An aneurysm is a localized ballooning or bulging in the wall of a blood vessel, typically an artery, caused by a weakness in the vessel wall. In the brain, this is known as an intracranial or cerebral aneurysm, primarily affecting arteries at the base of the skull. The most significant danger is the risk of rupture, which causes a subarachnoid hemorrhage by releasing blood into the space surrounding the brain. This event can lead to severe stroke, brain damage, or death. The decision to treat an unruptured aneurysm with surgery or a minimally invasive procedure is rarely based on size alone, requiring a detailed assessment of multiple patient and aneurysm-specific variables.
The Critical Role of Size and Location
The physical size of an unruptured intracranial aneurysm is generally considered the strongest predictor of its risk of rupture. As the aneurysm expands, the tension on the vessel wall increases, making a tear more likely. For this reason, neurosurgeons often use general size guidelines to determine whether to recommend observation or intervention for aneurysms that have not yet bled.
Aneurysms measuring less than five millimeters in diameter are typically managed conservatively with observation and regular imaging scans in most patients. The surgical risk of intervention for these smaller lesions often outweighs the annual risk of rupture, which is extremely low in the anterior circulation. Conversely, aneurysms exceeding ten millimeters in diameter are often strong candidates for treatment in younger, healthier patients, as the risk of rupture increases significantly with this size threshold.
However, the aneurysm’s location within the brain’s vascular network profoundly modifies the risk associated with its size. Aneurysms situated in the posterior circulation, which includes the basilar and vertebral arteries, or the posterior communicating artery (PComm) in the anterior circulation, are considered high-risk locations. These aneurysms are known to rupture at smaller sizes than those in the anterior circulation, meaning an aneurysm of three to five millimeters in these locations may warrant treatment. Aneurysms in the anterior cerebral artery or middle cerebral artery are generally considered lower-risk locations, where a physician might monitor a lesion up to seven or ten millimeters before recommending intervention.
Beyond Size: Other Factors Influencing Intervention Decisions
While size and location provide a framework for risk stratification, they represent only two pieces of the complex clinical puzzle. The physical characteristics of the aneurysm itself, known as its morphology, can signal an elevated risk of rupture regardless of its overall diameter. Aneurysms that are irregularly shaped, lobulated, or possess a small outpouching called a “daughter sac” or “bleb” are more vulnerable to rupture than those with a smooth, symmetrical contour.
Evidence of recent growth, demonstrated by sequential imaging scans, is another major factor pushing the decision toward treatment. An actively expanding aneurysm suggests an unstable wall structure, even if the current size is below the general treatment threshold. Patient-specific factors also influence risk and treatment tolerability.
Uncontrolled high blood pressure and a history of smoking are established risk factors that accelerate aneurysm wall degradation. A family history of intracranial aneurysm rupture suggests a genetic predisposition, increasing individual risk. The patient’s age and overall health status introduce a trade-off between the risk of rupture and the risk of intervention. Younger patients with a longer life expectancy are generally more likely to be treated, as they have more years for the aneurysm to potentially rupture.
Comparing Surgical and Endovascular Treatment Options
Once the clinical team determines that the risk of aneurysm rupture outweighs the risk of treatment, two primary methods are used to secure the lesion. The traditional approach is microsurgical clipping, which involves an open surgical procedure, a craniotomy, to access the brain. The neurosurgeon places a tiny titanium clip across the “neck” of the aneurysm, sealing it off from circulation. Surgical clipping is highly durable and often results in a complete and permanent obliteration of the aneurysm, providing a low risk of recurrence.
The alternative is endovascular treatment, a less invasive method performed from inside the blood vessel. A specialized catheter is threaded from an access point, typically in the groin or wrist artery, up to the aneurysm site. The most common technique, coiling, involves deploying tiny, soft platinum coils into the aneurysm sac to induce clotting and block blood flow. Newer endovascular techniques include flow diversion, which uses a mesh-like stent to redirect blood flow away from the aneurysm opening, promoting healing and clotting.
Endovascular procedures generally offer a shorter hospital stay and a faster initial recovery time than open surgery, making them a preferable option for older or medically fragile patients. However, coiling may carry a higher risk of recurrence and may require follow-up procedures to ensure the lesion remains fully closed. The ultimate choice between clipping and coiling depends on the aneurysm’s specific shape, size, neck width, and the patient’s individual medical profile and anatomy.