Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

ABDOMINAL VASCULATURE

by angiography is not identified by CT. At times splenic vein thrombosis reveals a hypodense splenic vein, collateral vessels, and a patent portal vein. Ultrasonography shows an echogenic thrombus in the splenic vein lumen.

Splenic vein aneurysms are rare. An occasional aneurysm regresses after resolution of splenomegaly.

Pancreatic pseudocysts occur in the general location of the splenic and portal veins. An occasional one erodes into the splenic vein.

Superior Mesenteric Vein

Superior mesenteric vein thrombosis is idiopathic, associated with an underlying malignancy, develops in hypercoagulable states, and occurs in a setting of pancreatitis, sigmoid diverticulitis, or appendicitis, and as a sequela after abdominal surgery such as a Whipple procedure or even a complicated appendectomy (Fig. 17.21). It occurs in young patients. Acute thrombosis, in the absence of collaterals, often progresses to intestinal infarction. A more chronic course leads to pain, diarrhea, and malabsorption.

The major issue with superior mesenteric vein thrombosis is that the clinical presentation is usually sufficiently vague and the diagnosis is initially overlooked. The mortality from this condition has not changed significantly over the

Figure 17.21. Superior mesenteric vein thrombosis after appendectomy. Contrast-enhanced CT reveals an enlarged vein, an intraluminal clot and an enhancing vein wall (arrow). (Source: Schmutz GR, Benko A, Billiard JS, Fournier L, Péron JM, FischPonsot C. Computed tomography of superior mesenteric vein thrombosis following appendectomy. Abdom Imaging 1998;23: 563–567, with permission from Springer-Verlag.)

Figure 17.22. Superior mesenteric vein thrombosis. Contrastenhanced CT shows an enlarged, enhancing superior mesenteric vein and an intraluminal clot (arrow). (Courtesy of Gérard Schmutz, M.D., Centre Hospitalier Universitaire, Caen, France.)

years. Complicating the picture is the frequent presence of other intraabdominal disease.

Occasionally an acute thrombus resolves spontaneously. In most patients, however, prompt diagnosis and therapy with thrombolytic, anticoagulant, antiplatelet, or antispasmodic agents is warranted to prevent complications and reduce mortality. At times selective superior mesenteric artery infusion of urokinase is helpful.

Computed tomography shortly after clot formation reveals a thrombus to have a higher density than does soft tissues. The superior mesenteric vein is often enlarged. Later, a characteristic CT appearance consists of a hypodense central thrombus surrounded by a higher density wall (Fig. 17.22). At times a thrombus extends into the portal vein. With incomplete lumen obstruction, contrast-enhanced blood surrounds a lower density thrombus. The vein wall enhances after contrast. Surrounding mesenteric edema is not uncommon.

Inferior Mesenteric and Pelvic Veins

Septic thrombophlebitis of the inferior mesenteric vein is most often secondary to sigmoid diverticulitis. It is readily detected with CT.

Contrast-enhanced MR venography using a blood-pool contrast agent outlines major pelvic veins and evaluates for pelvic vein thrombosis. Superimposed arterial structures can be minimized by a subtraction technique.

Budd-Chiari Syndrome

Clinical

Budd-Chiari syndrome consists of obstruction to the venous outflow from the liver. The reported prevalence of Budd-Chiari syndrome varies throughout the world, in part due to different reporting criteria. The older literature considered only acute obstruction under BuddChiari syndrome, although currently the definition has been expanded to also include a more common chronic form. It is caused by hepatic vein obstruction or, less often, obstruction in the adjacent inferior vena cava (Table 17.4). In Japan, most patients with Budd-Chiari syndrome have a chronic form; the etiology is idiopathic in the majority, and over 90% have obstruction in the intrahepatic portion of the inferior vena cava. The most common direct underlying cause of Budd-Chiari syndrome in Western countries is either hepatic vein or inferior vena caval thrombosis. Patients developing a Budd-Chiari syndrome believed to be idio-

Table 17.4. Conditions associated with Budd-Chiari syndrome

Idiopathic

Vascular conditions Veno-occlusive disease

Inferior vena cava web or membrane

Inferior vena caval narrowing induced by right hemidiaphragm elevation

Behçet’s disease Polycythemia vera

Thrombotic thrombocytopenic purpura in a pregnant patient

Drug induced

Oral contraceptives

Neoplasms

Hepatocellular carcinoma

Inferior vena cava or hepatic vein sarcoma Adjacent neoplasm invading inferior vena cava

Pregnancy

Infection

Trauma

Other

Hypereosinophilic syndrome

Systemic lupus erythematosus

ADVANCED IMAGING OF THE ABDOMEN

pathic in origin should be investigated for an undetected latent coagulation disorder. Other causes include a web-like membranous obstruction of the inferior vena cava, a condition more common in the Orient. Trauma is a rare cause of Budd-Chiari syndrome. Major hepatic surgery with compromise of the hepatic veins or intrahepatic portion of the inferior vena cava is an occasional predisposing factor. A renal cell carcinoma, a rare adrenal neoplasm with inferior vena cava invasion, retroperitoneal sarcoma, and malignancies involving the inferior vena cava also result in Budd-Chiari syndrome. Hepatic vein thrombosis is a complication of Behçet’s disease, and Budd-Chiari syndrome should be suspected if hepatomegaly and ascites are detected in this disease. A rare cause of acute Budd-Chiari syndrome is percutaneous insertion of a transhepatic inferior vena cava catheter.

Regenerative liver nodules are more prevalent in chronic Budd-Chiari syndrome than expected. The question of whether this syndrome is linked to an increased risk for hepatocellular carcinoma has been raised. Complicating this question, some of these patients have underlying cirrhosis.

The onset of symptoms in Budd-Chiari syndrome ranges from acute to insidious. Initially liver dysfunction is mild, but gradually hepatomegaly, ascites, and complications of portal hypertension ensue. The presentation is complicated by associated portal vein, splenic vein, or superior mesenteric vein thrombosis, which also develop in some patients. A majority of these patients have limited therapeutic options and a poor prognosis.

From a clinical viewpoint it is useful to subdivide Budd-Chiari syndrome into acute and chronic presentations. From an anatomic viewpoint, a subdivision into a partial obstruction limited to thrombosis of one hepatic vein and a more complete obstruction involving more of the liver drainage is more informative. Complete obstruction of the hepatic veins is rare; drainage of the caudate lobe tends to be preserved unless the adjacent inferior vena cava is obstructed. Especially in chronic obstruction, accessory hepatic veins enlarge and become evident. At times the obstruction involves mostly smaller veins, with the main hepatic veins being patent.

ABDOMINAL VASCULATURE

Imaging

Imaging findings differ between acute and chronic liver outflow obstruction. Hepatomegaly, especially with caudate lobe enlargement and central liver contrast enhancement are common with acute obstruction. Left lobe, and especially caudate lobe hypertrophy and an irregular liver surface develop on a more chronic basis. Liver enhancement varies in chronic obstruction depending on degree of subcapsular collaterals and portal blood stasis. Homogeneous enhancement is found with less severe chronic obstruction, probably due to extensive collateral drainage.

The gold standard in diagnosing Budd-Chiari syndrome is inferior cavography and selective hepatic venography, yet the diagnosis can often be suspected with CT, US, MRI, or scintigraphy. Occasionally the condition is misdiagnosed as an infiltrating tumor. In particular, portal vein blood flow abnormalities are common in Budd-Chiari syndrome, and a misdiagnosis of primary portal hypertension should be avoided.

Computed tomography in patients with acute Budd-Chiari syndrome reveals hepatomegaly, a heterogeneous hypodense liver showing decreased central postcontrast enhancement, and marked ascites. Caudate lobe enlargement is common even with an acute onset, at times compressing the adjacent inferior vena cava. Veins draining the caudate lobe dilate. An irregular liver outline develops, but the liver is not nodular as found in cirrhosis.

Ultrasonography suggests Budd-Chiari syndrome if gray-scale US identifies hepatic veins and Doppler US detects either no blood flow or a reversal of flow. Likewise, the syndrome should be suspected if both gray-scale and Doppler US fail to identify hepatic veins. Normal flow in the inferior vena cava varies with respirations and cardiac cycle, but in some patients with Budd-Chiari syndrome and partial vena caval obstruction either reversed or continuous caval flow is detected by Doppler US. Still, the specificity with US is low.

Not only does the caudal lobe enlarge in Budd-Chiari syndrome, but the caudate vein also becomes prominent. In fact, US detection of a caudate vein equal to or >3mm in diameter, in the appropriate clinical setting, should suggest Budd-Chiari syndrome (93).

Both morphological and perfusion abnormalities are defined with MR angiography. The MR postcontrast appearance varies depending on chronicity. Acute Budd-Chiari syndrome results in early homogeneous enhancement of an enlarged caudate lobe and heterogeneously decreased enhancement of the rest of the liver; central liver portions enhance considerably more than the periphery. Magnetic resonance imaging identifies ascites, major hepatic vein thrombi, and alternate venous pathways; it detects either hepatic vein thrombosis or simply the absence of hepatic venous flow (94).

During the subacute obstruction phase, postcontrast CT enhancement differences between the central and peripheral liver become less pronounced than during the acute phase. Computed tomography of chronic Budd-Chiari syndrome results in caudate lobe and usually left lobe hypertrophy and varying degrees of right lobe atrophy, with the hypertrophic regions having heterogeneous postcontrast enhancement. Ascites is not as prominent as with an acute syndrome. Computed tomography arterial portography reveals similar heterogeneous liver contrast enhancement in a setting of vascular congestion.

Fibrosis ensues during the chronic phase, identified by its hypointense signal on both T1and T2-weighted images. Major venous thrombosis is not a prominent feature of chronic Budd-Chiari syndrome; instead, detected are extensive collaterals, caudate lobe enlargement, and regenerative nodules in less affected portions of the liver. These nodules are hyperintense on T1and isoto hypointense on T2-weighted images and enhance during the arterial phase. Collateral vessels are best identified during the portal vein phase.

Inferior vena caval involvement ranges from obstruction by a thrombus, to a web resulting in partial obstruction, to the vena cava being compressed by an enlarged caudate lobe. Occasionally vena cavography and hepatic venography fail to identify the full extent of venous outflow obstruction; in such a setting percutaneous transhepatic venography appears useful to define the proximal and distal ends of an occlusion.

With a simple obstruction the portal venous system is not affected, but an extensive obstruction results in flow reversal. Alternate venous

ABDOMINAL VASCULATURE

such a finding should suggest a partial BuddChiari syndrome.

Therapy

Short stenotic segments are dilated with balloon angioplasty; shunt placement, TIPS, or thrombolytic therapy are employed with thrombosed segments. Therapy in some patients is surgical.

Hepatic Vein Thrombosis

The primary problem in patients with hepatic vein thrombosis is to control the underlying hematologic abnormalities; otherwise recurrent thrombosis is common. The type of therapy varies with the underlying lesion. Thus an intraluminal thrombus in an isolated hepatic vein is amenable to fibrinolytic infusion or balloon thrombectomy, more extensive obstruction is treated with venoplasty or stenting, and TIPS should be considered for diffuse intrahepatic venous obstruction.

Although angioplasty is safe and usually successful, recurrent stenosis is common. Therapy often involves several sessions. A stent placed across a stenotic hepatic vein segment reduces the pressure gradient. These stents should be monitored every 6 months or so because intimal fibrosis leads to lumen obliteration.

Transjugular intrahepatic portosystemic shunt placement is effective therapy for some patients with acute and even chronic BuddChiari syndrome (Fig. 17.23). Using a transjugular access, puncture from a hepatic vein stump is performed into an engorged intrahepatic vein. At times, puncture is feasible even in the absence of any patent residual hepatic vein, with a stylet perforating from the inferior vena cava through the liver into a portal vein; dilatation and stent placement then establish a venous communication. For unknown reasons stent occlusion is a common complication after TIPS placement for Budd-Chiari syndrome.

A percutaneous transhepatic approach is feasible to recanalize the hepatic veins but is not often employed.

Inferior Vena Cava Obstruction

Percutaneous transluminal angioplasty and stent placement is an option in patients with Budd-Chiari syndrome caused by intrahepatic

inferior vena caval obstruction. Percutaneous transluminal angioplasty alone results in about a 50% patency rate; stenting after primary percutaneous transluminal angioplasty decreases the restenosis rate considerably. A residual narrowing is often evident after stent insertion, but a stent keeps the vena cava patent and decreases the risk of Budd-Chiari syndrome recurrence.

Renal Vessels

Hypertension

Nonrenal Causes

Most hypertension is idiopathic. Identified causes can be subdivided into nonrenal, nonvascular renal, and renovascular. Nonrenal causes of hypertension include primary aldosteronism and the presence of a pheochromocytoma, Cushing’s syndrome, vasculitis, and other disorders. An unusual cause of nonrenal malignant hypertension was gallbladder hemobilia (95). A rare patient has several conditions responsible for hypertension.

Nonvascular Renal Causes

The final pathway for nonvascular renal causes of hypertension is increased renin production.

Nephroptosis is a rare cause of renal hypertension. Technetium-99m-renography identifies diminished perfusion and excretion on the affected side.

In general, renal cysts are not associated with hypertension. Patients with cysts, however, tend to have systolic and diastolic blood pressures significantly higher than those without cysts, either due to underlying renal disease that is responsible for both or due to renal artery compression caused by an expanding cyst, thus leading to increased renin release. Compression by an adjacent extrarenal tumor, typically an adrenal one, is occasionally implicated in unilateral renal ischemia and the resultant hypertension.

A thrombus or an atheromatous plaque embolizing distally can result in renal hypoperfusion and hypertension, or the involved renal parenchyma infarcts.

A number of glomerular and interstitial renal diseases result in hypertension. Occasionally ureteric obstruction, generally on an acute