Imaging Evaluation of Common Pediatric Emergencies

Published on 12/06/2015 by admin

Filed under Radiology

Last modified 12/06/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 3636 times

CHAPTER 6 Imaging Evaluation of Common Pediatric Emergencies

Limiting radiation exposure to young patients, while at the same time acquiring diagnostic information, is a primary goal of the pediatric radiologist. The imaging evaluation of common pediatric emergencies relies heavily on plain radiographs, fluoroscopy, and ultrasound. While computed tomography (CT) is useful in the evaluation of some children, CT is often reserved for cases where ultrasound and plain films are nondiagnostic. This chapter illustrates the imaging manifestations of some of the most common pediatric emergencies.

NEONATAL EMERGENCIES

Neonates make up a small subset of pediatric patients with their own unique diagnoses. Even seemingly healthy, full-term babies may become distressed and require immediate imaging evaluation. With ongoing advances in prenatal imaging a number of conditions that require urgent treatment are diagnosed before birth, such as posterior urethral valves and esophageal atresia. In many infants the diagnosis may be suggested for the first time on the basis of imaging performed in the first days and weeks of life.

Air Block Complications

Infants on mechanical ventilators are at risk of developing complications secondary to barotrauma or volutrauma. This is especially true for infants with underlying lung disease such as SDD. Air may rupture outside of the alveoli, entering the perivascular or peribronchiolar spaces and giving rise to interstitial lucencies termed pulmonary interstitial emphysema (Fig. 6-2). The air may also dissect into the mediastinum, pleural space, or pericardium. On a supine chest radiograph, a pneumothorax tends to collect anteriorly along the heart border or inferiorly above the diaphragm (Fig. 6-3), and is manifested as a paracardiac lucency (for a medial pneumothorax) or a deep costophrenic sulcus (for an inferior pneumothorax). A cross-table lateral view is often necessary to demonstrate the anterior location of the air (Fig. 6-4). Pneumomediastinum causes anterior displacement of the thymus in a “spinnaker sail” configuration (Fig. 6-5). Pneumopericardium outlines the heart border without extending superior to the great vessels (Fig. 6-6).

image

Figure 6-4 A cross-table lateral chest radiograph in the same infant as in Figure 6-3 demonstrates the anterior location of the pneumothorax (arrow).

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 and should be suspected in any male patient who fails to void within 24 hours of birth.

Ultrasound (US) examination of patients with PUVs reveals dilated renal collecting systems and ureters bilaterally (Fig. 6-8). 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 ultrasound. 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 to reach capacity. Bladder wall trabeculations and diverticula are often present. When reflux is present, the refluxed contrast 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. 6-9). 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 urethra just proximal to the valves.

image

Figure 6-9 Lateral-oblique voiding fluoroscopic image acquired during voiding cystourethrogram (VCUG) on same patient as in Figure 6-8 demonstrates a distended urinary bladder with several small diverticulae (black arrowhead). There is an abrupt change in caliber from the dilated posterior urethra to the anterior urethra (white arrowhead). The enlarged verumontanum can be identified as a filling defect in the posterior urethra (arrow).

The treatment for PUVs is 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, as massive vesicoureteral reflux (VUR) in utero may cause severe renal dysplasia that leads to renal failure. 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.

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 mid-ileum. Babies 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 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. 6-10). In a baby with intestinal obstruction the correct interpretation of the plain film is critical in directing the most appropriate next course of action.

A high intestinal obstruction may be secondary to duodenal atresia or stenosis, jejunal atresia or stenosis, or malrotation. An upper gastrointestinal (UGI) examination is often requested in order to identify the level of obstruction. The UGI also assists the surgeon in determining the urgency of surgery. In duodenal atresia or stenosis, the obstruction almost always occurs at the level of the ampulla of Vater. Abdominal radiographs in duodenal atresia often reveal the classic “double bubble” sign in an otherwise gasless abdomen. The dilated gas-filled “bubbles” represent the stomach and duodenal bulb. This is virtually diagnostic of duodenal atresia in the correct clinical setting. In cases of incomplete duodenal obstruction, radiographs will reveal gas in distal bowel loops. At UGI examination, there is often a focal area of narrowing within the duodenum through which a tiny amount of contrast may pass. In some cases, the stenosis may be secondary to a duodenal web, which appears as a curvilinear filling defect extending across the duodenal lumen (Fig. 6-11). Coexistent congenital anomalies are not uncommon, most often in the form of congenital heart disease. Approximately 30% of babies with duodenal atresia or stenosis have Down syndrome.

Low intestinal obstruction may be secondary to meconium ileus, ileal atresia, small left colon syndrome (functional immaturity of the colon), colonic atresia, and Hirschsprung’s disease. A contrast enema is the study of choice to elucidate the diagnosis. Meconium ileus almost always occurs in infants with cystic fibrosis. On contrast enema, a microcolon is present. 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 the distal small bowel allows the colon to achieve a normal caliber. In patients with meconium ileus refluxed contrast in the distal ileum reveals multiple filling defects within the distal small bowel. These filling defects represent inspissated meconium. In ileal atresia, on the other hand, it is not possible to reflux contrast past the atresia into the terminal ileum. In functional immaturity of the colon, the descending colon and sigmoid colon are small in caliber compared with the normal ascending and transverse colon. These patients are often infants of diabetic mothers. In colonic atresia the visualized colon at contrast enema will be a microcolon, and contrast will be unable to pass proximal to the atresia. In Hirschsprung’s disease there is a transition from normal bowel that is of normal caliber to aganglionic bowel that is small in caliber (Fig. 6-12). Classically, this transition occurs at the level of the rectosigmoid. In normal infants the rectal diameter should be greater than the sigmoid diameter, although in Hirschsprung’s disease this relationship is reversed. Ultimately, a biopsy is required to confirm the diagnosis.

Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is a gastrointestinal condition that occurs in 2% to 3% of neonates, most commonly premature infants. 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 for monitoring its progression. An early sign of NEC is a mildly dilated and fixed loop (or loops) of bowel on serial abdominal radiographs. Pneumatosis intestinalis, or air within the wall of the bowel, is considered a hallmark of NEC (Fig. 6-14). Pneumatosis represents by-products of the metabolism of bacteria within the intestinal wall. While the bubbly lucencies of pneumatosis can often be mistaken for stool on an abdominal radiograph, it is uncommon for an infant younger than 2 weeks of age to have stool within the colon. The presence of portal venous gas and pneumoperitoneum on an abdominal radiograph usually indicates more severe disease and/or perforation of the bowel. Portal venous gas appears on abdominal radiographs as lucent, branching, linear structures within the liver (Fig. 6-15). When pneumoperitoneum is present the falciform ligament is visualized on a radiograph secondary to the presence of outlining air on both sides (Fig. 6-16). Both sides of the bowel wall are also visible (Fig. 6-17). The abdomen may appear abnormally lucent when large amounts of gas collect anteriorly within the abdomen on a supine film (Fig. 6-18). A cross-table lateral view of the abdomen is helpful in confirming the presence of free air under the diaphragm (Fig. 6-19).

image

Figure 6-18 Abdominal radiograph in the same patient as in Figure 6-16 taken several hours later demonstrates hyperlucency of the liver (white arrow) and free air under the diaphragm (arrowhead), as well as air outlining loops of bowel (black arrow).

Most cases of NEC are treated medically, usually with bowel rest, gastric suction, antibiotics, and hydration. Surgery is usually not performed unless there is evidence of bowel perforation (i.e., free air). 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, as this often coincides with clinical improvement.

GASTROINTESTINAL

Abdominal pain and vomiting are common chief complaints in children presenting to the emergency department. Very young patients are unable to verbalize symptoms of pain, and the presence of vomiting may be the first and only sign that there is an underlying gastrointestinal problem. In young infants a history of vomiting invokes a relatively short differential diagnosis that includes hypertrophic pyloric stenosis and intestinal malrotation. In the older child, a history of emesis, especially bilious emesis, invokes a more complicated differential diagnosis, which includes intussusception, appendicitis, bowel obstruction, and gastroenteritis, as well as many other conditions. Further imaging investigation largely depends on other clinical signs and symptoms as well as laboratory markers.

Hypertrophic Pyloric Stenosis

Hypertrophic pyloric stenosis (HPS) is a condition that affects young infants usually between 3 and 6 weeks of age. While normal at birth, these infants develop gradual onset of nonbilious emesis over the first several weeks of life, commonly described as “projectile” in quality. In severe cases dehydration and malnourishment will ensue that 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 etiology for HPS is unclear.

Abdominal radiographs in infants with HPS often 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. 6-20). Ultrasound is the imaging study of choice for the diagnosis of HPS. When HPS is present, the pyloric channel is nearly always visualized and appears both thickened and elongated (Fig. 6-21). 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 pyloric channel is variable and can range between 14 and 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. It is important to document the passage of fluid from the gastric antrum into the duodenal bulb. In normal patients the normal pyloric ring bridges these two structures (Fig. 6-22). Careful observation of this region over the course of the examination reliably allows differentiation between a normal and an abnormal study.

Prior to the advent of ultrasound, UGI examination was performed to confirm the diagnosis of HPS. While the thickness of the pylorus muscle is not discernible at UGI examination, there are other imaging signs that suggest the diagnosis. The pyloric channel appears elongated at UGI examination as wispy, linear tracts of contrast pass through the canal, a finding called the “string” sign (Fig. 6-23). The “shoulder” sign is the name given to the appearance of the thickened pylorus impression on 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, as contrast is never able to pass through the channel in more than tiny wisps at a time.

The current treatment for HPS is 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., the muscle is not sutured over). 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’s” 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 well infant presenting with bilious emesis midgut volvulus is a primary clinical concern. While symptoms related to malrotation may theoretically 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 of paramount importance in making the diagnosis of malrotation. The bowel gas pattern is often normal in the presence of malrotation. A normal 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 duodeno-jejunal junction (DJJ). In normal patients the duodenum has a characteristic C-sweep that consists of four portions: the duodenal bulb and the descending, transverse, and ascending portions (Fig. 6-24). 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. 6-25). There are a few rare exceptions to this rule, which include 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. 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. Despite all of these findings, the diagnosis of malrotation remains difficult to make with certainty in up to 30% of cases in light 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 distally to the colon. When malrotation is present, the small bowel is commonly located on the right side of the abdomen, and the cecum may also be abnormally positioned.

The preferred treatment for infants or children with malrotation is a Ladd’s procedure. This surgery, often performed laparoscopically, entails reduction of a midgut volvulus, division of obstructing peritoneal bands, placement of the bowel in a state of nonrotation, and appendectomy. The urgency of surgery depends on whether a midgut volvulus is present at the time of diagnosis. Since all patients with malrotation are at risk of volvulus, even those patients without a volvulus proceed to surgery semi-urgently. Recurrence of volvulus after Ladd’s procedure 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 occurs most often in children between 6 months and 3 years of age. Common presenting symptoms are vomiting, lethargy, and bouts of irritability where 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 the minority of cases and should not be relied on 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 less than half of all cases. The implications of delayed diagnosis may be devastating, as a persistent intussusception leads to bowel ischemia and perforation. Most intussusceptions in children are idiopathic, while the remaining cases are secondary to the presence of a pathologic lead point (PLP). PLPs include Meckel’s diverticulum, colonic polyps, lymphoma, duplication cyst, or underlying diseases of the bowel such as Henoch-Schönlein purpura. 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 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. 6-26). Small bowel dilatation and air–fluid levels may be present signifying the presence of a small bowel obstruction (Fig. 6-27). Despite these signs, half of all abdominal radiographs are diagnostically unhelpful in the diagnosis of intussusception, and further evaluation is required.

For confirmation of intussusception, ultrasound 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 the transverse plane, an intussusception appears as a mass usually between 3 and 5 cm in diameter that has a “target” or “doughnut” configuration of alternating hypo- and hyperechoic layers (Fig. 6-28A). In the sagittal plane the mass assumes a “pseudo-kidney” configuration. Color Doppler is used to evaluate the vascularity of the bowel and is a promising predictor of bowel viability (Fig. 6-28B). 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 proceed to 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 (less than 2 cm), and short-segment involvement (less than 5 cm).

Fluoroscopically guided contrast 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 different contrast agents are used, air is preferred at our institution. After inserting a rectal catheter into the buttocks air is insufflated into the colon. The intussusception is visualized fluoroscopically as it is reduced back through the colon (Fig. 6-29). 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 is successful in over 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 requiring emergent abdominal surgery. Acute appendicitis is caused by obstruction of the appendiceal lumen that leads to accumulation of fluid and secondary inflammation and infection. Luminal obstruction is most often secondary to the presence of a fecalith, but may also be secondary to lymphoid hyperplasia, foreign bodies, or mass lesions (i.e., 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 in order to avoid complications of perforation, such as abscess formation, bowel obstruction, and sepsis.

Graded-compression sonography is an appealing imaging modality for the initial evaluation of children suspected to have acute appendicitis. The advantages of ultrasound include the lack of ionizing radiation, the lack of patient preparation, and the lack of sedation. The patient is often able to precisely localize the site of pain in order to direct the sonographer to the appropriate area. Disadvantages of ultrasound are that US is highly operator dependent, the accuracy of the study is largely impacted by the body habitus of the patient, US is poorly accurate at diagnosing a normal appendix, and US is often not able to determine if the appendix is perforated.

CT examination is highly sensitive for the diagnosis of acute appendicitis. In many cases, CT may also provide an indication that the appendix has perforated. In many institutions, a negative or equivocal ultrasound for appendicitis prompts a CT scan. Depending on the institution, various combinations of oral and intravenous contrast are used. Although CT is more highly accurate at diagnosing appendicitis than US, US is often performed initially in children in an effort to avoid or reduce the inherent risks of radiation exposure. Therefore, it is important to be familiar with the imaging features of acute appendicitis with US as well as CT.

On US, the inflamed appendix appears as a blind-ending, tubular structure with bowel signature. 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. While no exact measurement defines an acutely inflamed appendix, in general practice an appendix with a diameter greater than 6 mm is considered abnormal. Secondary signs of appendicitis include free fluid in the abdomen or pelvis, enlarged mesenteric lymph nodes, and fluid collections adjacent to the appendix that represent abscesses.

Many of these same imaging features for US also apply to CT. CT findings of acute appendicitis include a maximal outer luminal diameter greater than 6 mm, lack of oral contrast within the lumen (if oral contrast was administered), an appendicolith, and periappendiceal inflammatory changes. 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 hemorrhagic ovarian cyst. Likewise, a subhepatic appendix may present with right upper quadrant pain simulating gallstones, or flank pain concerning for pyelonephritis.