Introduction
Unruptured intracranial aneurysms (UIAs) are one of the most common and anxiety-inducing findings on Neuroradiology studies. Your role, both on call and during the board exam, is to clearly define the aneurysm and recommend the appropriate management, balancing the risk of rupture against the risk of intervention.
This guide provides a comprehensive Intracranial Aneurysm Protocol for residents, covering the essential imaging features, key size thresholds, and follow-up recommendations to ensure you pass the test and protect your patients.
The Imaging Protocol: Defining the Aneurysm
A UIA is typically found incidentally on CTA or MRA performed for other reasons (e.g., headache, stroke workup). Your report must clearly communicate three key characteristics:
1. Location and Morphology
- Location: Specify the vessel and segment (e.g., Left Middle Cerebral Artery bifurcation, Right Posterior Communicating Artery). Location is crucial for rupture risk calculation.
- Shape (Morphology): Is it Saccular (most common, berry-shaped) or Fusiform (dilatation of the entire vessel wall)? Saccular aneurysms are usually what require intervention.
- Neck/Dome: Define the neck size (the width of the attachment to the parent artery) and the dome (the widest part). A wide neck can make endovascular coiling more challenging.
2. Size and Growth
- Measure: Accurately measure the maximum diameter in the largest plane (e.g., 4.5 mm).
- Small: <7 mm
- Medium: 7 mm to 12 mm
- Large: 13 mm to 24 mm
- Giant: ≥25 mm
- Growth: Compare with prior studies. Any documented growth is a strong indication for treatment.
3. The Imaging Modality of Choice
| Modality | When to Use | Key Strength |
| CT Angiography (CTA) | Initial detection, looking for acute hemorrhage (SAH), or urgent cases. | Fast, excellent for bony anatomy and acute hemorrhage. |
| MR Angiography (MRA) | Long-term follow-up, especially for small aneurysms. | Avoids ionizing radiation. Excellent for follow-up and detecting subtle changes. |
| Digital Subtraction Angiography (DSA) | Pre-treatment planning and confirmation of difficult cases. | Gold standard. Provides real-time flow dynamics and the clearest morphology. |
Export to Sheets
Management Protocol: When to Follow Up vs. Treat
The decision to treat a UIA is complex, often relying on the PHASES Score or the ISUIA (International Study of Unruptured Intracranial Aneurysms) data. For board purposes, remember the size and location cut-offs.
1. Follow-Up (Conservative Management)
- General Rule: Aneurysms in lower-risk locations (e.g., anterior circulation) that are less than 7 mm are typically followed.
- Frequency: Usually follow-up MRA at 6 months, then yearly if stable. If the patient has significant risk factors (smoking, family history), they may be followed more closely.
2. Treatment (Intervention Required)
- Size Threshold: Aneurysms ≥7 mm are generally considered for intervention, especially if the location is high-risk.
- High-Risk Locations (Higher Rupture Rate):
- Posterior Circulation (Basilar Artery, Vertebral Artery)
- Posterior Communicating Artery (PCOM)
- Anterior Communicating Artery (ACOM)
- Other Indications for Treatment:
- Documented aneurysm growth on follow-up imaging.
- Symptomatic aneurysm (causing mass effect on cranial nerves).
3. Treatment Options
- Endovascular: Coiling, stenting, or flow diversion (less invasive).
- Surgical: Clipping (more definitive but invasive).
🚨 Board Exam Tip: Treatment recommendations often hinge on the location-size combination. A 3 mm PCOM aneurysm may be treated, whereas a 3 mm MCA aneurysm is almost always followed.
Conclusion
Understanding the UIA protocol is about knowing the difference between a monitorable finding and a surgical time bomb. Master the imaging characteristics, know the size thresholds, and understand the location-based risk factors.
This knowledge will not only guide you on your Neuroradiology rotation but will guarantee points on your certification exam.
