The decision-making process in the management of unruptured intracranial aneurysms is complex and multifactorial. Our study evaluates the natural history of intracranial aneurysms and identifies risk factors for aneurysm rupture.

All patients who first presented to our department between 1984 and 2020 with the diagnosis of at least one unruptured intracranial aneurysm were included. Of 4715 patients, 661 patients (14%) with 767 aneurysms were assigned conservative management for various reasons. To identify risk factors for aneurysm rupture and growth both patient-based and aneurysm-based analyses were performed.

Of the overall study population, 66% were female. Aneurysms were predominantly located in the anterior circulation and had a median size at diagnosis of 5mm. For 23 patients, aneurysm rupture was reported during the conservative observation period, leading to a calculated annual hemorrhage rate of 0.6%. Patients who experienced rupture were significantly older and their aneurysms larger, symptomatic, and often partially thrombosed at diagnosis compared to patients without aneurysm rupture. ELAPSS-score and PHASES-score were found to reliably predict aneurysm growth and rupture.

The natural history of unruptured intracranial aneurysms may vary substantially depending on several patient- and aneurysm-specific risk factors. Thus, in the management of unruptured intracranial aneurysms, the patient"s individual aneurysm rupture risk has to be carefully weighed against the center specific treatment outcome and complication rates.