Portal-Mesenteric Venous Thrombosis

Etiology

At present, causative factors are detected in most cases, although 8% to 20% are still considered idiopathic. In children, up to 50% of cases of PVT are secondary to abdominal infection (chiefly omphalitis), followed by congenital abnormalities of the portal vein (20%), which are generally associated with cardiac and biliary abnormalities (e.g., hypoplasia of the portal vein associated with biliary atresia). In adults, PVT is basically associated with cirrhosis, and its incidence increases as the disease progresses. The prevalence in patients with cirrhosis and hepatocarcinoma is as high as 44%.

Other frequent causes in adults are neoplasm other than hepatocarcinoma, coagulation disorders, and inflammatory-infectious abdominal causes. When the cause is infectious, septic phlebitis of the portal vein associated with thrombosis may occur, and the process is known as pylephlebitis .

Even when local causes of PVT are present, the existence of a prothrombotic predisposition or a latent myeloproliferative disorder should always be ruled out because these are frequent causes of PVT that respond to treatment.

Clinical Presentation

The acute and chronic forms of PVT are differentiated by the length of time over which they develop. The dividing line between the two has not been defined, and the criteria vary from author to author. Some consider cases diagnosed less than 40 to 60 days from the onset of symptoms to be acute in the absence of radiologic findings or complications related to chronic PVT. Others hold that acute cases in noncirrhotic patients must meet three conditions: (1) recent onset of symptoms, (2) no signs of chronic portal hypertension, and (3) absence of portoportal collaterals as shown by computed tomography (CT) or color Doppler ultrasound.

The chronic form is diagnosed more frequently because PVT hardly ever produces symptoms or liver function disorders in the acute stage. The absence of acute clinical manifestations is mainly because in most cases, the PVT is not complete and is associated with rapid development of portoportal collaterals (cavernomatosis) and an increase in hepatic arterial blood supply. Consequently, diagnosis in these cases is made in the chronic phase when complications resulting from chronic portal hypertension appear (chiefly variceal bleeding) or abdominal images are taken for some other reason.

At the acute stage, symptoms are generally nonspecific. If the thrombus extends as far as the distal mesenteric branches, it may cause ischemia or infarction of the mesenteric veins, which produces abdominal pain, nausea, vomiting, and slight ascites. The pain is characteristically disproportionate to the findings on clinical exploration of the abdomen. The existence of peritoneal irritation and ascites indicates necrosis of the wall and perforation. In such cases the prognosis is poor, and urgent surgery is required; 13% to 50% mortality is reported. In cases of pylephlebitis, fever, leukocytosis, and abdominal pain are also present.

In the chronic form, complications proper to portal hypertension occur, principally bleeding through varices, splenomegaly, and hypersplenism. The varices are most commonly esophageal (90%–95%), followed by anorectal (80%–90%) and gastric (35%–40%). Portal hypertensive gastropathy is rare in the absence of cirrhosis. In cirrhotic patients, portal hypertension and preexisting poor liver function make PVT and bleeding varices a serious complication that is poorly tolerated. PVT also renders liver transplantation difficult.

In contrast, portal cavernomatosis frequently causes secondary biliary disorders, mainly stenosis and dilation, known by the generic name of portal biliopathy . However, only a small proportion of patients with cavernomatosis have clinical manifestations in the form of jaundice, cholangitis, choledocholithiasis, or cholecystitis.

In children, the clinical findings are similar, except that growth is also retarded for reasons not fully understood.

Prognosis

The prognosis for patients with PVT is determined by the pathologic conditions that cause it and, in acute cases, by the degree of mesenteric involvement. The prognosis is good in the absence of malignancy, cirrhosis, sepsis, or thrombosis of the mesenteric vein.

Bleeding from varices is the principal complication of PVT, but recent studies have observed no significant changes in survival of these patients as a result of variceal bleeding at diagnosis in the absence of preexisting hepatic pathology, probably because of the efficacy of endoscopic treatment. However, in patients with bleeding from digestive varices in the presence of cirrhosis, mortality is much higher (30%–70% vs. 1%–5%). In cases of suppurative pylephlebitis, the prognosis is poor, and mortality may be as high as 80%.

Imaging

Imaging techniques are essential for the diagnosis of PVT because the clinical manifestations are minor and unspecific. Imaging is also indispensable for planning treatment, be it medical, surgical, or percutaneous. Careful evaluation must be made of the vascular anatomy, size of the thrombus, and the presence or absence of signs of intestinal ischemia and portoportal collaterals (cavernomatosis) or portosystemic collaterals, as well as possible local causes of PVT.

Four anatomic categories related to the extent of PVT have been defined and have clinical relevance for both prognosis and treatment: grade I, thrombus limited to the portal vein that does not reach the splenomesenteric confluence; grade II, thrombus extending to the superior mesenteric vein but with free mesenteric vessels; grade III, thrombus spreading diffusely through the splanchnic venous system but with the presence of large collaterals; and grade IV, thrombus spreading diffusely through the venous system but without large collaterals.

The extent of thrombosis in mesenteric venous branches (grade III–IV) raises the risk for intestinal infarction and consequently the risk for death.

Portal Vein Anatomy

The main portal vein is formed by the confluence of the splenic and superior mesenteric veins, and in the absence of anatomic variants, it divides at the porta hepatis into a right and left branch, which lead, respectively, to the right and left hepatic lobes. The left branch is divided into branches for each segment, whereas the right branch is divided into an anterior right branch for segments V and VIII and a posterior branch for segments VI and VII. This normal distribution is present in 65% to 90% of individuals.

Anatomic variants usually occur in the right branch, and the most frequent ones are portal trifurcation (absence of the common right branch with direct division of the main portal vein into a left branch and two right branches [ Fig. 71.1 ]), an anterior or posterior right branch originating from the left branch, and a posterior right branch originating from the main portal vein. The presence of one such variant can have considerable repercussions on surgery and interventional procedures.

Imaging Techniques

Color Doppler Ultrasound

Color Doppler ultrasound is an effective technique for assessing PVT, and it is also fast, economical, and safe. This technique is generally used for detecting and monitoring PVT. If ultrasound brings the presence of PVT to light, the investigation must be completed with another technique (CT or magnetic resonance imaging) to more accurately evaluate how far it extends into the mesenteric area and detect signs of intestinal ischemia or abdominal causes of PVT.

The most specific ultrasound finding for PVT on grayscale imaging is the presence of an echogenic thrombus in the lumen of the portal vein, which is present in the case of both chronic thrombi and tumors, but not in acute PVT. An increase in the diameter of the portal vein may also be observed (>15 mm), as may the presence of portoportal collaterals and the absence of an identifiable main portal vein.

At the acute stage, the thrombus may be markedly hypoechoic and pass unnoticed if color Doppler ultrasound is not performed. This study will show an area of absence of flow inside the lumen of the portal vein, with high sensitivity and specificity. Occasionally if portal flow is very slow, it may not be detected in the color Doppler study and might be confused with thrombosis. In such cases, pulsed Doppler imaging may reveal the existence of continuous slow flow.

Color Doppler ultrasound also enables the specialist to assess whether arterial flow is present inside the echogenic thrombi. The presence of such arterial flow is highly specific for tumor thrombosis, although its sensitivity is variable.

Regarding the use of intravenous contrast material in ultrasound, several studies have shown better sensitivity in the detection of PVT and in the characterization of tumor thrombi than with conventional and color Doppler ultrasound.

Ultrasound assessment may be limited in obese patients, in patients with small livers, or if intestinal gas is present (this affects mainly assessment of the mesenteric branches).

Contrast-Enhanced Computed Tomography

Many authors consider this study to be the technique of choice, particularly if multislice CT is available. Images obtained with CT offer the advantage of great anatomic detail, which makes them useful for evaluating intraluminal and extraluminal abnormalities, calcification of the portal vein intima, mural thrombosis, existence and distribution of portosystemic collaterals, and any mesenteric edema. Disadvantages include the use of radiation and intravenous iodinated contrast material.

In general, a nonenhanced phase is performed first, followed by injection of 125 to 150 mL of intravenous contrast material at 3 to 5 mL/s to obtain an arterial and a venous phase (portal, with a delay of 55–70 seconds from the beginning of injection of the contrast material). The initial phase without contrast medium makes it possible to detect very recent thrombi, which are hyperdense, as well as calcifications in the intima of the portal vein, and allows analysis of whether the thrombus enhances after the administration of contrast medium.

After injection of intravenous iodinated contrast medium, the thrombus appears as a persistently hypodense area in the lumen of the portal vein, which is generally associated with peripheral enhancement corresponding to the vein wall ( Fig. 71.2 ). If the thrombus is due to a tumor, it may enhance after administration of contrast medium, especially in the arterial phase. Conversely, the coexistence of hypodensity in the superior mesenteric vein, thickening of the wall of bowel loops, and ascites indicates transmural intestinal infarction, and urgent surgery may be necessary.

Another common finding is heterogeneity in enhancement of the hepatic parenchyma. In general, clearly defined areas of hypodensity are observed and attributable to alterations caused by an irregular decrease in the uptake of contrast medium or the glycogen or fat content. On other occasions, areas of greater enhancement can be seen in the arterial phase because the arterial blood that supplies these areas of decreased portal flow increases more markedly.