Hirschsprung Disease

Hirschsprung disease is a developmental disorder of colonic function, in which spasm and lack of normal peristalsis interfere with normal distal passage of the bowel contents. The length of colon involved and the severity of symptoms vary between patients. The prevalence of Hirschsprung disease is approximately 1 in 6000 livebirths. There is a male predominance for the short-segment form, but the gender incidence is equal with long-segment disease. Hirschsprung disease occurs with an increased prevalence in individuals with trisomy 21; approximately 10% of patients with Hirschsprung disease have trisomy 21. Other associations include genitourinary abnormalities (especially megaureter), cardiac anomalies, congenital deafness, ileal atresia, intestinal malrotation, multiple endocrine neoplasia type 2, Bardet–Biedl syndrome, and cat eye syndrome. Approximately 10% of children with total colonic Hirschsprung disease have a neurocristopathy such as congenital central hypoventilation syndrome. A familial association is present in up to 10% of patients with Hirschsprung disease, particularly the total colon variant.¹

Hirschsprung disease is due to absence of myenteric and submucosal plexus ganglion cells within the wall of the affected portion of the intestine. The embryogenesis apparently involves the failure of appropriate migration of neural crest cells and/or failure of appropriate craniocaudal extension of the myenteric plexus and submucosal plexus of the intestine. Because neural crest cell migration occurs in a proximal-to-distal fashion within the developing intestine, the aganglionic segment extends from the rectum for a variable distance proximally, that is, the portion of the intestine distal to the site of arrested migration is aganglionic. The transition from innervated to aganglionic bowel (i.e., the “transition zone”) is within the rectosigmoid portion of the colon in approximately 75% to 80% of patients, in the descending segment of the colon in 10% to 15%, in the ascending or transverse portions in 5% to 10%, and the entire colon is involved in 1% to 3%; extension of the aganglionic segment into the small intestine is rare. Localized or skip areas of congenital aganglionosis either are extremely rare or do not occur.²

There is overexpression of intrinsic parasympathetic and sympathetic nerve activity within the aganglionic segment of bowel, and lack of activity of enteric nerves that cause relaxation. The aganglionic segment, therefore, remains abnormally contracted, thereby causing functional obstruction. This segment is narrow and spastic. Normal peristaltic activity is lacking. The bowel wall is thickened due to muscular hypertrophy. The intestine proximal to the transition zone typically becomes somewhat distended with feces and gas.

The presentation of Hirschsprung disease is variable, ranging from complete intestinal obstruction in the neonate to nonspecific chronic constipation in the older child. Approximately 80% of patients with Hirschsprung disease present in the neonatal period. Those patients who present during the neonatal period most often exhibit abdominal distention during the first day of life and fail to pass meconium appropriately. Approximately 90% of normal infants pass meconium within the first 48 hours of life, whereas 90% of children with Hirschsprung disease do not.

In addition to abdominal distention, infants with Hirschsprung disease may develop vomiting during the first several days of life. Although often mild initially, the vomiting usually increases in severity with time, and occasionally is bile-stained. The obstructive symptoms diminish in severity if feedings are stopped, but recur when oral intake resumes. With low-segment Hirschsprung disease, rectal examination may result in expulsion of a copious amount of fecal material; this provides temporary relief of symptoms for some patients. If there is delay in diagnosis and treatment, infants with Hirschsprung disease often suffer failure to thrive. In an older infant, the combination of failure to thrive, abdominal distention, intermittent vomiting, and constipation strongly suggests the diagnosis of Hirschsprung disease.

Infants and young children with Hirschsprung disease may have alternating episodes of constipation and diarrhea. These patients are at risk for the development of enterocolitis, which is precipitated by stasis of gas and fecal material in the distended portion of the colon proximal to the aganglionic segment. Clostridium difficile is an important pathogen in many patients. Potential clinical findings in children with enterocolitis include diarrhea, hematochezia, abdominal distention, dehydration, acidosis, and shock. There is marked passage of fluid and electrolytes into the intestinal lumen. Enterocolitis in a Hirschsprung disease patient requires aggressive treatment, and is potentially fatal.

Patients with Hirschsprung disease who do not present until later in childhood often have a history of chronic constipation that can sometimes be traced to the first weeks of life. The stools are usually pellet- or ribbon-like. Fecal soiling is uncommon in these patients and, when present, is suggestive of functional constipation. Manifestations of failure to thrive in older children with Hirschsprung disease vary with the severity of colonic dysfunction. Some children have a history of normal growth and development at the time of diagnosis, whereas others have findings that mimic those of a malabsorption syndrome, with wasting, a protuberant abdomen, and anemia. The similarity to malabsorption syndrome is most pronounced in those Hirschsprung disease patients who have chronic diarrhea. Frank enterocolitis is uncommon in older children with Hirschsprung disease. Rectal examination usually reveals a small anal canal and absence of stool within the rectal vault.

Diagnostic imaging studies of children with suspected Hirschsprung disease serve to suggest or confirm the diagnosis, exclude mimicking conditions, evaluate complications, and help localize the transition zone. Rectal suction biopsy is a simple procedure that is highly sensitive and specific for the diagnosis of Hirschsprung disease, and should be performed in any patient for whom the diagnosis is entertained. The patient history, clinical evaluation, and findings on abdominal radiographs and contrast enema are helpful in selecting those patients who should be biopsied. However, when there is a suspicious clinical history, a normal contrast colon examination should not preclude performance of a biopsy.

Infants with Hirschsprung disease usually have a functional distal bowel obstruction. Therefore, abdominal radiographs show dilated loops of bowel, sometimes containing air–fluid levels (Figure 37-1). The dilated loops in these patients tend to vary in length and distribution. There is distension of both small bowel and the portion of colon proximal to the transition zone (Figure 37-2). In most instances, the greatest degree of bowel dilation occurs in the transverse colon. The rectum is usually empty or contains only a small amount of gas. Abdominal radiographs of older children with untreated Hirschsprung disease typically show an empty rectum and a prominent amount of stool in the colon proximal to the transition zone. Substantial small bowel dilation is uncommon in these older children.

The contrast enema is the most useful radiographic technique for the diagnosis and characterization of Hirschsprung disease. Because the narrowing of the aganglionic segment is predominantly due to spasm, interventions that may artificially increase the caliber of this segment should be avoided before and during the contrast enema.

The child should not receive a bowel preparation prior to the examination. A small rectal tube, without a balloon, should be utilized. Digital rectal examination should be avoided prior to the procedure.

Because the manifestations of Hirschsprung disease on contrast enema are sometimes subtle or transient, careful fluoroscopic technique is essential for appropriate diagnosis. The study is initiated with the child in the lateral projection. Images of the rectum are obtained prior to and during the slow introduction of contrast medium. The patient is then turned into the supine position as the remainder of the colon is slowly filled. The classic appearance of Hirschsprung disease on contrast enema is that of a narrow rectum that is devoid of stool, and an abrupt caliber change (i.e., the transition zone) to dilated bowel proximally (Figure 37-3). With short-segment disease, the rectosigmoid ratio is less than 1 (maximum diameter of the rectum divided by the maximum diameter of the sigmoid colon).

In the newborn infant with distal-segment disease, the classic contrast enema features of Hirschsprung disease are frequently lacking. This is in part due to a lack of sufficient time for the development of substantial bowel distention proximal to the transition zone. In addition, there can be artificial distention of the short aganglionic segment in these infants by impacted meconium or feces, or by too rapid filling with contrast material. In this situation, there may be only minimal rectal narrowing, and the junction with the sigmoid colon tends to have a gradual funnel-shaped configuration, rather than an abrupt change in caliber. In some of these infants, a foreshortened appearance of the sigmoid colon provides a clue to the diagnosis; the normal sigmoid colon in infants is relatively long, uniform in caliber, and courses beyond the margin of the pelvis. As viewed on the lateral projection, the rectum of an infant with Hirschsprung disease usually has an abnormal tubular configuration, with lack of the normal posterior outpouching just above the anorectal sling. It is important to recognize that the radiographic transition zone in infants with Hirschsprung disease may underestimate the true length of the aganglionic segment, due to passive distention of the proximal aspect of the aganglionic bowel.^3,4

Several types of irregular colonic contractions can be present on contrast enemas of children with Hirschsprung disease; these tend to be most prominent in infants. (1) Irregular, deep, saw-toothed contractions are common within the aganglionic segment. (2) Fine serrations may be present in the colon proximal to the transition (Figure 37-4). These fine, transversely oriented, parallel folds are apparently caused by circular muscle contractions. Similar findings are sometimes present in the aganglionic segment (Figure 37-5). Serrations of the colonic wall are not specific to Hirschsprung disease, as they may be identified in any condition that causes excessive workload of the infantile colon. (3) Irregular spiculation within the mucosa of the aganglionic segment and, occasionally, elsewhere in the colon suggests the presence of enterocolitis.

Aganglionosis of the entire colon, that is, total colonic Hirschsprung disease or total colonic aganglionosis, is an uncommon entity that often presents a clinical and radiographic diagnostic challenge. There is considerable variability in the clinical presentations of these children. Radiographs of the infant with total colon involvement may show some degree of small bowel dilation. A contrast enema sometimes demonstrates an abnormal reduced colonic diameter throughout; however, the caliber of the colon is normal in most instances. The colon often is foreshortened, with loss of normal redundancy of the splenic flexure, hepatic flexure, and sigmoid segment. The colonic wall is sometimes irregular or spastic. Images obtained after reflux of contrast into the small intestine show mild to moderate dilation of the distal portion of the ileum. In neonates with total colonic Hirschsprung disease, meconium plugs are often present throughout the colon. Because the radiographic findings of total colonic Hirschsprung disease are often nonspecific, rectal suction biopsy should be performed in any child for whom the diagnosis is included in the differential.

The categorization of the extent of colonic involvement in Hirschsprung disease is into short- and long-segment varieties, based on whether or not the aganglionic segment extends proximal to the inferior aspect of the descending colon (Figure 37-6). The transition is in the rectosigmoid colon in about three-fourths of patients, and proximal to this level in about one-fourth. Total colon disease, usually with a distal ileal transition, accounts for less than 4% of Hirschsprung disease cases. An accurate estimate of the length of the aganglionic segment is important for appropriate surgical planning. Short-segment disease is usually approached with a single-stage transanal pull-through procedure. Long-segment disease may require laparoscopic mobilization of the colon as part of a transanal pull-through procedure. If there is extensive involvement of the colon, a formal laparotomy is usually required, with removal of the aganglionic bowel, placement of a colostomy, and a second-stage procedure for the anastomosis.⁵

A contrast enema performed preoperatively helps to determine the length of the aganglionic segment in patients with Hirschsprung disease. The most important finding on contrast enema is a transition zone from small to larger caliber in the colon. Caution is required in the use of this finding, however, as the location of the transition zone on contrast enema can substantially overestimate or underestimate the length of the aganglionic segment of bowel. Contrast enema is more reliable for determining the length of short-segment disease than for determining that of long-segment disease.^6–9

The surgical approach to Hirschsprung disease involves resection of the aganglionic segment of bowel and bringing innervated bowel to the distal aspect of the rectum. With the Swenson procedure, ganglion-containing bowel is anastomosed at the level of the sphincter. With the Duhamel modification, the anterior aspect of the aganglionic rectum is left in place and innervated bowel is anastomosed to the posterior wall. The transanal or endorectal pull-through procedure (Soave procedure) is the most common technique in modern practice. After removal of the mucosa and submucosa of the aganglionic bowel, the innervated bowel is pulled through the remaining rectal muscular sleeve. Imaging studies are helpful in selected patients after surgical repair of Hirschsprung disease to detect a stricture, residual aganglionic segment, or leak. MR and CT provide depiction of the puborectalis muscle mass and the location of the pulled-through intestine in relation to the puborectalis sling and external sphincter (Figure 37-7).^10,11

Clinical Presentations: Constipation

Functional constipation (psychogenic constipation) refers to pathologic constipation without an identifiable underlying anatomic or physiological abnormality. These children often have a history of normal stooling during infancy, with symptoms developing in the early childhood years. Potential symptoms include abdominal pain, diminished frequency of defecation, anal fissures, and painful defecation. Encopresis is common in these patients, in contradistinction to Hirschsprung disease. Other clinical features of functional constipation overlap those of distal-segment Hirschsprung disease.¹²

Abdominal radiographs of children with functional constipation show generalized distention of the colon with stool. There is usually a capacious rectum. Secondary small bowel dilation can occur. Barium enema typically shows prominent stool in an otherwise normal colon. Some children with long-standing constipation have an elongated sigmoid colon. Without appropriate treatment, the child with severe functional constipation may develop a markedly dilated, tortuous, and dysfunctional colon.

The most common organic cause of constipation in infants and older children is Hirschsprung disease. Lack of normal innervation of a portion of the colon in these patients results in functional obstruction. In addition to Hirschsprung disease, there are various other disorders that involve abnormalities of the submucosal plexus and myenteric plexus of the intestine. These generally present with manifestations of intestinal dysmotility. Unlike Hirschsprung disease, various portions on the GI tract in addition to the colon can be involved.

Hypoganglionosis refers to a segmental or generalized paucity of intestinal myenteric neurons. Congenital and acquired (i.e., due to toxic or autoimmune mechanisms) forms are recognized. Affected patients may present with intractable constipation or intestinal pseudoobstruction. A full-thickness biopsy is required for diagnosis.^13,14

Intestinal neuronal dysplasia is a visceral neuropathy that occurs in 2 forms. Type A is characterized by sympathetic aplasia, myenteric plexus hyperplasia, and a variable degree of colonic inflammation. With type B, the predominant abnormality involves the submucosal plexus; sympathetic aplasia is lacking. Both types of intestinal neuronal dysplasia can involve the intestine in a localized or diffuse manner. The clinical findings are sometimes indistinguishable from those of Hirschsprung disease. Other patients have manifestations of generalized intestinal dysmotility. Rectal bleeding can occur. Contrast studies of the colon demonstrate a flaccid megacolon. Involvement of portions of the small intestine results in delayed transit of contrast, and focal areas of small bowel dilation.^15,16

Intestinal pseudoobstruction is a motility abnormality of the intestine in which there is chronic/recurrent dilation of the intestine in the absence of a mechanical obstruction or Hirschsprung disease. This can occur as an idiopathic disorder or in association with various systemic abnormalities that affect intestinal smooth muscle function, such as scleroderma, hypothyroidism, and drugs (Table 37-1).

Potential clinical manifestations of intestinal pseudoobstruction include abdominal distention, constipation, vomiting, and weight loss. Most children with the primary form become symptomatic during the first year of life. In some patients with this disorder, there are associated abnormalities of urinary tract function, such as urinary retention. Abdominal radiographs demonstrate dilated loops of bowel that contain air–fluid levels. There is poor bowel motility on contrast studies. The colon is dilated, redundant, and hypoperistaltic. Volvulus is a potential complication. Dilation of variable severity also occurs in the small bowel and stomach.^17–19

Approximately 95% of newborn infants pass at least 1 meconium stool during the first 24 hours of life and 70% during the first 12 hours. Absent or delayed passage of meconium can occur in association with Hirschsprung disease, meconium plug syndrome, small left colon syndrome, colonic inertia of prematurity, sepsis, hypothyroidism, narcotic therapy, and maternal magnesium sulfate therapy (Table 37-2). A contrast enema is indicated for further evaluation of many of these infants, depending on the specific clinical situation. Water-soluble contrast sometimes facilitates disimpaction of meconium, and leads to symptomatic improvement. Those infants with clinical or radiographic findings that raise the possibility of Hirschsprung disease should undergo a suction rectal biopsy.

Meconium Plug Syndrome

Meconium plug syndrome refers to distal colonic obstruction by 1 or more plugs of tenacious meconium. Many infants with meconium plug syndrome are otherwise normal, and symptoms resolve after dislodgment of the plug. However, up to 25% of infants with manifestations of meconium plug syndrome have Hirschsprung disease. Other underlying conditions that can be associated with a meconium plug include cystic fibrosis, prematurity (colonic inertia of prematurity), neonatal small left colon syndrome, and maternal magnesium sulfate therapy.^20–22

Contrast enema examination of an infant with meconium plug syndrome shows 1 or more pellet-like filling defects in the distal portion of the colon (Figure 37-8). Despite the obstruction, the caliber of the colon is usually normal throughout. In infants with meconium plug syndrome due to cystic fibrosis, prominent meconium is often present throughout the colon; the findings are distinct, however, from meconium ileus, which includes microcolon and meconium plugging of the terminal ileum. The use of water-soluble contrast material frequently relieves the obstruction in infants with meconium plug syndrome. Occasionally, additional contrast or saline enemas are required. Careful clinical follow-up is essential to detect those instances in which underlying pathology, such as Hirschsprung disease, is present.

Small Left Colon Syndrome

Small left colon syndrome is a form of neonatal functional colonic obstruction that apparently is caused by immaturity of the neural plexus in the distal portion of the colon. Because maturation of the ganglion cells in the wall of the colon of the fetus progresses in a cranial-to-caudal manner, functional maturation of the distal portion of the colon may lag that of the more proximal aspect. The clinical presentation of small left colon syndrome typically occurs during the first 3 days of life, with abdominal distention, bilious vomiting, and failure to pass meconium. Many of these patients are infants of diabetic mothers. The differential diagnosis includes other forms of functional colonic obstruction, such as meconium plug syndrome, Hirschsprung disease, colonic inertia of prematurity, and ileus due to sepsis, medications, or hypothyroidism (Table 37-3). Some authors include small left colon syndrome and meconium plug syndrome as forms of neonatal functional immaturity of the colon.^23–25

Contrast enema of infants with small left colon syndrome shows a mildly to moderately diminished caliber of the left colon (Figure 37-9). Long filling defects are usually present in this portion of the colon, due to meconium. The rectum may be somewhat small or is normal in caliber. Unlike Hirschsprung disease, the diameter of the rectum is usually greater than that of the sigmoid colon, and the rectal morphology at the level of the anorectal sling is normal. The ascending and transverse portions of the colon are more prominent in size than the contracted left colon. This caliber change between the transverse and descending portions of the colon may result in an appearance that mimics that of Hirschsprung disease. A suction rectal biopsy should be performed if the clinical and/or radiographic features are equivocal.

Colonic Atresia

The colon is a relatively uncommon site of intestinal atresia. The estimated prevalence of colonic atresia is 1 in 20,000 livebirths; fewer than 10% of intestinal atresias involve the colon. Many patients with colonic atresia have concomitant atresias elsewhere in the bowel. Other concomitant anomalies can also occur, such as gastroschisis, Hirschsprung disease, intestinal neuronal dysplasia, defective mesenteric fixation, and anorectal anomalies. As with other intestinal atresias, the classification of colonic atresias is into 3 types: a membranous diaphragm, a fibrous cord between blind-ending lumina, and complete separation with a defect in the mesentery. The prevalence distribution of atresia is uniform throughout the colon. The likely pathophysiology of colonic atresia is a localized fetal vascular insult.^26–28

The clinical manifestations of colonic atresia are nonspecific. Bilious vomiting develops early in the neonatal period. Because the obstruction is relatively distal, the child may tolerate several feedings before becoming symptomatic. The degree of abdominal distention tends to be more extensive than that associated with atresias in the proximal aspect of the bowel. All infants with colonic atresia have obstipation, although passage of some meconium may occur initially.

Abdominal radiographs of a newborn with colonic atresia show distention of the small intestine and lack of gas in the rectum. Air–fluid levels are present on upright or decubitus views. The distended loops of bowel often fill the entire abdomen. Atresia of the proximal aspect of the colon has imaging findings similar or identical to those of ilial atresia. With more distal colonic lesions, there typically is massive distention of the segment of colon proximal to the obstruction (Figure 37-10). These radiographic findings are similar to those of other low GI tract obstructions, such as atresia of the distal portion of the ileum and meconium ileus. There is sometimes a mottled admixture of gas and meconium in dilated bowel proximal to the obstruction, but the classic “soap bubble” appearance of meconium ileus is lacking. In addition, air–fluid levels tend to be more prominent and better defined in patients with colonic or distal ileal atresia than in those with meconium ileus.

As with other low intestinal obstructing lesions of the newborn, the oral administration of contrast material is of no diagnostic use and carries some risk. A contrast enema is often helpful, however. The study is optimally performed with warmed nonionic water-soluble contrast medium, because of the risk of peritoneal spillage. The portion of the colon at the distal margin of the atretic segment sometimes communicates with the peritoneal cavity. In other patients, there is an abrupt termination of the contrast column at the site of the atresia (Figure 37-11). If there is a membranous obstruction, the head of the contrast column may assume a windsock configuration. The colon distal to the site of obstruction is abnormally small in infants with colonic atresia; however, the severe microcolon of meconium ileus is not a feature of this disorder.^29,30

Sonography can aid in the diagnosis of colonic atresia in the neonate, particularly early in the course when there is insufficient passage of bowel gas to allow an accurate radiographic estimation of the level of obstruction. It shows markedly dilated distal small intestine. There is also dilation of the colon proximal to the atresia. The dilated colon often has a complex echogenic pattern, due to retained meconium.³¹

Congenital Colonic Stenosis

Congenital stenosis of the colon is a rare cause of neonatal bowel obstruction. The pathogenesis likely is similar to that of atresia; an intrauterine vascular insult interferes with development of a short segment of the colon. Acquired stenoses of the colon can also occur in young infants, most often as a complication of necrotizing enterocolitis.

The radiographic features of infantile colonic stenosis are determined by the severity and location of the obstruction. Abdominal radiographs of these patients may be normal or show manifestations of a distal bowel obstruction. The diagnosis is established with a contrast enema, which shows 1 or more foci of narrowing. The margins are usually tapered. The bowel is sometimes somewhat dilated proximal to the narrowed segment. Careful fluoroscopic observation is required to avoid misdiagnosing localized spasm as a fixed stricture.

Anorectal Malformations

The spectrum of anorectal anomalies ranges from anal stenosis to persistent cloaca. Formation of the urorectal septum at 6 weeks gestation results in division of the cloaca into the urogenital sinus anteriorly, and the anorectal canal posteriorly. Failure of normal formation of the urorectal septum may result in a fistula between the distal colon and the urethra in males and the vagina in females. A fistula is present in approximately 70% of anorectal malformations. Major anomalies on the rectum and anus occur in approximately 1 in 5000 livebirths. The prevalence is slightly higher in males. Additional developmental lesions frequently accompany anorectal anomalies; potential associations include vertebral segmentation anomalies (half of infants with high anorectal malformations), cardiovascular anomalies (10%), anomalies of the esophagus or duodenum, and anomalies of the trachea, kidneys, spinal cord, or extremities (e.g., hypoplasia or absence of the radius). Anorectal malformations are part of the VACTERAL association (vertebral anomalies, anal atresia, cardiac anomalies, tracheoesophageal fistula, renal anomalies, and limb anomalies).^32–36

Anorectal malformations are classified into 4 types. Type 1 is a stenotic, but patent, anus and rectum. When located at the level of the anus, this is likely due to incomplete rupture of the fetal anal membrane. A stenosis within the rectum may be caused by incomplete development of the bulbus terminalis. Type 2 is an imperforate anus with a thin septal partition, due to persistence of the anal membrane.

Type 3 anorectal malformations consist of a blind-ending rectum and imperforate anus. Type 3 anomalies are classified into high and low subtypes, based on the relationship to the puborectalis muscle, that is, supralevator and infralevator malformations. This group encompasses approximately 90% of all anorectal malformations. Fistulae to the urethra, vagina, bladder, cloaca, vulva, or perineum are common with type 3 malformations; fistulae occur in approximately 90% of girls and 70% of boys.

High type 3 anorectal malformations are more common in boys than in girls, and are usually accompanied by a fistula that connects the blind-ending rectum to the urethra or bladder neck. A fistula between the rectum and the posterior vaginal fornix most often accompanies those uncommon instances of a high lesion in a girl. At least 90% of low type 3 anorectal malformations have an external fistula to the perineum or vestibule. The tract is usually quite small and may not be clinically apparent at birth. Meconium is eventually forced through the opening, however. In boys, the presence of a low lesion is indicated when meconium is observed exiting at or anterior to the imperforate anus. Cystic accumulations of green or white mucus, that is, “perineal pearls,” anywhere in the midline anterior to the anus or along the scrotal raphe always indicate a low lesion. In girls, it is sometimes possible to pass a small catheter through the fistula, and thereby establish the diagnosis.

In female infants with a high type 3 anorectal malformation, there may be separate vaginal and urethral orifices or a single urogenital sinus (cloaca) with bowel entering posteriorly and the urethra anteriorly. A rectovaginal fistula in these infants can result in the accumulation of gas and meconium in the vagina/and or uterus. Vaginal obstruction is common in this situation, and imaging studies may demonstrate hydrometrocolpos.

With type 4 anorectal malformations, the anus and the lower portion of the rectum are completely separated from the upper rectal segment. The rectum terminates as a blind pouch. This malformation is due to obliteration of the lumen at the proximal portion of the bulbous annulus, and may be the result of an ischemic event; this entity, therefore, likely represents a form of colonic atresia.

The clinical manifestations of anorectal malformations vary somewhat according to the specific anatomy. When the anus is imperforate, the presence of an anomaly is discovered during the newborn physical examination. A patent anus with rectal atresia is occasionally missed on the initial examination, but a presumptive diagnosis is subsequently established by the inability to pass a catheter into the rectum. The uncommon type 2 lesion (imperforate anal membrane) may result in the appearance of dark meconium visible through a bulging thin anal membrane. Type 3 lesions are detected clinically by observation of material exiting via a fistula, or the initial presentation may be related to manifestations of intestinal obstruction. These infants nearly always have signs of a bowel obstruction within the first 24 hours of life, unless there is a fistula of sufficient size to allow decompression. Infants with a type 4 anorectal malformation present with manifestations of a high-grade distal bowel obstruction.

Diagnostic imaging studies of the infant with an anorectal malformation serve to determine the level of the patent portion of the hindgut in relationship to the puborectalis muscle, to identify a fistula, and to detect associated anomalies. Within several hours after birth, abdominal radiographs of these infants usually show manifestations of a distal bowel obstruction. There is dilation of the colon, and to a lesser extent the small intestine (Figure 37-12). Vertebral anomalies, such as hemivertebrae and butterfly vertebrae, are common in these infants. Absence of a portion of the sacrum is also common. Multiple foci of calcified intracolonic meconium (meconium enteroliths) are sometimes visible on abdominal radiographs. The calcification usually indicates mixing of meconium and urine; therefore, this radiographic finding suggests the presence of a fistula to the urinary tract.³⁷

Lateral abdominal radiographs of the infant with an anorectal malformation sometimes provide information concerning the level of the obstruction (Figure 37-13). Useful localization of the lesion requires extension of bowel gas to the termination of the bowel. However, inspissated meconium may block the passage of air and result in an overestimation of the length of obstruction. Historically, the lateral radiograph was obtained with the infant in a head-down position to facilitate passage of air into the blind-ending rectal pouch; however, this has largely been supplanted by the prone cross-table technique, which is safer and technically easier to perform.

The location of the puborectalis sling is inferred on lateral radiographs by the “M line of Cremin.” The M line is located midway between a line drawn from the center of the pubic bones to the sacrococcygeal interspace (pubococcygeal line) and a line tangential to the inferior margin of the ischial bones (the ischial line). Proper patient positioning for the radiograph is essential for accurate measurement. Reliability of the examination is also compromised by variation in the location of the intestine as the infant cries or strains. Extension of rectal gas below the M line provides strong evidence of a low lesion; lack of this finding can indicate a high lesion or impaction of the inferior aspect of the rectum with meconium. Gas in the bladder suggests the presence of a high lesion with a fistula to the bladder or urethra.

In patients with a visible perineal or posterior four-chette fistula, injection of water-soluble contrast with a small catheter may allow visualization of the distal rectal pouch for preoperative planning. In a male infant with a suspected high lesion, a voiding cystourethrogram or retrograde urethrogram sometimes allows visualization of the rectum via a fistula. The fistula is often thin and only opacifies intermittently (Figure 37-14). In males, accurate characterization of the fistulous communication to the bladder neck or posterior, membranous, bulbus, or penile segment of the urethra has important prognostic and surgical implications. Injection of contrast through a percutaneously placed needle through the imperforate anus into the blind-ending rectal pouch is a simple and safe technique for the evaluation of anorectal malformations that cannot be opacified by other methods. This allows accurate demonstration of the level of the obstruction and the presence of associated fistulae. Following colostomy, an antegrade colostogram utilizing water-soluble contrast material is a simple technique for assessing the rectal anatomy (Figure 37-15).

Transperineal sonography is an additional technique to define the pathologic anatomy in infants with anorectal malformations. However, an inability to directly demonstrate the relationship of the rectum to the puborectalis sling is an important limitation of sonography. The presence of a low lesion is inferred when the distance between the perineum and the rectal termination is less than 1.5 cm. The rectal pouch appears on sonography as a fluid- or meconium-containing structure anterior to the sacrum (Figure 37-16).

MR imaging allows direct evaluation of the relationship of the rectum to the puborectalis muscle and the anatomy of the external sphincter. The size of the puborectalis muscle is also demonstrated. In those patients with a sacral anomaly, the puborectalis muscle and external anal sphincter may be located in eccentric positions. In general practice, however, MR is usually not performed prior to the initial surgical repair of anorectal malformations, but does serve an important role for the evaluation of older symptomatic patients who had repairs during infancy. Suboptimal surgical placement of the rectum with respect to the sphincter muscles is an important cause of incontinence in patients with anorectal malformations. MR of the pelvis provides information concerning the anatomy of the sphincter muscles as well as the relative location of the rectum. The sphincter muscles in these patients are nearly always hypoplastic and somewhat irregular. The most common site of misplacement of the rectum is anteriorly in the external anal sphincter and laterally in the puborectalis muscle.^38,39

Urinary tract abnormalities other than fistulae occur in approximately 25% of infants with a low anorectal malformation and 40% of those with high lesions. Therefore, urinary tract evaluation with avoiding cystourethrogram and renal sonography should routinely be performed for these patients. Potential associated urinary tract lesions include horseshoe kidney, renal ectopia, unilateral renal agenesis, renal hypoplasia, prune belly syndrome, hydronephrosis, and ureteral anomalies. Vesicoureteral reflux can also occur. In boys with an anorectal malformation, the urethrographic demonstration of sharp posterior angulation of the poster urethra often indicates the presence of a fistula even if contrast opacification of the tract is lacking.

Ectopic anus and anal stenosis are relatively minor types of anorectal malformation. The rectal pouch forms completely, and traverses the levator ani appropriately, but the opening through the cloacal membrane is incomplete (stenosis) or is displaced anteriorly (ectopic anus). The appropriate location of the anus can be determined by the following measurement: the distance from the base of the external genitalia to the anus divided by the distance from the base of the external genitalia to the coccyx (the anogenital index) should be greater than 0.46 in male infants and greater than 0.34 in female infants.

Ulcerative Colitis

Ulcerative colitis (nonspecific chronic ulcerative colitis) is a chronic idiopathic inflammatory condition of the colon. It most often presents in older children and young adults; the mean age at diagnosis in the pediatric age group is 12 years. There is a rare infantile form that is often fatal. Ulcerative colitis is particularly common among Jews, and is rare in individuals of African and Asian descent. This disorder occurs more frequently in first-degree relatives of affected patients and in individuals with the HLA-B237 histocompatibility antigen. There is no gender predilection. The overall prevalence of ulcerative colitis in children is estimated to be 2 to 14 per 100,000.^40–42

The hallmark pathological features of ulcerative colitis are extensive ulceration and diffuse inflammation of the mucosa. Involvement of the colon is continuous, beginning in the rectum and extending proximally for a variable distance. The disease is confined to the portion of the colon distal to the splenic flexure in 15% to 25% of patients and involves the transverse colon in approximately 50%. The entire colon is affected in 20% of patients with ulcerative colitis. Approximately 10% of patients with pancolitis have involvement of the distal ileum, that is, backwash ileitis.⁴³

The primary inflammatory process of ulcerative colitis is limited to the mucosa and submucosa; any changes in the deeper portions of the bowel wall are due to secondary involvement. The mucosa is infiltrated with inflammatory cells and is ulcerated. The ulcers initially arise from lateral extensions of crypt abscesses that eventually coalesce to form shallow mucosal ulcerations. Late in the disease, the colon may become foreshortened and the wall fibrotic.

At the time of clinical onset, about two-thirds of children with ulcerative colitis have diarrhea, often containing copious mucus. There is a history of hematochezia in about one-third of patients at presentation. Other potential findings include lower abdominal crampy pain, tenesmus, and fecal urgency. The onset of symptoms is often gradual; those patients with chronic disease may have anorexia and weight loss. About one-third of patients, however, experience a fulminant presentation, with explosive bloody diarrhea, fever, and manifestations of systemic toxicity. There is a potential for progression to perforation and peritonitis. Rectal bleeding in children with ulcerative colitis is nominally due to mucosal ulcerations and leakage of blood from engorged mucosal vessels. Diarrhea in these patients is predominantly due to sodium loss into the gut and disruption of the water-resorptive mechanism of the colon because of mucosal destruction.⁴⁴

Extraintestinal symptoms and signs are common in children with ulcerative colitis. Loss of protein through the inflamed bowel can lead to hypoalbuminemia. In young children, dehydration, anorexia, anemia, hypocalcemia, and osteoporosis may occur. Clubbing sometimes develops with long-standing disease. Arthralgia or frank arthritis coexists with colonic symptoms in 10% to 20% of children with ulcerative colitis. This most often is a migratory, nondeforming arthritis of the large joints of the lower extremities; spondylitis occurs occasionally. Manifestations of hepatobiliary disease are present in less than one-quarter of patients with ulcerative colitis; potential forms of liver involvement include fatty infiltration, pericholangitis, chronic hepatitis, autoimmune hepatitis, cirrhosis, and sclerosing cholangitis. Patients with ulcerative colitis are at elevated risk for deep venous thrombosis and thromboembolic disease.

There is a strong association between chronic ulcerative colitis and carcinoma of the colon. The likelihood of neoplastic degeneration increases with the duration of disease. Patients with pancolitis are at greatest risk. The cumulative risk for carcinoma of the colon among these patients is approximately 5% in those patients who have the disease for 10 years and 25% in those who have the disease for 20 years; the risk continues to increase thereafter. Ulcerative colitis patients with disease confined to the left side of the colon have a cancer risk that is only slightly greater than that of the general population. Colon carcinoma in patients with ulcerative colitis is often poorly differentiated and metastasizes early. Multiple lesions may be present. On contrast studies, these tend to be flat, poorly defined lesions; therefore, any contour irregularity of the bowel wall in a patient with ulcerative colitis should be considered suspicious.

The usual method for establishing a definitive diagnosis of ulcerative colitis is endoscopy and biopsy. Contrast studies of the colon are helpful for selected patients. In general, the air-contrast technique should be utilized. However, contrast studies of the colon should be avoided in patients with clinical manifestations of severe colitis, due to the risk of perforation or inducement of toxic megacolon. An important characterizing feature of ulcerative colitis on imaging studies and endoscopy is contiguous and circumferential involvement of the colon; skip areas do not occur in untreated patients (Figure 37-17).⁴⁵

Early in the course of ulcerative colitis, edema, hyperemia, and abnormal mucin production result in a fine granular mucosal pattern on barium studies and colonoscopy (Figure 37-18). Slight haustral thickening, due to edema, is sometimes present. The mucosa may have a stippled appearance due to retention of contrast in superficial ulcers. If there is granulation tissue interspersed between ulcerations, a coarse granular mucosal pattern occurs. Deeper, more advanced, ulcerations produce a serrated contour of the bowel wall. With severe progressive disease, ulceration can denude portions of the colonic wall, resulting in inflammatory pseudopolyps. The pseudopolyps represent islands of inflamed mucosa or granulation tissue that are surrounded by bowel that is devoid of mucosa. Other potential imaging findings during the acute phase of ulcerative colitis include perforation, mural thinning, and pneumatosis (due to toxic megacolon).

During the subacute and chronic phases of ulcerative colitis, mural thickening and luminal narrowing are the predominant imaging features. These changes result from hypertrophy of the muscularis mucosae, and are distinct from the transmural fibrosis that occurs in Crohn disease. Contraction of the hypertrophied longitudinal muscle may pull the mucosa away from the submucosa and thereby produce luminal narrowing. Additional factors responsible for luminal narrowing in these patients are thickening of the submucosa due to edema or fat deposition and thickening of the lamina propria due to round cell infiltration. On contrast studies, the involved portion of the colon lacks haustra, the mucosa is atrophic, and pseudopolyps may be present. Pseudopolyps in the chronic phase often have a branching or worm-like appearance. Fibrosis can lead to incompetency of the ileocecal valve and backwash ileitis (reflux ileitis). This appears on contrast studies as mucosal irregularity or frank ulceration in the terminal ileum. There is often dilation of the involved portion of small bowel, and normal mucosal folds are absent (Figure 37-19). Widening of the anorectal contour on lateral images can occur in both the acute and chronic phases of ulcerative colitis.

Ulcerative colitis is 1 of several inflammatory conditions of the colon in children (Table 37-4). Many of these disorders are acquired abnormalities that are recognized clinically. Therefore, the most important consideration in the differential diagnosis is the other major idiopathic inflammatory bowel disease, Crohn disease. There is overlap in the clinical and radiographic appearances of ulcerative colitis and Crohn disease. In ulcerative colitis, inflammation is restricted to the mucosa of the colon, the rectum is universally involved in untreated patients, the mucosal inflammation is continuous and homogeneous, and the small intestine is not involved (except in the form of backwash ileitis). Crohn disease, in contradistinction, is a transmural inflammatory process that can involve any segment of the bowel from the oral cavity to the anus. The intestinal manifestations of Crohn disease are nearly always asymmetric and segmental; the terminal ileum is the most commonly affected site. Disease is restricted to the large intestine in only 10% to 15% of children with Crohn disease. Table 37-6 summarizes some of the imaging features that are helpful in differentiating ulcerative colitis from Crohn disease.

The pathological changes in the colonic wall during the subacute and chronic phases of ulcerative colitis result in a target or halo appearance on contrast-enhanced CT. A ring of soft tissue attenuation that represents the mucosa, lamina propria, and hypertrophied muscularis mucosae surrounds the lumen. The submucosa with fatty infiltration forms a surrounding lower attenuation zone. More peripherally is a ring of soft tissue attenuation that represents the muscularis propria. This layered appearance of the colonic wall is termed mural stratification (the “fat halo sign”). Mural stratification is not specific to ulcerative colitis, as it can occur in any condition that causes fatty infiltration of the submucosa. The differential diagnosis for this finding includes ulcerative colitis, Crohn disease, and graft-versus-host disease. With ulcerative colitis, the outer surface of the colonic wall is typically smooth, whereas wall irregularities are present in most patients with Crohn colitis. Wall thickening tends to be more severe in patients with Crohn colitis than in those with ulcerative colitis. Mural stratification is a more common finding with ulcerative colitis (approximately 60% of patients) than with Crohn colitis (8% of patients).^46,47

Rectal involvement is a consistent feature of ulcerative colitis. Mural thickening results in narrowing of the lumen of the rectum, and axial CT shows a target appearance. Proliferation of perirectal fat causes widening of the presacral space; on CT, this fat has slightly higher attenuation than normal. There may be nodular or irregular soft tissue densities intermixed with the fat. The proliferation of perirectal fat is related to ex vacuo expansion of the presacral space as the rectum becomes narrowed, as well as lipodystrophy that is caused by edema and inflammation. The inflammation also results in enlargement of perirectal lymph nodes.