The portal vein carries approximately 1.5 L/min of blood from the splanchnic circulation to the liver. Unlike most veins in the body, the portal vein lacks valves. It constitutes around 80% of blood supply to the liver, with portal venous flow estimated to be around 2 cm/sec. The normal pressure of the portal vein is around 5 mm Hg, and pressure greater than 10 mm Hg results in the development of portosystemic shunts such as gastroesophageal varices.

Normal Anatomy

The portal vein is approximately 8 cm in length, and the normal diameter is around 1 cm. It is formed by the confluence of the superior mesenteric vein (SMV) and the splenic vein. It runs behind the neck of the pancreas and the first part of the duodenum and enters the hepaticoduodenal ligament to the right of common hepatic artery and behind the common bile duct. The retropancreatic portion of the portal vein/SMV lacks venous tributaries and allows dissection of the pancreas off the venous structure during pancreaticoduodenectomy. Several important venous branches, such as anterior and posterior superior and inferior pancreaticoduodenal veins, drain directly into the portal vein/SMV. On the other hand, the left gastric vein and the inferior mesenteric vein usually join the portal vein via the splenic vein. Left gastroepiploic vein and middle colic vein can either drain directly to the SMV or sometimes form the so-called gastrocolic trunk of Henle before joining the SMV.

In the porta hepatis, the portal vein bifurcates into the right and left portal veins. The right portal vein usually receives the cystic vein before dividing into right anterior and right posterior branches. The right anterior branch divides further to supply portal venous blood to liver segments V and VIII, and the right posterior branch bifurcates into segmental branches to supply liver segments VI and VII. The left portal vein takes more horizontal course under the hilar plate, and it runs below the quadrate lobe before it enters the umbilical fissure. It usually supplies the caudate lobe (segment I) before dividing into segmental branches of the lateral segments (II and III) and medial segment (IV) of the left hepatic lobe.

The left portal vein is connected by a fibrous remnant of the obliterated umbilical vein called the ligamentum teres to the systemic periumbilical veins. This connection can sometimes recanalize in cases of portal hypertension. The left portal vein is also connected to the inferior vena cava (IVC) by the obliterated ductus venosus called the ligamentum venosum.

Normal Variations

The most common variation of the portal vein is trifurcation of the main portal vein.¹ It usually takes the form of the right anterior and posterior portal veins arising directly from the main portal vein at the same time as the left hepatic vein without forming right portal vein. It occurs up to 10% of the population. The next common variation is when the right posterior portal vein arises from the main portal vein before the remaining portal vein divides into right anterior portal vein and the left portal vein. In 2% to 4% of patients, the right anterior portal vein can also directly arise from the left portal vein.

Congenital Anomalies

At around 4 to 5 weeks of gestation, two vitelline veins are connected by three anastomoses near the septum transversum. Complex interconnections and involution of these primitive vessels result in the future portal vein, the hepatic veins, and the IVC. Anomalous development of this complex embryonic process leads to congenial malformation of the portal vein.

Congenital agenesis of the main portal vein itself or congenital agenesis of the right or left portal veins is the most common developmental anomaly of the portal vein encountered in clinical practice. When there is complete absence of the main portal vein, there usually exists a congenital portosystemic shunt, which directs portal venous flow to the IVC or the left hepatic vein. It is more common in the females, and it is associated with other congenital anomalies such as congenital heart defects and biliary atresia. Agenesis of major branches of the main portal vein leads to absence or atrophy of the corresponding lobe. The gallbladder may be located behind the liver in the case of agenesis of the right hepatic vein, or it may migrate superiorly under the diaphragm in the case of agenesis of the left hepatic vein.

The portal vein can be thrombosed de novo or secondary to various causes (Table 26-1). Prothrombotic states such as inherited hypercoagulable conditions or myeloproliferative disorders can present with portal vein thrombosis (PVT). In addition, malignancies such as pancreatic cancer and hepatocellular carcinoma may be associated with PVT as a result of tumor invasion of the portal vein or secondary to the hypercoagulable state of malignancy. Liver cirrhosis is found in around 25% of patients with PVT. Conversely, approximately 11% of patients with liver cirrhosis develop PVT in their lifetime, either primarily or as a result of hepatocellular carcinoma. Septic PVT, also called pyelophlebitis, may present in association with inflammatory or infectious intraabdominal processes such as appendicitis or diverticulitis.

The incidence of PVT is estimated to be around 0.05% to 0.5%, and it affects both genders equally. Thrombotic occlusion of the portal vein can be complete or partial and acute or chronic. The timing of and the degree to which the portal vein is thrombosed influence the way it presents.² Acute PVT can result in prehepatic portal hypertension and lead to acute abdominal pain, nausea or vomiting, or upper gastrointestinal bleeding. PVT-induced portal hypertension can also induce mesenteric venous congestion and culminate in intestinal infarction if not recognized and treated promptly. Slower onset of PVT, however, allows development of portosystemic shunts to decompress portal hypertension up to a degree. However, gastroesophageal varices, splenomegaly, and ascites can develop if portal hypertension persists. Collateral veins can also form around structures in close proximity to the occluded portion of the portal vein such as the gallbladder, pancreas, or bile ducts. This can be sometimes mistaken for inflammatory or neoplastic process of the affected structure.

Diagnosis is usually made with color Doppler ultrasonography, contrast-enhanced computed tomography angiography (CTA), or magnetic resonance angiography (MRA).³ Ultrasonography demonstrates a hyperechoic material inside the portal vein lumen, and duplex images confirm absent or reduced hepatopedal portal venous flow. Proximal part of the occluded portal vein can be dilated as a consequence of venous obstruction, and a portal vein diameter greater than 13 to 15 mm is commonly associated with PVT. Therefore, the dilatation of the portal vein can serve as a useful marker of PVT.

CTA is another useful diagnostic modality, and a filling defect inside the portal venous lumen is easily visualized and is characteristic of PVT (Figure 26-1). The presence of periportal collateral vessels around the thrombosed portal vein is termed cavernous transformation of the portal vein and indicates chronicity. Additionally, CT scan may show secondary effects of PVT, such as extension of thrombus into the mesenteric venous system, venous congestion, and bowel wall thickening. It may also demonstrate signs of portal hypertension such as gastroesophageal varices, venous collateralization, splenomegaly, and ascites. Venous collaterals around the biliary tree may impede bile flow and lead to obstructive jaundice; this is termed portal cholangiopathy.

Treatment depends on acuity, secondary effects, and the cause of PVT. Anticoagulation is recommended for patients with acute PVT of infectious or inflammatory causes, inherited or acquired hypercoagulable states, and those associated with postsplectomy or postcolectomy. PVT can extend into mesenteric venous branches, and associated venous congestion can be complicated by venous infarction of the bowel. It requires emergent laparotomy and resection of the necrotic bowel segment. Postoperative anticoagulation is recommended in such cases. The duration of anticoagulation is generally for 6 months for reversible etiologies and lifelong for irreversible causes such as inherited hypercoagulable states.

Chronic PVT can lead to gastroesophageal varices, and treatment such as endoscopic banding of variceal vessels may be necessary. β-blockers, which are commonly used in portal hypertension associated with liver cirrhosis, have been used to treat chronic PVT, but they lack proven efficacy. Anticoagulation in cases of chronic PVT has sometimes been recommended, but its efficacy is also not conclusively demonstrated because venous collateralization may compensate for the portal hypertension induced by PVT. Biliary stenting may be necessary when PVT is associated with biliary outflow obstruction.

Pylephlebitis is septic thrombophlebitis of the portal vein. It usually occurs in association with intraabdominal infectious or inflammatory processes such as appendicitis, diverticulitis, or inflammatory bowel disease. PVT can also be secondarily infected.