1. Establish the rates of aneurysm re-treatment and haemorrhage from aneurysms treated with endovascular treatment or microsurgical treatment in a large, multi-centre cohort.

2. Establish factors associated with requirement for re-treatment and haemorrhage from treated aneurysms following endovascular treatment or microsurgical treatment of intracranial aneurysms.

• The proposed study will be an international, multicentre, retrospective cohort study.

• Phase 1 of data collection will start in November 2024, and for each centre, any intracranial aneurysm meeting inclusion criteria treated endovascularly or microsurgically treatment between 1st January 2013 and 31st December 2013 will be included.

• Following interim data review, Phase 2 of data collection may be initiated to permit inclusion of aneurysms treated between 1st January 2014 and 31st December 2016.

Adult patients (≥18 years old) who have undergone endovascular or microsurgical treatment (including clipping, wrapping or procedures requiring bypass) of an intracranial saccular aneurysm (ruptured or unruptured) during the study period.

• Extradural (e.g. cavernous carotid) aneurysms

• Fusiform, dissecting or mycotic aneurysms

• Arteriovenous malformation (AVM) flow aneurysms

• Previously treated aneurysms.

• Trainee lead (Neurosurgery or Neuroradiology trainee): Single lead point of contact for data collection at each site.

• Consultant neurosurgeon with a subspecialty neurovascular interest (project lead): Ensures that local guidelines are adhered to by all members and oversee validity by ensuring a complete, accurate dataset is returned.

• Independent data validator (trainee or consultant): Review a random selection of submitted cases to calculate data accuracy and ascertainment.