The Natural History of Cerebral Aneurysms

The natural history of any disease is defined as the outcome of the disease in the absence of any intervention. This information therefore provides benchmarks to assess the effectiveness of treatment options. This is especially relevant with cerebral aneurysms because the correct management of this potentially lethal lesion is still unclear even in the 21st century and depends upon the accurate knowledge of its natural history. The present chapter discusses the natural history of both unruptured and ruptured cerebral “berry” aneurysms, but not infectious (“mycotic”), dissecting, and fusiform and traumatic aneurysms. Our evaluation considers intact and ruptured aneurysms separately because epidemiologic studies have determined that the natural histories of unruptured and ruptured cerebral aneurysms are distinctly different. For unruptured aneurysms, we focus on prevalence and rupture rates, both of which vary widely. Prevalence of intact aneurysms correlates with gender, age, smoking history, nationality, hypertension, wall stress/hemodynamic aspects, genetic influences, and lifestyle. Factors that correlate with rupture include size, morphology, growth, wall stress/hemodynamic influences, location, multiplicity, inflammation, and sentinel headaches. The natural history of ruptured aneurysms is divided into short term (0-6 months) and long-term (≥6 months) outcomes and correlated with both patient and aneurysm factors. Short-term outcome for hospitalized patients is related to patient factors such as grade on admission, time from hemorrhage, rebleeding, gender, age, hypertension, and possibly genetic and molecular factors. Aneurysmal factors that correlated with outcome include aneurysm location, size, rebleeding, and the development of vasospasm. With respect to long-term outcome, patients who survive for more than 6 months can achieve a functional recovery but face persistent rebleeding (3%-5% per year) and fatal rebleeding (2%-3% per year), cardiovascular disease, and the development of epilepsy.