In a 31‑year‑old patient with a left thalamic hemorrhage and digital subtraction angiography showing a 6–7 mm focal irregular narrowing of the left posterior cerebral artery P2‑B segment, what is the most likely diagnosis?

Most Likely Diagnosis: Arterial Dissection of the Posterior Cerebral Artery

The most likely diagnosis in this 31-year-old patient with left thalamic hemorrhage and focal irregular narrowing of the P2B segment is arterial dissection of the posterior cerebral artery. 1, 2

Key Diagnostic Reasoning

Age and Demographics Strongly Favor Dissection

• Arterial dissection accounts for 10-15% of ischemic strokes in younger patients (under 45 years), making it the predominant vascular pathology in this age group 3
• Isolated PCA dissection, though rare, characteristically involves the P2 segment and occurs predominantly in younger patients, particularly affecting women but also seen in young men 2
• The patient's age of 31 years essentially excludes atherosclerotic disease as a primary consideration 1

Angiographic Features Are Pathognomonic

• The 6-7mm focal irregular narrowing with irregular vessel appearance on DSA is the classic angiographic signature of arterial dissection 1, 2
• Dissection typically manifests as an irregular, tapered narrowing that may extend 6-7mm and is often accompanied by a "string sign" or double-lumen on high-resolution imaging 1
• The irregular appearance reflects intramural hematoma causing luminal compromise, distinguishing it from the smooth narrowing of vasospasm or the eccentric appearance of atherosclerosis 1

Hemorrhagic Presentation Fits Dissection Pattern

• Intracranial dissections, particularly in the posterior circulation, carry a higher risk of rupture and hemorrhage compared to extracranial dissections 4
• The left thalamic location corresponds to the vascular territory supplied by P2 segment branches, making hemorrhage from P2B dissection anatomically consistent 5, 6
• Dissecting aneurysms of the PCA P2 segment can present with hemorrhagic complications, as documented in multiple case reports 7, 2

Alternative Diagnoses to Exclude

Vasospasm (Essentially Ruled Out)

• The presence of subarachnoid hemorrhage within the prior two weeks strongly supports vasospasm, whereas its absence essentially excludes this diagnosis 1
• The patient presented with thalamic hemorrhage, not subarachnoid hemorrhage, making post-hemorrhagic vasospasm extremely unlikely 1
• Vasospasm typically occurs 3-14 days after subarachnoid hemorrhage and usually appears smooth rather than irregular 1

Atherosclerosis (Age Makes This Unlikely)

• Posterior-circulation atherosclerosis is strongly linked to traditional cardiovascular risk factors (hypertension, diabetes, dyslipidemia, smoking) and typically affects older patients 1
• At age 31 without mentioned risk factors, atherosclerotic stenosis would be extraordinarily rare 1

Moyamoya Disease (Wrong Pattern)

• Moyamoya features progressive stenosis of terminal internal carotid arteries and proximal circle-of-Willis vessels, with characteristic collateral "moyamoya" vessels on angiography 1
• The isolated focal P2B narrowing without bilateral involvement or characteristic collaterals argues against moyamoya 1

CNS Vasculitis (Less Likely Without Systemic Features)

• CNS vasculitis can produce focal or multifocal irregular arterial narrowing but typically presents with multifocal lesions, systemic inflammatory markers, or known autoimmune disease 1
• The isolated focal finding without mentioned systemic features makes vasculitis less probable 1

Critical Next Steps

Confirm Diagnosis with Vessel-Wall Imaging

• High-resolution vessel-wall MRI is essential to differentiate among atherosclerotic plaque (eccentric wall thickening with enhancement), dissection (intramural hematoma or intimal flap), and vasculitis (concentric wall thickening with enhancement) 1
• This imaging will definitively demonstrate the intramural hematoma characteristic of dissection 1

Assess Full Vascular Territory

• CT angiography or MR angiography should be performed to evaluate the full vascular territory and detect additional stenoses in other cerebral arteries 1
• Always assess the contralateral circulation, as failure to image both sides can miss bilateral disease 1

Obtain Focused Clinical History

• Inquiry about recent trauma, vigorous neck manipulation, or chiropractic procedures aids in identifying arterial dissection 1
• Screen for connective tissue disorders (Ehlers-Danlos type IV, Marfan syndrome, fibromuscular dysplasia) which are associated with dissection 3

Management Approach

Antithrombotic Therapy

• The American College of Cardiology recommends initiating antithrombotic therapy immediately for 3-6 months using anticoagulation or antiplatelet therapy for patients with carotid dissection, with the choice based on individual hemorrhagic risk 4
• Given the hemorrhagic presentation (thalamic bleed), antiplatelet therapy is strongly preferred over anticoagulation to avoid hemorrhage expansion 3
• Anticoagulation may adversely influence the outcome of subarachnoid hemorrhage in the event of intracranial extension of cervical artery dissection 3

Critical Contraindication

• Never anticoagulate if there is intracranial extension of the dissection with subarachnoid hemorrhage, as intracranial vertebrobasilar dissections have a higher risk of rupture 4
• The presence of thalamic hemorrhage in this case makes anticoagulation contraindicated 4

Monitoring Protocol

• Perform non-invasive imaging of the intracranial arteries at 1 month, 6 months, and annually to assess patency and exclude the development of new lesions 4
• Reserve endovascular revascularization exclusively for patients with persistent or recurrent ischemic symptoms who do not respond to optimal antithrombotic therapy 4

Common Pitfalls to Avoid

• Do not presume atherosclerosis solely on the basis of an irregular narrowing; vessel-wall imaging is necessary because dissection, vasculitis, or other etiologies may mimic atherosclerotic appearance 1
• Prompt evaluation is critical in younger patients, as dissection and moyamoya disease are more prevalent in this age group and require timely intervention 1
• Do not delay vessel-wall MRI, as this is the definitive test to confirm intramural hematoma and establish the diagnosis of dissection 1
• Integrate the clinical presentation (stroke symptoms, timing, associated headache or pain) with imaging findings to narrow the differential diagnosis 1