Saccular posterior cerebral artery aneurysm encased within a lipoma

We present a case of a man in his 80s with an incidental posterior cerebral artery aneurysm encased within a lipoma. The literature surrounding the incidence and intricate relationship of lipomas to cerebral aneurysms is reviewed. Lipomas are proposed to be derived from maldifferentiated subarachnoid space. For this reason, lipomas are often associated with vascular malformations and may develop in conjunction with vascular malformations such as cerebral aneurysms. Hypothesised theories include the impediment of smooth muscle nutrient diffusion and the secretion of factors that weaken the arterial wall thereby predisposing to aneurysm formation. When lipomas neighbour cerebral vasculature, careful evaluation of the adjacent vessels should be conducted.