What is a fetal-type posterior circulation on Computed Tomography Angiography (CTA) of the brain?

Fetal-Type Posterior Circulation on CTA of the Brain

A fetal-type posterior circulation on CTA of the brain is an anatomical variant where the posterior cerebral artery (PCA) is primarily supplied by the internal carotid artery (ICA) rather than the vertebrobasilar system, representing a persistence of embryological circulation patterns.

Anatomical Definition

The fetal-type posterior circulation represents a common embryological variant of the cerebral vasculature, occurring in approximately 4-29% of the population 1. This variant is characterized by:

• A normal-sized patent posterior communicating artery (PComA)
• Hypoplasia or aplasia (underdevelopment or absence) of the ipsilateral P1 segment of the PCA 2
• Blood supply to the PCA territory primarily coming from the anterior circulation (internal carotid artery) rather than the vertebrobasilar system

Types of Fetal Posterior Circulation

There are two main types of fetal posterior cerebral artery (FPCA) variants:

1. Partial fetal PCA (p-FPCA): The PCA is mostly supplied by the ICA, but there is still some contribution from the vertebrobasilar system
2. Full/complete fetal PCA (f-FPCA): The PCA is exclusively supplied by the ICA with no contribution from the vertebrobasilar system 3

These variants can be:

• Unilateral (more common, occurring in approximately 7.18% of cases)
• Bilateral (less common, occurring in approximately 2.14% of cases) 3

Clinical Significance

The presence of a fetal-type posterior circulation has several important clinical implications:

1. Stroke patterns: In patients with FPCA, emboli from the anterior circulation can cause paradoxical PCA territory infarction, with or without concurrent infarction in the middle cerebral artery (MCA) or anterior cerebral artery (ACA) territories 4

2. Aneurysm management: Internal carotid artery aneurysms originating at the takeoff of fetal PCA vessels pose a greater risk for ischemic injury during surgical or endovascular treatment. These aneurysms are more commonly encountered in women and require careful consideration of treatment approach 5

3. Diagnostic considerations: When evaluating posterior circulation ischemia, recognizing this variant is crucial as the source of emboli may be from the carotid circulation rather than the vertebrobasilar system 2

4. Stroke mechanism understanding: In cases of PCA territory infarction, identifying a fetal PCA variant may redirect the search for embolic sources from the vertebrobasilar system to the carotid circulation 4

Imaging Detection

CTA is highly accurate in detecting fetal-type posterior circulation:

• Sensitivity: 100%
• Specificity: 97%
• Positive predictive value: 77%
• Negative predictive value: 100% 2

CTA is considered a valid diagnostic tool for assessing fetal origin of the PCA in patients with cerebral ischemic events in the PCA territory 2. The high accuracy of CTA makes it an excellent non-invasive method for detecting this variant, though catheter angiography remains the gold standard for detailed vascular assessment 6.

Embryological Origin

The fetal-type posterior circulation represents a persistence of embryonic circulation patterns. Two developmental pathways have been observed:

1. Extension of the embryonic posterior communicating artery
2. Developmental transformation of the anterior choroidal artery into a fetal PCA 4

Understanding these embryological origins helps explain the variable anatomy observed in different patients with this variant.

Clinical Approach When Encountering This Variant

When a fetal-type posterior circulation is identified on CTA:

1. Recognize the potential for emboli from the anterior circulation to cause posterior circulation strokes
2. Consider carotid sources of emboli when evaluating PCA territory infarctions
3. Exercise extra caution when planning surgical or endovascular treatment of aneurysms at the origin of fetal PCAs
4. Understand that this variant may be associated with other circle of Willis variations that could affect collateral circulation

This anatomical variant, while common, requires careful consideration in the clinical setting to ensure appropriate diagnostic workup and treatment planning for patients with cerebrovascular disease.