Distal Small Bowel Obstruction
Ileal atresia is an important cause for low intestinal obstruction. It represents approximately 50% of small bowel atresias and the etiology is similar to that of jejunal atresia. As jejunal atresias, they are believed to result from an intrauterine vascular injury. Approximately 25% have a history of polyhydramnios (Sweeney et al. 2001).
Plain film shows numerous dilated loops of bowel occupying the entire abdominal cavity, including the pelvic portion, and multiple air-fluid levels in upright film (Fig. 1.19). With this degree of distension the mucosal pattern of the small bowel is effaced and it is impossible to differentiate the small bowel from the colon (Winters et al. 1992). Examination of the colon is then warranted to disclose the presence or absence of a colonic lesion. In ileal atresia, the colon is normally placed but has an abnormally small caliber, the so-called functional microcolon typical of distal small bowel obstruction (Dalla Vecchia et al. 1998) (Fig. 1.19d,e). The presence of pneumoperitoneum indicates that perforation has occurred and a colon examination is contraindi-cated. Intraperitoneal calcifications, indicative of meconium peritonitis, are not uncommon in ileal atresia.
Meconium ileus is a low intestinal obstruction produced by impaction of abnormal meconium in the distal ileum. It almost always occurs in patients with cystic fibrosis, in which a deficiency of pancreatic enzymes causes the meconium to become abnormally viscous and thick. The effects on the gastrointestinal tract range from temporary meconium retention with delayed but spontaneous evacuation of the meconium, to complete obstruction. When obstruction occurs, the distal ileum is narrowed and contains concretions of grey inspissated meconium pellets (Garza-Cox et al. 2004). Proximal to this the ileum is grossly distended by thick, tenacious, chewing-gum-like meconium due to the viscid, abnormal mucus. Meconium ileus is the earliest manifestation of cystic fibrosis and occurs in 10%-15% of patients. A family history is often elicited. Symptoms usually commence on the first day of life and consist of bile-stained vomiting and abdominal distension. The abdomen may have a doughy feel and it is sometimes possible to indent the bowel contents on pressure. The diagnosis may be confirmed by finding an increased concentration of sodium chloride in the sweat (Burge and Drewett 2004).
Meconium ileus can be complicated or uncomplicated. The plain radiograph in uncomplicated meco-nium ileus usually demonstrates low obstruction. The abdomen is filled with air-distended loops, and occasionally there is a relative absence of air-fluid levels (Fig. 1.20). In general, patients with meconium ileus have fewer air-fluid levels than patients with small bowel atresias. The sticky, viscid meconium prevents rapid movement of air shadows during changes of posture so that fluid levels do not easily develop. Sometimes, a mixture of air and meconium may be visualized as coarse, granular, ground-glass shadows, giving rise to a "soap bubble" appearance similar to the fecal pattern of the colon in older patients. However, a similar faucial pattern may also be seen in ileal atresia and aganglionosis of the terminal ileum, so these findings are not of much use in the differential diagnosis of low obstruction (Leonidas 1976). On ultrasound, abnormal intestinal contents are hyperechogenic and thick, which is clearly different from that of the ileal atresia, where the bowel is filled with hypoechoic fluid and air (Barki and Bar-Ziv 1985; Sigalas et al. 2003) (Fig. 1.21).
Contrast enema classically demonstrates a microcolon, and multiple small filling defects consisting of meconium pellets may be seen within the microcolon and in the collapsed distal ileum, with dilated small bowel proximal to the obstruction (Fig. 1.22). These findings are diagnostic of meconium ileus. The type of contrast to be used in this examination should be considered carefully, especially given the fact that the etiology of the distal obstruction is not known prior to this examination. Barium is not ideal in patients with meconium ileus and impacted meconium pellets. It is preferable to begin the enema examination with a water-soluble contrast medium. Low osmolar, non-ionic water-soluble agents are the best choice to avoid large fluid shifts into the bowel. These agents are expensive, so a relatively dilute isosmolar ionic water-soluble contrast medium may also be acceptable. Once the diagnosis is made, these patients should undergo a therapeutic enema to help the passage of the sticky meconium and so relieve the obstruction avoiding surgery. The choice of contrast media is controversial, but a hyperosmolar agent such as Gastrografin continues to be used by many radiologists (Burke et al. 2002). Gastrografin is meglumine diatrizoate; a water-soluble, radiopaque solution containing 0.1% polysorbate 80 (Tween 80) and 37% organically bound iodine. The solution's osmolarity is 1900 Osm/L. This very hyperosmolar agent decreases the tenacity of the a c
Fig. 1.19a-e. Ileal atresia. a-c Supine, lateral, and upright abdominal radiographs show multiple dilated air-filled bowel loops occu pying the entire abdominal cavity, with air-fluid levels in the upright radiograph. Note in (b) the absence of air in the rectum. With this gree of distension it is impossible to differentiate the small bowel from the colon. d,e Contrast enema outlines the minute size of the colon corresponding to an unused colon
Fig. 1.19a-e. Ileal atresia. a-c Supine, lateral, and upright abdominal radiographs show multiple dilated air-filled bowel loops occu pying the entire abdominal cavity, with air-fluid levels in the upright radiograph. Note in (b) the absence of air in the rectum. With this gree of distension it is impossible to differentiate the small bowel from the colon. d,e Contrast enema outlines the minute size of the colon corresponding to an unused colon b a d c
meconium by drawing water into the bowel lumen. Before proceeding with this therapy, the infant must show signs of uncomplicated meconium ileus and no clinical or radiologic evidence of complicating factors (e.g., volvulus, gangrene, perforation, peritonitis). Under fluoroscopic control, a 25%-50% solution of Gastrografin should be infused slowly at low hydrostatic pressure through a catheter inserted into the rectum. Balloon inflation must be avoided to minimize the risk of rectal perforation.
An expert group convened by the Cystic Fibrosis Foundation Consensus Conference found no scientific evidence that Gastrografin was superior to other low osmolar water-soluble contrast media in the
Fig. 1.22a,b. Meconium ileus. a Water-soluble contrast enema showing a microcolon with scattered filling defects that correspond to inspissated meconium. b The enema was continued with reflux of the contrast medium into the terminal ileum, showing filling defects that represent meconium pellets. The patient was discharged within 48 h of successful treatment
Fig. 1.22a,b. Meconium ileus. a Water-soluble contrast enema showing a microcolon with scattered filling defects that correspond to inspissated meconium. b The enema was continued with reflux of the contrast medium into the terminal ileum, showing filling defects that represent meconium pellets. The patient was discharged within 48 h of successful treatment treatment of meconium ileus. As Gastrografin may cause hypotension or circulatory collapse in newborn infants, many radiologists currently favor the use of low osmolar agents in these infants. In any case, the flow of the contrast through the thick meconium is slow, and usually several enemas are required. Efforts must be taken to reflux contrast into the ileum and if possible back into the dilated proximal bowel (Fig. 1.23). Care should be taken that the patient is well hydrated because of fluid shifts. The patient's electrolyte balance must be attended to before, during, and after each enema (Kao and Franken 1995).
Meconium ileus may be complicated by volvulus of a distal intestinal loop, perforation, or atresia. All patients with complicated meconium ileus require surgical intervention (Fig. 1.23), therefore radiographs need to be read carefully for the presence of calcification or pneumoperitoneum which indicates in utero perforation and the need for surgical intervention without further imaging (Rescorla and Grosfeld 1993).
Meconium peritonitis is a condition that is the result of in utero perforation of the fetal gastrointestinal tract during the last 6 months of pregnancy. Sterile meconium escapes through the perforation into the peritoneal cavity producing a marked reaction with dense adhesions, which usually calcify rap idly, in some cases as early as 24 h after it leaves the intestinal lumen. Yellow-green nodules form on the bowel wall and the perforation may be sealed off so well that at birth there may not be any macroscopic evidence of the leak. However, if the perforation is still present after birth and meconium still escapes into the peritoneal cavity, secondary septic peritonitis will develop. Very often a pseudocyst is formed by adjacent loops of the intestine, which tend to wall off the perforation. The wall of this pseudocyst is lined by a thick plaque of greenish-yellow material with areas of calcification (Reynolds et al. 2000) (Fig. 1.24). The intrauterine perforation may be due to any obstructing lesion such as atresia, meconium ileus, volvulus, Meckel's diverticulum, internal hernia, or bands. In such cases the causative lesion will be found at laparotomy. Frequently, however, no obvious cause for the perforation can be found. Such idiopathic perforations may be produced by localized areas of intestinal infarction due to minute emboli from the placenta (Eckoldt et al. 2003).
The diagnosis is usually obvious on the plain radiographs of the abdomen and is characterized by linear or punctate calcifications over the serosal surfaces of the abdominal viscera. The calcification may consist of a few irregular scattered areas or may be more extensive, consisting of continuous
Fig. 1.24a,b. Meconium peritonitis. a Abdominal radiograph demonstrates a small amount of gas in a markedly distended abdomen. A calcified mass within the peritoneal cavity is observed (arrows). There is no pneumoperitoneum. b Sonography of the same patient shows a pseudocyst containing debris with peripheral calcification
Fig. 1.24a,b. Meconium peritonitis. a Abdominal radiograph demonstrates a small amount of gas in a markedly distended abdomen. A calcified mass within the peritoneal cavity is observed (arrows). There is no pneumoperitoneum. b Sonography of the same patient shows a pseudocyst containing debris with peripheral calcification b a linear depositions of calcium localized underlying the anterior or posterior abdominal walls, in the flanks, or beneath the diaphragm. Calcifications may also be observed in the scrotal sac of males owing to communication through a patent processus vaginalis (Salman et al. 1999) (Fig. 1.25). Pseudocysts also may be seen as a solid mass on plain radiographs. In addition, distended loops of the bowel with air-fluid levels may be present due to the underlying intestinal obstruction. Where the perforation is still patent, free air will be seen in the peritoneal cavity (Fig. 1.26) or trapped in a walled-off loculus or pseudocyst. Decubitus radiographs to determine the presence or absence of free air with a persistent perforation are essential. The typical appearances of pneumoperitoneum, how-
Fig. 1.25a-c. Meconium peritonitis with calcified meconium in the scrotum. a Plain radiograph at birth showing scattered areas of calcification in the scrotum (arrowhead). Small flakes of intraabdominal calcifications are also observed. b Sonography of the same patient shows an in-trascrotal pseudocyst containing fluid and echogenic debris and calcium. c Clinical picture of the same patient showing the enlarged scrotum b
Fig. 1.25a-c. Meconium peritonitis with calcified meconium in the scrotum. a Plain radiograph at birth showing scattered areas of calcification in the scrotum (arrowhead). Small flakes of intraabdominal calcifications are also observed. b Sonography of the same patient shows an in-trascrotal pseudocyst containing fluid and echogenic debris and calcium. c Clinical picture of the same patient showing the enlarged scrotum a c b ever, are not usually seen because of the presence of adhesions, which obliterate most of the peritoneal cavity.
Ultrasound may be useful in these patients, especially in the presence of a relatively airless abdomen. Meconium peritonitis predominantly presents two ultrasound appearances: general or cystic abnormality. With the generalized condition, highly echo-genic material spreads throughout the abdomen and around the bowel loops (Fig. 1.27), producing a
Fig. 1.26. Meconium peritonitis with intrauterine bowel perforation. Free air within the peritoneal cavity is observed (arrows) in this neonate with ileal atresia. No bowel distension is observed. A patent bowel perforation was found at surgery
characteristic "snowstorm appearance" (LawrencE and Chrispin 1984). The cystic form is characterized by localized, cystic collections of meconium (meconium pseudocysts) ranging from a few centimeters in size to huge cysts occupying most of the abdominal cavity. The cyst is usually well defined and predominantly echogenic, but may also be less well defined and markedly heterogeneous. The walls are echogenic and may be thick or thin (Bowen et al.
1984; Foster et al. 1987) (Figs. 1.24 and 1.28). Calcified foci may be demonstrated with posterior shadowing in many cases.
Meconium peritonitis may also be diagnosed by prenatal ultrasound. It can present as fetal meconium ascites, giant pseudocysts, small pseudocysts, and calcifications. Associated polyhydramnios is a common finding (Eckoldt et al. 2003; Kamata et al. 2000).
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