An endovascular approach to the treatment of intracranial aneurysms

Although aneurysms occurring anywhere in the body can be problematic, aneurysms within the cerebrovasculature are especially problematic because the brain is the end organ. Subarachnoid hemorrhage (SAH) caused by rupture of an intracranial aneurysm occurs at a rate of 6 to 8 per 100,000 population,¹ and aneurysms that rupture cause significant disability or mortality in up to 50% of affected patients.² The effects of blood flow disruption within the brain vary according to the vascular territory affected. Aneurysm size and location are important variables in that morbidity and mortality rates are higher when the aneurysm is larger or when it is located within the posterior circulation.³

Aneurysms arising within the intracranial circulation occur in the general population with an incidence of
0.2% to 8.1%^.4-7 Men and women are affected almost equally.⁸ Most aneurysms are diagnosed when patients are 40 to 59 years old.9 Intracranial aneurysms are uncommon in the pediatric population and rare in those younger than 5 years.¹⁰

The etiology is not well understood, but epidemiologic studies have identified certain factors that may contribute both to incidence and to risk of rupture. Uncontrolled hypertension and current cigarette smoking are associated with an increased risk of rupture.⁸ There appears to be a genetic predisposition as well, especially in persons with two or more family members who have had an intracranial aneurysm.¹¹ Diseases such as Ehlers-Danlos syndrome, Marfan syndrome, and polycystic kidney disease can increase the likelihood.¹² Patients who have had one aneurysm have a 15% to 45% chance of having additional aneurysms.¹³

Treatment is aimed at removing the aneurysm while preserving blood flow in the parent vessel. The primary objective is to prevent aneurysm rupture, which can be accomplished using a traditional neurosurgical approach (craniotomy followed by placement of a clip across the base of the aneurysm) or using an endovascular approach (filling the aneurysm sac with metallic coils). This article discusses the endovascular treatment of intracranial aneurysms as an emerging option.

Pathophysiology and diagnosis

The structural composition of the arterial wall in vessels within the dura is different from that of vessels elsewhere in the body. Vessels within the dura lack an external elastic lamina, and the adventitia and media are thin. Congenital thinning may be present at vessel branch points. These structural differences render vessels within the dura at increased risk for aneurysm formation due to an altered hemodynamic state, including those resulting from hypertension, arteriovenous malformation, and polycystic kidney disease. Other risk factors for aneurysms include smoking, IV drug use, damage caused by radiation therapy, atherosclerosis, trauma, and infection.²

Approximately 90% of aneurysms occur within the anterior (carotid) circulation; the remainder occur within the posterior (vertebrobasilar) circulation. In the anterior circulation, ruptured aneurysms are most commonly found at the anterior communicating artery (see Figure 1), then at the posterior communicating artery, the middle cerebral artery bifurcation, and the internal carotid artery bifurcation. In the posterior circulation, most aneurysms are seen at the basilar artery bifurcation (see Figure 2,), then at the vertebrobasilar junction and the posterior-inferior cerebellar artery.^14-16

The most common clinical manifestation of a ruptured aneurysm is SAH; 70% to 80% of cases of nontraumatic SAH are due to a ruptured aneurysm.^2,17 Patients may present with severe headache, neurologic deficits, or seizures. Emergent noncontrast CT of the head is usually ordered because this test has a high sensitivity (around 90%) for subarachnoid blood.¹³

Once SAH is diagnosed, the suspicion of intracranial aneurysm is high and further neuroimaging is done. Catheter cerebral angiography is the gold standard for the diagnostic assessment of aneurysmal SAH; it provides a high degree of anatomic resolution and allows the angiographer to obtain the best angles and images to sort out the aneurysmal morphology. Noninvasive CT angiography (CTA) or magnetic resonance angiography (MRA) may be useful as well. CTA can easily be used in the hyperacute setting when surgery is urgently required and provides images comparable to those obtained with conventional angiography (see Figure 3).

Endovascular treatment options

Until a little more than 10 years ago, the only treatment option for an intracranial aneurysm was open neurosurgery with placement of a clip on the aneurysm neck. With new interventional neuroradiology techniques, however, many aneurysms can now be treated using an endovascular approach. Digital fluoroscopy, microcatheters, and coils are increasingly being used to embolize aneurysms.

Morphology must be determined before an aneurysm is treated using an endovascular technique. Aneurysms are most commonly saccular, with a single, definable opening to the parent vessel, referred to as the neck of the aneurysm. The ratio of the top, or dome, of the aneurysm to the neck determines the stability of the coils after they are placed into the aneurysm. If the neck is too wide, it may be difficult to prevent prolapse of the coils into the parent artery. A soft balloon may be employed to serve as a temporary neck to hold coils in position during placement. Once the coils are properly positioned, the balloon is deflated and is removed when the procedure is completed. More recently, stents have been used to create a scaffold that maintains the coils positioned in the aneurysm and keeps the parent artery patent, even in cases of wide-necked aneurysms. After the morphology, size, location, and number of aneurysms are known, treatment is planned.