular enlargement.

Results 21 physicians at 17 centers were identified and contacted for participation. A total of 12 centers agreed to participate, obtained IRB approval and provided the necessary data for the study. 42 patients were collected with a total of 43 aneurysm coiling procedures performed. Two patients (control group) had critical data points missing at the time of analysis and were not included. Three patients had a history of ruptured aneurysms or the target aneurysm was ruptured and were also excluded.

25 patients with 26 aneurysm procedures had postprocedure events and 12 patients without were evaluated. Mean aneurysm size in the event group was 13.7 mm (vs 9.8 mm in the non-event group). The average time from treatment to onset of symptoms was 8.5 months (2 weeks–30 months; median 6 months).

Of the 25 patients who had events post-coiling, seven were asymptomatic and did not require additional intervention. Delayed hydrocephalus was the most common symptomatic postprocedural event.

Frequency and histograms were evaluated to determine if packing density of hydrogel, packing density overall and aneurysm size were normally distributed. The results demonstrate that all variables were normally distributed except for aneurysm size, which is skewed to the larger dimensions. T tests were performed for the same variables, all of which were not significant; however, there was a trend for large aneurysm size. Non-parametric testing of the aneurysm size demonstrates that there is a significant difference between the size of the aneurysm treated between the groups that had events and those that did not.

Conclusion Patients who undergo endovascular coiling of unruptured aneurysm may be at risk of developing delayed events which may or may not be symptomatic. It appears that this risk is very small and is more likely to occur in larger aneurysms. These events can be difficult to detect due to delayed presentation.