Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Table 17.1. Causes of portal hypertension (due to increased vascular resistance)

Prehepatic

Portal venous system thrombosis

Extrinsic portal compression or tumor invasion Congenital atresia or thrombosis

Intrahepatic Presinusoidal

Conditions leading to diffuse hepatic fibrosis Gaucher’s disease type 1

Polycystic kidney disease and hepatic fibrosis Mixed connective tissue (collagen vascular)

disease

Primary biliary cirrhosis Schistosomiasis

Postsinusoidal Laënnec’s cirrhosis

Tumor infiltration, such as by a hepatocellular carcinoma

Amyloidosis

Systemic lupus erythematosus Hepatitis and hepatic failure

Diffuse intrahepatic portal venous system thrombosis

Distal biliary obstruction

Biliary atresia patients post-portoenterostomy (Kasai operation)

After renal transplantation Systemic mastocytosis Idiopathic

Posthepatic

Budd-Chiari syndrome

Congestive (right) heart failure

Inferior vena caval obstruction

Blood flow velocity decreases in a number of liver diseases, and duplex Doppler US measurements of blood flow are useful in evaluating disease progression. Increased intrahepatic resistance to flow in patients with chronic liver disease is due not only to intrahepatic morphologic changes but also to a dynamic constriction of the intrahepatic portal drainage bed, believed to be due to decreased synthesis of nitrous oxide in the intrahepatic circulation. Complicating this issue, patients with portal hypertension have increased portal blood flow due to splanchnic arteriolar vasodilation. Endothelins and poorly understood neural and humoral regulation, in part mediated by vasodilators, appear to play a role in increasing intrahepatic vascular resistance. Plasma endothelins, potent systemic and portal vasoconstrictors, are elevated in

ADVANCED IMAGING OF THE ABDOMEN

patients with bilharzial and postviral chronic liver diseases with portal hypertension; a positive correlation exists between plasma endothelin levels and portal vein diameter. On a systemic basis, peripheral arterial vasodilation and an increase in cardiac output ensue in patients with chronic liver disease.

Patients with mixed connective tissue disease (collagen vascular disease) can develop sufficient periportal fibrosis to cause portal hypertension and esophageal varices. Patients with advanced Gaucher’s disease type I and a noncirrhotic liver develop portal hypertension. These patients have extensive confluent central hepatic fibrosis, which presumably is responsible for their portal hypertension (64), although Gaucher’s cells compressing liver sinusoids and thus increasing resistance to flow is probably also a factor. Extensive perisinusoidal amyloid infiltration will also lead to portal hypertension.

Liver involvement in cystic fibrosis increases with age.With increasing survival,some of these individuals develop biliary cirrhosis and eventual portal hypertension. A liver transplantation is a viable option provided that adequate pulmonary function has been maintained.

A rare patient with sclerosing peritonitis and extensive liver capsule fibrosis develops portal hypertension even with a patent portal vein; the capsule fibrosis presumably prevents hepatomegaly and any liver disease leads to a sufficient increase in intrahepatic pressure to compress intrahepatic portal vein and hepatic vein branches and results in portal hypertension. Some patients with autosomal dominant polycystic kidney disease and extensive hepatic fibrosis also develop portal hypertension; in some, distortion of intrahepatic portal vein branches by extensive hepatic cysts is sufficient to produce portal hypertension.

An association of portal hypertension and pulmonary hypertension exists in patients with underlying liver cirrhosis and those with mixed connective tissue disease. The pulmonary findings are similar to those found in primary pulmonary hypertension.

In long-term surviving neonates with biliary atresia who undergo hepatic portoenterostomy (Kasai operation), about half develop portal hypertension (65); the incidence of subsequent portal hypertension is significantly lower in those with a serum bilirubin <2mg/dL at 3 months postsurgery than in those with a biliru-

ABDOMINAL VASCULATURE

bin level >2mg/dL. Among those developing portal hypertension, esophageal varices were discovered between 11 months and 5 years of age in more than 70% of children, while thrombocytopenia tended to develop at a slightly older age.

The etiology of Banti’s disease, consisting of anemia, splenomegaly, and portal hypertension, is unclear. Portal hypertension is considered to be idiopathic. A rare patient develops portal hypertension several years after renal transplantation; their portal venous pressure decreases and esophageal varices clear after splenectomy.

In some studies of noncirrhotic portal hypertension an idiopathic etiology continues to be a prominent feature.

Clinical Aspects

Portal hypertension is assessed by measuring hepatic venous pressure gradient, a technique that is the gold standard in planning subsequent hemodynamic therapy. Nevertheless, portal hypertension is often assumed to be present if portal vein collaterals are detected. Thus the presence of distal esophageal varices is generally taken to be presumptive evidence of portal hypertension. Splenomegaly may or may not be present in adults, although splenomegaly is more common in children.

A typical scenario with Laënnec’s cirrhosis is an increase in intrahepatic resistance to portal blood flow, opening of collateral vessels, and a decrease in portal blood flow to the liver.A compensatory increase in hepatic artery blood flow develops, but due to postsinusoidal obstruction some of this arterial blood is diverted to the portal vein and eventually portal vein blood flow reverses direction (hepatofugal flow). Doppler US in patients with cirrhosis reveals a significant decrease in portal flow with a worsening Child’s grade of cirrhosis; patients with ascites and encephalopathy also have a significantly lower portal blood flow rate compared to those without these abnormalities.

Some patients with portal hypertension develop large esophageal varices, while others have small varices or none. Likewise, the prevalence of nonesophageal portosystemic collaterals varies. Some develop both large esophageal varices and nonesophageal portosystemic collaterals.

Physical exercise in patients with liver cirrhosis and portal hypertension increases portal pressure and reduces hepatic blood flow and thus appears to increase risk of variceal bleeding.

Splenic artery occlusion will mask underlying portal hypertension.

A portosystemic shunt reverses gastropathy in most patients with noncirrhotic portal hypertension; in these patients it is presumably venous congestion that causes gastropathy, realizing that gastric mucosal capillary dilation does not signify portal hypertension.

Imaging

The thoracic duct caliber increases in cirrhosis. No direct relationship is apparent between the degree of portal hypertension and the caliber of this duct. The distal end of this duct in the left supraclavicular region can be visualized with US in most patients.

CT is insensitive in detecting hepatofugal flow in the main portal vein in patients with cirrhosis. On the other hand, a main portal vein diameter of <1cm is highly specific for hepatofugal flow (66).

Doppler US provides a measure of hepatic artery and portal vein pulsatility. Normally the hepatic artery has pulsatile flow, but portal venous blood is nonpulsatile with only minor cardiac and respiratory effects. A significant increase in hepatic artery pulsatility occurs in patients with end-stage liver disease. The hepatic veins, on the other hand, are dampened in cirrhosis and their appearance approaches that of the portal vein.

Detection of portosystemic collateral vessels is often used as proof for portal hypertension (Fig. 17.12). A collateral vein within the ligamentum teres is relatively common and is detectable with Doppler US. One should keep in mind, however, that in a minority of normal individuals Doppler US detects blood flow in a paraumbilical vein; velocity increases in portal hypertension and flow extends anterior to the liver surface, a finding not seen normally. Inferior vena cava dilation is also often found in patients with cirrhosis and portal hypertension.

In a setting of portal hypertension, contrastenhanced 3D MRA is currently the imaging modality of choice in evaluating portal blood flow and portal vein anatomy. Subtracting the

Figure 17.12. Major pathways of portosystemic venous shunting in portal hypertension. E, esophageal varices; P, paraumbilical veins; R, rectal veins; S, splenorenal veins. Reversal of flow occurs in the coronary vein (C) and inferior mesenteric vein (I).

arterial phase from venous phase data aids in visualizing blood flow patterns and shunts using several viewing projections.

Discrepancies exist between Doppler US and contrast-enhanced MRA in assessing portal vein anatomy. Detection of portal vein patency, especially of intrahepatic portal vein branches, is more accurate with MRA. Splenorenal shunts and varices are better detected with MRA.

Portal Vein Obstruction/Thrombosis

A rare cause of portal hypertension is partial obstruction by a portal vein web or membrane. The etiology of these webs is not clear.

Clinical Aspects

Infection, neoplasm, and a hypercoagulable state are associated with portal vein thrombosis. Blood stasis due to decreased flow presumably plays a role in thrombosis developing in a setting of portal hypertension. The reverse is also true, namely, portal vein thrombosis leads to portal hypertension. At times splenic vein thrombosis extends into the portal vein.

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The most common cause of massive gastrointestinal bleeding in children is from portal hypertension–induced varices secondary to portal vein thrombosis. Splenomegaly is common in children with portal vein thrombosis. Their liver is normal. In most children thrombosis is considered to be idiopathic, although prior umbilical vein catheterization or omphalitis play a role. Prior umbilical sepsis, even at birth, should be suspected in the child or young adult who develops portal hypertension with no obvious underlying cause. In an interesting US study of 100 neonates with umbilical vein catheterization, clinically silent portal venous thrombosis was detected in 43% (67); follow-up US revealed complete or partial resolution in only about half, with a correlation found between initial thrombus size and subsequent clot resolution. Significant risk factors for thrombosis were catheterization for >6 days and blood transfusion.

Some of the conditions associated with portal vein thrombosis are listed in Table 17.2. Patients

Table 17.2. Conditions associated with portal vein thrombosis

Children

Idiopathic

Prior umbilical vein catheterization

Prior omphalitis

Homocystinuria

Adults Idiopathic Inflammation

Pancreatitis Ascending cholangitis Ulcerative colitis Crohn’s disease Adjacent abscess

Tuberculosis involving porta hepatis lymph nodes Primary hepatic actinomycosis

Penetrating peptic ulcer Neoplasm

Pancreatic and liver carcinomas Gastric carcinoma

Bladder carcinoma Posttherapy

After hepatocellular carcinoma therapy After splenectomy

Gastric variceal therapy Other

Hypercoagulation state Behçet’s disease Postpartum

Myeloproliferative syndromes

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with a hepatocellular carcinoma are prone to developing portal vein thrombosis, with the thrombus often consisting of tumor encroaching into portal vein branches rather than being nonneoplastic. Thrombi develop after hepatocellular carcinoma therapy with percutaneous ethanol injection. Fine-needle biopsy of a portal vein thrombus is useful in some patients with hepatocellular carcinoma to identify a neoplastic thrombus if results will influence patient management.

Portal, splenic, and superior mesenteric vein thrombosis is an occasional complication after splenectomy. Most of these patients are symptomatic and an occasional one even develops an acute abdomen. Some authors suspect that endoscopic variceal sclerotherapy predisposes to portal vein thrombosis, although controlled evidence is lacking.

Portal vein thrombosis should be suspected if a patient with Behçet’s disease develops splenomegaly. These patients often have cavernous transformation of the surrounding vessels. Patients with homocystinuria, a rare, inherited metabolic disease, are at risk for arterial and venous thromboemboli, including portal vein thrombosis.

In a setting of portal vein thrombosis, an anomalous insertion of the right gastric vein maintains hepatopedal blood flow if the insertion is at the portal vein bifurcation or intrahepatically (distal to the thrombus). Such an anomalous right gastric vein insertion is a pathway for TIPS placement.

Ultrasonography mass screenings can detect extrahepatic portal venous obstruction in asymptomatic patients.

Imaging

Portal vein thrombosis can be assessed with contrast-enhanced CT, gray-scale and Doppler US, contrast-enhanced MRI, and angiography (Fig. 17.13). Most benign thrombi do not widen the portal vein caliber, while a malignant thrombus often does. Also, blood flow (neovascularity) is present in about half of malignant thrombi but not in benign ones.

Contrast-enhanced CT detection of portal vein thrombosis approaches 100% by visualizing an intraluminal thrombus. These thrombi often extend into the splenic and superior mesenteric veins. Coronal or sagittal 3D recon-

struction often provides an overall view of these thrombi.

Established portal vein thrombosis often results in a hypodense liver on precontrast CT scans, presumably secondary to fat accumulation. Segmental atrophy develops in an involved segment. The proportion of blood supplied by the hepatic artery increases and as a result postcontrast liver CT shows increased enhancement during the late arterial phase and decreased enhancement during the venous phase. Segmental intrahepatic portal vein thrombi result in transient wedge-shaped parenchymal defects showing increased enhancement during the arterial phase.

Postcontrast CT reveals a benign thrombus as a tubular low-density intraluminal tumor. At times enhancing collateral vessels are evident (cavernous transformation). A thrombus in a nonoccluded portal vein is seen as a nonenhancing tumor surrounded by contrast enhanced venous blood. Mesenteric edema and mesenteric varices are evident in some patients even without a thrombus extending into the superior mesenteric vein. On a chronic basis, CT identifies a cord-like sclerotic portal vein or portal vein calcifications.

Some of these patients develop arterioportal shunts, seen as segmental intrahepatic portal vein enhancement during arterial CT phase.

Controversy surrounds the role of US. Gray-scale US reveals a portal vein thrombus as an intraluminal tumor or abnormal intraluminal echoes. A thrombus ranges from focal to diffuse. Gray-scale US does not detect an anechoic clot, and Doppler US is necessary for these.With complete extrahepatic portal thrombosis, Doppler US reveals an absent portal vein lumen except for a hyperechoic band from which no flow is detected. Usually the site of obstruction can be established. Doppler US evaluates residual blood flow in patients with partial obstruction. Ultrasonography is more problematic in differentiating a benign from a malignant portal vein thrombus in a setting of cirrhosis. Doppler US detection of pulsatile flow in the thrombus is rather specific in diagnosing a malignant thrombus but at the expense of a lower sensitivity, although published results vary considerably. In general, the presence of pulsatile arterial flow, detected by Doppler US, is assumed by some to obviate a need for percutaneous biopsy of the thrombus to estab-

lish a definitive diagnosis. Continuous flow, on the other hand, is seen with both benign and malignant thrombi. With slow portal vein blood flow Doppler US reveals no flow, but no thrombus is detected by gray-scale US. In

patients with portal vein thrombosis Doppler US reveals a significantly lower mean hepatic artery resistive index than controls; such a lower resistive index is a secondary sign of portal vein thrombosis.

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Published studies favor MR over US. MRI detects more occlusion or encasement of smaller portal vein branches than US, although occasionally the reverse is true. In general, MRI provides additional information over US in preoperative assessment of the portal venous system and MRA achieves a sensitivity and specificity similar to intraarterial DSA in assessing portal venous system patency or thrombosis in patients with portal hypertension (68). In fact, contrast enhanced 3-D MRA is emerging as the method of choice for studying the portal venous system in patients with portal hypertension, and thus potentially could replace DSA for this application.

Flowing blood within the portal vein appears as a void on both T1and T2-weighted MR images. A thrombus appears similar to intraluminal soft tissue, although slow flow has a similar appearance (Fig. 17.14). On contrastenhanced MR the normally hyperintense intraluminal blood is replaced by a hypointense thrombus. At times enhanced collateral circula-

tion is identified. Any enhancing vessels within the thrombus suggest malignant infiltration.

Occlusion of an intrahepatic portal vein branch results in a peripheral wedge-shaped hyperintense segment on immediate contrastenhanced MR due to a compensatory increased arterial blood supply. This segment gradually becomes isointense. One should keep in mind, however, that contrast-enhanced MRI reveals a signal-intensity decrease in some right (8%) and left (9%) portal vein branches and portal vein (6%) during the equilibrium phase (69); these flow artifacts tend to mimic a portal venous thrombosis.

Portal Vein Cavernous Transformation

Cavernous transformation represents formation of venous channels either within a thrombosed portal vein or in surrounding extraperitoneal tissues. Some fresh portal vein thrombi recanalize within days, but cavernous transformation of surrounding vessels, consist-