What does a diminutive basilar artery secondary to persistent fetal circulation of bilateral Posterior Cerebral Arteries (PCAs) indicate?

Diminutive Basilar Artery with Persistent Fetal Circulation of Bilateral PCAs

What This Anatomical Variant Means

A diminutive (hypoplastic) basilar artery associated with bilateral fetal-type posterior cerebral arteries (PCAs) represents a congenital anatomical variant where the PCAs arise directly from the internal carotid arteries rather than the basilar artery, resulting in reduced basilar artery caliber due to decreased flow demand. This configuration fundamentally alters posterior circulation hemodynamics and creates a significant risk factor for posterior circulation ischemia 1, 2.

Anatomical Configuration

The fetal-type posterior circulation variant occurs when:

• The PCAs originate directly from the internal carotid arteries (via enlarged posterior communicating arteries) rather than from the basilar artery terminus 1, 2, 3
• The P1 segment of the PCA is absent or severely hypoplastic, while the posterior communicating artery is the dominant vessel supplying the PCA territory 3
• The basilar artery becomes diminutive (hypoplastic) because it no longer needs to supply the PCA territories, resulting in reduced flow and smaller caliber 1, 2
• This variant can be unilateral or bilateral, with bilateral fetal-type PCAs creating the most pronounced basilar hypoplasia 1, 3

Clinical Significance and Stroke Risk

The combination of basilar artery hypoplasia with bilateral fetal-type PCAs significantly increases the risk of posterior circulation ischemic events 1:

• Patients with this anatomical configuration have decreased flow velocity and increased pulsatility index in the basilar artery, indicating compromised hemodynamics 1
• The hypoplastic vertebrobasilar system supplies a smaller territory (brainstem and cerebellum only, without PCA territories), but paradoxically has reduced flow capacity 1, 2
• This variant predisposes to ischemic stroke or TIA in the posterior circulation, particularly affecting brainstem structures 1

Protective vs. Risk Factors

There is nuanced evidence regarding outcomes:

• When acute basilar artery occlusion occurs in patients with fetal-type PCAs, the variant may act as a safeguard by allowing retrograde filling through the carotid circulation and reducing the territory at risk 3
• However, the chronic presence of basilar hypoplasia with bilateral fetal PCAs increases baseline stroke risk due to compromised vertebrobasilar flow reserve 1, 2

Diagnostic Imaging Findings

CT angiography and MR angiography will demonstrate 2:

• A basilar artery with reduced caliber (typically <2mm diameter) compared to normal (3-4mm) 2
• Bilateral posterior communicating arteries that are larger than normal, directly supplying the PCAs 2, 3
• Absent or thread-like P1 segments of the PCAs 3
• Potentially hypoplastic vertebral arteries as well 1

Clinical Management Implications

Patients with this anatomical variant require heightened vigilance for posterior circulation symptoms 1:

• Any symptoms of vertebrobasilar insufficiency (vertigo, ataxia, diplopia, dysarthria, drop attacks) warrant urgent vascular imaging 4, 1
• The hypoplastic basilar artery has limited collateral capacity, making these patients vulnerable to even minor hemodynamic perturbations 1
• Aggressive vascular risk factor modification is essential, as the compromised baseline flow reserve leaves little margin for additional insults 1

Critical Pitfall

Do not assume this variant is purely protective based on the fetal PCA configuration alone—the chronic hypoplastic state of the vertebrobasilar system creates ongoing ischemic vulnerability that requires proactive management 1, 2.