Residency · Residency · Neurosurgery

Cerebral Aneurysms: Microsurgical Clipping vs. Endovascular Coiling

Overview

The treatment of intracranial aneurysms has evolved dramatically since the publication of the International Subarachnoid Aneurysm Trial (ISAT) in 2002. Both microsurgical clipping and endovascular coiling are effective treatments, and the choice depends on aneurysm morphology, location, patient factors, and institutional expertise. The emergence of flow diverters, intrasaccular devices, and stent-assisted coiling has expanded endovascular options, while microsurgical clipping remains essential for specific anatomic configurations.

Aneurysm Anatomy and Morphology

Classification by Type

Saccular (berry) aneurysms are the most common (80-90%) and present as focal outpouchings at vessel bifurcations. Fusiform aneurysms are circumferential dilations of the vessel wall associated with atherosclerosis or dissection. Mycotic (infectious) aneurysms are distal branch aneurysms from septic emboli, typically in endocarditis. Dissecting aneurysms result from intimal tears with subintimal or subadventitial hematoma and are common in the vertebral artery. Traumatic aneurysms arise from penetrating or blunt vascular injury. Blister aneurysms are small, fragile, broad-based lesions on the non-branching surface of the ICA that are notoriously difficult to treat.

Common Locations

The anterior circulation accounts for approximately 85% of aneurysms. The anterior communicating artery complex is the most common location at 30-35%, followed by the posterior communicating artery at 25%, MCA bifurcation at 20%, and ICA (ophthalmic, superior hypophyseal, cavernous) at 10%. The posterior circulation accounts for approximately 15%, with the basilar tip at 5-8%, PICA/vertebral at 3-5%, and SCA and AICA being rare.

Rupture Risk Factors (Unruptured Aneurysms)

Size is a major factor, with aneurysms larger than 7 mm in the anterior circulation and larger than 4-5 mm in the posterior circulation carrying higher risk. Posterior communicating artery and posterior circulation locations carry higher risk. Morphologic features including irregular shape, daughter sac, and aspect ratio greater than 1.6 increase risk. Clinical risk factors include Japanese or Finnish descent, smoking, hypertension, family history, and multiple aneurysms. The PHASES score (Population, Hypertension, Age, Size, Earlier SAH, Site) estimates 5-year rupture risk. ISUIA data showed that small (under 7 mm) anterior circulation aneurysms carry an annual rupture risk of approximately 0.5%, with posterior circulation and larger aneurysms carrying higher risk.

Microsurgical Clipping

Principles

Microsurgical clipping achieves permanent obliteration of the aneurysm by placing a titanium or cobalt-alloy clip across the neck, preserving the parent vessel and branch arteries. Complete obliteration is achieved in 90-95% of cases, with a retreatment rate of only 2-4%, which is lower than coiling.

Surgical Approach

The pterional or frontotemporal craniotomy is the workhorse approach for most anterior circulation aneurysms. The orbitozygomatic approach provides additional exposure for complex basilar tip or high-riding aneurysms. The interhemispheric approach accesses distal ACA (pericallosal) aneurysms. The subtemporal approach reaches basilar trunk and SCA aneurysms. The far-lateral approach accesses vertebral artery and PICA aneurysms.

Surgical Technique

The procedure begins with positioning and craniotomy, using the pterional approach with head rotation and extension to optimize Sylvian fissure access. Wide opening of the Sylvian fissure provides access to the carotid cistern and circle of Willis. Cisternal dissection identifies the parent artery proximal and distal to the aneurysm, with proximal control obtained first. The aneurysm neck is then exposed through circumferential dissection, identifying all branch vessels and perforators. An appropriate clip (straight, curved, bayonet, fenestrated, or angled) is applied across the neck, confirming no kinking of parent or branch vessels. Confirmation uses intraoperative micro-Doppler, ICG videoangiography, or intraoperative DSA to verify complete obliteration and patent vessels.

Temporary Clipping

A temporary clip on the parent artery softens the aneurysm during dissection and permanent clip application. Duration should be minimized to less than 5-10 minutes per application, with intermittent use preferred. Burst suppression with propofol or etomidate may provide neuroprotection during temporary occlusion. Blood pressure should be augmented during temporary clipping.

Intraoperative Rupture

Intraoperative rupture occurs in 5-15% of cases. Management involves suction, temporary clip placement on the parent artery, rapid permanent clip application, and bipolar coagulation. Premature rupture before proximal control is obtained is more dangerous. Adenosine-induced cardiac pause can provide a brief period of flow arrest for critical moments, though this remains controversial.

<image> Microsurgical view of an anterior communicating artery aneurysm clipping via a pterional approach. The sylvian fissure has been opened and the optic nerve, ICA, A1 segment, and AComA complex are exposed. A curved aneurysm clip is shown applied across the aneurysm neck with the dome of the aneurysm collapsing. Both A2 segments and the recurrent artery of Heubner are preserved. ICG videoangiography inset shows patent parent vessels and excluded aneurysm. Clean microsurgical illustration with labeled structures. </image>

Endovascular Coiling

Principles

Endovascular coiling uses catheter-based delivery of platinum coils into the aneurysm sac to promote thrombosis and obliteration. The procedure is performed via femoral or radial artery access under general anesthesia or conscious sedation. Packing density of coils correlates with durability of occlusion.

Technique

The procedure begins with diagnostic angiography using 3D rotational angiography for optimal working projections. A microcatheter is navigated into the aneurysm dome. Sequential coils of decreasing size fill the aneurysm. Final angiography confirms complete or near-complete obliteration and assesses parent vessel patency.

Adjunctive Endovascular Techniques

Balloon-assisted coiling uses temporary balloon inflation across the aneurysm neck during coil deployment to prevent coil herniation into the parent vessel, particularly useful for wide-necked aneurysms. Stent-assisted coiling deploys an intracranial stent across the aneurysm neck to scaffold coils, requiring dual antiplatelet therapy and carrying higher risk in ruptured aneurysms.

Flow diverters (Pipeline Embolization Device, FRED, Surpass) are dense-mesh stents placed across the aneurysm neck that redirect flow away from the aneurysm and promote thrombosis. They require dual antiplatelet therapy for 3-6 months and are best suited for large or giant, wide-necked, fusiform, or sidewall ICA aneurysms. Occlusion rates reach 75-85% at 6 months and over 90% at 1-3 years. Risks include delayed rupture (rare), perforator occlusion, and in-stent stenosis or thrombosis.

Intrasaccular flow disrupters (WEB device) treat wide-necked bifurcation aneurysms without requiring antiplatelet therapy.

Endovascular Complications

Thromboembolic events occur in 5-10% of cases and are managed with intra-arterial abciximab or mechanical thrombectomy. Aneurysm perforation occurs in 2-4% and can be catastrophic. Coil migration or herniation into the parent vessel may occur. Access site complications include groin hematoma, pseudoaneurysm, and retroperitoneal hemorrhage.

ISAT and Other Landmark Trials

ISAT (International Subarachnoid Aneurysm Trial, 2002)

ISAT randomized 2,143 aSAH patients suitable for either clipping or coiling. Coiling reduced the risk of death or dependence at 1 year (23.7% versus 30.6%, absolute risk reduction 7%, p=0.0019). Long-term follow-up showed the survival benefit of coiling maintained at 10 years, though the rebleeding rate was higher with coiling (12 of 1,073 coiled versus 7 of 1,070 clipped). Important criticisms include that only 22% of screened patients were randomized (suggesting selection bias), the study predominantly included small anterior circulation aneurysms, and it was conducted at predominantly European centers with endovascular expertise.

BRAT (Barrow Ruptured Aneurysm Trial, 2012/2015)

This single-center RCT of 408 patients showed no significant difference in poor outcomes between clipping and coiling at 6 years (33.7% versus 23.2%, not significant after crossover analysis). The retreatment rate was higher for coiled aneurysms. Posterior circulation aneurysms fared significantly better with coiling, while MCA aneurysms were better treated with clipping.

Decision-Making: Clipping vs. Coiling

Factors Favoring Clipping

MCA aneurysms are the strongest indication for clipping because they frequently have broad necks, incorporate branches, and may be associated with Sylvian hematoma requiring evacuation. Other factors favoring clipping include wide-necked aneurysms not amenable to adjunctive techniques, aneurysms with incorporated branch vessels that cannot be preserved with coiling, large intracerebral hematoma requiring surgical evacuation, young patients (due to the lower long-term retreatment rate), failed endovascular treatment, and contraindication to antiplatelet therapy needed for stent-assisted coiling or flow diverters.

Factors Favoring Coiling

Posterior circulation aneurysms are a strong indication for coiling because surgical morbidity is significantly higher in this location. Other factors favoring coiling include elderly patients over 70 (lower procedural morbidity), poor medical condition precluding craniotomy, small narrow-necked aneurysms with favorable anatomy, aneurysms suitable for both treatments in centers with endovascular expertise (per ISAT data), and prior craniotomy making surgical access difficult.

Multidisciplinary Decision

All aneurysms should ideally be discussed in a neurovascular conference with cerebrovascular neurosurgeons and interventional neuroradiologists. Treatment should be individualized based on aneurysm anatomy, patient factors, and team expertise.

<image> Comparative illustration of microsurgical clipping and endovascular coiling of a posterior communicating artery aneurysm. The left panel shows the open surgical view with a clip applied across the aneurysm neck via a pterional craniotomy with the PComA preserved. The right panel shows the endovascular approach with a microcatheter within the aneurysm dome, platinum coils filling the sac, and a final angiogram demonstrating aneurysm occlusion with patent parent vessel. Key anatomical landmarks (ICA, PComA, CN III) are labeled in both panels. Clean side-by-side comparison illustration. </image>

Management of Unruptured Aneurysms

Management requires a careful risk-benefit analysis weighing natural history rupture risk (PHASES score) against procedural risk. Observation is reasonable for small (under 5-7 mm) anterior circulation aneurysms in older patients without risk factors. Treatment is indicated for large size, posterior circulation location, symptomatic aneurysms (cranial nerve palsy, SAH history), family history, morphologic risk factors, or significant patient anxiety. Serial imaging with CTA or MRA every 6-12 months monitors observed aneurysms, with intervention indicated if growth exceeds 1 mm per year. Risk factor modification includes smoking cessation and blood pressure control.

Clinical Pearls

Proximal control is the cardinal rule of aneurysm surgery: always identify and prepare the parent artery for temporary clipping before approaching the aneurysm dome. MCA bifurcation aneurysms remain the strongest indication for microsurgical clipping because branch vessels are frequently incorporated into the neck and the Sylvian fissure provides a natural surgical corridor. ICG videoangiography should be used after every clip application to confirm aneurysm obliteration and vessel patency, having largely replaced intraoperative DSA at many centers. Flow diverters have revolutionized treatment of large and giant ICA aneurysms but require prolonged dual antiplatelet therapy, limiting their use in ruptured aneurysms. ISAT showed coiling is superior for patients suitable for both treatments, but only 22% of screened patients were eligible for randomization, meaning many complex aneurysms are best treated surgically. Retreatment rates are higher for coiling (approximately 15-20% versus 2-4% for clipping), which is particularly relevant for young patients with long life expectancy. Blister aneurysms of the ICA are among the most dangerous to treat because they lack a true wall and cannot be simply clipped; wrapping, trapping with bypass, or flow diverter placement may be needed.

References

• Molyneux AJ, et al. "International Subarachnoid Aneurysm Trial (ISAT) of Neurosurgical Clipping versus Endovascular Coiling in 2,143 Patients." Lancet. 2002;360(9342):1267-1274.
• Spetzler RF, et al. "The Barrow Ruptured Aneurysm Trial: 6-Year Results." J Neurosurg. 2015;123(3):609-617.
• Wiebers DO, et al. "Unruptured Intracranial Aneurysms: Natural History, Clinical Outcome, and Risks of Surgical and Endovascular Treatment (ISUIA)." Lancet. 2003;362(9378):103-110.
• Becske T, et al. "Pipeline for Uncoilable or Failed Aneurysms: Results from a Multicenter Clinical Trial." Radiology. 2013;267(3):858-868.
• Greving JP, et al. "Development of the PHASES Score for Prediction of Risk of Rupture of Intracranial Aneurysms." Lancet Neurol. 2014;13(1):59-66.

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