Imaging Evaluation of Common Pediatric Emergencies

Neonatal Intestinal Obstruction

Intestinal obstruction is one of the most common neonatal abdominal emergencies. The obstruction is classified as either a high or low obstruction depending on whether it occurs above or below the level of the mid ileum. Infants typically present with abdominal distention, vomiting, and failure to pass meconium within 24 to 48 hours. The distinction between a high and a low obstruction is often made on the basis of plain abdominal radiographs. If the radiograph demonstrates one or only a few dilated loops of bowel, the obstruction is likely a high obstruction. If the radiograph reveals multiple dilated loops of bowel, the obstruction is a low obstruction ( Fig. 6-1 ). In an infant with intestinal obstruction, the correct interpretation of the plain film is critical in directing the most appropriate next course of action.

High Obstruction

A high intestinal obstruction may be the result of duodenal atresia or stenosis, jejunal atresia or stenosis, or malrotation with volvulus. Esophageal atresia will be discussed separately. An upper gastrointestinal (UGI) examination is often requested to identify the level of obstruction. The UGI examination also assists the surgeon in determining the urgency of surgery. In children with duodenal atresia or stenosis, the obstruction almost always occurs at the level of the ampulla of Vater. Abdominal radiographs in children with duodenal atresia often reveal the classic “double bubble” sign in an abdomen in which gas is otherwise absent ( Fig. 6-2 ). The dilated gas-filled “bubbles” represent the stomach and duodenal bulb. This sign is virtually diagnostic of duodenal atresia in the correct clinical setting. More than 50% of babies with duodenal atresia will have additional associated anomalies, and approximately 30% will have trisomy 21. In cases of incomplete duodenal obstruction, radiographs reveal gas in the distal bowel loops. In this case a UGI examination would show a focal area of narrowing within the duodenum through which a tiny amount of contrast material may pass. In some cases the stenosis may be the result of a duodenal web, which appears as a curvilinear filling defect extending across the duodenal lumen ( Fig. 6-3 ). Annular pancreas also can coexist with duodenal stenosis and may result in luminal narrowing of the proximal duodenum. Jejunal atresia and stenosis are less common but should be included in the differential diagnosis of congenital high intestinal obstruction. Malrotation with volvulus is a surgical emergency and will be discussed later in this chapter.

Low Obstruction

Low intestinal obstruction may be a result of meconium ileus, ileal atresia, functional immaturity of the colon, colonic atresia, and Hirschsprung disease. A contrast enema is the study of choice to confirm the diagnosis. Meconium ileus almost always occurs in infants with cystic fibrosis. When a contrast enema is performed, a microcolon is present ( Fig. 6-4 ). A microcolon is a colon of universally small caliber (1 cm or less in diameter). Its presence implies that the colon has never been used. Bowel atresia proximal to the distal ileum does not lead to a microcolon because the succus entericus produced by distal small bowel allows the colon to achieve a normal caliber. In patients with meconium ileus, refluxed contrast material in the distal ileum reveals multiple filling defects within the distal small bowel. These filling defects represent inspissated meconium and help distinguish meconium ileus from ileal atresia.

In functional immaturity of the colon, previously known as “small left colon syndrome” or “meconium plug syndrome,” the descending colon and sigmoid colon are small in caliber compared with the normal ascending and transverse colon. The incidence of functional immaturity of the colon is increased in infants of mothers with diabetes and of mothers who received magnesium sulfate. Functional immaturity of the colon will resolve on its own without intervention.

Colonic atresia results in complete obstruction of a segment of colon, likely as a result of an ischemic event in utero. As such, a contrast enema will reveal a microcolon distal to the atresia, the inability to pass contrast material into the proximal colon, and multiple dilated bowel loops proximal to the obstruction.

In infants with Hirschsprung disease, a transition occurs from normal bowel that is of normal caliber to aganglionic bowel that is small in caliber ( Fig. 6-5 ). Classically, this transition occurs at the level of the rectosigmoid. In healthy infants the rectal diameter should be greater than the sigmoid diameter, although in infants with Hirschsprung disease, this relationship is reversed. Boys are affected four times as often as girls. Allergic colitis related to milk allergy occasionally can mimic Hirschsprung disease. Ultimately a biopsy is required to confirm the diagnosis. Total colonic aganglionosis is a rare form of Hirschsprung disease that occurs in approximately 2% to 13% of cases. Total colonic aganglionosis is often familial, affects boys and girls equally, and can be difficult to diagnose radiographically because of the diffuse nature of the disease.

Esophageal Atresia

Congenital atresia of the esophagus is a rare abnormality that may occur as an isolated abnormality or as part of a larger spectrum of abnormalities—for instance, in children with vertebral, anal, cardiac, tracheal, esophageal, renal, and limb (VACTERL) association. In most cases an associated fistula to the trachea is present. The diagnosis is almost always suspected clinically when an infant has difficulty feeding or when difficulty is encountered in passing a nasogastric tube into the stomach. Radiographs help confirm the diagnosis when no gas is identified within any bowel loops in the abdomen. If a tracheal-esophageal fistula is present, a normal bowel gas pattern may occur in the abdomen. If placement of a nasogastric tube has been attempted, the tube will be identified above the diaphragm on the radiograph, often in a dilated, gas-filled, esophageal pouch ( Fig. 6-6 ). An esophagram with use of contrast material or a UGI examination is not necessary to make the diagnosis of esophageal atresia.

Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is a gastrointestinal condition that occurs in 2% to 3% of neonates, most commonly in premature or very low birth weight infants. In term infants, NEC is associated with congenital heart disease, such as aortic coarctation, which results in intestinal hypoxia. Mortality rates in infants with NEC range from 15% to 40%. The pathogenesis of the disease is thought to be related to relative immaturity of intestinal motility and barrier function, as well as abnormal bacterial colonization. Neonates with NEC often present within the first 2 weeks of life with feeding intolerance, vomiting, abdominal distention, blood in the stool, and/or generalized lethargy. Abdominal radiographs are the modality of choice for the initial evaluation of neonates with suspected NEC and to monitor its progression. An early sign of NEC is a mildly dilated and fixed loop (or loops) of bowel on serial abdominal radiographs. Although this finding is sensitive, it is not specific. Pneumatosis intestinalis, or air within the wall of the bowel, is considered a hallmark of NEC ( Fig. 6-7 ). Pneumatosis represents by-products of metabolism of bacteria within the intestinal wall. Occasionally the bubbly lucencies of pneumatosis can be mistaken for stool on an abdominal radiograph. It is important to know if the baby is being fed, because stool is not expected in babies who are receiving total parenteral nutrition. Portal venous gas appears on abdominal radiographs as lucent, branching, linear structures within the liver ( Fig. 6-8 ). The presence of pneumoperitoneum indicates perforation of the bowel. When pneumoperitoneum is present in a supine infant, the falciform ligament is visualized on a supine radiograph as a result of the presence of air around it, also known as the “football sign” ( Fig. 6-9 ). Both sides of the bowel wall are also visible, which is called the Rigler sign ( Fig. 6-10 ). The abdomen may appear abnormally lucent when large amounts of gas collect anteriorly within the abdomen on a supine film ( Fig. 6-11 ). A cross-table lateral or lateral decubitus view of the abdomen is helpful in confirming the presence of free air ( Fig. 6-12 ).

Most cases of NEC are treated medically, usually with bowel rest, gastric suction, antibiotics, and hydration. Surgery is indicated if evidence is found of bowel perforation. Plain radiographs are critical in evaluating the progression of the disease. A changing bowel gas pattern, as opposed to dilated bowel loops that remain fixed on serial abdominal radiographs, is an important radiographic sign, because this changing pattern often coincides with clinical improvement. Sequelae of NEC include short gut syndrome and intestinal strictures, which typically present 3 to 8 weeks after the acute illness.

Hypertrophic Pyloric Stenosis

Hypertrophic pyloric stenosis (HPS) is a condition that affects young infants, usually between 3 to 12 weeks of age, with 4 weeks the peak age at diagnosis. Although these infants are normal at birth, they display the gradual onset of nonbilious emesis during the first several weeks of life, commonly described as projectile in quality. In severe cases, dehydration and malnourishment ensue, which may be accompanied by marked electrolyte imbalances. The disease is caused by overgrowth of the muscularis layer of the pylorus. The mucosal layer also becomes hypertrophied and redundant, which causes obstruction of the lumen. The precise cause of HPS is unclear, although there is a familial predisposition. Males are affected four times as frequently as females.

US is the imaging study of choice for the diagnosis of HPS, and use of a high-resolution linear array transducer is recommended. When HPS is present, the pyloric channel is nearly always visualized and appears both thickened and elongated ( Fig. 6-13 ). The hypertrophied pyloric channel fails to relax despite vigorous peristaltic contractions in the stomach. The hyperechoic mucosa is redundant and crowded and often protrudes into the antrum, a finding termed the “nipple sign.” The length of the hypertrophic pyloric channel is variable and can range between 14 to 20+ mm. Measurements of muscle thickness are more reliable for the diagnosis of HPS. A muscle thickness greater than 3 mm is consistent with pyloric stenosis. Occasionally transient pylorospasm can mimic hypertrophic pyloric stenosis, and therefore it is important to assess for the passage of fluid from the gastric antrum into the duodenal bulb. In healthy patients the normal pyloric ring bridges these two structures ( Fig. 6-14 ). Careful observation of this region over the course of the examination reliably allows differentiation between a normal and an abnormal study. If the US findings are negative or borderline but the patient’s symptoms progress, a follow-up US examination may be necessary to evaluate for interval development of pyloric stenosis. Abdominal radiographs are not necessary to make the diagnosis of HPS but typically demonstrate a dilated, gas-filled stomach (unless the stomach has been decompressed with a nasogastric tube). Peristaltic contractions within the stomach give the appearance of a “caterpillar stomach” ( Fig. 6-15 ).

In the past, UGI examination was performed to confirm the diagnosis of HPS. Although the thickness of the pylorus muscle is not discernable at UGI examination, other imaging signs suggest the diagnosis. The pyloric channel appears elongated at UGI examination as wispy, linear tracts of contrast material pass through the canal, a finding called the “string sign” ( Fig. 6-16 ). The “shoulder sign” is the name given to the appearance of the thickened impression of the pylorus upon the dilated stomach antrum. Vigorous peristaltic contractions in the stomach occur without associated relaxation of the pyloric channel. Imaging over time allows reliable diagnosis of pyloric stenosis because contrast material is never able to pass through the channel in more than tiny wisps at a time.

The current treatment for HPS is a pyloromyotomy, which can be performed laparoscopically. In this procedure the hypertrophied muscularis is divided and the mucosa is allowed to bulge through the incision (i.e., suturing is not performed over the muscle). This procedure has been extremely effective at reducing the mortality rate in infants with HPS, which is now well below 1%.

Malrotation

“Malrotation” is a term used to describe an arrest in the development of the midgut that may occur at various stages of embryologic growth. This developmental arrest leads to a spectrum of abnormal rotation patterns of the intestinal tract. In patients with malrotation, peritoneal or “Ladd” bands form as a result of disorganized embryologic attempts to fixate the malrotated bowel. These bands may lead to bowel obstruction by crossing over a loop of bowel and compromising the integrity of its lumen. The malfixated intestines have a propensity to twist around the mesentery, leading to a midgut volvulus. The compromised blood supply to the bowel can lead to bowel infarction, sepsis, and eventually death. In a previously healthy infant presenting with bilious emesis, midgut volvulus is a primary clinical concern. Although symptoms related to malrotation theoretically may occur at any age, they are more likely to occur in the first month of life. In most cases malrotation is an isolated defect with no predisposing genetic susceptibility or associated syndromes. Some conditions, however, are nearly universally associated with malrotation, including gastroschisis, congenital diaphragmatic hernia, and omphalocele. The heterotaxy syndromes also are associated with malrotation.

Abdominal radiographs and gastrointestinal contrast studies (UGI) are paramount in making the diagnosis of malrotation and must be performed on an emergent basis. The bowel gas pattern is often normal in the presence of malrotation. Normal findings of an abdominal radiograph should not preclude further investigation in a child with clinical concern for malrotation. A UGI examination evaluates the course of the duodenum and the position of the duodenojejunal junction (DJJ). In healthy patients the duodenum has a characteristic C-sweep that consists of four portions: the duodenal bulb and the descending, transverse, and ascending portions ( Fig. 6-17 ). The descending through ascending portions of the duodenum are fixed in the retroperitoneum. The DJJ is normally located to the left of midline at the level of the duodenal bulb. When the DJJ is inferior or medial to this position, it is considered abnormal ( Fig. 6-18 ). A few rare exceptions to this rule exist, including previous surgery in the abdomen, abdominal masses that displace bowel, and marked gastric or colonic distention. A “corkscrew” configuration of the duodenum implies that a midgut volvulus is present ( Fig. 6-19 ). Midgut volvulus causes a closed loop bowel obstruction. The bowel may appear tapered or “beaked” at the level of obstruction, with proximally dilated bowel loops. Midgut volvulus is a surgical emergency. Despite all of these findings, the diagnosis of malrotation (without midgut volvulus) remains difficult to make with certainty in up to 30% of cases because of the subtlety of the findings and the overlap with normal anatomy. If the course of the duodenum is not straightforward, serial abdominal radiographs can be performed to follow the contrast material distally to the colon. When malrotation is present, the small bowel is commonly located on the right side of the abdomen, and the cecum also may be abnormally positioned.

The preferred treatment for infants or children with malrotation is a Ladd procedure. This surgery, often performed laparoscopically, entails reduction of a midgut volvulus, division of obstructing peritoneal bands, placement of the bowel in a position of nonrotation, and appendectomy. The urgency of surgery depends on whether a midgut volvulus is present at the time of diagnosis. Because all patients with malrotation are at risk of volvulus, even patients without a volvulus proceed to surgery semiurgently. Recurrence of volvulus after a Ladd procedure has been performed is rare but can occur.

Intussusception

Intussusception is one of the most common causes of bowel obstruction in young children. Intussusception occurs when one segment of the bowel (the intussusceptum) telescopes into the bowel immediately distal to it (intussuscipiens). This phenomenon occurs most often in children between 6 months and 3 years of age. Common presenting symptoms are vomiting, lethargy, and bouts of irritability in which the child draws his or her legs to the chest. The more classic symptoms of a palpable abdominal mass and “red currant jelly stool” are present in only a minority of cases and should not be relied upon to suggest the diagnosis. Bloody stool is a manifestation of sloughed intestinal mucosa, which is a late finding in the disease and is present in fewer than half of all cases. Delayed diagnosis may lead to bowel ischemia and perforation. Most intussusceptions in children are idiopathic, and approximately 5% of cases are the result of the presence of a pathologic lead point (PLP). PLPs include Meckel diverticulum, colonic polyps, lymphoma, duplication cyst, or underlying diseases of the bowel such as Henoch-Schönlein purpura. PLPs should be considered in children younger than 1 month or older than 5 years or who have had multiple recurrent intussusceptions. The most common site for intussusception to occur is at the hepatic flexure, of which ileocolic intussusceptions are the most common type. Small bowel intussusceptions also may occur. Small bowel intussusceptions are usually transient phenomena that may not cause symptoms. They are likely to reduce spontaneously, and an air reduction enema is not therapeutic.

Imaging evaluation of the child with suspected intussusception begins with abdominal radiographs. Supine and left lateral decubitus views are recommended. The decubitus film, by allowing the cecum and ascending colon to assume a nondependent position in the abdomen, allows air to fill the cecum when an ileocolic intussusception is not present or if the cecum is not filled with stool. The sigmoid colon is often located within the right lower quadrant in young children, and one should be careful when excluding intussusception based on gas-filled bowel loops in this location. The “meniscus sign” and “target sign” are terms used to describe the radiographic appearance of a soft tissue mass in the colon ( Fig. 6-20 ). Small bowel dilatation and air-fluid levels may be present, signifying the presence of a small bowel obstruction ( Fig. 6-21 ).

For confirmation of intussusception, US is the preferred modality. When the sonographer is familiar with its imaging appearance, intussusception is identified in all cases. Intussusception has a characteristic appearance in both transverse and sagittal planes. In transverse planes, an intussusception appears as a mass, usually between 3 to 5 cm in diameter, that has a “target” or “doughnut” configuration of alternating hypo- and hyperechoic layers ( Fig. 6-22, A ). In the sagittal plane the mass assumes a “pseudokidney” configuration. Color Doppler imaging is used to evaluate the vascularity of the bowel and is a promising predictor of bowel viability ( Fig. 6-22, B ). Enlarged mesenteric lymph nodes may be present at the site of intussusception, which may function as a lead point. Imaging pitfalls in the diagnosis of intussusception include stool in the cecum, thickened loops of small bowel in the right lower quadrant, and the normal psoas muscle. Each of these structures may be mistaken for an intussusception. It is also important to appreciate the difference between small bowel intussusceptions and ileocolic intussusception. Small bowel intussusceptions are usually treated conservatively, whereas ileocolic intussusceptions are treated with enema reduction or surgery. The diagnosis of small bowel intussusception is suggested by the location of the intussusception (outside of the right lower quadrant), small diameter (<2 cm), and short-segment involvement (<5 cm).

Fluoroscopically guided air reduction enema is a widely accepted and utilized method of treatment for ileocolic intussusception. Contraindications to performing an enema include shock, peritonitis, or radiographic evidence of perforation. Although contrast enema reductions can be performed, air is generally preferred. After inserting a rectal catheter into the buttocks with the patient in the prone position, air is insufflated into the colon, ensuring a tight seal at the level of the buttocks to prevent an air leak. The intussusception is visualized fluoroscopically as it is reduced back through the colon ( Fig. 6-23 ). A successful reduction involves the reduction of the intussusception beyond the ileocecal valve (often with a visible “pop”), with reflux of air into the small bowel. Air reduction enema procedures are successful in more than 80% of cases. When the enema is not successful, surgery is curative. A rare but serious complication of the air reduction enema is bowel perforation leading to tension pneumoperitoneum, which necessitates emergent needle decompression. Perforation occurs in approximately 1.5% of cases.

Acute Appendicitis

Acute appendicitis is the most frequent condition in children that requires emergent abdominal surgery. Acute appendicitis is caused by obstruction of the appendiceal lumen, which leads to the accumulation of fluid and secondary inflammation and infection. Luminal obstruction can be the result of the presence of a fecalith, lymphoid hyperplasia, foreign bodies (FBs), or mass lesions (e.g., lymphoma). Perforation is common in pediatric patients. The frequency of perforation increases as the age of the patient decreases. Children who are suspected of having acute appendicitis warrant imaging evaluation that is both expeditious and accurate to avoid complications of perforation, such as abscess formation, bowel obstruction, and sepsis.

Graded-compression sonography is the imaging modality of choice for the initial evaluation of children suspected of having acute appendicitis. Advantages of US include the lack of ionizing radiation, lack of the need for patient preparation, and lack of sedation. The patient often is able to precisely localize the site of pain to direct the sonographer to the appropriate area. On US, the inflamed appendix will appear as a blind-ending tubular structure with bowel signature ( Fig. 6-24 ). When pressure is applied, the inflamed appendix is noncompressible. On transverse images the appendix will have a “target sign” configuration that is made up of the alternating layers of the appendiceal wall. Although no exact measurement defines an acutely inflamed appendix, in general practice an appendix with a diameter greater than 6 mm with compression is considered abnormal. Secondary signs of appendicitis include hyperemia, increased echogenicity of the periappendiceal fat, suggesting inflammation, free fluid in the abdomen or pelvis, and fluid collections adjacent to the appendix that may represent abscesses and suggest perforation. Disadvantages of US are that it is highly operator dependent, the accuracy of the study is largely affected by the body habitus of the patient, and the appendix can be difficult to locate, particularly if it is in a retrocecal position.

CT examination is highly sensitive for the diagnosis of acute appendicitis and can be used if US is nondiagnostic and there is strong clinical concern for appendicitis. Complications from a ruptured appendix, such as the development of abscesses, will also be visible on CT. At our institution, CT for appendicitis is performed with intravenous contrast material only, and the field of view is restricted to below L3 to perform the examination without delay while minimizing radiation exposure.

CT findings of acute appendicitis include an enlarged (greater than 6 mm in diameter), fluid-filled appendix with or without an appendicolith, with periappendiceal inflammatory changes ( Fig. 6-25 ). Nonspecific associated findings include enlarged mesenteric lymph nodes, free fluid, and inflammatory changes in the adjacent bowel. The presence of free air and abscess formation are important signs that the appendix is likely perforated.

Although appendicitis is common, its imaging features are not always straightforward. Given the variability in the location and position of the appendix, the clinical symptoms can be misleading. If the appendix is located within the pelvis in a young woman, appendicitis may simulate an ovarian process such as torsion of the ovary or a hemorrhagic ovarian cyst. Likewise, a subhepatic appendix may present with right upper quadrant pain simulating gallstones or flank pain, representing a concern for pyelonephritis. Imaging the uterus and ovaries in female patients at the time of their appendix US is important to exclude ovarian causes of acute pain. This imaging is best achieved by imaging through a full bladder to provide a window to the uterus and ovaries, because transvaginal US in not an option in most pediatric patients who are not sexually active.

Meckel Diverticulum

Meckel diverticulum, the most common congenital anomaly of the gastrointestinal tract, is found in 2% to 3% of the general population. A Meckel diverticulum is a true diverticulum along the antimesenteric border of the distal ileum that results from incomplete atrophy of the omphalomesenteric duct in fetal development. Its lining consists of normal small bowel mucosa, although in some cases ectopic gastric or pancreatic mucosa may be present. It is most often located in the right lower quadrant. No known association with other congenital malformations exists. Asymptomatic Meckel diverticulum occurs with nearly equal frequency in boys as in girls, although symptomatic Meckel is more common is males.

Approximately 40% of children with vitelline duct anomalies such as Meckel diverticulum are symptomatic. The most common symptoms are abdominal pain, nausea, vomiting, and rectal bleeding. Hemorrhage and obstruction are the most common complications. In patients presenting with hemorrhage related to a Meckel diverticulum, ectopic gastric mucosa is almost always present. The bleeding is caused by peptic ulceration within the gastric mucosa and/or the adjacent ileal mucosa, and it is typically painless. The presence of ectopic gastric mucosa serves as the basis for the “Meckel scan,” or technetium 99m pertechnetate scintigraphy. Given the relatively low sensitivity of this examination, however, a negative result often has little impact on clinical management when a Meckel diverticulum is strongly suspected. Arteriography and contrast studies may be helpful in these instances, although these evaluations may be superseded by colonoscopy or laparoscopy.

Bowel obstruction as a result of Meckel diverticulum can occur as a result of either volvulus, intussusception, inversion of the diverticulum into the bowel lumen, inclusion of the diverticulum into a hernia, or diverticulitis. Persistent attachments of the diverticulum to the umbilicus may be the cause of bowel obstruction. A transition point may be identified on CT where proximal, dilated bowel loops abruptly change caliber to collapsed, distal loops, although the obstructing band itself is not visible. The diverticulum also can serve as a lead point for intussusception and should be suspected in any child presenting with intussusception who is outside the typical age range for idiopathic intussusception ( Fig. 6-26 ). Acute Meckel diverticulitis may mimic acute appendicitis in a child presenting with abdominal pain, fever, and vomiting. Visualization of a normal appendix is helpful in distinguishing these two entities and identifying the ileum at the base of the diverticulum. This assessment is much more easily made with CT than with US. Identification of an enterolith on abdominal radiographs or CT is useful for suggesting the diagnosis of Meckel diverticulum in the setting of obstruction, especially when it is located in the right lower quadrant. Mixed-attenuation fecal material may be entrapped within the blind-ending diverticulum.

Mesenteric Adenitis

Not all children presenting to the emergency department with abdominal pain have an abnormality requiring emergent diagnosis and treatment. In the absence of other abnormalities, a child’s symptoms are often attributed to an entity called “primary mesenteric lymphadenitis.” Enlarged mesenteric lymph nodes on CT or US may be the only imaging abnormality present. The cause of the lymphadenopathy is unclear, although a nonspecific gastrointestinal inflammation may be the underlying cause. Although no precise size criteria define an abnormally enlarged mesenteric lymph node in children, a node with a short-axis diameter greater than 8 mm is considered enlarged. Although these measurements are based on data acquired with CT, the diagnosis also may be made on the basis of US findings.

Omental Infarction

Omental infarction most often occurs in obese children. This entity is best recognized with CT, although it also can be detected with US. Patients often present with right lower quadrant or diffuse abdominal pain, and often fever and an elevated white blood cell count are not present. Imaging studies (CT and US) demonstrate a normal-appearing bowel, including the appendix. The presence of hazy, increased attenuation within the anterior abdomen in the region of the omentum is a hallmark of this diagnosis ( Fig. 6-27, A ). The diagnosis also may be suggested at US by detection of an echogenic, masslike structure just deep to the anterior abdominal wall in the area of pain ( Fig. 6-27, B ). If no other abnormalities are appreciated, this entity can be treated conservatively with prophylactic antibiotic and analgesic drugs.

Posterior Urethral Valves

Posterior urethral valves (PUVs) are the most common congenital cause of bilateral renal obstruction. Their embryologic development is complex and has been previously described in the literature. Briefly, PUVs consist of an obstructing membrane, or persistent urogenital membrane, at the level of the verumontanum in the posterior urethra of male infants. This membrane causes varying degrees of bladder outlet obstruction and bilateral hydroureteronephrosis. The diagnosis is often made in utero. The presence of PUVs is associated with oligohydramnios, which can result in pulmonary hypoplasia if it is severe.

US examination of patients with PUVs reveals dilated renal collecting systems and ureters bilaterally ( Fig. 6-28 ). The bladder is often greatly distended, and the bladder wall may appear sacculated, thickened, or trabeculated. The dilated posterior urethra has a “keyhole” configuration at US. A voiding cystourethrogram (VCUG) is the standard of care for the diagnosis of PUVs. The bladder is usually of large caliber and may require a larger than expected volume of contrast material to reach capacity. Bladder wall trabeculations and diverticula are often present. When reflux is present, the refluxed contrast material will be diluted by the preexisting urine within the dilated ureter and renal collecting system. The degree of hydronephrosis is often massive. Images of the urethra acquired during voiding demonstrate an abrupt change in caliber between the dilated posterior urethra and the normal-caliber anterior urethra ( Fig. 6-29 ). In some cases the obstructing membrane is identified at the point of transition. The verumontanum is often enlarged and is identified as an intraluminal filling defect along the posterior wall of the urethral just proximal to the valves.

The PUVs are treated with ablation. Even after the valves are ablated, the long-term sequelae of the bladder outlet obstruction in utero can be catastrophic, with renal failure developing by early childhood. Ultimately, the degree of renal function is of primary importance in determining patient outcome, because massive vesicoureteral reflux (VUR) in utero may cause severe renal dysplasia that leads to renal failure. In utero, calyceal rupture and urinoma formation is protective of renal function. PUVs must be differentiated from other causes of bilateral hydroureteronephrosis in infancy, including prune-belly syndrome, bilateral ureterovesicular junction obstruction, transient bilateral VUR, and the rare “megacystis-microcolon-intestinal hypoperistalsis-syndrome.” The findings at VCUG should readily differentiate PUVs from these other entities.

Urinary Tract Infection

The diagnosis of a urinary tract infection (UTI) in children is based on symptoms, bacterial growth in the urine, and other laboratory markers. Distinguishing between upper tract infection (i.e., pyelonephritis) and lower tract infection (i.e., cystitis) is based on clinical criteria such as the presence of a fever. The goal of imaging in children with a UTI is to exclude an underlying structural abnormality that predisposes the child to develop infection, evaluate for an obstructed urinary system or other disease that would require urgent intervention (e.g., an abscess), and evaluate for evidence of renal damage such as scarring. Ultrasonography of the kidneys and bladder is recommended to assess for any underlying anatomic abnormality. Although cortical renal scintigraphy is the most sensitive imaging technique for the detection of acute pyelonephritis, it is rarely performed emergently. Pyelonephritis in children can be a result of VUR. Therefore, many young children with a febrile UTI are evaluated for VUR with a VCUG or radionuclide cystogram. Depending on the severity of reflux, the child may warrant treatment with prophylactic antibiotics to decrease the risk of renal scarring related to pyelonephritis. Currently, the American Academy of Pediatrics recommends VCUG in the setting of an abnormal renal US or recurrent febrile UTI, but not after a single febrile UTI. VCUG can be performed several days or weeks after the initial infection. Children presenting to the emergency department with a complicated UTI (i.e., persistent fevers despite administration of antibiotic or clinical signs and symptoms that cause concern for severe illness or sepsis caused by a UTI) warrant evaluation with US.

In patients with acute pyelonephritis, US may reveal focal areas of decreased attenuation. These same areas demonstrate decreased vascularity with color Doppler US. Urothelial thickening is often present. Complications of pyelonephritis include acute focal bacterial nephritis (AFBN) and, rarely, renal or perinephric abscess formation. AFBN is a pseudotumoral inflammatory mass that can be mistaken for a tumor. On imaging it may be heterogeneous with areas of both enhancement and necrosis, but it should decrease in size over time with treatment. A frank abscess will appear as a hypoechoic or anechoic mass with increased through-transmission ( Fig. 6-30, A ). A renal abscess may simulate a solid mass at US. At CT an abscess will appear as a focal, hypoattenuating mass ( Fig. 6-30, B ) with peripheral but no central enhancement. A perinephric abscess is suggested by fluid or soft tissue attenuation within the perinephric space. These collections may penetrate through Gerota fascia and involve surrounding structures, such as the psoas muscle. Urine cultures in patients with renal abscess are negative in up to 20% of cases.

Hydronephrosis

Hydronephrosis refers to dilatation of the urinary collecting system caused by an obstruction to the normal flow of urine. The obstruction may occur anywhere along the urinary tract from the renal calyces to the urethra. Stasis of urine and increased pressures within the collecting system may have deleterious effects on the kidney over time and lead to progressively worsening renal function. A dilated urinary collecting system is not always obstructed. VUR, for example, causes transient dilatation of the urinary collecting system. An obstructing ureteral calculus or stricture may lead to proximal hydroureteronephrosis (dilatation of both the renal collecting system and the ureter), which is indistinguishable from VUR. Determining the cause for a dilated collecting system is not always straightforward. Often other clinical parameters are necessary to make this determination, along with additional imaging evaluation.

US is the modality of choice for evaluating for hydronephrosis. Communication between multiple cystic-appearing, dilated renal calyces and the renal pelvis is an important finding at US. If these cystic spaces do not communicate with each other or with the renal pelvis, it is more likely that the patient has cystic renal disease rather than a dilated collecting system. Color Doppler US is helpful in demonstrating prominent vessels in the renal hilum so that they are not mistaken for a dilated renal pelvis. The ureters are not typically visualized at US unless they are dilated. A dilated distal ureter often can be visualized posterior to the bladder as the level of the ureterovesicular junction (UVJ). If a single ureter is dilated at this level, the obstruction is likely at the UVJ; if both ureters appear dilated at this level, the obstruction is likely within or below the bladder (although bilateral UVJ obstruction can occur). Imaging of the bladder may reveal wall thickening and/or trabeculations in patients with an underlying abnormality, such as a neurogenic bladder. Intraluminal debris or calculi may be present within the bladder of patients with concurrent UTI.

Ureteropelvic Junction Obstruction

A common cause of hydronephrosis in children is ureteropelvic junction obstruction (UPJO). UPJO may be the result of either intrinsic or extrinsic factors. In many children prenatal US detects intrinsic UPJO, and therefore approximately half of all children presenting later in childhood with intermittent UPJO have an extrinsic abnormality such as a crossing vessel. UPJO is often an intermittent phenomenon. Children with intermittent UPJO present with episodic abdominal pain, nausea, and vomiting. The severity of pain correlates with the rapidity of the onset of symptoms rather than the degree of distention of the collecting system. Hematuria is present in approximately half of patients and aids in elucidating the urinary system, rather than other abdominal organs, as the source of symptoms. UPJO is more common in males and occurs most often on the left side.

The US appearance of UPJO is a dilated renal collecting system on the patient’s symptomatic side during the acute pain crisis ( Fig. 6-31 ). The proximal ureter appears dilated with an abrupt tapering at the level of the crossing vessel. US should be performed during the pain crisis because the hydronephrosis is present only when the patient is experiencing symptoms. Unilateral hydronephrosis is not specific for UPJO, nor does US evaluate the functional status of the kidney. Although additional investigation is often required before treatment can be initiated, US is critical in the preliminary imaging of patients with suspected urinary tract obstruction.

Urolithiasis

Although less common in children than in adults, urinary tract calculi are an important cause of flank pain and UTI. The majority of urinary tract calculi in children are located in the kidneys or ureters. Boys and girls are affected with nearly equal frequencies. Abdominal or flank pain is the most common presenting symptom but is present in fewer than half of children with renal stones. Other signs and symptoms include pyuria, hematuria, nausea, vomiting, fever, and urinary retention. Smaller children who are not able to verbalize their symptoms more commonly present with symptoms of infection rather than specific complaints of pain. Most pediatric patients with urolithiasis have an underlying predisposition to forming stones. These predisposing conditions include congenital structural disorders of the urinary system (e.g., UPJO or horseshoe kidney), metabolic disorders (e.g., hypercalciuria or cystinuria), prolonged states of immobilization leading to urinary stasis, enteric disease causing intestinal malabsorption, or recurrent UTIs.

The imaging evaluation of a child with suspected urolithiasis is performed with US, CT, or both. US is an appealing first-line imaging modality because it detects complications of urolithiasis such as hydronephrosis or perinephric fluid without the use of ionizing radiation. At US, renal calculi appear as echogenic foci with posterior acoustic shadowing ( Fig. 6-32 ). US also has the advantage of evaluating the renal parenchyma for changes of medical renal disease or evidence of scarring. US is less sensitive than CT for detection of urinary tract calculi and is limited in diagnosing calculi that are confined to the ureter. CT is often performed in children who have persistent urolithiasis symptoms despite normal results of a US examination.

Unenhanced CT is highly sensitive for the detection of urolithiasis in pediatric patients. CT protocols that include a lower dose of radiation have been developed, thus minimizing radiation risk to patients. Phleboliths, which often mimic ureteral calculi in older patients, are rarely encountered in children, which facilitates diagnosis of urolithiasis on CT. Secondary CT signs of urolithiasis are helpful in confirming the diagnosis and include proximal ureteral dilatation, renal enlargement, hydronephrosis, decreased renal attenuation, the tissue rim sign, and perinephric stranding. The tissue rim sign refers to the circumferential rim of soft tissue attenuation surrounding a ureteral calculus ( Fig. 6-33 ). These secondary signs of urinary tract calculi are less commonly encountered in pediatric patients than in their adult counterparts. Proximal ureteral dilatation and renal enlargement are the most commonly detected secondary signs and occur in approximately half of patients. Perinephric or periureteral stranding is rarely encountered in the pediatric patient, which may be a result of the decreased amount of perinephric fat in this population.