Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Allergic Granulomatous Vasculitis

(Churg-Strauss Syndrome)

Allergic granulomatous angitis (Churg-Strauss syndrome) is a necrotizing vasculitis leading to ischemia due to arterial occlusion. Some patients develop ulcerative colitis–like findings. Symptoms tend to improve with steroid therapy, but disease recurs when steroids are discontinued.

Degos’ Disease

Degos’ disease (malignant atrophic papulosis) is a rare, progressive disease leading to occlusion of small and medium-size arteries. Bowel involvement results in perforations, peritonitis, and fistulas.

Degos’ disease is in the differential diagnosis of small bowel ulcerations.

Systemic Lupus Erythematosus

Some patients with systemic lupus erythematosus develop a gastrointestinal vasculitis with resultant symptoms of diarrhea and pain. The final end point is ischemic bowel disease. In affected patients with acute abdominal pain, CT reveals bowel wall thickening, a target-like appearance and mesenteric vascular engorgement and haziness, findings suggesting ischemic bowel disease (51). In distinction from more common bowel ischemia, bowel wall thickening tends to be multifocal, varies in length, and is not confined to a single vascular field. Unless irreparable damage ensues, the changes should revert to normal after therapy (Fig. 17.9).

A three-phase Tc-99m–pyrophosphate scintigram appears useful in these patients; a positive scan suggests a vasculitis.

Antiphospholipid Antibody

Syndrome

Study of antiphospholipid antibody syndrome belongs mostly in the hematologist’s domain. Abdominal imaging enters the picture when venous and arterial thromboses develop in patients with serum antiphospholipid antibodies and thrombocytopenia. Abdominal pain and abdominal distention are the most common clinical presentations. Spontaneous abortions also occur.

Both antiphospholipid antibodies and lupus anticoagulant alter hemostasis and induce a hypercoagulable state. This syndrome exists as a primary form and also in association with systemic lupus erythematosus. Both forms manifest similar clinical findings, although lupus usually leads to venous thrombosis only.

Patients with antiphospholipid syndrome develop major vascular thromboses, including the aorta, inferior vena cava, portal vein, superior mesenteric vein, and splenic vein (52). Resultant ischemia is either localized or diffuse and can progress to infarction and necrosis and involve kidneys, liver, spleen, stomach, and bowel. Adrenal infarction can develop without evident hemorrhage (53). Pancreatitis and hepatic dysfunction associated with portal hypertension develop in some. Inferior vena cava or hepatic vein thrombosis can lead to Budd-Chiari syndrome. In some patients thromboemboli recur at the same site; in others they tend to be limited to either the arterial or venous side. Imaging detection of unusual or recurrent thrombi, especially in a younger patient, should suggest this entity. Histology simply reveals extensive small artery, arteriolar, and venous occlusive disease.

Other Vasculitides

Pseudoxanthoma elasticum is a connective tissue disorder consisting of characteristic skin lesions, angioid streaks in the eyes, and occlusive vascular disease. Presumably the latter findings are responsible for the occasional gastric hemorrhage developing in this condition. Some patients develop ischemic bowel perforations before manifesting peripheral findings.

Dermatomyositis is a vasculitis also affecting the gastrointestinal tract. Underlying ischemia

ADVANCED IMAGING OF THE ABDOMEN

leads to edema, ulcers, gangrene, and, if severe enough, eventual bowel perforation.

A distinct form of vasculitis consisting of a giant cell phlebitis in a 16-year-old girl led to an ischemic colonic stricture (54); arterioles and arteries were not involved.

Ischemia

Clinical

Adults

Acute intestinal ischemia is a result of arterial thrombosis, arterial embolism, venous thrombosis, a vasculitis, or a nonocclusive low blood flow state. (Ischemic enterocolitis is discussed in Chapters 4 and 5, and gastrointestinal tract bleeding is discussed Chapter 4). From a clinical viewpoint, intestinal ischemia is best subdivided into acute and chronic types; some authors also include a subacute category.

Acute intestinal ischemia often presents as an abdominal catastrophe, evolving rather swiftly into a major infarction. Mortality remains high even with rapid diagnosis and ready availability of surgical consultation.

Nonocclusive mesenteric ischemia is difficult to detect and is probably underdiagnosed. Presenting symptoms tend to be vague and wide-ranging. The jejunum is involved in most patients. At its extreme, nonocclusive ischemia leads to necrosis, gangrene, and perforation. Computed tomography, US, and MRI are not reliable in the early detection of this condition, and mesenteric angiography should be considered in a clinical setting of suspected acute mesenteric ischemia.

Mesenteric ischemia can ensue with an aortic dissection involving the origin of the superior mesenteric artery or even compressing it; at times stenting relieves the ischemia.

Cholesterol embolization is associated with atheromatous plaques, angiography, vascular surgery, and even thrombolytic therapy. Such embolization can lead to mesenteric ischemia, gastrointestinal hemorrhage and bowel infarction and perforation (55). Clinically, these patients often also develop extremity ischemia and renal insufficiency. Eosinophilia is detected in some.

Although fibromuscular dysplasia is common in the renal arteries, it is unusual in visceral

ABDOMINAL VASCULATURE

arteries; it is an occasional cause of ischemia, especially in a younger patient. Angiography reveals a characteristic “string of beads” appearance seen with fibromuscular dysplasia.

Mesenteric ischemia is more common in patients undergoing chronic hemodialysis than in the general population, probably due to atherosclerosis.

Heroin-related mesenteric ischemia is well known. Ischemia also develops after intranasal cocaine use.

Most chronic ischemia, especially in the elderly, is caused by either arterial stenosis due to atherosclerosis or a vasculitis. It is more common in women. Patients with chronic ischemia present with intestinal angina, weight loss, anemia, and diarrhea. Stenoses and occlusions range from a single visceral artery being involved to a combination of vessels. Collateral flow from the internal iliac or other arteries is not unusual. On a chronic basis, at times the arc of Buehler enlarges to the point that it provides most of the arterial blood supply to the liver and spleen.

Infants and Children

Gastric ischemia in a neonate is usually a complication of acute anoxia or shock. Intestinal ischemia in children is often superimposed on underlying congenital vascular or metabolic abnormalities. Thus vascular thromboemboli are a known complication in a setting of homocystinuria, a rare inborn error of amino acid metabolism manifesting as a multisystemic disease associated with mental retardation and vascular disease, consisting of premature arteriosclerosis and thrombosis.

Does the use of umbilical artery catheters in newborns impair mesenteric blood flow? Doppler US performed before and after removal of umbilical artery catheters found that after catheter removal mean peak celiac artery systolic blood flow velocity increased from 50cm/sec to 62cm/sec and superior mesenteric artery flow from 52cm/sec to 72cm/sec (56); end diastolic blood flow velocity and vessel diameters did not change significantly.

Umbilical arteriovenous fistulas are uncommon. They can be either congenital or acquired. Even in a neonate an arteriovenous fistula between the umbilical artery and umbilical vein can result in bowel ischemia. Bowel perfu-

sion improves in some after umbilical vein ligation.

Imaging

Angiography is the historic gold standard in suspected acute mesenteric ischemia. It is more time-consuming and often less available than CT, which is the current examination of choice in many centers. Mesenteric CTA detects superior mesenteric artery embolism or thrombosis and superior mesenteric vein thrombosis with a sensitivity approaching that of angiography. In addition to acute ischemia, CT also evaluates other causes of an acute abdomen.

Conventional radiography findings in patients with intestinal ischemia reflect the underlying spectrum of pathologic changes. They are normal initially or reveal variable degrees of gaseous bowel distention and fluid levels. Dynamic CT findings range from early ischemic ileus, bowel wall edema, and hematoma manifesting as bowel wall thickening, major vessel stenosis, or occlusion (either arterial or venous) to eventual bowel wall necrosis identified as intramural gas and lack of bowel wall contrast enhancement. In patients requiring surgery for acute mesenteric ischemia, detection of at least one of these CT findings achieves a specificity of over 90% but a considerably lower sensitivity. At times a contrast-enhanced CT target sign is evident during the arterial phase, especially if ischemia is due primarily to venous obstruction.

Gas–fluid levels and dilated bowel loops, both nonspecific signs of acute mesenteric ischemia, are seen equally well with conventional radiography and CT. Major ischemia leads to gas within the bowel wall (pneumatosis intestinalis) and mesenteric and portal vein gas. With few exceptions, in adults detection of mesenteric and portal venous gas implies mesenteric infarction. Such gas occasionally migrates to the internal spinal venous plexus, presumably from the pelvic veins. Pneumoperitoneum and even pneumoretroperitoneum are uncommon manifestations of bowel ischemia and presumably reflect a perforation. Some authors preach the superiority of CT in detecting portal venous gas and probably rightly so, although a formal comparison with conventional radiography is lacking. Nevertheless,

ABDOMINAL VASCULATURE

sition in the steady state (GRASS) MRA identifies any stenosis in celiac and superior mesenteric arteries.

Deoxyhemoglobin in blood cells is paramagnetic, but oxyhemoglobin is not. This difference can be used to measure superior mesenteric vein blood oxygen saturation in hemoglobin, obtained using flow-independent MR T2 data; it appears useful in confirming suspected chronic mesenteric ischemia. In patients without ischemia, superior mesenteric vein blood oxygen saturation increases after a meal, but in symptomatic patients with chronic mesenteric ischemia blood saturation tends to decrease. In spite of research on this topic for over a decade, the clinical relevance of such measurements is yet to be established.

In an occasional patient positive uptake of Tc-99m–HMPAO–labeled leukocytes appears to reflect underlying chronic ischemia rather than primary inflammation.

Therapy

Acute Ischemia

Different therapy is employed for acute and chronic ischemia. Without therapy, acute nonocclusive ischemia often progresses to infarction. The treatment of choice in some patients with nonocclusive ischemia is papaverine infusion via a vascular catheter. Resection and anticoagulation are therapies for infarcted bowel. Recurrent bowel ischemia is a complication of surgical bypass grafting for acute ischemia. Fibrinolytic therapy using urokinase, rather than surgical embolectomy, appears effective in a majority of patients with a mesenteric embolus and without evidence of intestinal infarction; others require laparotomy. The best sign of successful therapy is pain abatement; persistent pain suggested intestinal infarction.

Rather than surgical embolectomy, in selected individuals mechanical thrombolysis of mesenteric and portal vein thrombosis, using a jugular vein approach, is an alternate approach.

Chronic Ischemia

Either percutaneous transluminal angioplasty or a surgical bypass graft is employed in a setting of chronic mesenteric ischemia. At times transaortic endarterectomy is performed. All of

these techniques have their associated complications. The published success rates of percutaneous transluminal angioplasty are difficult to put in perspective because different criteria are used (e.g., size of postprocedure luminal diameter, restenosis rate, improved clinical status) and are operator dependent.

Similar to surgical mesenteric vascular graft placement, percutaneous transluminal angioplasty has a technical procedure success rate of about 90% and short-term clinical success of about 80%. Procedure-related mortality and the major complication rate appear similar for operative bypass grafting and percutaneous transluminal angioplasty; long-term pain relief is similar or better with grafting.

Splanchnic Aneurysms

Many splanchnic aneurysms contain only a portion of the vessel wall; therefore, strictly speaking these outpouchings should be called pseudoaneurysms, yet the term aneurysm has been adopted in most of the medical literature and is used here.

The most common site for abdominal aneurysms is in the splenic artery, although they occur widely. Detection of a splanchnic aneurysm in a young patient should suggest Ehlers-Danlos syndrome (57). Some patients with multiple visceral artery aneurysms have connective tissue fragility suggesting cystic medial necrosis, even without clinical features of Marfan syndrome; they are prone to excessive hemorrhage during angiography and appear to be at increased risk for developing complications during vascular catheterization procedures.

Pancreatitis and resultant pseudocysts lead to aneurysms in adjacent vessel. Other causes include trauma, such as biopsy, and infection resulting in mycotic aneurysms. Intestinal tuberculosis is an occasional cause of a mesenteric artery aneurysm, generally manifesting as massive gastrointestinal hemorrhage.

Many aneurysms calcify and are identified with conventional radiography. In general, the presence of rim calcifications suggests an atheromatous origin rather than mycotic or traumatic. Some of these aneurysms become quite large. Noncontrast imaging reveals a fluidfilled cavity similar to a cyst or abscess. Thus prior to attempted drainage of a suspected cyst