OBJECTIVE Delayed bleeding of unruptured intracranial aneurysms (UIAs) after coil embolization is rare; this study aimed to analyze the occurrence of delayed bleeding of UIAs after coil embolization. METHODS We retrospectively analyzed patients with UIAs after coil embolization between January 2002 and December 2018 and assessed the features of UIAs with delayed bleeding after coil embolization. RESULTS Analysis included 307 patients with 335 UIAs. Mean follow-up was 7.1 ± 4.9 years, and total follow-up was 2365 aneurysm-years. There were 271 (80.9%) aneurysms located in the anterior circulation and 64 (19.1%) aneurysms located in the posterior circulation. Significant differences were observed between the 2 groups in terms of maximum size of the aneurysm (P < 0.01), width of the aneurysm neck (P < 0.01), and number of retreatment cases (P < 0.01). During the follow-up period, delayed bleeding occurred in 4 aneurysms (annual bleeding rate of 0.17%); all were located in the posterior circulation. The original size was not relatively large (mean 8.6 ± 2.4 mm). All aneurysms bled within 5 years (mean 35 ± 9.6 months) after the initial treatment. Two were de novo aneurysms that developed adjacent to the coiled aneurysms and were not detected on follow-up magnetic resonance angiography. CONCLUSIONS Cautious follow-up of UIAs with digital subtraction angiography is important, articularly within the first 5 years after the procedure. If there are changes in the anatomic outcomes, short-term reassessment or additional treatment should be actively considered, particularly for aneurysms in the posterior circulation.
