Where do the blood goes if there is no caudal vena cava?

Computed tomographic and magnetic resonance imaging features of canine segmental caudal vena cava aplasia

T Schwarz 1, F Rossi, J D Wray, B Ablad, M W Beal, J Kinns, G S Seiler, R Dennis, J F McConnell, M Costello

Background
Segmental caudal vena cava (CVC) aplasia is a rare congenital vascular anomaly in dogs, characterized by interruption or absence of a segment of the CVC. The missing postrenal caval segment results in venous blood being shunted to an azygos vein. While previous case reports have described this anomaly, there is limited data on computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) features. This study aimed to describe the imaging characteristics of CVC aplasia and its associated vascular anomalies in affected dogs.

Methods
Study Design: Retrospective review of CT and MRI records from eight veterinary referral institutions between 1999 and 2007.

Inclusion Criteria:
-A diagnostic-quality abdominal CTA or MRA study.
-Supportive clinical and follow-up data.

Data Collection:
-Signalment, clinical history, and imaging findings.
-CTA and MRA studies were evaluated for:
-Size, shape, location, and course of abdominal veins.
-Presence of shunt vessels, thrombosis, and abnormal drainage patterns.

Results
Population:
-10 dogs met the inclusion criteria.
-Mean age: 2.6 years (range: 3 months to 8 years).
-70% of affected dogs were female.

Imaging Findings:
-All dogs had a missing postrenal CVC segment, with venous drainage redirected to the right azygos vein (8/10) or left azygos vein (2/10).
-Seven distinct angiographic patterns of cavo-azygos shunting were identified.
-Aneurysmal dilation of the shunt vessel was observed in 40% of cases, with thrombus formation in one dog.
-Two cases had additional porto-azygos shunts, where portal blood drained into the azygos vein instead of the liver.
-One dog exhibited multiple vascular anomalies, including an intrahepatic portocaval shunt and complete azygos continuation of the CVC.

Clinical Associations:
-Most dogs were asymptomatic, with the anomaly detected incidentally during imaging for unrelated conditions.
-Two dogs presented with episodic collapse, possibly related to impaired venous return through an aneurysmal, collapsing shunt vessel.
-One dog developed thrombosis in the shunt vessel, highlighting the potential for vascular complications.

Limitations
Small sample size (n = 10), limiting generalizability.
Retrospective study design, dependent on the availability and quality of archived imaging data.
Lack of histopathology or genetic analysis, preventing a deeper investigation into the developmental basis of CVC aplasia.

Conclusions
Segmental CVC aplasia results in postrenal venous blood shunting to an azygos vein, with multiple anatomic variations observed on CTA and MRA. Although often an incidental finding, some cases may present with thrombosis, aneurysmal dilation, or collapse-related venous insufficiency. CTA and MRA provide excellent visualization of complex vascular anomalies, making them essential for preoperative planning in dogs with concurrent portosystemic shunts or thromboembolic disease.

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