Introduction Unexplained post-procedural events such as cerebral edema, inflammation, aseptic meningitis and hydrocephalus have been reported following unruptured cerebral aneurysm coiling. However, understanding of the etiology for these occurrences is limited due to their rare occurrence. A multicenter registry was developed to investigate further the occurrence of these events.

Method This registry consisted of a retrospective analysis of unruptured aneurysms treated with hydrocoil that evolved to develop focal cerebral edema, inflammation, aseptic meningitis, or ventricular enlargement/hydrocephalus following uncomplicated coil embolization. Data points included pre, intra, and postoperative imaging, patient demographics, aneurysm demographics, procedural details such as coils used, medications administered, and intraprocedural complications, and all post-procedure follow-up including clinical status of the patients and all adverse events.

Results Twenty-five patients (26 aneurysm coiling procedures) were found at 12 centers over an 8-year period. The mean aneurysm size was 13.7 mm. The average time from treatment to onset of symptoms was 8.5 months (2 weeks to 30 months, median 6 months). Delayed hydrocephalus was the most common clinical presentation. Six of the 25 patients were asymptomatic and did not require treatment.

Conclusion Patients undergoing endovascular coiling may be at risk of developing delayed complications, which may or may not be symptomatic. This risk appeared low and was restricted mostly to larger aneurysms. These events can be difficult to detect due to delayed presentation.