Colon: Non Neoplastic Disease: Radiologic Evaluation of the Acute Abdomen in AIDS

Karen M. Horton, MD and Elliot K. Fishman, MD

Introduction

Many patients with AIDS develop abdominal symptoms, usually related to opportunistic infections or neoplasms. Approximately 4.2 percent of these patients will require abdominal surgery, often in an emergency setting . Radiological evaluation of AIDS patients with acute abdominal complaints is essential for two main reasons. First, AIDS patients are usually immunosuppressed and thus the classic physical signs and symptoms of peritonitis and sepsis are often absent . For instance, many AIDS patients with acute intraabdominal pathology will have a normal or low white blood cell count, often without a "left shift". Since the physical exam in these patients is often not reliable, radiological imaging plays a crucial role in the diagnosis of acute abdominal conditions in AIDS patients who present with vague and nonspecific symptoms. Second, patients with AIDS are often debilitated with very poor nutritional status and low albumin . These factors undoubtedly contribute to the high morbidity and mortality rates associated with emergency abdominal operations in patients with AIDS . Radiological examinations can help distinguish non-surgical disorders such as acute colitis or enteritis from those which will require surgical intervention, thus avoiding unnecessary high risk operations.

This chapter will review the radiological evaluation and findings of the acute abdomen in patients with AIDS, with an emphasis on the value of computed tomography. Both infectious and neoplastic conditions affecting the gastrointestinal tract, liver and biliary tract will be discussed and illustrated.

Radiologic Examinations

Plain radiographs of the abdomen can be sometimes be useful as a quick inexpensive initial study in AIDS patients who present with acute abdominal complaints. They may reveal pneumoperitoneum, pneumotosis or bowel obstruction. However, in most cases the plain film will be normal or will demonstrate a nonspecific bowel gas pattern and further imaging will be necessary.

Barium studies such as the small bowel series or the contrast enema can be helpful in patients with suspected bowel obstruction. These studies may detect the presence and level of obstruction, but typically provide little information regarding the cause of the obstruction, especially if there is extraintestinal disease. Also, barium studies allow visualization of the small bowel and colonic mucosa and can be useful for evaluation of patients in the non-acute setting with nonspecific complaints such as nausea or diarrhea.

Sonography is particularly valuable for imaging the gallbladder and hepatobiliary disease in evaluation of the acute abdomen in patients with AIDS . Sonography is superior to CT for the evaluation of acalculous cholecystitis and AIDS cholangitis which are well-described conditions in patients with AIDS . Compression-graded sonography is also a reliable technique for the evaluation of suspected appendicitis, especially in children and thin patients . Sonography may be the initial imaging modality of choice when these conditions are suspected. However, ultrasound plays little role in evaluation of the gastrointestinal tract or in patients with vague complaints without localizing signs and symptoms.

Although plain radiographs, barium studies and sonography each may be useful in evaluation of the acute abdomen, CT remains the diagnostic modality of choice . CT is fast and accurate. On modern spiral scanners, the entire abdomen and pelvis can be imaged in less than a minute. CT is the most sensitive modality for the detection of pneumoperitoneum and pneumatosis and in most cases will be able to determine the cause. CT is also valuable in evaluation of the gastrointestinal tract. CT can detect bowel wall thickening in patients with enteritis and colitis, and in many cases can suggest the correct etiology such as infection vs. ischemia vs. tumor. CT is both sensitive and specific for the detection of bowel obstruction and for identifying the cause . CT is also excellent for the evaluation of the solid viscera such as the liver, spleen, and pancreas as well as lymph nodes, all of which may be involved in infectious or neoplastic processes.

Infections

Cytomegalovirus

Cytomegalovirus (CMV) is one of the most common opportunistic infections which occur in patients with AIDS. It can affect any portion of the gastrointestinal tract, although the colon is the most common site of involvement. Patients with CMV colitis can present with crampy abdominal pain and high fever and in cases of colon involvement, severe watery or bloody diarrhea. With colonic involvement, the CT will characteristically demonstrate circumferential wall thickening with inflammatory stranding in the pericolonic fat . Ulcerations may also be identified . The entire colon may be affected, or involvement can be limited to the terminal ileum and right colon . Segmental involvement of the colon can also occur . Occasionally the colonic wall with demonstrate a target appearance, due to severe submucosal edema with stratification of the bowel wall layers. This target appears is not specific for CMV colitis, and does occur in other conditions such as ischemia and inflammatory bowel disease. In addition to detecting the colonic pathology in an AIDS patients with acute abdominal signs and symptoms, CT is helpful for identifying complications such as signs of ischemia, obstruction, abscess or perforation.

In patients with CMV enteritis, the CT may reveal nonspecific small bowel thickening, usually confined to the distal ileum, although the proximal ileum and jejunum can also be involved . . The wall thickening is presumably as result of small vessel vasculitis resulting in hemorrhage and ischemia. Ascites and small nodes may also be present.

CMV can effect the esophagus and stomach, resulting in acute abdominal complaints . CMV esophagitis is best demonstrates on contrast esophagogram which will reveal discrete superficial ulcers, primarily involving the mid and distal esophagus . In severe cases the esophagus may also show fold thickening with only one large flat ulcer. CMV gastritis typically involves the gastroesophageal junction or antrum and results in fold thickening and/or ulcerations .

CMV as well as cryptosporidium infection can cause cholangitis . Patients present with right upper quadrant pain and fever, and may be jaundiced. CT may demonstrate intra and or extrahepatic ductal dilatation, which typically is irregular and simulates the appearance of sclerosing cholangitis . The gallbladder wall may also appear thickened. Ultrasound demonstrates wall thickening of the bile ducts in addition to irregular dilatation. Also, the papilla of Vater can appear echogenic, due to edema . ERCP can also demonstrates the beading and pruning or the bile ducts, also similar to findings in patients with sclerosing cholangitis. Microabscesses can occur in the liver in AIDS patients with ascending cholangitis. These can be detected on US or CT.

Crytosporidium

Cryptosporidiosis is a major cause of diarrhea in patients with AIDS and results in significant morbidity. Patients typically present with profuse watery diarrhea and severe abdominal pain. Although radiological studies are not necessary for diagnosis, they may be the first to suggest the diagnosis in patients presenting with acute abdominal pain. Cryptosporidiosis usually affects the proximal jejunum although gastric involvement has also been reported. On CT, jejunal wall thickening is characteristic. On barium studies, the proximal jejunum will be dilated with fold thickening. The appearance can resemble Sprue. If severe, there may be complete effacement of the jejunal folds resulting in a "tooth paste" appearance. Gastric involvement is usually characterized by antral narrowing . Pneumatosis has been reported as a rare benign complication of intestinal Cryptosporidiosis .

Cryptosporidiosis can cause cholangitis in patients with AIDS . The radiological findings are identical to the cholangitis caused by CMV. In fact, the two organisms are probably co-existent in most patients.

MAI

MAI is a very common opportunistic infection in AIDS patients . AIDS patients with abdominal involvement by MAI can present with severe abdominal pain, fever, diarrhea, and weight loss. MAI involvement of the gastrointestinal tract is common. It typically will involve the duodenum and the small bowel . The appearance on barium studies is non-specific, demonstrating fold thickening and nodularity as well as dilatation. Due to increased intra-luminal fluid, there may be segmentation and flocculation of the barium, simulating Whipples disease. Disseminated infection from MAI has been recognized as a common and severe complication of AIDS. On CT, significant retroperitoneal and mesenteric adenopathy is common, occurring in up to 82-100% of patients . Typically, the nodes are large and bulky and may demonstrate decreased attenuation due to necrosis. Splenomegaly and hepatomegaly may also be present. MAI can also result in small micro-abscesses in the liver and spleen, which appear as small foci of decreased attenuation on CT . On ultrasound, the small lesions can also be seen and may appear as echogenic or hypoechoic foci. CT or US can be also used as imaging guidance to sample enlarged nodes to confirm the diagnosis of MAI in difficult cases .

Tuberculosis

Patients with AIDS are also susceptible to tuberculosis, which is often disseminated. Tuberculosis can effect the gastrointestinal tract, usually resulting from ingestion of contaminated food products or resulting from lymphatic or hematogenous dissemination . Tuberculosis typically involves the distal ileum and cecum producing wall thickening and occasionally ulcerations . This can be demonstrated on barium studies or CT. The radiographic findings of tuberculosis are not specific, as other organisms can effect the ileum and cecum. However, findings such as significant asymmetric thickening of the cecum, predominantly the medial wall or the association of bowel thickening and low-density adenopathy can be suggestive of tuberculosis. Low density nodes can be seen in up to 89% of patients . In addition to involvement of the gastrointestinal tract, tuberculosis can result in peritonitis. On CT, ascites and soft tissue thickening of the mesentery and omentum can be present in association with adenopathy . The CT appearance can mimic carcinomatosis or peritoneal mesothelioma. Tuberculosis, when disseminated, can also effect the liver and spleen resulting in small low-density lesions, occurring in 73% of patients in one series .

Clostridium Difficile

Pseudomembranous colitis results from the production of an endotoxin by the overgrowth of C-difficile organisms. It is commonly seen in patients treated with antibiotics. Patients can present with severe abdominal pain and may sometimes require surgical resection.

CT findings of pseudomembranous colitis include marked colonic wall thickening, often measuring greater than 1.5 cm in thickness . Other infections, which can cause colitis, usually do not result in this degree of wall thickening. In addition, patients with pseudomembranous colitis can demonstrate stranding in the pericolic fat often much less than would be expected with the degree of colonic involvement. Ascites has been reported in up to thirty percent of cases and can help suggest the diagnosis of pseudomembranous colitis, although this is not specific . Although typically thought of a pancolitis, pseudomembranous colitis can be segmental or even focal, mimicking a soft tissue mass . In addition to wall thickening and ascites, the colonic wall may demonstrate enlargement of the haustral folds, which can trap the oral contrast producing an appearance similar to an accordion. Although this can be suggestive of pseudomembranous colitis it is neither sensitive nor specific. CT may be the first modality to suggest the diagnosis, which can be confirmed with toxin assays of the stool. In a series of patients by Kawamoto et al., although CT demonstrated a variety of findings in patients with pseudomembranous colitis, it was unable to predict which patients would require surgical intervention . Surgery is usually considered if there is associated ischemia, pneumatosis, or pneumoperitoneum or significant obstruction. It may also be considered in patients who do not respond to antibiotic therapy.

Other infections

Amebiasis has also been reported in AIDS patients. In contrast to Cryptosporidiosis, Amebiasis usually effects the distal small bowel and colon, producing wall thickening and inflammation which can be detected on barium studies or CT scan . In addition, Giardia infection can also result in abdominal complaints and diarrhea. The radiologic appearance on barium studies and CT can be identical to that of Cryptosporidiosis, resulting in proximal small bowel thickening and dilatation.

Urinary Tract Calculi

Indinavir Sulfate is a protease inhibitor, which is used in the treatment of HIV infection. This medication is associated with a significant incidence of crystallization and stone formation in the urinary tract. In one study by Reiter et. al., 105 patients were treated with the drug. The incidence of symptomatic nephrolithiasis during the indinavir treatment was 12.4% . These patients can present with the usual symptoms of urinary colic including flank pain and/or nausea and vomiting. Patients may also describe dysuria, urgency, hematuria, and/or proteinuria. Recently, spiral CT has been used in patients with suspected ureteral calculi to detect radiopaque stones. However, the urinary stones associated with this protease inhibitor are radiolucent and cannot be identified on unenhanced abdominal CT scans . Secondary signs of obstruction including hydronephrosis and/or perinephric stranding can be detected on CT although the stones themselves will not be visualized.

Pneumatosis

Pneumatosis intestinalis refers to the presence of air within the bowel wall. This can be due to a variety of etiologies; the most worrisome of which is ischemia with bowel ischemia/necrosis. This has been reported in an AIDS patient with necrotizing enterocolitis and in these cases, the patient will be symptomatic. However, a more indolent form of pneumatosis intestinalis has also been described in asymptomatic patients with AIDS in association with various infections including CMV and cryptosporidiosis and therefore the clinical significance of pneumatosis in AIDS patients is not certain. In one series of 6 cases by Wood et. al., 5/6 patients with AIDS associated pneumatosis intestinalis were managed conservatively without surgical resection . Similarly, in another series of 5 patients by Gelman et al, all five AIDS patients with pneumatosis intestinalis were managed . When present, pneumatosis typically involves the right colon or cecum and in patients with AIDS . Asymptomatic pneumatosis intestinalis along with free abdominal air has been described in a patient with AIDS . Therefore, the finding of pneumatosis intestinalis in patients with AIDS should be correlated carefully with the clinical symptoms, as it may indicate bowel necrosis in patients with severe intestinal infections or may simply be an incidental finding which can be treated conservatively in asymptomatic patients .

Neoplasms

Patients with AIDS are not only predisposed to various infections but are also at risk for certain neoplasms. The two most important neoplasms associated with AIDS are non-Hodgkin's Lymphoma and Kaposi's Sarcoma. There are several other cancers, which appear to have a slighter increased incidence in patients with AIDS including anal cancer, testicular cancer, and cervical cancer. However, we will limit our discussion to Kaposi's Sarcoma and non-Hodgkin's lymphoma.

Non-Hodgkin's Lymphoma

It is estimated that the incidence of intermediate and high-grade B-cell non-Hodgkin's lymphoma in HIV infected individuals is almost 60 times greater than in the general population . These AIDS related lymphomas tend to be a late manifestation of HIV infection and have unique clinical pathologic features, which are different from classical non-Hodgkin's lymphoma occurring in the general population. In contrast to non-Hodgkin's lymphoma in the general population, lymphomas in patients with AIDS tend to be more aggressive and have a preponderance for extra nodal involvement . The central nervous system, bone marrow, and abdomen are the most common sites of involvement. Also, response to treatment is not as good in patients with AIDS in comparison to immunocompetent patients.

AIDS patients with abdominal involvement by non-Hodgkin's lymphoma can present acutely with pain, palpable mass, or gastrointestinal bleeding. Also, lymphoma involving the gastrointestinal tract can cause acute complications such as perforation, obstruction, or intussusception. In these cases, the patients will also present with acute abdominal symptoms, requiring radiologic evaluation. In most cases, CT is considered the imaging modality of choice for detecting, staging, and planning therapy in patients with lymphoma . CT is sensitive for the detection of bulky abdominal and pelvic adenopathy as well as for lymphoma involving solid organs such as the liver or spleen or gastrointestinal tract . Hepatic or splenic involvement may appear as small low density masses on CT, which may or may not demonstrate enhancement . Alternatively, lymphoma can appear as a diffuse infiltrative process in the liver or spleen.

Patients with gastrointestinal tract involvement by lymphoma can present with acute abdominal symptoms requiring radiologic evaluation. The stomach is the most common site of involvement . The CT appearance of gastric lymphoma can vary from diffuse or segmental wall thickening to a discreet mass with or without ulceration . The CT appearance can resemble adenocarcinoma of the stomach. One differentiating feature is despite extensive gastric involvement, lymphoma vary rarely will result in gastric outlet obstruction. Alternatively, involvement of the stomach by adenocarcinoma will often cause gastric outlet obstruction. Also, in the AIDS population lymphoma of the stomach is much more common than adenocarcinoma.

The small intestine is the next most likely site of involvement of non-Hodgkin's lymphoma in AIDS patients . In a series of 42 consecutive patients with small bowel lymphoma, Balthazar determined that more than half of the individuals with small bowel lymphoma had AIDS . However, there was no significant difference in the radiologic features of patients with AIDS versus immunocompetent patients. In the patients with lymphoma and AIDS, 22% demonstrated solid organ involvement such as involvement of the liver, spleen, kidney or adrenal glands. Only 10% of the immunocompetent patients with lymphoma demonstrated solid organ involvement. Two main CT patterns of small bowel involvement were observed . In the first pattern, there were single or multiple segments of circumferential wall thickening which appeared homogenous in attenuation (1.5-7 cm in thickness). The second pattern of involvement observed was single or multiple cavitary lesions associated with bowel wall thickening. Mesenteric or retroperitoneal lymphadenopathy was demonstrated in 45% of the patients with AIDS in comparison to 60% of immunocompetent patients . The distribution and pattern of CT presentation was similar for the two groups.

Lymphomas effecting the colon and rectum are uncommon, but have been reported with increased incidence in patients with AIDS . Colonic lymphoma can appear as a large polypoid mass or as diffuse thickening. Although the CT appearance of the primary tumor can be similar to that of colon cancer, lymphomas are typically associated with extensive lymphadenopathy.

Kaposi's Sarcoma

Kaposi's sarcoma is the most common AIDS-associated cancer in the United States, although its incidence has decreased from 40% of American men with AIDS in 1981 to less than 20% in 1992 . This aggressive and frequently fatal variant of Kaposi's sarcoma effects homosexual men with AIDS twenty times as frequently as male patients with AIDS who are not homosexual and have a similar degree of immunosuppression. As with the indolent form of Kaposi's sarcoma, which occurs in non-HIV, infected elderly men, Kaposi's sarcoma in AIDS patients typically involves the skin. However, patients with AIDS who develop Kaposi's sarcoma also can progress rapidly to include wide-spread cutaneous involvement as well as involvement of lymph nodes and the gastrointestinal tract. These patients with abdominal involvement may also present with acute abdominal symptoms requiring radiologic evaluation. The most common site of gastrointestinal tract involvement is the duodenum, along gastric involvement has also been reported . On upper GI series, or small bowel series, gastrointestinal tract involvement can appear as small submucosal masses, often with ulcerations. This gives a characteristic target or "bulls eye" appearance. Although this appearance is not specific for Kaposi's sarcoma, it is very suggestive in the right population. More extensive involvement of the gastrointestinal tract can appear as fold thickening, plaque, or masses on barium studies. Intestinal perforation has been described as a complication of gastrointestinal Kaposi’s sarcoma . CT often demonstrates larger masses involving the gastrointestinal tract as well as any associated adenopathy. Hepatic or splenic involvement has also been reported. The radiographic appearance can be similar to lymphoma or MAI infection and may require biopsy for definitive diagnosis.

Occasionally, the only radiographic finding in patients with Kaposi's sarcoma may be adenopathy, simulating lymphoma or MAI infection. One published study by Herts in 1992 suggested that when the lymphadenopathy in AIDS patients have a high attenuation, this is more suggestive of Kaposi's sarcoma than lymphoma or MAI which often have low density adenopathy. Typically, in these circumstances, biopsy, often with imaging guidance if necessary, will be needed to make the diagnosis .

Conclusions

Radiological examinations play a valuable role in the evaluation of AIDS patients. Although plain films, barium studies and sonography all play a defined role in certain patients, CT is considered the modality of choice in patients presenting with acute or nonspecific abdominal complaints. CT allows evaluation of the both the gastroinestinal tract and other intraabdominal organs which are commonly involved with infectious or neoplastic diseases. CT can also help direct patient management, by identifying those patients requiring surgical intervention.