The embryologic site of origin and the pathophysiology of these anomalies are unknown. Several theories have been proposed, but none of them, if considered individually, can explain all the different combinations of type, location, and associated anomalies of duplications.

There are four major theories concerning the origin of enteric duplications:

The partial or abortive twinning theory states that alimentary tract duplications are the result of incomplete twinning. This hypothesis can apply in particular for the foregut and hindgut duplications that are associated with doubling of the mouth, genitourinary tract, and lower bowel. The extent of the twinning depends on the moment the process starts [8].

The aberrant luminal recanalization theory, postulated by Bremer in 1944, proposed that duplication is due to the persistence of outpouching of the fetal bowel. This theory may apply to those duplications of the segment of the gastrointestinal tract that goes through a “solid stage,” such as the esophagus, small bowel, and colon. After this phase, which occurs between 6 and 8 weeks of intrauterine life, the progressive craniocaudal growth allows the development of a lumen. According to Bremer’s theory, duplication may be the result of an incomplete or defective vacuolization of the intestine and/or the persistent embryological diverticula with the formation of two channels, either communicating with each other or not. This theory can apply to simple duplications, i.e., not associated with other anomalies, and provides a functional explication of the highest occurrence of duplications in the ileal tract, which is the main site of diverticula; however, it does not provide proper explanation for the heterotopic mucosa found in some duplications and why the lesions are located more frequently on the mesenteric side while most of diverticula are found instead on the antimesenteric side [9].

Bentley and Smith in 1960 proposed the split notochord theory to describe the duplications that develop in the chest and are associated with spinal defects and skin anomalies. During the third to fourth week of gestation, the notochord starts to close and separate from the endoderm. If an error occurs during the separating phase, an abnormal adhesion between the neural tube ectoderm and the gut endoderm forms, with the development of a gap and a secondary herniation of endodermal cells; furthermore, endodermic tissue can act as a barrier to the anterior fusion of the vertebral mesoderm resulting in vertebral defect. This mechanism may explain the long duplication cysts and foregut duplication, their dorsal location, and the association with spinal malformation (15 % of the cases). It does not explain, however, the entire range of abnormalities (such as heterotopic gastric mucosa) [10].

The environmental factors theory suggests that stress, hypoxia, and trauma can be involved as cause factors of the malformation as described by Mellish and Koop in 1961. Although the actual mechanism that induces the malformation is not clear, an intrauterine vascular accident and/or a compression from nearby organs could explain the anomalies and the association with other malformations as intestinal atresia [11].

Foregut duplications need to be discussed separately. Foregut duplications include a wide spectrum of anomalies, the esophageal, the bron-chogenic, and the neurenteric cysts, subdivided according to their embryologic origin, the anatomopathological characteristics, and the anatomical district concerned. It is believed that bronchogenic and the esophageal duplication cysts result from an altered budding of the embryonic foregut between the fifth and the eighth week of gestation; notochord subdivision alterations can explain the origin of the neurenteric cysts. About 50–60 % of foregut duplications are bronchogenic; they are usually located close to the trachea but can be found in many locations (mediastinum, intraparenchymal, paraesophageal, paratracheal, perihilar) and are frequently associated with congenital pulmonary airway malformations (congenital cystic adenomatoid malformations, pulmonary sequestration), forming hybrid lesions. An enteric cyst may be lined by ciliated respiratory epithelium, but the presence of bronchial wall structures, particularly cartilage, but also smooth muscle and glands, is necessary for diagnosis of bronchogenic cyst; a neurenteric cyst can also be lined by enteric-type mucosa and has a pedicle that extends to the spinal canal [12, 13].

As described by Ladd, enteric duplications have three characteristics: (1) an intimate anatomical connection with any part of the gastrointestinal tract, (2) an epithelial lining representing some portion of the alimentary tract, and (3) a well-developed coat of smooth muscle [3]. The lesion tends to locate on the antimesenteric side of the alimentary tract with which it frequently shares the muscular coat and blood supply. The epithelial lining is usually the same as the mucosa native to the lesion, but in 35 % of the cases, an ectopic tissue is present, most commonly gastric followed by pancreatic mucosa, which predisposes to complication as ulceration, hemorrhage, and perforation; rarely, in the thoracic duplication, a respiratory epithelial lining can be present. Generally duplications are classified as two entities, the tubular and the cystic type. Cystic lesions are more common (65–90 %); they are more frequently found in the small intestine and can have big size; and they are closed at their two ends and normally covered with the same mucosa as the native intestine. They do not usually communicate with the intestinal lumen. The tubular type (10–35 %) can often be remarkably long; it may communicate with the adjacent alimentary tract, usually in the caudal end or at both ends, and can contain heterotopic mucosa more frequently than the cystic type [1, 5, 14].

Li et al. have classified small intestinal duplications in two types based on the vascular pattern (Fig. 18.2).

In their study, Li and colleagues found that vertebral defects were more frequently associated with the type 2 lesion (91.6 %) than the type 1 lesion (5.5 %), hypothesizing a different embryologic cause as the origin of the two anomalies. Furthermore, their study shows how the knowledge of the vascular anatomy of small intestinal duplications may have a surgical implication, allowing excision of these lesions without resection of the adjacent bowel. The type 1 duplication could be excised by dividing the mesentery; the type 2 duplication may be enucleated by tying off the short branches from the main vessels [15].

Clinical manifestations are extremely variable, depending on the site, type, and size of the duplication and if it contains gastric mucosa. Most duplications (80 %) are detected in the first two years of age, in particular in half of the cases in the first six months of age. They are frequently asymptomatic and diagnosed incidentally on routine X-ray or ultrasound examination performed for other malformations or rarely may be an incidental intraoperative finding. In recent years the diagnosis has been increasingly made with prenatal ultrasonography.

When located in the mediastinum, they could lead to pneumonia, wheezing, cough, or dysphagia. Symptoms such as respiratory distress and failure to thrive are more common in small infants, whereas chest pain occurs more frequently in older children.

The most common presentations of an abdominal duplication are abdominal pain and distension, vomiting, and abdominal mass. Complications include bleeding and perforation due to an ectopic gastric mucosa, with peptic ulceration or intestinal occlusion due to the development of volvulus, intussusception, or extrinsic compression frequently for an acutely enlarged cystic mass. Other rare complications are pancreatitis and cholecystitis in the gallbladder, duodenal and pancreatic duplication, cyst infection, and malignancy, the latter more frequent in adulthood and in hindgut duplications. Volvulus and intussusceptions are more frequent in midgut duplications, whereas in hindgut duplications symptoms by mass effect with obstruction of both the urinary tract and the bowel tract prevail. Bleeding, hemorrhage with melena, and perforation are frequent both in midgut and hindgut duplications [4, 14, 16].

The high resolution modern imaging techniques allow physicians to identify enteric duplications in the prenatal age, in particular those located in the chest and in the upper abdomen, in approximately 30 % of cases. Since duplications are often associated with other malformations, if an enteric duplication is found during a prenatal ultrasound, a fetal magnetic resonance imaging (MRI) is advisable. The fetal MRI also allows physicians to identify fetuses at risk who might require invasive procedures (i.e., thoraco-amniotic shunt for fetal hydrops or mediastinal shift) and to establish the best therapeutic strategy in the postpartum so as to avoid complications [14, 16].

The clinical history and the physical examination are the first step for the diagnosis of a duplication, whereas the laboratory exams may detect only anemia in case of bleeding due to heterotopic mucosa or higher serum amylase and lipase levels in the rare case of pancreatitis.

Postnatally ultrasound can be useful to demonstrate the nature (solid or cystic) and the location of the mass as well as to evaluate any connection with the adjacent intestine. The cystic lesion appears as an anechoic structure (in case of no bleeding) surrounded by a 2–3 mm thick wall that determines a characteristic and pathognomonic echogenic signal defined as “gut signature” or “double layer,” which is made up of an hyperechoic inner mucus layer and a hypoechoic muscular outer layer.

An abdominal X-ray can show a mass effect in the event of a large cyst or signs of intestinal obstruction or perforation in complicated cases. A thorax X-ray can show a mass usually located in the medium or posterior mediastinum and eventually associated vertebral anomalies.

Computerized axial tomography scan (CT scan) and MRI are more accurate in showing the anatomical features and the relationship with nearby organs; MRI is advisable in the childhood and useful to detect any spinal involvement.

Gastrointestinal contrast studies can demonstrate a filling defect as the duplication does not usually communicate with the intestinal lumen. The endoscopic ultrasonography, the esophagogastroduodenoscopy, and the wireless capsule endoscopy can show ulcers or stenosis and can help defining better the anatomy before a surgery. The endoscopic ultrasound (EUS) can provide further information, detecting the cystic nature of the lesion with its characteristic wall, its location, and its anatomical relationships with adjacent structures; it can also be useful as a guide for fine needle aspiration, which is used only in selected cases.

In the case of a patient presenting anemia and lower gastrointestinal bleeding, a scintigraphy with technetium-99m pertechnetate can be useful to identify ectopic gastric mucosa and make a differential diagnosis with the Meckel’s diverticulum.

Definitive diagnosis is based on histopathological findings after surgical excision.

Differential diagnosis includes intrathoracic mass or tracheoesophageal fistula in the foregut duplications, appendicitis, Meckel’s diverticulum and other causes of intussusception in the small bowel duplications, and constipation or Hirschsprung’s disease in colonic and hindgut duplication. In the rare case of a gastric or duodenal duplication, the sign and symptoms can mimic a hypertrophic pyloric stenosis or a gastroesophageal reflux [1, 4, 8, 14].

Management depends on the presentation. Intestinal duplications often require an urgent surgical intervention due to the onset of complications such as perforation, intestinal occlusion, or severe bleeding. This is why, although the treatment of asymptomatic lesions remains controversial, most authors currently recommend surgery even with no symptoms in order to avoid any complication at a later moment [16, 17]. The type of surgery varies depending on the cases [6, 7]: