Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Behçet’s syndrome. In these settings caval obstruction should be suspected if hepatosplenomegaly, ascites, and lower extremity dependent edema develop.

Imaging detects a thrombus as an intraluminal filling defect or simply as lack of caval filling with contrast (Fig. 17.7). A primary thrombus and a neoplastic thrombus have a similar imaging appearance and can be differentiated only if thrombus neovascularity or other evidence of a neoplasm is detected. Noncontrast CT reveals an acute thrombus to be isodense or slightly hyperdense to blood. With age, a thrombus gradually becomes hypodense. The involved vessel diameter tends to be expanded focally, regardless of etiology. Gas within a thrombus is rare and suggests infection. Calcifications develop in some chronic thrombi.

With incomplete obstruction, postcontrast CT identifies most thrombi as a hypodense tumor surrounded by contrast-opacified blood. A bland thrombus does not enhance postcontrast, while a tumor thrombus does. Complicating the issue is the occasional bland thrombus attached to a tumor thrombus. Still, caution is needed to differentiate a thrombus from incomplete mixing of opacified and nonopacified blood and a resultant transient artifact.

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Collateral vessels are common with obstruction of the inferior vena cava. Lower extremity radionuclide venography reveals collateral flow; an occasional patient has diffuse hepatic uptake of the radionuclide. With superior vena caval obstruction, the azygos vein, hemiazygos vein, internal mammary veins, vertebral venous plexus, and lateral thoracic and some superficial thoracoabdominal veins enlarge and become collateral vessels. Because of these collaterals, after contrast injection into an upper extremity CT can reveal enhancement of a liver segment or the inferior vena cava. Early and dense contrast enhancement of liver segment IV occurs due to segmental liver perfusion from epigastric and paraumbilical veins; such a pseudolesion in segment IV is a potential pitfall seen occasionally during both arterial portography and helical CT.

Magnetic resonance readily detects a caval thrombus. With SE sequences the vena cava contains a tumor rather than a signal void as seen with flowing blood. Slow-flowing blood, however, also results in loss of the caval signal void. With GRE sequences flowing blood appears hyperintense, and this technique appears more reliable in detecting a thrombus than a SE technique. In some patients incom-

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plete obstruction simply results in hepatosplenomegaly and follows a relatively benign course, while others develop chronic liver disease and esophageal varices. Caudate lobe and left lobe hypertrophy, right lobe atrophy, and a nodular liver outline develop with intrahepatic inferior vena caval obstruction; peripheral linear or wedge-shaped hypodense defects are detected in some patients. Some chronic thrombi are associated with caval wall thickening; MR contrast enhancement extends from the vena cava into surrounding soft tissues, probably reflecting thrombophlebitis.

Occasionally a caval thrombus regresses spontaneously.

Membranous Obstruction

One of the causes of Budd-Chiari syndrome is inferior vena cava obstruction by a membrane (web). This condition is more common in East Asia than in the West and predominates in young adult males. Some of these patients also have hepatic vein membranes.

A congenital inferior vena cava web is rare. After therapy some of these webs restenose and require several dilations. Why an acquired web forms in some patients is not clear. A number of affected patients have suffered prior abdominal trauma. An underlying hypercoagulable state has been suggested; in fact, some patients with membranous obstruction have both an underlying hypercoagulable condition and also had recent trauma.

A membranous obstruction can be detected by either US or cavography but not by CT. Doppler US is useful in evaluating membranous obstruction of both the inferior vena cava and main hepatic veins.

With some membranous obstructions the hepatic veins act as an alternate pathway for blood flow. Thus intrahepatic collaterals can develop between inframembranous and supramembranous hepatic veins; because of these pathways, a Budd-Chiari syndrome does not develop.

Other Obstructions

A rare inferior vena caval obstruction is associated with a diaphragmatic hernia; liver herniation results in torsion and narrowing of the inferior vena cava.

Therapy

Most membranous obstructions are amenable to transfemoral balloon dilatation with excellent results. An occasional membrane, however, is relatively thick and resists balloon dilatation. Endoluminal recanalization and stent insertion are viable options in patients with chronic inferior vena caval obstruction

Unique percutaneous thrombectomy of floating iliocaval thrombi has been performed with an occluding balloon sheath. Using a transjugular access route, the sheath was positioned in the inferior vena cava, a balloon inflated to prevent central thrombi embolization, mechanical fragmentation performed through a working channel using a rotating basket, and residual thrombus fragments then aspirated (41).

Tumor

Primary

Primary inferior vena caval malignancies are rare; they are readily confused with adjacent extraperitoneal tumors. Primary leiomyomas and sarcomas should be distinguished from secondary ones, the latter often representing intravenous leiomyomatosis from a uterine leiomyoma. Some of these women have had a previous hysterectomy for uterine leiomyomas. For example, cavocardiac leiomyomatosis was discovered in a woman who had a hysterectomy 16 years previously for a hemorrhagic fibroma (42); her present tumors were believed to be uterine in origin.

Leiomyosarcoma

The most common primary vena caval malignancy is a leiomyosarcoma. For some reason authors have an urge to publish their experience with vena cava leiomyosarcomas, but these extensive case reports belie the rarity of this condition.A number of these patients have been enrolled in the International Registry of Inferior Vena Cava (IVC) Leiomyosarcomas. Out of 218 patients in this study, over half developed tumor recurrence after radical resection (43). Recurrence consisted of local spread, distant metastases, or both.

Leiomyosarcomas range from an intraluminal tumor obstructing the inferior vena cava, to

a tumor extending primarily outside the caval wall, to a complex tumor having features of both; CT shows a lobulated well-defined heterogeneous tumor, while MRI reveals a tumor having an isointense signal on T1and an isoto hyperintense signal on T2-weighted images. Regions of hemorrhage result in a hyperintense T1-weighted signal. These tumors usually enhance markedly with contrast. With a complex appearance, both CT and MRI are useful in differentiating between a neoplasm and a simple thrombus. At times a tumor is so extensive that identification of the site of origin is not possible radiologically, surgically, or even pathologically.

Histiocytoma

Only a few inferior vena caval malignant fibrous histiocytomas have been reported. Contrastenhanced CT of one revealed a minimally enhancing intraluminal tumor expanding the lumen considerably, findings not seen with a simple thrombus (44); hyperand hypointense regions were identified on T1and T2-weighted sequences. Small serpiginous enhancing tubular structures on immediate postgadolinium SGE images were believed to represent feeding vessels.

Secondary

Extraperitoneal tumors readily compress the inferior vena cava or invade and progress to an intraluminal tumor thrombus. Invasion by renal cell carcinoma is familiar to most radiologists. Some of the more unusual invasive tumors include a liposarcoma and adrenal carcinoma. Intracaval extension of these tumors can be determined by transesophageal US.

Magnetic resonance imaging and MRA identify both intraluminal tumors and extrinsic caval compression by a tumor. Collateral vessels are also detected. Magnetic resonance imaging generally provides more information than CT.

Caval Fat

True intracaval fat-density tumors range from a lipoma to renal angiomyolipoma extending through a renal vein.

A fat-density intraluminal caval tumor is rare; a more common reason for such a CT appearance is juxtacaval fat, located medially at the

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hepatic vein level or slightly superior, artificially projecting into the caval lumen. This fat is contiguous to fat surrounding the subdiaphragmatic esophagus. Either coronal reconstruction images or contrast-enhanced images should suggest a correct diagnosis. Such fat is generally considered to be a normal variant, although it appears to be more common in patients with chronic liver disease (45).

Filters

Clinical

From initial open surgical placement, insertion of inferior vena cava filters has evolved into a percutaneous technique using an introducer sheath. Indications for inserting inferior vena cava filters have expanded from patients who had a pulmonary embolus and could not be anticoagulated to those at risk for emboli. Ease of filter insertion and reduced associated morbidity have expanded the indications further, although in a setting of a pulmonary embolus anticoagulation is generally the first step. Recurrent emboli in the face of adequate anticoagulation or a complication of anticoagulation, such as bleeding, indicates the need for filter insertion. Contraindications to anticoagulation include a recent hemorrhage, with other contraindications often relative and at times subjective.

A number of vena cava filters are available. They differ in their appearance, such as cone, basket,or net; in their construction material and thus radiodensity; and in whether they are readily removable. In broad terms, all filters trap most clots, but some pulmonary emboli are unavoidable. At times vena caval filters are inserted temporarily, such as in the presence of iliac vein or caval thrombi and or in the risk of embolization during thrombolysis.

Carbon dioxide is an alternate contrast agent for vena cavography during filter insertion when iodinated contrast agents are contraindicated or in those patients with renal insufficiency (46).

Complications

Complication rates for radiologic and surgical placement of inferior vena cava filters appear comparable although in one institution radiologists achieved a higher success rate and fewer

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complications (47); part of the reason may be that radiologists tended to perform cavography prior to filter placement and surgeons did not. A review of over 1700 implanted filters at one institution (up to 1998) found a 6% prevalence of observed postfilter pulmonary emboli, fatal in 4% of patients, with most fatal pulmonary emboli occurring a median of 4 days after filter insertion (48); the prevalence of observed postfilter caval thrombosis was 3%. Intrafilter clots are not uncommon. Benefit must be balanced against risk.

Other filter complications include malposition, filter tilting, and insufficient opening. Ideally, a filter is positioned between the ileal vein confluence and renal veins. A filter cephalad to the renal veins risks renal vein thrombosis. Mild filter tilting is common, but the ability to trap clots is reduced with excessive tilting. Likewise, insufficient filter opening reduces a filter’s clot trapping ability. A filter is designed to prevent pulmonary emboli, and the evidence of such an embolus after filter placement represents filter failure. In such a situation the filter should be investigated for migration, strut failure, or inferior vena cava thrombosis. Filter migration occurs either caudal or cranial.Vena caval thrombosis is a recognized filter complication, regardless of filter design. Some caval thrombi are asymptomatic. Even without a thrombus, extremity venous stasis tends to accentuate after filter placement. Rare complications include myocardial infarction due to filter migration and pericardial tamponade.

Filter perforation is rare. Anecdotal reports describe caval penetration resulting in pancreatitis and even biliary obstructive. Filter struts can fracture and migrate, including to the kidneys. Rare reports describe penetration of a vertebral body and caval perforation leading to small bowel volvulus.

Caval filters are recognized on contrastenhanced SGE MRI because of their symmetry and magnetic artifacts.

Incidentally, J-tipped guidewires should be used with care around filters; these guidewires are at risk for entrapment.

Aneurysms

Inferior vena cava aneurysms are very rare. Most are believed to be congenital. Acquired

ones are secondary to trauma or an arteriovenous fistula. Some are asymptomatic and are incidentally discovered when CT or MR are performed for other reasons. Sagittal reconstruction is helpful in their visualization.

Visceral Vessels

Vasculitides

Systemic vasculitis often involves small visceral vessels, with resultant ischemia leading to ulcerations, perforation, or evolving into a stricture. Vasculitis can involve any part of the gut, even the duodenum, which has a rich blood supply. It often affects multiple bowel segments, in distinction to thromboemboli, which tend to affect continuous segments. De novo duodenal ischemia is almost always secondary to a vasculitis. Concomitant splenic and renal involvement also suggest a vasculitis.

Behçet’s Syndrome

A chronic multisystemic disorder, Behçet’s syndrome is of unknown etiology. Whether Behçet’s syndrome has a primary ischemic basis is debatable; some authors classify it not under ischemia but in a separate category. Also, some authors separate Behçet’s syndrome into two forms: complete and incomplete. In Japan, Behçet’s colitis patients typically have no eye involvement, and these patients are classified under the incomplete form of Behçet’s syndrome.

Resected bowel in these patients reveals multiple punched-out inflammatory ulcers, mostly in the ileocecal region; most ulcers are on the antimesenteric border. Histopathology reveals a nonspecific vasculitis. Granulomas are not identified. Clinically, many patients develop erythema nodosum, arthralgias, oral and genital ulcers, and a relapsing iritis. Gastrointestinal symptoms are common, with abdominal pain and recurrent aphtha mimicking Crohn’s disease, ulcerative colitis, or another vasculitis. An acute presentation with bowel perforation and peritonitis is not uncommon. An occasional patient presents with prolonged fever. The diagnosis can be difficult; in 1990 the International Study Group for Behçet’s Disease proposed specific diagnostic criteria consisting of oral ulcers and at least two of the following: genital ulcers, specific eye lesions, skin lesions, and a

skin pustule test. A diagnosis of Behçet’s syndrome is established mostly on clinical grounds rather than on histopathologic findings.

The most common site of gastrointestinal involvement is the ileocecal region, followed by the colon, small bowel, and, least often, esophagus. Gastric disease is rare. Imaging in affected individuals reveals inflammatory polyps and either aphtha or deep ulcers. In general, deep or punched-out ulcers in either the distal ileum or right colon should suggest Behçet’s syndrome.A perforation is not an uncommon acute event. A retrospective CT study of patients with intestinal Behçet’s syndrome found intestinal polyps in 36% and thickened bowel wall in 32% (49); enhancement ranged from mild to marked. Both polyps and a thickened bowel wall were more common in patients with complications. Also, severe perienteric infiltration was seen only in patients with complications. Adjacent lymph nodes can be deformed by chronic ileocecal ulcerations.

Most therapy is unsatisfactory in patients with Behçet’s syndrome. Some patients’ clinical symptoms improve and imaging abnormalities clear after the start of a low residue diet. Even thalidomide treatment has been helpful.

Patients with Behçet’s syndrome are prone to develop aortic and arterial aneurysms (50). Stent-grafts are helpful, although artery occlusion is a complication. Thromboses of inferior vena cava, portal vein, and smaller veins also develop.

Postoperative recurrence after bowel resection is common and generally is close to an anastomosis.

Periarteritis Nodosa

Periarteritis nodosa (polyarteritis nodosa), a necrotizing inflammation (arteritis) of small and medium-sized artery walls, eventually results in organ ischemia, with renal arteries more often involved than liver vessels. Hypertension is common. These patients develop arterial aneurysms and stenoses, generally at sites of arterial bifurcation—characteristic but not pathognomonic findings; these findings also occur in other arteritides. Aneurysms tend to be small, saccular, at times having an irregular outline.

Contrast CT shows patchy renal uptake bilaterally due to scarring; wedge-shaped defects

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develop secondary to focal ischemia. The imaging appearance is nonspecific.

Henoch-Schönlein Purpura

Henoch-Schönlein purpura (nonthrombocytopenic purpura) is a vasculitis of unknown etiology affecting mostly children. Usually a characteristic lower extremity purple rash precedes abdominal symptomatology, with some exceptions. Abdominal pain and bleeding in these patients presumably is secondary to intramural bleeding or a serohemorrhagic effusion.

Imaging reveals a predilection for proximal small bowel involvement. With extensive involvement, a small bowel study reveals intramural fluid as fold thickening and thumbprinting (Fig. 17.8). An intramural hematoma occasionally acts as a lead point for an intussusception. Computed tomography identifies segmental small bowel mural thickening, a nonspecific finding.

In children with Henoch-Schönlein purpura, US detects dilated small bowel segments, decreased motility, and eccentric bowel wall thickening.

Figure 17.8. Henoch-Schönlein purpura resulting in bowel wall edema (arrows) and fold infiltration.