A portosystemic shunt (PSS), commonly known as a liver shunt, is an abnormal connection that allows blood intended for the liver to be diverted directly into the main circulation of the body. The hepatic portal vein system normally collects blood rich in nutrients and absorbed substances from the digestive tract, pancreas, and spleen. A shunt creates a bypass, allowing this unpurified blood to skip the necessary filtration and detoxification steps the liver provides before it enters the general bloodstream. This vascular anomaly affects the body’s metabolic and neurological functions.
Mechanisms: Why Portosystemic Shunts Form
Portosystemic shunts are categorized into two primary types: congenital and acquired. Congenital shunts are vascular malformations present from birth, resulting from a failure of fetal blood vessels to close after delivery. The ductus venosus, which normally shunts blood past the liver in the fetus, sometimes remains open and functions as an intrahepatic shunt within the liver tissue. Other congenital shunts, termed extrahepatic, form when a single large vessel bypasses the liver, connecting the portal vein or one of its tributaries directly to the systemic vena cava.
Acquired shunts develop later in life and are almost always a consequence of severe, chronic liver disease, such as cirrhosis. Scarring of the liver tissue increases resistance to blood flow through the portal vein, leading to portal hypertension. To relieve this high pressure, the body redirects blood away from the liver by expanding small, pre-existing collateral vessels. These multiple and often tortuous vessels are the acquired portosystemic shunts. Unlike congenital shunts, acquired shunts are typically numerous and form a complex network of collateral channels.
How Shunting Affects the Body
The primary consequence of a portosystemic shunt is the loss of the liver’s detoxification function. Substances absorbed from the digestive tract, including neurotoxins produced by gut bacteria, bypass the liver’s metabolic machinery and are delivered directly to the systemic circulation. The most prominent unfiltered substance is ammonia, which the liver normally converts into harmless urea.
The accumulation of ammonia and other toxins in the bloodstream directly affects the central nervous system, leading to hepatic encephalopathy (HE). Symptoms of HE range from subtle behavioral changes, disorientation, and slowness to severe confusion, ataxia, or even coma. While ammonia is a primary factor, other compounds like mercaptans and altered neurotransmitters also contribute to the brain dysfunction associated with shunting.
The liver is also deprived of nutrient-rich portal blood, which is essential for its normal growth and function. In congenital cases, this lack of hepatotrophic factors can lead to an underdeveloped or small liver, known as hepatic atrophy. This insufficient size, combined with chronic toxin exposure, can cause stunted growth and malnutrition, especially in younger individuals. The chronic metabolic derangement also affects other systems, potentially leading to altered blood clotting factors or specific urinary stones.
Identifying and Treating Portosystemic Shunts
Identifying a portosystemic shunt begins with blood tests measuring liver function and toxin concentration. An elevated blood ammonia level, particularly after consuming a meal high in protein, is a strong indicator of shunting. Specialized tests, such as measuring pre- and post-prandial bile acid concentrations, also help confirm that the liver is being bypassed.
Once metabolic tests suggest a shunt, imaging studies are used to locate and characterize the abnormal vessel. Abdominal ultrasound is often the initial screening tool, but cross-sectional imaging like computed tomography (CT) or magnetic resonance angiography (MRA) provides the detailed anatomical map necessary for treatment planning. These advanced scans confirm whether the shunt is a single, isolated vessel (typical of congenital cases) or a network of multiple, smaller collateral vessels (characteristic of acquired shunts).
Treatment is dependent on the type of shunt identified. For a single congenital shunt, the definitive therapy is an interventional procedure or surgery to close the vessel, known as occlusion or ligation. Techniques involve placing specialized devices, such as coils or cellophane bands, to gradually or completely shut down the shunt and force blood back into the liver. Gradual occlusion is preferred to prevent a sudden spike in portal pressure.
Acquired shunts form to decompress the high pressure of portal hypertension and generally cannot be surgically closed without causing severe complications. Management focuses on controlling the symptoms of hepatic encephalopathy through medical therapy. Medications like lactulose, a non-absorbable sugar, work by drawing ammonia into the colon and trapping it for excretion. Rifaximin, a non-absorbable antibiotic, is also used to reduce ammonia-producing bacteria within the gut. In cases of large, spontaneous acquired shunts causing refractory encephalopathy, specialized interventional radiology procedures, such as Balloon-Occluded Retrograde Transvenous Obliteration (BRTO), may be performed to deliberately close the shunt to improve neurological symptoms.