Recurrence of endovascularly and microsurgically treated intracranial aneurysms—review of the putative role of aneurysm wall biology

Although endovascular therapy has been proven safe and has become in many centers the primary method of treatment for intracranial aneurysms, the long-term durability of endovascular embolization remains a concern; at least for some aneurysms despite initial good result. While healing after clipping relies on mechanical occlusion, restoration after endovascular occlusion mainly requires the induction of a biological response. Healing after embolization depends on the growth of new tissue over the thrombus formed by the embolization material, or alternatively, on the organization of thrombus into fibrous tissue. This review highlights the fundamental importance of aneurysm wall biology on the healing process and long-term occlusion after intracranial aneurysm (IA) treatment. It seems likely that the effect of luminal thrombus on the IA wall, as well as the IA wall condition at the time of thrombosis, determine if thrombus organizes into scar tissue (neointima formation by infiltration of cells originating from the IA wall) or if the wall undergoes continuous remodeling, which is primarily destructive (loss of mural cells). In the latter, intraluminal thrombus organization fails and the impaired healing increases the chance of recurrence. Mechanisms underlying IA reopening, the influence of intraluminal thrombosis on the IA wall, and clinical implications of the IA wall condition are discussed in detail, along with how knowledge of IA wall biology can offer new solutions for IA treatment and affect the patient selection for and follow-up after endovascular treatment.