Abernethy Malformation Type Ib in a Patient With Trisomy 21: A Rare Case of Portal Vein Absence, Mesenteric Thrombosis, and Bowel Perforation.

Congenital portosystemic shunts (CPSSs), also known as Abernethy malformations, are rare vascular anomalies in which blood from the portal circulation bypasses the liver and drains directly into the systemic circulation. They are classified into type I, involving complete diversion of portal blood with absent intrahepatic portal branches, and type II, where partial shunting occurs with preserved intrahepatic perfusion. While often asymptomatic, these malformations can cause diverse complications, including hepatic encephalopathy, pulmonary hypertension, and gastrointestinal bleeding. We present the case of a 21-year-old woman with trisomy 21 who developed a severe and multifaceted clinical course due to an undiagnosed Abernethy malformation type Ib. She was initially referred for transjugular intrahepatic portosystemic shunt (TIPS) placement after imaging revealed thromboses of the mesenteric and splenic veins, splenomegaly, and signs of portal hypertension. Endoscopic findings confirmed grade II esophageal varices and portal hypertensive gastropathy. Imaging showed thrombosis of the splenic vein (SV) and superior mesenteric vein (SMV), with drainage into the inferior vena cava (IVC) and absence of a native portal vein, consistent with Abernethy malformation type I. The planned intervention was not feasible due to anatomical constraints and extensive thrombosis. The patient was started on full anticoagulation. Three days later, she developed acute abdominal symptoms. Imaging revealed dilated, thickened bowel loops with signs of obstruction. An exploratory laparotomy uncovered a conglomerate mass with intestinal perforation and purulent peritonitis. Resection and double anastomosis were performed, but due to massive edema, initial closure was impossible, and open abdomen treatment was required. She developed systemic inflammatory response syndrome, acute kidney injury, and bilateral pleural effusions. Targeted antimicrobial therapy was initiated based on microbiological cultures. After 26 days in intensive care, she stabilized and was transferred to the general ward. Histological evaluation of the resected specimen revealed transmural colonic damage, intestinal thromboses, and IgG4-positive lymphadenopathy without evidence of malignancy or systemic IgG4 disease. Follow-up imaging showed persistent thromboses without progression. Compared to published cases, this is the only report describing the combination of trisomy 21, Abernethy malformation type Ib, mesenteric thrombosis, bowel perforation, sepsis, and IgG4-positive lymphadenopathy in one patient. No interventional treatment options were available, and management was limited to surgical and supportive care. This case illustrates a rare and severe manifestation of CPSS with complex vascular and immunologic features. The risk of future complications remains high, and liver transplantation may represent the only curative approach in selected patients with Abernethy malformation type I.