What are the differential diagnoses for a 6–7 mm focal irregular narrowing of the left posterior cerebral artery P2‑B segment?

Differential Diagnosis of Focal Irregular Narrowing of the Left PCA P2B Segment

Primary Differential Diagnoses

The most likely diagnoses for a 6-7 mm focal irregular narrowing of the left posterior cerebral artery P2B segment are atherosclerotic stenosis, arterial dissection, vasospasm (if recent subarachnoid hemorrhage), and less commonly vasculitis or moyamoya disease. 1

1. Atherosclerotic Stenosis (Most Common)

• Large-artery atherosclerosis causing ≥50% stenosis or occlusion of major cerebral arteries is a leading cause of ischemic stroke, accounting for approximately 20% of cases 2, 3
• The irregular appearance of the vessel wall strongly suggests atherosclerotic plaque with surface irregularity, which can lead to artery-to-artery embolism or hemodynamic insufficiency 2
• Atherosclerotic disease in the posterior circulation can present with focal stenosis and is associated with risk factors including hypertension, diabetes, dyslipidemia, and smoking 1
• Key distinguishing feature: Look for evidence of atherosclerotic disease in other vascular territories and traditional cardiovascular risk factors 3

2. Arterial Dissection

• Cervical and intracranial arterial dissection can cause focal irregular narrowing and is classified as a stroke of "other determined etiology" 2
• Dissection typically presents with an irregular, tapered narrowing that may extend for several millimeters, matching the 6-7 mm length described 1
• The posterior circulation, including the PCA, can be affected by spontaneous or traumatic dissection 2
• Key distinguishing features: Recent trauma or neck manipulation, younger age, presence of headache or neck pain, and the characteristic "string sign" or double lumen on high-resolution imaging 1

3. Vasospasm (Post-Subarachnoid Hemorrhage)

• Angiographic vasospasm (aVSP) describes transient radiological narrowing of cerebral arteries following subarachnoid hemorrhage 1
• Vasospasm typically occurs 3-14 days after SAH and can affect any intracranial vessel, including the PCA 1
• The narrowing is usually smooth rather than irregular, but severe vasospasm can appear irregular 1
• Key distinguishing feature: Recent history of subarachnoid hemorrhage within the past 2 weeks; absence of SAH essentially excludes this diagnosis 1

4. Moyamoya Disease/Syndrome

• Moyamoya disease involves progressive stenosis or occlusion of the terminal internal carotid arteries and proximal circle of Willis vessels, including the posterior cerebral artery 1
• The posterior circulation involvement in moyamoya is associated with worse clinical presentation and higher risk of hemorrhage 1
• Characteristic findings include narrowing of major cerebral arteries with development of collateral "moyamoya" vessels at the base of the brain 1
• Key distinguishing features: Bilateral involvement (though can be asymmetric), presence of moyamoya collaterals on angiography, younger age (though adult-onset exists), and Asian ethnicity as a risk factor 1

5. Vasculitis (Less Common)

• Central nervous system vasculitis can cause focal or multifocal arterial narrowing with irregular appearance 2
• The irregular vessel appearance may reflect inflammatory changes in the arterial wall 1
• Key distinguishing features: Multifocal involvement of multiple vascular territories, systemic inflammatory markers, presence of autoimmune disease, and response to immunosuppression 2

6. Intracranial Aneurysm-Related Changes

• Fusiform aneurysms of the PCA can present with focal arterial dilation and adjacent narrowing, though the P2 segment has a relatively low incidence of aneurysms 4
• The P2 segment accounts for approximately 28 of 135 PCA aneurysms in one large series, with a high incidence (24%) of fusiform morphology 4
• Key distinguishing feature: Presence of focal dilation rather than pure narrowing; fusiform aneurysms show both narrowed and dilated segments 4

Critical Next Steps for Diagnosis

Immediate Imaging Evaluation

• High-resolution vessel wall MRI is essential to differentiate between atherosclerotic plaque (eccentric wall thickening with enhancement), dissection (intramural hematoma, intimal flap), and vasculitis (concentric wall thickening with enhancement) 1
• CT angiography or MR angiography should be performed to assess the full extent of vascular involvement and identify additional stenoses in other territories 1, 3
• Digital subtraction angiography (DSA) provides the highest spatial resolution for evaluating vessel morphology and may be necessary if non-invasive imaging is inconclusive 1

Clinical Context Assessment

• Review for recent subarachnoid hemorrhage (within 2 weeks) to assess for vasospasm 1
• Assess cardiovascular risk factors (hypertension, diabetes, hyperlipidemia, smoking) that favor atherosclerotic etiology 1, 2
• Evaluate for trauma or neck manipulation that could suggest dissection 2
• Screen for systemic inflammatory or autoimmune conditions if vasculitis is suspected 2

Common Pitfalls to Avoid

• Do not assume atherosclerosis without vessel wall imaging: The irregular appearance could represent dissection, vasculitis, or other etiologies that require different management 1, 3
• Do not overlook bilateral evaluation: Moyamoya disease often has bilateral involvement, and failure to image the contralateral circulation may miss the diagnosis 1
• Do not delay evaluation in young patients: Dissection and moyamoya are more common in younger populations and require prompt diagnosis 1, 2
• Do not ignore the clinical presentation: The presence or absence of stroke symptoms, their temporal pattern, and associated features significantly narrow the differential 1, 2