A liver shunt, medically known as a portosystemic shunt (PSS), is an abnormal vessel connection that allows blood from the digestive system to bypass the liver entirely or in part. This bypass prevents the liver from performing its intended function of filtering and detoxifying the blood before it reaches the rest of the body. The liver is responsible for processing nutrients and neutralizing harmful substances absorbed from the intestines. When a shunt is present, this essential detoxification process is compromised, allowing toxins and unprocessed compounds to circulate throughout the systemic bloodstream.
Understanding the Disruption of Liver Blood Flow
The digestive organs, including the stomach, spleen, pancreas, and intestines, drain their blood supply into the hepatic portal vein system. This specialized circulation is designed to transport blood, which is rich in newly absorbed nutrients and various metabolic byproducts, directly to the liver. Once inside the liver, this blood passes through the filtering cells, called hepatocytes, which metabolize nutrients and break down toxins, such as ammonia. The cleansed blood then exits the liver through the hepatic veins and flows into the systemic circulation, which carries it to the heart and the rest of the body.
A portosystemic shunt creates an unnatural shortcut between the portal vein and a systemic vein. This anatomical flaw diverts the portal blood away from the liver’s filtering system and into the general circulation. Consequently, the liver receives less blood flow, which can cause it to remain underdeveloped or shrink over time, a condition known as hepatic hypoplasia or atrophy. The shunting of blood means that toxins, particularly ammonia produced by gut bacteria, are not neutralized by the hepatocytes and instead accumulate in the bloodstream.
Types of Portosystemic Shunts
Liver shunts are categorized into two main groups based on their origin: congenital and acquired. Congenital shunts are rare vascular malformations present from birth, typically due to developmental issues during fetal development. These shunts are often single, large vessels that connect the portal vein to the systemic circulation either inside (intrahepatic) or outside (extrahepatic) the liver.
Acquired shunts develop later in life, usually as a consequence of severe liver disease, such as cirrhosis. This chronic damage leads to portal hypertension, which is high blood pressure within the portal vein system. To relieve this excessive pressure, the body creates multiple, smaller blood vessels that act as bypasses, forming a network of collateral circulation. These acquired shunts are typically numerous and function as pressure-release valves, contributing to the bypass of the liver’s detoxification function.
Clinical Signs and Manifestations
The most significant clinical manifestation is a neurological condition called Hepatic Encephalopathy (HE). This brain dysfunction occurs because toxins, primarily ammonia, bypass the liver and travel through the systemic circulation to the brain. Symptoms of HE can range from subtle cognitive changes to severe, life-threatening confusion.
Patients may experience difficulty concentrating, fatigue, and changes in personality. A characteristic physical sign is asterixis, an involuntary, jerky movement of the hands when the arms are outstretched. In congenital cases, the lack of proper liver function can also lead to developmental issues, including poor growth and muscle development. Other complications can include digestive issues and serious pulmonary vascular disorders.
Management and Treatment Options
Treatment for a portosystemic shunt is highly dependent on the type of shunt and the patient’s overall liver health, often requiring a multidisciplinary approach. Initial management focuses on medical and dietary interventions aimed at reducing the level of ammonia in the bloodstream. This includes a restricted protein diet to limit ammonia production and medications like lactulose, a sugar that traps ammonia in the gut for excretion. Rifaximin, a non-absorbable antibiotic, also reduces the number of ammonia-producing bacteria in the gut.
For single, congenital shunts, the curative treatment involves closing the abnormal vessel, either through surgery or minimally invasive endovascular techniques. Interventional radiology procedures, such as coil embolization or balloon-occluded retrograde transvenous obliteration (B-RTO), place a device inside the shunt to block blood flow and redirect it back to the liver.
In cases of acquired shunts resulting from irreversible, advanced liver disease like cirrhosis, or when shunt closure is not feasible, liver transplantation is the definitive treatment. Transplantation removes the diseased liver and the underlying cause of the acquired shunts, often leading to the regression of the portosystemic vessels. For patients with severe portal hypertension, a Transjugular Intrahepatic Portosystemic Shunt (TIPS) may be placed to relieve pressure, although this is a different procedure that creates a shunt to manage complications of liver disease.