**4. The rupture rate of recurrent aneurysms after endovascular coil embolization is low. What are the characteristics of the recurrence with a high risk of rupture?**

The rupture rate of recurrent aneurysms after endovascular coil embolization is low. A systematic review by Arnaout OM et al. reported the risk of recurrent cerebral aneurysms after initial coil embolization of 2.3 to 8.3% or 0.8% per year [22]. A study by the CARAT Investigators of 1010 patients (299 after coil embolization) reported that the rate of delayed rebleeding from the recurrent aneurysm after coil embolization was 0.11% per year [26]. As these indicate, the risk of rupture of recurrent cerebral aneurysms after initial coil embolization is low. Thus, these are relatively stable lesions, and not all are indications of retreatment.

On the other hand, we have experienced cases of death or serious illness due to the rupture of these recurrent cerebral aneurysms, and we would like to call for caution.

We present a case of our own experience (**Figure 1**). The patient was a 63-year-old woman with a subarachnoid hemorrhage due to a ruptured basilar bifurcation aneurysm and underwent coil embolization at another hospital 3 years earlier. She was discharged home with a modified Rankin Scale score (mRS) of 0. During outpatient follow-up, a recurrence of the aneurysm, which had been treated for 3 years, was found, and the patient was referred to our outpatient clinic. Her neurological imaging study showed marked re-enlargement of the aneurysm due to coil compaction (**Figure 1a** and **b**), and we were considering coil embolization for retreatment of the aneurysm. However, 1 week after the visit, she suddenly developed a loss of consciousness and was brought into our emergency department. When she came to our hospital, she was in a coma with bilateral dilated pupils, and a head CT scan showed marked intraventricular hemorrhage and acute hydrocephalus (**Figure 1c**). She underwent emergency ventricular drainage, but the neurological findings did not improve even after the operation, and she died and was discharged from the hospital the day after arrival.

#### **Figure 1.**

*(a)-(c), (a): Lateral view of a plain X-ray image of the head: The coil mass is divided into two pieces due to coil compaction. (b): Magnectin resonance imaging (MRI) coronal section of the head shows the recurrent partially thrombosed basilar bifurcation aneurysm. (c): A head computed tomography (CT) scan shows marked intraventricular hemorrhage and ventricular dilatation.*

As this case demonstrates, clinicians must be fully aware of the possibility of recurrent cerebral aneurysms that can rupture and cause death or serious clinical consequences. So what are the characteristics of recurrent cerebral aneurysms are most likely to lead to rupture?

What are the characteristics of recurrent cerebral aneurysms after endovascular treatment with a high risk of bleeding?

