Chapter 17: The gastrointestinal tract
Many gastrointestinal organs develop simultaneously and hence a given defect warrants investigation for lesions in other organs.
While agenesis of the esophagus can occur atresia—replacement of a segment (usually near the corina) with a non-canalized cord—is comparatively more common. Atresias are often associated with fistulae, which connect the malformed esophagus to the trachea or bronchus, though fistulae can occur without atresia. Fistulae between the respiratory tract and the upper or lower esophageal pouches can result in aspiration, suffocation, pneumonia, or fluid and electrolyte imbalances. Atresia of the intestines tend to involve the duodenum more than other segments however an imperforate anus is the most common form of intestinal atresia and is due to failed involution of the cloacal diaphragm. An incomplete atresia is termed stenosis—a fibrous thickening of the wall—resulting in partial or complete obstruction. Stenoses can be congenital or acquired (e.g. caustic injury, chronic reflux, systemic sclerosis, irradiation injury) and most often affect the esophagus and small intestine.
A diaphragmatic hernia describes presence of the abdominal viscera in the thoracic cavity due to incomplete formation of the diaphragm; sever herniation can cause lung hypoplasia.
Omphalocele describes herniation of abdominal viscera into a ventral membranous sac due to incomplete formation of the abdominal musculature, which is often repaired surgically but frequently associated with additional congenital defects. Gastroschisis involves all layers of the abdominal wall including the skin.
Ectopic tissue is common throughout the GI tract. The most common site for ectopic gastric mucosa is in the upper 3rd of the esophagus, which is typically asymptomatic but can cause dysphagia, esophagitis, barrett esophagus, or adenocarcinoma.
Ectopic pancreatic tissue found in the esophagus and stomach—and like ectopic gastric mucosa—is often asymptomatic but can also cause damaging inflammation and may lead scaring and obstruction (if present in the pylorus).
Gastric heterotopia, patches of gastric mucosa in the intestines, can mimic cancer appearing as mass and causing occult blood from peptic ulceration.
Diverticula are defined as outpouchings of the alimentary tract that communicates with the lumen and includes all three layer of the bowel wall. Meckel diverticula are congenital and occur when the vitelline duct fails to involute. The vitelline duct connects the lumen of the embryo’s gut to the yolk sac. The diverticula often resemble normal small intestine but sometimes contain ectopic tissue causing abdominal pain and occult bleed, which mimics acute appendicitis or obstruction.
Meckel diverticula occur in 2% of the population, most often occur within 2 feet (60 cm) of ileocecal valve, are 2 inches (5cm) long, twice as common in males, are become symptomatic at age 2.
Twin studies and associations between turner syndrome and trisomy 18 with congenital hypertorphic pyloric stenosis suggest that there hereditary and environmental factors contribute to the etiology. Acquired stenosis may occur in adults secondary to antral gastritis, peptic ulcers, or carcinomas of the distal stomach or pancreas.
Hyperplasia of the pyloric muscularis propria obstructs the gastric out flow tract and is often accompanied by inflammatory mediated cellular changes, which can contribute to narrowing of the orifice.
Congenital stenosis presents between the 3rd and 6th weeks of life as new onset projectile, nonbilious vomiting after feeding and demand for re-feeding. Individuals may have a 1-2cm firm, ovoid, abdominal mass. A left-to-right hyperperistalsis may be evident during feeding or immediately before vomiting. Surgical splitting of the muscularis is curative.
Agenesis of the enteric neuronal plexus results in compensatory dilation of the normally innervated proximal colon—diameters may reach 20cm—potentially causing rupture and ulcerations due to inflammation, the former thereof occurs most frequently at the cecum while the latter occurs anywhere along the innervated portion of the colon. The rectum is always involved but can involvement of the other segments varies widely.
Pathogenesis. Congenital aganglionic megacolon has two principle etiologies: failed migration of normal neuronal crest cells form the cecum to the rectum or premature death of the ganglion along segments of the colon. In both cases the result is aganglionosis: agenesis of the submucosal (Meissner) and myenteric (Auerbach) plexus and thus there is not mechanism for coordinated peristaltic to take place. The defective mechanism of neural crest cell migration is unknown however autosomal dominant loss-of-function pathogenic variants of RET, which codes for a tyrosine kinase receptor, accounts for the majority of familial cases and some sporadic cases. Pathogenic variants of RET ligand glia-derived-neurotrophic growth factor, endothelin, and endothelin receptor have also also been described as possible etiologies. While there are acquired forms of megacolon, the only one associated with death of enteric ganglion is due to Chagas disease.
Clinical features. Presentation is often immediately after birth with failure to pass meconium followed by obstruction; constepation; visable, ineffective peristalsis; abdominal distension; and bilious vomiting. Involvement of short segments may make diagnosis difficult. The most serious complications include: enterocolitis, fluid and electrolyte imbalance, perforation, and peritonitis. Treatment is surgical resection of aganglionic portions of the colon and may take years to develop normal bowl function.
The esophagus develops from the cranial portion of the foregut and is identifiable by the 3rd week of gestation.
Obstruction be structural or functional such as by a neoplasm or lack of peristaltic contractions, respectively. Esophageal dysmotility has three formal classifications:
Nutcracker esophagus: hyper-amplitude due to uncoordinated contractions of the inner circular and outer longitudinal distal muscular layers.
Diffuse esophageal spasm: repetitive simultaneous contractions of the distal muscular layers.
Hypertensive lower esophageal sphincter: high resting pressure of incomplete relaxation of the lower espohageal sphincter.
Small diverticula are likely to develop in all forms of esophageal dysmotility (increased wall stress) most commonly in the epiphrenic esophagus superior to the lower sphincter. Less commonly, pharyngoesophageal (Zenker) diverticula can develop immediately above the upper sphincter due to spasm or impaired relaxation of the cricopharyngeus muscle. Pharyngoesophageal diverticula uncommon to form before the 5th decade of life and when large enough cause regurgitation and chronic halitosis.
Mechanical obstructions are often insidious: individual may unconsciously develop preferences for softer foods before presenting. However individuals with mechanical obstructions typically maintain their weight and appetite.
Benign stenosis occurs as a result of atrophy of the muscularis propria and fibrous thickening of the submucosa. It can be congenital but is most often acquired through chronic inflammation caused be chronic reflux, irradiation, or caustic injury.
Idiopathic protrusions of the mucosa are rare cause of obstruction that tends to affect women over the age of 40 and are also associated with chronic graft-versus-host disease, reflux, and blistering skin diseases. These esophageal mucoasl webs are semi-circumferential, fibrovascular lesions with normal overlaid epithelium. Patterson-brown-kelly or plummer-vinson syndrome describes upper esophageal webs, iron-deficiency anemia, glossitis, and cheilosis. Presentation is often due to non-progressive dysphagia with incompletely chewed food.
Esophageal rings are fully circumferential protrusions that include the mucosal, submucosal and occasionally a hypertrophic mucularis propria layers. They are typically thicker than webs and occur anywhere above the gastroesophageal junction. There are two types: A-rings and B-rings—that are covered by squamous mucosa and cells at squamocolumnar junction, respectively.