A brain aneurysm, also known as a cerebral or intracranial aneurysm, is a localized dilation or ballooning that forms on a blood vessel within the brain. This bulge occurs because of a weakness in the vessel wall, which swells outward under the constant pressure of blood flow. When this fragile structure leaks or bursts, it releases blood into the surrounding space, causing a life-threatening type of stroke called a subarachnoid hemorrhage.
Understanding Brain Aneurysms
The most common aneurysm is the saccular aneurysm, often referred to as a “berry” aneurysm. This type appears as a rounded pouch or sac attached to the main artery. Saccular aneurysms are the form most frequently associated with rupture and typically develop at the junctions or forks of arteries, particularly in the Circle of Willis at the base of the brain, where the vessel walls face increased strain.
A less common shape is the fusiform aneurysm, which involves a uniform widening or bulging that extends along a segment of the artery wall. Unlike the saccular type, which protrudes from one side, the fusiform aneurysm affects the entire circumference of the vessel. The saccular morphology, with its defined neck and dome, is the structure involved in the majority of aneurysmal bleeds.
Prevalence of Unruptured Aneurysms
Estimates suggest that unruptured intracranial aneurysms affect about 3.2% to 5% of adults worldwide. Most aneurysms remain asymptomatic and do not cause problems unless they grow large enough to press on surrounding nerves or brain tissue. These silent aneurysms are often discovered incidentally when a person undergoes brain imaging, such as a CT or MRI scan, for an unrelated medical condition. The majority of these small, unruptured formations—estimated to be between 50% and 80%—will never rupture over a person’s lifetime.
Incidence of Rupture and Associated Mortality
The annual risk of rupture for any individual unruptured aneurysm is small, often cited to be less than 1%. However, when considering the entire population, the annual rate of a ruptured aneurysm event is approximately 8 to 10 per 100,000 people. In the United States, this frequency results in an estimated 30,000 people suffering a rupture each year.
A ruptured aneurysm causes a subarachnoid hemorrhage (SAH). Fatalities occur in about 50% of all cases, and approximately 15% of individuals who experience a rupture die before they can even reach the hospital for treatment. For those who survive the initial event, about two-thirds suffer some form of permanent neurological deficit.
Identifying Major Risk Factors
Several factors are known to increase the likelihood of an aneurysm forming and, more specifically, the risk of a rupture event. Chronic, uncontrolled high blood pressure, or hypertension, is a significant modifiable risk factor, as the constant forceful impact against the arterial walls weakens the vessel over time. Smoking is also highly detrimental, increasing the risk for both aneurysm formation and rupture.
Non-modifiable factors also play a substantial role in risk assessment. A family history of brain aneurysms, particularly having two or more first-degree relatives with an aneurysm, significantly elevates personal risk. Certain inherited connective tissue disorders, such as Ehlers-Danlos syndrome or Polycystic Kidney Disease, weaken blood vessels throughout the body, making aneurysm development more likely. The risk for both having an aneurysm and for a rupture increases with age, and aneurysms are generally more prevalent in women than in men, especially in women over the age of 55.