INTRODUCTION: Hepatic encephalopathy (HE) is a spectrum of neuropsychiatric abnormalities in patients with underlying liver dysfunction such as acute liver failure (type A) or cirrhosis (type C). Less commonly, it involves patients with acquired or congenital portosystemic shunt (PSS) in the absence of hepatocellular disease (type B). CASE DESCRIPTION/METHODS: A 70-year-old female with a past medical history of chronic pain, CHF, lymphedema and prior recurrent encephalopathy, presented with altered mental status and seizure-like activity. Vital signs were stable and laboratory studies including CBC, BMP and LFTs were normal except for an ammonia level of 272 umol/L. She denied any prior history of liver disease and was previously informed that the hyperammonemia was secondary to her use of narcotics for chronic pain. CT chest and abdomen was obtained which revealed normal appearing liver, prominent azygos vein and side-to-side porto-azygos shunt (Figures 1 and 2). A liver US indicated normal echotexture with patent vasculature. Liver biopsy showed normal portal triads with mild steatosis and minimal sinusoidal dilatation. She was treated with lactulose until resolution of HE and was eventually discharged. Given the absence of liver disease, the PSS likely represent a congenital malformation that has been perpetuating the HE. DISCUSSION: In the presence of a PSS, underlying liver disease should be excluded to ensure the etiology of the shunt is not a result of portal hypertension. Congenital PSS were first described in 1793 by J. Abernethy and are classified as type 1 which involves an end-to-side porto-caval shunt with absence of the portal vein (PV), and type 2 which consists in a side-to-side shunt with an intact PV. Congenital PSS may also be associated with other abnormalities including biliary atresia, polysplenia, in addition to cardiac, skeletal and renal malformations. HE, which occurs as a complication of the shunt bypassing the hepatic conversion of ammonia to urea, is the most common neurologic complication. In addition, acquired cardiopulmonary complications are also common due to volume overload. Patients with symptomatic congenital PSS should be considered for PSS closure, whether endovascular or surgical. In patients with type 2 shunt, obliteration of the PSS can reverse HE from hyperammonemia and potentially resolve heart failure and hepatopulmonary syndrome but rarely causes regression of pulmonary hypertension. Acute portal hypertension as well as thrombosis have been reported after closure.Figure 1.: Prominent Azygos vein on coronal view.Figure 2.: Side to side portal to azygos vein shunt.