What is the most likely diagnosis for a man with headaches and a 5mm berry-like appearance of a blood vessel at the arterial bifurcation in the proximal circle of Willis with degenerative changes?

Saccular (Berry) Aneurysm

The most likely diagnosis is a saccular (berry) aneurysm of the Circle of Willis, given the characteristic 5mm berry-like appearance at an arterial bifurcation in the proximal Circle of Willis. 1, 2

Diagnostic Reasoning

Anatomical Location Confirms Saccular Aneurysm

• The proximal Circle of Willis represents the classic location for saccular aneurysms, which preferentially occur at arterial branch points and bifurcations of this arterial ring 1
• Saccular aneurysms are the most common type of cerebral aneurysm, with the anterior portion of the Circle of Willis being the predominant site (approximately 65% of cases) 3, 4
• The "berry-like" morphology described is pathognomonic for saccular aneurysms, which have a characteristic rounded, berry-shaped appearance protruding from arterial bifurcations 2, 3

Distinguishing from Mycotic Aneurysm

• Mycotic (infectious) aneurysms are located distally, not proximally, with 55-77% occurring in the middle cerebral artery branches beyond the first bifurcation 5
• Mycotic aneurysms have a poorly defined wide base or fusiform shape with thin friable walls, contrasting with the well-defined neck of congenital berry aneurysms 5
• Without fever, known infective endocarditis, or systemic infection, mycotic aneurysm is highly unlikely 5

Clinical Significance of Headache

• Headache in the setting of an unruptured aneurysm may represent a sentinel warning sign requiring urgent evaluation 6, 7
• Severe, localized, unremitting headache can indicate impending rupture and warrants immediate intervention 5
• The 5mm size meets the threshold where treatment is recommended, particularly given symptomatic presentation 2

Immediate Management Algorithm

Diagnostic Confirmation Required

• Obtain catheter-based digital subtraction angiography (DSA) to definitively characterize neck morphology, neck-to-dome ratio, and precise relationship to parent vessels 2
• DSA remains the gold standard despite CT findings, as treatment planning requires detailed anatomic characterization that CT cannot fully provide 2

Treatment Decision Based on Aneurysm Characteristics

• If neck diameter <5mm and neck-to-dome ratio <0.5: Endovascular coil embolization is preferred, with procedural morbidity of 2.6% permanent neurological deficits 2
• If wide neck (≥5mm) or unfavorable neck-to-dome ratio (≥0.5): Surgical clipping provides more durable exclusion from circulation 2
• The 5mm size combined with symptomatic presentation (headaches) mandates treatment rather than observation, as cumulative lifetime rupture risk becomes significant 2

Critical Pitfalls to Avoid

• Do not delay angiography in patients with headache and suspected aneurysm, as sentinel headaches may precede catastrophic rupture 6
• Do not proceed with treatment based on CT alone without catheter angiography when endovascular treatment is considered 2
• Ensure treatment at high-volume centers with experienced neurovascular specialists, as operator experience significantly impacts complication rates 2

Additional Considerations

Associated Conditions to Screen For

• Berry aneurysms occur in up to 10% of patients with aortic coarctation, warranting screening for cerebrovascular aneurysms if symptoms develop 5
• Polycystic kidney disease is associated with berry aneurysms and should be evaluated 3

Post-Treatment Surveillance

• Angiographic follow-up at 6 months and 1-3 years is essential, particularly after endovascular coiling due to substantial recurrence rates 2
• MRA or CTA can substitute for catheter angiography in follow-up, with MRA sensitivity of 85-100% for aneurysms ≥5mm 2