Residency · Residency · Interventional Radiology
Intracranial Aneurysm: Endovascular Coiling Fundamentals
Overview
Intracranial aneurysms are focal dilations of cerebral arteries, most commonly saccular (berry) aneurysms. Prevalence: 2-5% of the general population; most are asymptomatic. Subarachnoid hemorrhage (SAH) from ruptured aneurysm: devastating — 30-day mortality ~40%. Endovascular coiling has become the primary treatment modality for most intracranial aneurysms since the ISAT trial (2002).
Aneurysm Classification and Morphology
Types
Saccular (berry): most common (~90%); focal outpouching from vessel wall at branch points. Fusiform: circumferential dilation of a vessel segment; associated with atherosclerosis. Dissecting: from intimal tear with subintimal hemorrhage; may cause SAH or ischemia. Mycotic (infectious): from septic emboli (endocarditis); typically distal vessel location. Traumatic: post-traumatic pseudoaneurysm; may involve any segment.
Common Locations
Anterior communicating artery (AComA): 30-35% (most common overall). Posterior communicating artery (PComA) origin: 25-30%. Middle cerebral artery (MCA) bifurcation: 15-20%. Basilar tip: 5-10%. Other: ophthalmic segment ICA, PICA, pericallosal, SCA.
Morphologic Assessment
Dome-to-neck ratio: fundus diameter / neck width. Ratio >2: favorable for coiling (narrow neck retains coils). Ratio <1.5: wide-neck, may require adjunctive devices (stent or balloon). Aneurysm size classification: Small: <7 mm. Medium: 7-12 mm. Large: 13-24 mm. Giant: ≥25 mm. Neck width: <4 mm (narrow) vs. ≥4 mm (wide). Incorporates branch vessels: complicates treatment strategy.
<image>3D rotational angiography reconstruction showing a saccular anterior communicating artery aneurysm with measurement of dome diameter, neck width, and dome-to-neck ratio for treatment planning</image>
Key Trials
ISAT (International Subarachnoid Aneurysm Trial, 2002)
Ruptured aneurysms randomized to endovascular coiling vs. surgical clipping. Primary outcome: death or dependency at 1 year. Result: coiling superior — absolute risk reduction of 7.4% (23.5% vs. 30.9%). Benefit maintained at 10-year follow-up. Higher retreatment rate with coiling (~17% vs. ~4% for clipping). Limitation: selected population (aneurysms amenable to both treatments).
BRAT (Barrow Ruptured Aneurysm Trial, 2012-2019)
Confirmed ISAT findings: coiling associated with better outcomes for posterior circulation aneurysms. MCA aneurysms: clipping may be preferred (better obliteration rates, lower retreatment). Long-term data: retreatment higher with coiling but overall outcomes comparable.
Unruptured Aneurysm Management
ISUIA (International Study of Unruptured Intracranial Aneurysms): Small (<7 mm) anterior circulation aneurysms in patients without prior SAH: very low rupture risk (~0.1%/year). Posterior circulation and larger aneurysms: higher rupture risk. Treatment risk must be weighed against natural history. PHASES score: predicts 5-year rupture risk based on Population, Hypertension, Age, Size, Earlier SAH, and Site. Treatment generally recommended for: ≥7 mm, symptomatic, posterior circulation, family history, documented growth.
Endovascular Coiling Technique
Access and Navigation
Femoral artery access (6-Fr sheath), guide catheter to ICA or vertebral artery. Microcatheter (1.7-2.3 Fr) and microwire navigated into the aneurysm dome. 3D rotational angiography for working angle selection. Roadmap technique for real-time catheter navigation.
Coil Types
Bare platinum coils: original design (Guglielmi detachable coils — GDC). Bioactive coils: coated with polymers to promote thrombosis and endothelialization (Matrix, Cerecyte). Hydrogel-coated coils (HydroCoil): expand with hydration to improve packing density. Coil sizing: first coil ("framing coil") matches the aneurysm dome; subsequent coils progressively smaller ("filling" and "finishing" coils). Detachment mechanism: mechanical, electrolytic, or hydraulic.
Coiling Procedure
Position microcatheter tip in the aneurysm dome. Deploy framing coil along the inner wall of the dome to create a basket. Sequentially deploy filling coils within the basket. Finishing coils (softer, smaller) packed into remaining spaces. Target packing density: >25-30% (higher packing density → lower recurrence). Final angiography: confirm aneurysm occlusion, parent vessel patency, and absence of complications.
Adjunctive Techniques for Wide-Neck Aneurysms
| Technique | Mechanism | DAPT Required | Use in Acute SAH | Best Indication |
|---|---|---|---|---|
| Simple coiling | Coils alone fill dome | No | Yes | Narrow neck (D:N >2) |
| Balloon-assisted | Balloon across neck during deployment | No | Yes | Moderate wide neck |
| Stent-assisted | Permanent stent scaffold at neck | Yes (3-6+ months) | Limited | Wide neck, unruptured |
| Flow diversion | Dense stent diverts flow from dome | Yes (3-6+ months) | No | Large/giant, wide neck |
Balloon-assisted coiling (remodeling technique): Compliant balloon inflated across aneurysm neck during coil deployment. Prevents coil herniation into parent vessel. Balloon deflated between coil placements to assess parent vessel. Temporary occlusion only — balloon removed at end of procedure. Stent-assisted coiling: Self-expanding intracranial stent (LVIS, Enterprise, Neuroform) deployed across the aneurysm neck. Provides scaffold to retain coils. Requires dual antiplatelet therapy (DAPT) — limits use in acute SAH. Indicated for wide-neck unruptured aneurysms.
<image>Intraprocedural fluoroscopic image showing sequential coil deployment within an anterior communicating artery aneurysm using balloon-assisted technique with the compliant balloon inflated across the aneurysm neck</image>
Flow Diverters
Pipeline Embolization Device (PED) and Others
Densely woven, self-expanding stent placed across the aneurysm neck. Diverts flow away from the aneurysm → promotes thrombosis within the dome. Promotes endothelial overgrowth across the neck (neo-intima formation). Aneurysm thrombosis occurs gradually over weeks to months (not immediate). Primary indication: large/giant wide-neck ICA aneurysms (paraclinoid, cavernous). Expanding indications: selected posterior circulation aneurysms, small wide-neck aneurysms.
Key Considerations
Requires DAPT (aspirin + clopidogrel) for 3-6+ months. NOT used in acute SAH (DAPT risk). Complete aneurysm occlusion rates: 75-85% at 6 months, 85-95% at 1 year. Complications: delayed rupture (2-3%), in-stent thrombosis, perforator occlusion.
Scope of Practice Considerations
IR vs. Neuroradiology vs. Neurosurgery
Endovascular aneurysm treatment requires dedicated neurointerventional training beyond standard IR residency. Performed by: interventional neuroradiologists, endovascular neurosurgeons, and select neurointerventionalists with appropriate training. IR residents gain foundational catheter and fluoroscopy skills applicable to neurointerventional procedures. Fellowship training in neurointerventional surgery/radiology provides the specific competencies required.
Complications
Procedure-Related
Intraprocedural rupture (2-4% for ruptured aneurysms, <1% for unruptured): Immediately halt coil deployment, complete coil packing to tamponade. Reverse anticoagulation (protamine). EVD placement if hydrocephalus develops. Thromboembolic events (5-10%): Prevented by systemic heparinization during the procedure. Treated with intra-arterial tPA, glycoprotein IIb/IIIa inhibitors, or mechanical retrieval. Coil migration/herniation: into parent vessel; may require stent deployment as rescue. Parent vessel occlusion: from thrombus or coil mass. Vasospasm: from catheter manipulation.
Disease-Related (SAH)
Cerebral vasospasm (peaks days 4-14 after SAH). Hydrocephalus (acute or chronic). Rebleeding (before treatment). Delayed cerebral ischemia.
Follow-Up
MRA or DSA at 6 months, 1 year, then annually for 3-5 years. Assess for aneurysm recurrence/remnant growth. Coil compaction and aneurysm recurrence: 20-30% develop some degree of recanalization. Retreatment (re-coiling or flow diversion) needed in 10-15%. Contrast-enhanced MRA is the standard non-invasive follow-up modality (platinum coils cause minimal MRI artifact).
<image>Follow-up MRA at 6 months post-coiling showing complete occlusion of a previously treated posterior communicating artery aneurysm with patent parent ICA</image>
Clinical Pearls
ISAT established coiling as the first-line treatment for most ruptured aneurysms — clipping is reserved for MCA bifurcation aneurysms, very wide-neck aneurysms not amenable to endovascular treatment, and cases with concurrent hematoma requiring surgical evacuation. Dome-to-neck ratio is the single most important morphologic factor in determining whether simple coiling vs. adjunctive techniques are needed. Flow diverters have revolutionized treatment of large/giant paraclinoid ICA aneurysms — previously some of the most challenging aneurysms to treat. Dual antiplatelet therapy is mandatory with stent-assisted coiling and flow diversion — this limits their use in acute SAH where DAPT increases hemorrhagic risk. The higher retreatment rate with coiling compared with clipping is the trade-off for lower initial procedural morbidity — long-term follow-up imaging is essential. Intraprocedural rupture is the most feared complication — continue coil deployment to achieve tamponade; stopping and withdrawing the coil worsens the situation. For unruptured aneurysms, the decision to treat must balance procedural risk (1-2% morbidity) against the natural history of rupture (often <1%/year for small anterior circulation aneurysms). IR trainees should be familiar with aneurysm morphology, treatment principles, and SAH management even if they will not independently perform coiling — these cases are frequently discussed in multidisciplinary neurovascular conferences.
References
- Molyneux A, et al. International Subarachnoid Aneurysm Trial (ISAT): Overall Effect of Endovascular Coiling vs. Neurosurgical Clipping. Lancet. 2002;360(9342):1267-1274.
- Molyneux AJ, et al. ISAT: Long-Term Follow-Up at 18 Years. Lancet. 2015;385(9969):691-697.
- Spetzler RF, et al. The Barrow Ruptured Aneurysm Trial (BRAT): 6-Year Results. J Neurosurg. 2015;123(3):609-617.
- Becske T, et al. Pipeline for Uncoilable or Failed Aneurysms: Results from a Multicenter Clinical Trial (PUFS). 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.
- Thompson BG, et al. Guidelines for the Management of Patients with Unruptured Intracranial Aneurysms (AHA/ASA). Stroke. 2015;46(8):2368-2400.
