Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Granular cell myoblastomas are more common in the esophagus. They are rare in the colon. Most are single, but occasional reports describe multiple tumors scattered throughout the large bowel.

Dilatation

Mechanical Obstruction

The two most common causes of colonic obstruction in adults, namely colon cancer and diverticulitis, have already been discussed. Hernias are covered in Chapter 14. Colon obstruction in a setting of cystic fibrosis was covered in a previous section (see Cystic Fibrosis).

Primary causes of intestinal obstruction in elderly patients requiring surgery are an incarcerated hernia and colonic neoplasms.

Figure 5.34. Cecal volvulus. CT identifies a massively dilated cecum displaced to the left of midline (arrows). Dilated loops of small bowel on the right are secondary to small bowel obstruction. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

Volvulus

Cecal

Traditionally, cecal volvulus was suggested with conventional radiography and confirmed either with a barium enema or colonoscopy. Most often cecal volvulus is idiopathic (Fig. 5.33), but occasionally it is induced by a more distal

partial obstruction. Computed tomography appears to be more accurate than conventional radiography in suggesting the diagnosis (187), and although CT is often performed for suspected cecal volvulus,few studies have evaluated whether it is superior or even equal to a barium enema (Fig. 5.34).

COLON AND RECTUM

In distinction to sigmoid volvulus, a successful therapeutic barium enema or colonoscopy is achieved only in a minority of these patients, and most undergo surgery.

Sigmoid

Sigmoid volvulus ranges from an acute condition, often associated with strangulation, to a chronic setting, with the patient presenting with a gradual onset or intermittent obstruction. An immediate concern is to ascertain that this is indeed idiopathic sigmoid volvulus rather than a sigmoid or rectal cancer-induced colonic obstruction. Sigmoid volvulus occasionally develops during pregnancy and after gynecologic and other abdominal surgery.

Imaging findings of sigmoid volvulus are familiar to most radiologists (Fig. 5.35). A CT whirl pattern consists of a twisted, dilated sigmoid loop and its associated vessels around the mesocolon. If the transverse colon can be identified on radiographs (with the patient supine), a dilated sigmoid colon located cephalad to the transverse colon is an accurate finding of sigmoid volvulus (188).

The preferred therapy for acute sigmoid volvulus is decompression either by endoscopy or barium enema, followed, if indicated, by elective sigmoid resection. Simple sigmoid decompression does relieve obstruction but volvulus

Figure 5.35. Sigmoid volvulus. A lateral view from a barium enema identifies a typical twist (arrow), shows barium in a dilated sigmoid and excludes a carcinoma as etiology for the obstruction.

tends to recur if no resection or fixation is performed. A surgical nonresective procedure consists of extraperitonealization of the sigmoid colon by placing it in the infraumbilical abdominal wall.

Transverse Colon

Transverse colon volvulus is rare. It is more common in women. Patients with Chilaiditi’s syndrome appear more prone to developing a transverse colon volvulus; lax colonic ligaments predispose to such torsion.

In adults, conventional radiographs are rarely diagnostic of transverse colon volvulus. Barium enema findings vary; even a coil-spring appearance mimicking an intussusception has been reported.

Other

Only rare reports describe splenic flexure volvulus. It occurs in association with systemic sclerosis and has developed in patients with a wandering spleen, generally around the splenic pedicle. It can be associated with small bowel obstruction.

Descending colon volvulus can develop in a setting of an anomalous mesocolon and a redundant bowel.

Intussusception

In an intussusception, a segment of bowel, the intussusceptum, invaginates into the lumen of an adjacent intussuscipiens. Any part of bowel can intussuscept, although a mobile intraperitoneal bowel loop and its associated mesentery are most often involved. The intussusceptum usually invaginates distally, although occasional proximal invagination does occur (for example, a jejunogastric intussusception after a Billroth II operation). Intussusceptions range from transient to fixed. As discussed below, some are reduced with pressure.

By its bulk, an intussusception should obstruct the bowel lumen, although in distinction to pediatric patients, bowel obstruction is not a prominent feature of adult intussusceptions. A more serious consequence, especially in the younger patient, is vascular occlusion of the intussusceptum, generally venous, and resultant ischemia.

Adults

Intussusceptions in most adults have an identifiable lead point and range from enteroenteric, to ileocolic, to colocolic, to rectal prolapse. The most common lead point is a cecal adenocarcinoma (Fig. 5.36); less common is cecal lymphoma or a benign polyp. Rarer lead points in adults consist of pseudomembranous colitis, Meckel’s diverticulum, a rare duplication, endometrioma (Fig. 5.37), or even calcified cecal fecaliths. An appendiceal polyp in a patient with Peutz-Jeghers syndrome acted as a lead point for intussusception (189). Although most colonic lipomas are intramural and sessile, they are prone to becoming pedunculated and act as lead points for an intussusception. Not all of these are at the ileocecal region; a number of sigmoid lipoma-associated sigmoidorectal intussusceptions have been reported.

A rectal intussusception is usually a transient phenomenon occurring during straining, is idiopathic, and is associated with constipation. Proctography shows circular infolding of the rectal wall during straining. The criteria defining when such infolding is abnormal are not well established, and minor changes probably are best considered normal variants, but solitary rectal ulcer syndrome (discussed later) is in the differential diagnosis.

Figure 5.37. Colocolic intussusception (arrows). The lead point was an endometrioma, a highly unusual source for an intussusception.

Computed tomography and MR detect most adult ileocolic intussusceptions but, aside from a lipoma, identification of a lead point is difficult. At times even endoscopic biopsy fails to provide an etiology, and the diagnosis is established only after a right hemicolectomy. Computed tomography findings of an intussus-

COLON AND RECTUM

ception consist of a target or sausage-shaped inhomogeneous soft tissue tumor. The appearance varies depending on the relative orientation of the x-ray beam and intussusception. Colocolic intussusceptions caused by a colonic lipoma can be suggested by US; CT is diagnostic if fat is detected in the lead point, although the lack of fat in the lead point due to infarction and necrosis of an intussuscepted tumor does not exclude a lipoma.

Unenhanced CT has a role if ischemia is suspected in adults with an intussusception; CT findings of a hypodense layer in the intussusceptum or surrounding fluid or gas should suggest vascular compromise (190); lumen obstruction is not always present in an ischemic or necrotic intussusception.

Overlying pneumatosis cystoides intestinalis and enteritis cystica profunda are uncommon associated finding of a colocolic intussusception.

Magnetic resonance imaging also readily identifies intussusceptions, with findings similar to those found with CT. Magnetic resonance imaging reveals concentric bowel rings.

Pediatrics

Clinical

An acute ileocolic intussusception in a young child is a common emergency. Most intussusceptions occur before the age of 2 years and are idiopathic in origin. The rare identifiable lead points, more common in older children, consist of a Meckel’s diverticulum, polyp, or even a duplication. Why the reported prevalence of intussusception is greater in some parts of the world is puzzling.

The typical clinical presentation and conventional radiographic findings are well known. Occasionally encountered, however, is an atypical presentation, for instance, bilious vomiting due to an ileocolic mass resulting in extrinsic duodenal obstruction.

One variant is an ileoileocolic intussusception. Prereduction findings are similar to those of an ileocolic intussusception. Once the intussusception was reduced to the cecum, air enemas in nine children with ileoileocolic intussusceptions identified the intussusceptum as two or more separate polypoid components, in contrast to ileocolic intussusceptums, which

tend to be either smoothly marginated or somewhat lobular in appearance (191).

Sigmoidorectal intussusceptions also occur in infants and children. In some, the typical clinical presentation of a palpable abdominal mass and colicky pain is absent. These intussusceptions can be misdiagnosed as simple rectal prolapse.

Presumably a surgical consultation has been obtained and a surgeon has examined the child prior to attempted intussusception reduction. The child should be in stable condition, and both the surgeon and the radiologist should be confident that no contraindication exists to a therapeutic enema. Contraindications for reduction include bowel perforation, peritonitis, and hypovolemic shock.

A long-term outcome study in children found an overall recurrence rate of 9%, with about two thirds of children having a single recurrence (192); reducibility was 95% for recurrent intussusceptions, with no perforations. Also, recurrence did not predict an abnormal lead point.

Imaging

Although conventional abdominal radiographs are often obtained first, their value has been questioned. Even experienced observers often differ whether in children with clinically suspected intussusception it is indeed present or absent; the best predictor of intussusception is a soft tissue mass and decreased large bowel gas (Fig. 5.38).

In some centers, US is the initial imaging modality of choice when suspecting an intussusception (Fig. 5.39). In experienced hands US has a high sensitivity and specificity in detecting an intussusception and a contrast enema is then limited to therapy. Viewed in a transverse section, prereduction US shows an intussusception as a doughnut or target lesion; it has a reniform shape (pseudokidney is the term often used) when viewed in longitudinal section. Scans close to the lead point of an intussusception reveal the intussusceptum as a hypoechoic central structure; scans away from the lead point have a hyperechoic crescent appearance due to mesentery and related vessels being drawn in by the intussusceptum. Although such a US appearance should suggest an intussusception, neither a target nor reni-

form appearance is pathognomonic. Necrotizing enterocolitis, volvulus, or even stool may mimic this appearance.

At times, because of obscure symptomatology, these patients are studied with CT. Findings of intussusception are straightforward in most. Computed tomography reveals an intraluminal tumor and a target sign–like appearance of alternating layers of high and low attenuation. With obstruction, more proximal bowel loops distend with fluid. Necrosis manifests as inflammation, loss of tissue planes, and presence of intraperitoneal fluid.

An extensive ileocolic intussusception distorts normal superior mesenteric vessel anatomy. Thus with the lead point of an intussusceptum at the sigmoid colon or distally, the superior mesenteric vein is located to the left of the superior mesenteric artery.

Published successful intussusception reduction rates range between 70% and 85%, with an occasional report of 90%, regardless of whether a liquid or air is used. A comparison of different contrast agents used is difficult unless the procedure used is standardized. A major factor influencing success rates is the intraluminal pressure achieved rather than any other techni-

cal factor. A barium enema bag at 1-m elevation produces greater intraluminal pressure than a typical water-soluble contrast agent or water at the same height. Pressure during pneumatic reduction varies considerably.

At times US identifies fluid within an intussusception, representing trapped peritoneal fluid, seen on axial images as an anechoic crescent between the intussusceptum and intussuscipiens.

Ultrasonography during reduction of an ileoileocolic intussusception reveals a complex frond-like appearance. The intussuscepted small bowel is also surrounded by cecal fluid. These intussusceptions are likewise difficult to reduce.

Contrast Agents

Historically, a barium enema was performed in the pediatric patient suspected of an intussusception. The study not only established a diagnosis but also was therapeutic.

Some radiologists still use barium for reducing intussusceptions, although teaching and pediatric hospitals have changed to a pneumatic technique.

A pneumatic reduction of intussusception is safe and successful in most children. Although fluoroscopy is useful for this procedure and is employed by many radiologists, a lead point is difficult to identify. Thus even with successful reduction, the presence of a tumor lead point is not excluded. Still, many radiologists believe that pneumatic reduction is quicker, safer, and more effective than hydrostatic reduction.

Reduction success rate varies with the duration of signs and symptoms. Thus success rate for air reduction was 89% for those symptomatic for <12 hours, 83% for those with symptoms for 12 to 24 hours, and 74% for those symptomatic for >24 hours (193). Several trials of air reduction increase the success rate. A success rate of 70% with one trial of air reduc-

tion increased to 91% after a policy of up to three trials was instituted (193).

Some radiologists perform pneumatic reduction under US control. In patients who underwent 52 US-guided pneumatic intussusception reductions, the overall success rate was 92% (194); a pressure of 60mmHg was maintained for 30 seconds, and if an intussusception failed to reduce, the procedure was repeated at a pressure of 120mm Hg. Perforation occurred in two others. The published data of pneumatic reduction under US control are difficult to place in the proper perspective because of the subjective nature of many of these studies.

In some parts of the world pneumatic reduction is performed with no imaging, and the success of reduction is evaluated purely on clinical grounds.

Color Doppler US evaluates whether blood flow is present in an intussusception. The success rate for air reduction is significantly greater in those children with blood flow in the intussusception than in those with absent flow. Lack of Doppler evidence for blood flow, however, should not be a contraindication to attempted reduction, and practical application of such Doppler study remains to be established.

A more recent technique is US-guided intussusception reduction using a saline enema or Hartmann’s solution. The sonographic criteria of intussusception reduction include an initial target sign that is later no longer identified, visualization of the ileocecal valve, and fluid refluxing into small bowel; this technique has a success rate of over 90% in reducing an intussusception.

ADVANCED IMAGING OF THE ABDOMEN

the rectosigmoid without invasion. A barium enema is diagnostic.

A pregnant patient with an ileal pouch–anal anastomosis presented at 36 weeks’ gestation with bowel obstruction (196); the obstruction cleared after delivery.

Obstruction by Gallstones

Gallstone ileus most often obstructs in the ileum (discussed in Chapter 4). With a cholecystoduodenal fistula, if a stone manages to pass through the ileocecal valve, colonic gallstone ileus occurs almost always only proximal to a stricture.

Another scenario of colonic gallstone ileus occurs if a cholecystocolic fistula develops; obstruction by the gallstone most often is in the sigmoid colon. If the obstruction is incomplete, a barium enema identifies a cholecystocolic fistula.

Complications

In some infants only partial intussusception reduction is achieved. In these infants, instead of performing immediate laparotomy, in consultation with the surgeon and if the infant is clinically stable, another attempt at intussusception reduction may be appropriate.

The risk of bacteremia during intussusception reduction is low.

What are the sequelae of a perforation during an intussusception reduction? In 14 perforations (seven using barium and seven air) all children with barium reduction required bowel resection, but only four of the seven with air required resection (195); in addition, the anesthesia time was longer and hospital stay longer in the barium group. Of interest is that perforations are through necrotic bowel only in a minority of these children; presumably increased pressure plays a major role in these perforation. In general, all other factors being equal, the perforation rate is probably similar regardless whether barium or air is used.

Extrinsic Obstruction

Occasionally a distended bladder compresses the rectosigmoid against the sacrum and obstructs on either an acute or chronic basis. Similarly, some gynecologic tumors compress

Obstruction Due to

Motility Abnormalities

Pseudo-Obstruction (Ogilvie’s Syndrome)

Etiology

A number of etiologies have been proposed for acute colonic pseudo-obstruction (Ogilvie’s syndrome), including an imbalance between sympathetic inhibitory and parasympathetic excitatory colonic innervation. It occurs most often after surgery or trauma (Table 5.6).

Table 5.6. Conditions associated with Ogilvie’s syndrome

Common

Recent surgery

Recent trauma

Severe medical condition

Uncommon Postcesarean section Leukemia

von Recklinghausen’s disease

Multiple endocrine neoplasia (MEN) syndrome type 2 Botulism in infants

Herpes zoster infection Hypothyroidism Myotonic dystrophy Drug therapy

Imipramine Tocolytic therapy

COLON AND RECTUM

Some chronically hospitalized or bedridden patients have a chronic megacolon or megarectum, generally of idiopathic etiology. Manometry in these patients reveals abnormal colonic tonicity. Histology of resected specimens from patients with idiopathic megarectum and megacolon reveals hypertrophy of muscularis mucosae and muscularis propria; those with an idiopathic megarectum tend to have decreased innervation density of the longitudinal muscle.

Recurrent acute colonic pseudo-obstruction in a young patient with no evident risk factors was eventually ascribed to toxoplasmosis infection (197); adrenergic bowel denervation was believed to be caused by toxicity or crossreaction between a toxoplasma antigen and the patient’s immune system.

Imaging

The conventional radiographic appearance of Ogilvie’s syndrome mimics distal colonic obstruction. Thus if the diagnosis is in doubt, a limited barium enema is indicated and should differentiate between these conditions.

Instead of a barium enema, two additional conventional radiographs often suffice: a right lateral decubitus view of the abdomen followed by a prone lateral view of the pelvis. With these two additional views gaseous distention of the rectum can be achieved in most patients with pseudo-obstruction, while in patients with mechanical obstruction such distention is not found.

Although the cecum is generally most dilated in Ogilvie’s syndrome, occasionally some other colonic segment is involved. The rectum tends to be collapsed.

Therapy

Ogilvie’s syndrome has been treated successfully with a parasympathomimetic drug such as neostigmine. The success of such therapy suggests that Ogilvie’s syndrome is a result of excessive parasympathetic suppression rather than sympathetic overactivity. Colonoscopic decompression has a high success rate, although some patients required multiple decompressions. A decompression tube positioned in either the right colon or transverse colon appears to be equally successful.

Untreated, Ogilvie’s syndrome has progressed to perforation and an acute abdomen.

Chagasic

Chagas’ disease is a chronic infection caused by the parasite Trypanosoma cruzi, which is endemic in rural regions of Latin America. A chronic phase develops several decades after initial infection, most often manifesting through cardiac abnormalities. Colonic abnormalities consist of decreased motility and tonicity, identified as a megacolon. These patients develop small bowel bacterial overgrowth with resultant complications.

Systemic Sclerosis (Scleroderma)

Colorectal dysfunction is common in patients with systemic sclerosis. Hypotonia and stasis develop in some. Constipation is common, but, paradoxically, incontinence is also not uncommon.

T1and T2-weighted SE MRI magnetization transfer contrast-weighted and dynamic gadolinium-enhanced images in 11 of 14 patients with scleroderma and fecal incontinence revealed forward deviation of an atrophied internal sphincter that had decreased contrast enhancement (198); for comparison, patients with incontinence alone showed no internal sphincter deviation or decreased vascularity but did have significant reduction in external sphincter mass.

Diverticula

Colonic

Colonic diverticula represent outpouchings in the bowel wall. In the past, a distinction was made between true and false diverticula (pseudodiverticula), but common indiscriminate usage has made any such distinction moot.

Prevalence of colonic diverticula varies considerably throughout the world. Their prevalence is increasing in some populations. Right-sided colonic diverticula are more common in Asia than in the West.

Occasionally encountered are calcified stones within a diverticulum. Presumably these stones form as a result of stasis. Superficially such