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- Did you know that all porto-azygous shunt terminate in the thorax and pass through the esophageal hiatus?
Did you know that all porto-azygous shunt terminate in the thorax and pass through the esophageal hiatus?
Veterinary Surgery, 2016
Obi Veterinary Education
September 01, 2025
Matan Or, Kumiko Ishigaki, Hilde de Rooster, Kenji Kutara, and Kazushi Asano
Background
Porto-azygos shunts are congenital vascular anomalies in dogs that divert blood from the gastrointestinal venous system directly into the azygos vein, bypassing the liver. These shunts account for approximately 25% of all extrahepatic portosystemic shunts. While computed tomography (CT) angiography is a valuable tool for evaluating portosystemic shunts, the specific anatomical details of porto-azygos shunts, particularly their insertion sites, have not been well documented. This study aimed to characterize the morphology of porto-azygos shunts in dogs using CT angiography, with a focus on their anatomical variations and termination points in the azygos vein.
Methods
This retrospective study analyzed CT angiography scans from 36 dogs diagnosed with porto-azygos shunts between 2007 and 2015. The origin, pathway, and termination of the shunts were assessed in both native and three-dimensional reconstructed images. Vessel diameters were measured at key locations, including the shunt’s insertion into the azygos vein. Statistical analysis was performed to determine correlations between shunt size and age at diagnosis.
Results
All 36 porto-azygos shunts terminated in the thoracic part of the azygos vein and crossed the diaphragm via the esophageal hiatus. Most (33/36) originated from the left gastric vein (left gastro-azygos shunts), while three originated from the right gastric vein (right gastro-azygos shunts). The majority of shunts followed a straight pathway after traversing the diaphragm, though four had a tortuous course. The mean shunt diameter at insertion was 3.0 mm, with the insertion site consistently being the narrowest portion. Two left gastro-azygos shunts exhibited concurrent azygos continuation of the caudal vena cava, and two right gastro-azygos shunts had a caudal splenic loop. No significant correlation was found between shunt size and age at presentation.
Limitations
The study was retrospective, limiting control over data collection. The sample was confined to a single institution, and while diverse, the breed distribution may not fully represent the broader canine population. Additionally, the study did not assess clinical outcomes following surgical intervention.
Conclusions
CT angiography effectively characterized the anatomical variations of porto-azygos shunts, confirming their consistent thoracic termination and variable origins. This information is clinically relevant for surgical planning, particularly for optimizing the placement of occlusive devices. Findings suggest that preoperative CT imaging improves the understanding of shunt morphology, potentially reducing surgical time and complications.
Left gastro-azygos shunt conformation, with a straight-lined right intra-thoracic course. (A) Transverse native image showing the shunting vessel coursing through the esophageal hiatus dorsal to the esophagus. (B) Sagittal native image showing the insertion site of the shunting vessel, situated cranial to the diaphragm. (C) Dorsoventral 3D reconstruction image showing the shunting vessel coursing to the right of the thoracic part of the aorta. (D) Sagittal 3D reconstruction image, viewed from the right side of the dog, showing the shunt coursing to the right of the thoracic part of the aorta with a characteristic L shape at insertion clearly visible. AO, aorta; AZ, azygos vein; CVC, caudal vena cava; PV, portal vein; SH, shunting vessel.
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