Anatomy, Abdomen and Pelvis, Portal Venous System (Hepatic Portal System)

Anatomical variants of the following venous morphologies occur in up to 35% of individuals.

• Trifurcation of the main portal vein
• The right posterior branch, which arises from the main portal vein
• The right anterior branch which originates from the left portal vein

Duplication of the portal vein and non-branching portal vein variants exist as well but are rare.

Congenital anomalies of the portal venous system can occur. The most common congenital anomaly is agenesis of the left or right portal vein. This variant can lead to the development of collateral vessels of the portal venous system as a result of portal hypertension. The most common of these collaterals are gastroesophageal, paraumbilical, inferior mesenteric, and splenorenal collateral vessels.[4]

Knowledge of both normal and variant anatomy is of utmost importance, especially for surgery. The branching patterns of the portal venous system are often assessed prior to surgery via imaging techniques. Color Doppler Ultrasound (US) is the most useful imaging technique for identifying aneurysms and/or thrombosis of the portal vein. Helical CT is a valuable tool for visualizing the branching pattern of the portal venous system. [4] Anatomical variants of portal vein’s branching pattern are essential to recognize on imaging for liver surgery to maintain perfusion to the remaining liver segments. Additionally, the anatomy of the portal venous circulation is significant for liver transplantation in which both the donor’s venous anatomy and the recipient’s anatomy requires analysis to ensure appropriate donor and recipient selection.[2]

Portal vein thrombosis is a frequent occurrence in patients with hypercoagulable diseases, such as malignancy and cirrhosis, and can be a challenge at the time of liver transplantation. Due to the increased morbidity and mortality associated with this condition at the time of transplantation, this thrombotic state is considered a relative contraindication to a liver transplant.[5] Portal vein recanalization (PVR) and transjugular intrahepatic portosystemic shunts (TIPS) are safe options for patients with portal vein thrombosis in dire need of a liver transplant.[6]

Due to the large quantity of blood contained within the liver and flowing through the portal system, patients that suffer liver injuries are at increased risk for hemorrhagic shock and death from blood loss. Cross clamping the hepatic pedicle, known as the Pringle Maneuver, stops the blood flow to the liver and allows for better visualization of the surgical field and aids in preventing hemodynamic collapse.[7]

Portal hypertension is a disease that arises when blood flow through the liver is compromised, and blood pressures in the portal vein rise. This disease has many complications that result from increased pressure, including esophageal varices, ascites, and hepatorenal syndrome, which contribute to the high morbidity and mortality often seen in this condition.[8] The blood flow through the liver has a direct effect on renal function via a reflex mechanism. The liver detects the sensory afferent response via blood flow-dependent changes in the intrahepatic adenosine concentrations. The efferent response to changes in hepatic blood flow is affected by the renal sympathetic nerves, causing constriction of renal vasculature and release of renal vasoconstricting molecules resulting in renal dysfunction.[9] To date, the only definitive treatment of hepatorenal syndrome is a liver transplant.[10]

Additionally, if a patient is under consideration for partial liver resection, portions of the portal vein can be embolized to enable hypertrophy of the remaining liver segments that still receive flow from other segments of the portal venous system. In a majority of patients, portal vein embolization results in adequate liver hypertrophy to allow surgery to proceed. However, should the portal vein embolization result in insufficient hypertrophy of the remaining liver segments, hepatic vein embolization may be an option to augment the effects of portal vein embolization.[11]

Another clinical consideration aforementioned is portal vein thrombosis, which can be pathologically acquired, as seen in cirrhosis, malignancy, liver tumors, and patients with a certain JAK2 mutation. Portal vein thrombosis can present with varied symptoms, including abdominal pain, ischemic hepatitis, liver failure, and/or small intestine infarction. Patients generally receive treatment with anticoagulant medication. This treatment can incur further complications by the accompanying coagulopathy often seen in these patients. The surgeon may consider a transjugular intrahepatic portosystemic shunt (TIPS) procedure in patients who do not improve or continue to worsen despite anticoagulation.[12][13]