Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

ing of a sponge-like mass of collateral vessels around the main portal vein, takes several weeks. Collateral circulation develops via two pathways: First, portosystemic shunting such as through a left gastric vein and these portosystemic collaterals usually imply that portal hypertension is present. Second, portoportal channels develop and extend into intrahepatic portal venous branches. Cavernous transformation is associated with such conditions as chronic pancreatitis, tumor infiltration, and intraabdominal sepsis.

In a Spanish study of patients with cavernous transformation, predisposing factors were omphalitis, echinococcal cyst, major abdominal surgery, cirrhosis, Sjögren’s syndrome, and idiopathic (70). A hydatid cyst in the porta hepatis can lead to cavernous transformation. The extent of collateralization varies considerably but tends to be more prominent with more chronic portal vein thrombi. Thus patients with portal vein thrombosis during childhood can eventually develop an extensive cavernous transformation involving adjacent structures. Many patients with congenital hepatic fibrosis have cavernous transformation of the portal vein.

Biliary veins (cystic and paracholedochal) represent an alternate blood flow pathway in cavernous portal vein transformation, and thus it is not uncommon to see numerous serpiginous extrinsic bile duct indentations. These bile duct varices can compress the extrahepatic bile ducts to the point of inducing biliary obstruction.

Of interest is that in most patients with cavernous transformation of the portal vein the pancreatic duct is smaller than normal, presumably due to pancreatic venous congestion.

In patients with cavernous transformation of the portal vein confirmed by angiography, color Doppler US detected the transformation in 93%, B-mode US in 64%, and contrast-enhanced CT in 50% of patients (71); color Doppler US was superior to B-mode US in identifying collateral channels.

Therapy

Treatment with heparin alone can result in recanalization of acute portal venous thrombosis. A number of case reports suggest that either intraportal plasminogen infusion or such therapy as plasminogen activator and urokinase

ADVANCED IMAGING OF THE ABDOMEN

through a transjugular intrahepatic catheter approach to the portal vein have dissolved thrombi.

Transjugular intrahepatic portosystemic shunting and thrombolysis appear to be viable therapy in patients with noncavernomatous portal vein thrombosis in order to increase portal output and restore portal blood flow; patent shunts can be achieved, although those with an initial complete thrombosis require more frequent shunt revisions than those with incomplete obstruction. Some of these patients continue to be symptomatic despite a functioning TIPS.

Few surgical options exist for chronic portal vein thrombosis. Lysis and thrombus aspiration, portal vein stenting, and TIPS are potential options.

Collateral Veins

Portosystemic Shunts

Extrahepatic portosystemic shunts are rare in absence of portal hypertension and in such a setting a congenital shunt origin is likely. An extrahepatic shunt, together with intrahepatic portal venous hypoplasia, generally implies a congenital basis. Some of these shunts in patients without portal hypertension can be rather large. Some are associated with hepatic encephalopathy and after successful shunt embolization encephalopathy clears.

In portal hypertension, for significant collaterals to open, the portocaval pressure gradient needs to be >12mmHg. Initially, as develops, the portal vein caliber increases, but the caliber tends to decrease once shunts start to form.

Traditionally conventional angiographic portography was used to evaluate portosystemic shunts, but 3D helical CT portography appears to be equal to and at times even superior to angiography (72). Results of CT portography can be used to plan therapy.

Esophagogastric Shunts: Esophageal and gastric fundal varices represent the most common spontaneous portosystemic shunts. A common pathway is from the portal vein, through the left gastric vein (also called the coronary vein) and into the gastroesophageal veins (varices). A common drainage path for these paraesophageal varices is via veins around the aorta into the hemiazygous vein. An alternate path is via a vein located anterior to the

ABDOMINAL VASCULATURE

inferior vena cava (precaval vein) and into the inferior vena cava.

Doppler US in the presence of these shunts reveals reversal of flow in the left gastric vein. A parallel pathway is from the splenic vein via the short gastric veins,which tend to dilate. Imaging detection of a dilated coronary vein implies either portal hypertension or splenic vein obstruction. The latter condition, of course, does not result in esophageal varices.

Splenomegaly is common in portal hypertension and the presence of esophageal varices, but little or no correlation exists between splenic size and size of esophageal varices.

Therapy and the resultant imaging of esophagogastric shunts are discussed in Chapters 1 and 2.

Intrahepatic Shunts: Spontaneous intrahepatic portal vein-to-systemic vein shunts are rare in a nondiseased liver. If present, aside from paraumbilical veins, they do not necessarily imply that portal hypertension is present; these shunts develop in either the right or left lobe and consist of either a single dilated vessel or multiple small veins. In the liver the left portal vein forms anastomoses with veins in the ligamentum terres and ligamentum venosum.

Gray-scale US reveals shunts as typical anechoic structures. Color Doppler US establishes flow patterns, such as bidirectional flow. Some shunts have a continuous flat portal vein flow pattern in both the shunt and the related hepatic vein. Computed tomography and MRI simply show abnormal vascular channels.

Patients with large intrahepatic portosystemic venous shunts develop hepatic encephalopathy. In symptomatic patients these shunts can be embolized using various coils and detachable balloons (73); retrograde transcaval obliteration is least invasive in treating simple portosystemic venous shunts.

Paraumbilical Collaterals: Paraumbilical veins are relatively common collaterals (Fig. 17.15). Rather than a single vein, it is usually a collection of veins close to the original obliterated paraumbilical vein. Located in the ligamentum teres, it connects the left portal vein and systemic abdominal wall collaterals and, when present, it is detected by US and the flow is established with Doppler US. Spontaneous formation of dilated umbilical veins should suggest portal hypertension (CruveilhierBaumgarten syndrome). Some patients develop a venous hum and a caput medusae. Occasion-

Figure 17.15. Effect of shunting via the paraumbilical vein (P). Hepatopetal portal vein (PV) flow is still maintained, but flow is reversed in the intrahepatic portal vein branches.

ally esophageal variceal sclerotherapy in a patient with idiopathic portal hypertension leads to portal vein thrombosis at the origin of the umbilical vein and the disappearance of the venous hum and dilated abdominal wall veins characteristic of Cruveilhier-Baumgarten syndrome.

Why some patients develop markedly dilated paraumbilical veins rather than other collaterals is not known. In general, those having hepatofugal paraumbilical flow greater than hepatopetal portal vein flow tend not to develop esophageal varices. Turbulent flow is identified in some patients.

A dilated paraumbilical vein most often drains via the inferior epigastric vein into the external iliac vein. Less common is drainage via the superficial epigastric vein into either the internal thoracic vein or saphenous vein, pathways identifiable with color Doppler US.

Other Collaterals: The azygos vein, which drains esophageal varices, dilates in a setting of portal hypertension and esophagogastric varices. Endoscopic US visualizes this vein; its caliber is increased and maximal azygous vein blood velocity is greater in patients with varices compared to controls. An occasional large splenic vein-to-azygos vein shunt is detected in a noncirrhotic patient, presumably on a congenital basis. Similarly, spontaneous left gastric vein to left renal vein shunts can occur, often in a portal hypertension setting.

Less often encountered are gastroepiploic and splenorenal collaterals and shunting from the inferior mesenteric vein to the inferior hemorrhoidal veins (Fig. 17.16). A spontaneous shunt involving the right renal vein is rare. Likewise, portosystemic collaterals through duodenal and other extraperitoneal structures are uncommon.

Colonic variceal bleeding is rare, but at times is massive. Several studies suggest that colonic varices tend to become more prominent after successful sclerotherapy of esophageal varices and the resultant obliteration of coronaryazygous venous anastomoses.

The prevalence of mesenteric varices in portal hypertension is not known. Rupture of a mesenteric varix is a rare cause of hemoperitoneum. In patients with ascites and mesenteric varices, bleeding into the peritoneal cavity can follow large-volume paracentesis, possibly induced by the sudden decrease in intraperitoneal pressure due to fluid withdrawal.

In general, MRI detects more varices in more patients than does US.

In addition to endoscopic obliteration of esophagogastric varices (by sclerotherapy and other procedures), portosystemic shunts can be embolized. Such embolization generally helps control hepatic encephalopathy after the failure of medical management. In a setting of a large portosystemic shunt with considerable superior mesenteric venous blood flowing through the shunt, shunt obliteration tends to improve liver function.

ADVANCED IMAGING OF THE ABDOMEN

Therapy of Portal Hypertension

Medical Therapy

In a patient with cirrhosis who bled, what are the relative roles of propranolol and sclerotherapy in preventing rebleeding and on survival? A meta-analysis of nine randomized trials concluded that the mean percentage of patients free of variceal rebleeding was 39% in the propranolol group and 55% in the sclerotherapy group, but that adverse events were higher in the sclerotherapy group (74); the mean survival rate, however, did not differ significantly. In this subgroup of patients, although sclerotherapy is more effective than propranolol in preventing variceal rebleeding, the authors suggest that propranolol is the preferred therapy for preventing rebleeding.

Beta-blocking agents reduce portal venous pressure in cirrhotic patients, although the results are somewhat idiosyncratic. The goal is to reduce the portal pressure gradient below 12 mmHg. Beta-blockers do not achieve such pressure reductions in some patients, and their use is thus limited as prophylactic agents. A reduction in portal pressure appears to be potentiated by combining beta-blockers and isosorbide-5- mononitrate.

Both flow velocity and pulsatility index are obtained with Doppler US, although US is of limited value in discriminating good from poor responders to medical therapy (61).

Rectal Tc-99m–pertechnetate scintigraphy of collateral blood flow from the inferior mesen-

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teric vein shows that propranolol in cirrhotics does reduce shunting; this response to propranolol appears to depend on the severity of liver disease.

Esophageal and gastric variceal sclerotherapy or variceal ligation helps control variceal bleeding but, theoretically at least, should not reduce portal hypertension. Nevertheless, portal venograms performed before and after variceal ligation reveal that although in a majority of patients portal pressure does increase, in a minority the pressure decreases, presumably due to the opening of other major collaterals.

Surgical Therapy

The ideal therapy of portal hypertension due to cirrhosis is liver transplantation, a procedure both complex and controversial.

Any abdominal surgery in a patient with varices is fraught with bleeding complications. Preoperative CTA, including 3D reconstruction, is very useful in outlining collateral vascular channels, especially in unusual locations, prior to shunting.

A portocaval shunt is performed either end- to-side or side-to-side. The former diverts all portal blood away from the liver. Some surgeons prefer a mesocaval interposition shunt using a graft between the superior mesenteric vein and vena cava, but a distal splenorenal shunt (Warren shunt) is performed more often (Fig. 17.17). The number of these procedures has decreased considerably since the advent of TIPS.

Shunt stenosis or occlusion should be suspected if recurrent variceal bleeding occurs after a surgically constructed shunt. Doppler US evaluates the patency of these shunts in most patients by detecting flow in both limbs and through the anastomosis. Percutaneous transcatheter angioplasty, and if necessary stent insertion, is worthwhile if shunt stenosis or occlusion is detected, but keep in mind that angioplasty of stenotic surgical shunts carries a risk of encephalopathy.

The Sugiura procedure consists of esophageal transection and esophagogastric devascularization, with a splenectomy also included by some surgeons. Hepatic function tends to worsen immediately postoperatively after a modified Sugiura operation but then improves. In patients with previous variceal bleeding, a

Figure 17.17. Effect of splenorenal shunt. Splenic vein (SV) and part of superior mesenteric vein (SMV) blood are shunted into the left renal vein (RV). Hepatopetal flow is still maintained in the portal vein (PV) and intrahepatic branches, in spite of a patent paraumbilical vein (P). With further increase in intrahepatic resistance portal vein flow will eventually reverse.

modified Sugiura procedure results in somewhat greater survival rate than a portosystemic shunt. Esophageal transection does not cure esophageal varices, and in most patients they recur in time. New collaterals are also common at other sites.

Transjugular Intrahepatic Portosystemic

Shunting (TIPS)

Clinical Aspects: One reason why surgical portosystemic shunting is not performed more often is difficulty in predicting which patients will progress with their hepatic failure or develop significant encephalopathy. An orthotopic liver transplantation, on the other hand, although having its own morbidity and mortality, is not associated with subsequent hepatic failure or encephalopathy. In a setting of an acute variceal bleed, however, liver transplantation is often impractical and it is in this setting that TIPS evolved as a viable alternative to surgical portosystemic shunting. From a practical point of view, TIPS is less invasive than a surgical portosystemic shunt.

A relatively high prevalence of portal vein thrombosis is found in patients with portal