Gastroenterology

Duodenal atresia or stenosis

Atresias can occur at any point along the gastrointestinal tract, and may be singular or multiple, complete or incomplete. The duodenum is one of the most common sites of atresia; the incidence is 1 in 10,000–30,000 live births. Most occur at the ampulla of Vater, either due to a complete mucosal membrane or a blind-ending proximal loop, probably due to failure of canalization of the duodenum after the 7th week of life due to ischaemia. Associated anomalies occur in about 50% of cases, including Down's and Prader–Willi syndromes. Presentation may be antenatal with polyhydramnios, or a ‘double bubble’ on ultrasound scan. Bilious vomiting is a presenting feature, usually immediately after the first feed.

Exomphalos

With an incidence of 1 in 3000, this is a midline defect of the anterior abdominal wall, where some of the organs lie outside the abdominal cavity. The organs are encased in a membranous sac derived from the amniotic membrane. The organs that are contained within the sac vary but can include stomach, intestine, liver and spleen. The defect is classified according to size (major being >4 cm abdominal wall defect and minor <4 cm). Exomphalos is associated frequently with other congenital abnormalities, including trisomy 13, 18 and 21, and 75% of cases also have other associated structural problems such as cardiac anomalies. It is usually identified on antenatal scanning. Surgical closure is required but, depending on the size of the defect and whether the sac remains intact, this is generally not an emergency providing the intra-abdominal structures are well perfused.

Gastroschisis

The incidence is 4 per 10,000 births and risk factors include low maternal age, drug misuse, low socio-economic status, smoking and ethnic origin. The abdominal muscles are normal but the stomach and intestines herniate through the anterior abdominal wall on the right-hand side of the umbilicus. Associated atresias are found in up to 10% of cases (due to ischaemic events) but other congenital anomalies are not usually present.

The cause is spontaneous herniation of the intra-abdominal wall in utero (possibly at the site of the right omphalomesenteric artery) or from incomplete reduction of the abdominal contents following rotation during the first weeks of fetal life. The abdominal contents are not covered by a membrane and therefore are exposed to amniotic fluid. This results in serositis with matting together of the intestines, which may need resection after birth.

There is a high risk of intrauterine death (15%) due to ischaemia and thus serial antenatal ultrasound scans are recommended. Delivery can be either vaginal or operative, but should take place in a specialist paediatric surgical centre. A protective membrane is placed over the eviscerated abdominal contents to prevent them drying out and urgent surgery is usually required to close the defect. The surgery may be a primary closure, or secondary if the defect is too large to replace all the contents in one stage without causing respiratory compromise from splinting of the diaphragm. In this case, a silo may be used to preserve the intestinal contents while complete closure is awaited.

It often takes time for feeding to be established due to poor intestinal motility. A prolonged course of parenteral nutrition may be necessary. Overall survival is 90%.

Meckel's diverticulum

This is common (2–4% of newborn infants) and occurs when the remnant of the omphalomesenteric duct does not fully regress and remains attached to the ileal mucosa ( Fig. 14.3 ). The diverticulum contains gastric mucosa, which produces acid. It can often be detected using a technetium-99m scan (the sensitivity of this is 85% in children with a specificity of 95%).

A Meckel's diverticulum may present with lower gastrointestinal bleeding, intussusception, small bowel volvulus or it may never cause any problems, depending on the size of the sac. If problems occur, surgical resection is necessary.

Rarely, failure of the omphalomesenteric duct to regress results in an intestinal fistula that may present with an umbilical discharge. It may appear similar to an umbilical granuloma, but it is possible for a catheter to be inserted. Surgical resection is required.

Midgut malrotation

This occurs when the intestine lies in an abnormal position within the peritoneal cavity. It is subsequently at risk of torsion (volvulus) around the superior mesenteric artery axis ( Fig. 14.4 ). Malrotation is due to incomplete rotation of the gut contents during the first 11 weeks of embryogenesis. The most common defect results in the duodenum lying to the right of the vertebral column instead of the left, with the caecum lying in the upper abdomen to the left of the duodenum (Pattern A in Fig. 14.4 ). Bands of peritoneum (Ladd's bands) pass from the caecum, across the duodenum, to the posterior abdominal wall. This may cause extrinsic obstruction of the duodenum, resulting in bilious vomiting, usually in infancy. As a result, the midgut has a short attachment to the base of the superior mesenteric artery and so may twist clockwise around this pedicle, resulting in potentially fatal midgut ischaemia. Midgut volvulus should be suspected in any child with bilious vomiting, as the ischaemia can be potentially reversed if the Ladd's bands are divided and the pressure from the arterial supply to the midgut is relieved. Children may also present with subacute symptoms of intermittent abdominal pain and vomiting. A barium meal is then the investigation of choice as it can show the position of the duodenal–jejunal flexure. The true incidence is not known, as there are probably individuals with undiagnosed malrotation who never have any problems if the pedicle is long enough.

Meconium ileus

In cases of pancreatic insufficiency, the distal ileum becomes impacted with thick, viscous meconium. This is associated with cystic fibrosis in most cases, with 20% of newly diagnosed children developing meconium ileus.

Duplication cysts

Duplication cysts are congenital cysts attached to the gastrointestinal tract anywhere between the mouth and anus, but are found most commonly in the ileocaecal region. The histological nature of the cyst is related to the adjoining structure. Presentation may be with an abdominal mass, or obstruction. When problems occur, surgical excision is required.

Hirschsprung's disease

In Hirschsprung's disease, there is an absence of ganglion cells in a variable segment of bowel (aganglionosis). The incidence is 1 in 5000 live births, and is more common in boys. The sigmoid and rectum are the most commonly affected sites, although the whole colon and, very rarely, the entire large and small bowel can be affected. The lack of ganglion cells causes an inability of the bowel to relax, resulting in a functional bowel obstruction.

It should be suspected in infants who have not passed meconium in the first 48 hours of life or in infants who present with bowel obstruction. Dramatic decompression occurs on digital examination of the rectum, with so-called ‘explosive diarrhoea’. Digital examination should be performed by a senior doctor, and limited to only one examination, where possible.

Diagnosis is confirmed by rectal suction biopsy to confirm the absence of ganglion cells (in older children, a strip rectal biopsy). Surgery is needed to remove the aganglionic section of bowel. Enterocolitis is one of the major complications after surgery, when children can present with malaise, fever, diarrhoea and generalized sepsis. The pathophysiology of this is incompletely understood, but is thought to be due to gastrointestinal stasis, altered gut flora and impaired mucosal immunity.

Anorectal anomalies

These occur in 1 in 2500 births. The pathophysiology is poorly understood, but it is thought to be due to failure of the breakdown of tissue at the caudal end of the gastrointestinal tract in week 3 of gestation. Most often identified on routine post natal check, 60% have other congenital malformations of the gastrointestinal tract and/or genitourinary tract; in boys there may be a fistula to the urethra, in girls to the vestibule adjacent to the vagina. Other anomalies (e.g. VACTERL) should be considered.

Intussusception

Intussusception is not classically considered as an embryological abnormality and can occur in the presence of a structurally normal gut. However, even minor gut abnormalities, such as a polyp or Meckel's diverticulum, may predispose a child to this condition. It occurs when one section of the intestine invaginates into another section and can result in obstruction. The most common site is at the ileocaecal junction, but it can occur at other sites if there is a lead point such as a polyp or a Meckel's diverticulum. Patients usually present with acute onset abdominal pain, drawing up of the legs and pallor. They may have ‘redcurrant jelly’ stool, but this is a late sign. Diagnosis is by ultrasound scan, which shows a target-shaped mass. Treatment aim is to reduce the intussusceptions with an air enema, but surgical reduction may be necessary.

Secretions

The digestive juices that are secreted throughout the GI tract are a mixture of water, electrolytes, enzymes, bile salts and mucins, which aid digestion. When there is excess secretion or a failure to reabsorb enough of these secretions, either vomiting or diarrhoea results.

Digestion

This is the process by which complex food particles are broken down to smaller units that can be absorbed across the gut mucosa. Enzymes aid this breakdown.

• Carbohydrates must be broken down to monosaccharides before absorption. The starch, glycogen and cellulose, the main sources of dietary carbohydrates, must be broken down to fructose, glucose and galactose to allow absorption.

• Proteins are broken down to their constituent amino acids.

• Fats (mainly triglycerides) are broken down to monoglycerides and fatty acids.

Absorption

Most absorption takes place in the small intestine (see the later section on small intestine for details).

Motility

Motility is the function of the gastrointestinal tract that allows movement of the contents of the intestine in a craniocaudal direction. It is the food contents themselves that stimulate the contraction of the muscles above the food bolus and relaxation of the muscles below the bolus to produce craniocaudal progression.

Mouth

The smell, thought and presence of food stimulates production of saliva from the sublingual, submandibular, parotid and buccal glands. Saliva contains mainly water, but also contains small quantities of the salivary enzymes amylase (which converts polysaccharides to maltose), mucus (to allow lubrication) and lysozyme (that lyses certain bacteria). Chewing of food particles is the first stage of breakdown of food and allows easier passage into the oesophagus. Whilst some drugs can be absorbed through the buccal mucosa (e.g. midazolam), no absorption of nutrients takes place here.

Oesophagus

When a food bolus is moved to the back of the pharynx, the swallowing centre in the cerebral medulla is activated. This results in closure of the vocal cords and epiglottis, and facilitates safe passage of the bolus into the oesophagus. Sphincters at the lower and upper end of the oesophagus prevent food particles from refluxing back to the mouth.

The upper oesophageal sphincter remains tonically contracted and only relaxes to allow food particles through. It is under the control of cranial nerves V, IX and X. Peristalsis within the oesophagus is stimulated by swallowing of a food bolus. Inhibitory neurons induce lower oesophageal sphincter relaxation and coordinate proximal-to-distal peristaltic contraction. When the lower oesophageal sphincter fails to relax, achalasia results, and usually needs surgery to divide the muscles at the lower oesophageal sphincter to allow the passage of food into the stomach. Achalasia is a rare condition in children, but can still occur. No further digestion or absorption of food takes place here.

Stomach

The stomach has a number of functions including storage of food, secretion of the digestive juices and mixing up the contents of the stomach into chyme.

Gastric motility ( Fig. 14.5 )

The smooth muscle of the stomach wall has the unique property of ‘plasticity’. In fully grown children and young adults, this allows the stomach to hold between 50 mL and 1000 mL of stomach contents whilst the pressure or tone exerted on the contents remains the same. The folds of stomach mucosa, or ‘rugae’, stretch out as the stomach becomes fuller. Once the stomach has become fully distended, the individual will experience discomfort and usually this will prompt them to stop eating.

The cells in the fundus generate slow-wave potentials that move down the length of the stomach towards the pylorus at a rate of 3 per minute. This contraction occurs rhythmically and does not cause contraction of the circular muscles. The muscle layers in the fundus of the stomach are significantly thinner than the antrum, so the strength of these contractions is lower at the proximal end.

The antral peristaltic contractions are responsible for the mixing of food. The pylorus will only open enough to allow the very watery chyme through; the thicker chyme is not able to pass through, so as the peristaltic wave meets the pylorus, the majority of the contents of the antrum are actually propelled backwards against the pylorus and back into the antrum for further mixing.

Pancreas, gall bladder and liver

Intestinal absorption and secretion

Large volumes of fluid are secreted into and reabsorbed by the intestine. In adults, the daily fluid exchange is:

• Input into small intestine: 9 L/day (diet: 1.8 L; endogenous secretions: 7.2 L)

• Input into colon: 1.5–2.0 L/day

• Output of fluid in faeces: 100–200 mL/day

Small intestine

The intestinal mucosa is a complex epithelium in which absorption and secretion occur simultaneously, with the majority of total water and electrolyte absorption occurring in the small intestine. The mucosa of the intestine acts as a semi-permeable membrane with pores in the membrane at intercellular junctions. Water movement is entirely passive, the majority passing paracellularly in response to osmotic gradients created by the transcellular absorption of solutes, particularly sodium.

Mechanism of water and solute absorption in the small intestine

Overall, water absorption is dependent on the movement of electrolytes, especially sodium. The primary mechanism of sodium absorption is by the glucose-sodium transporter 1. This promotes the active absorption of sodium, allied to the absorption of glucose, with water moving down the electrochemical gradient that is created as jejunal contents are broken down. This is the scientific basis for the use of oral rehydration solutions used in the management of gastro­enteritis ( Fig. 14.6 ).

In gastroenteritis, giving oral saline solution is not helpful because normal sodium absorption is impaired in the diarrhoeal state and if the sodium is not absorbed, neither is water. In fact, excess sodium in the lumen of the intestine causes increased secretion of water and the diarrhoea worsens. If glucose is added to a saline solution then glucose molecules are absorbed through the intestinal wall. This is unaffected by the diarrhoeal disease state. In conjunction, sodium is carried through by a cotransport coupling mechanism. This occurs in a 1 : 1 ratio, one molecule of glucose cotransporting one sodium ion. Glucose does not cotransport water – rather, it is the relative increase in concentration of sodium ions across the intestinal wall which pulls water through after it. Several other molecules apart from glucose have a similar capacity to cotransport Na ⁺ including:

• Amino acids (e.g. glycine)

• Dipeptides

• Tripeptides

The absorption of these molecules may occur independently of each other at different sites – thus, their effect can be additive. Research is currently being carried out to utilize these additive effects to develop a multi-component ‘super ORS’.

The movement of sodium provides energy for the active transport of amino acids, glucose and galactose across the membrane. Di- and tripeptide amino acid transport over the brush border is coupled with hydrogen ion reabsorption, and so helps to create the electropotential across the brush border, which again aids the transport of sodium.

The ATPase sodium/potassium pump is located in the basolateral membrane of the intestinal crypt and villus tip cells. The epithelial cells at the tips of the villi are active in net absorption, whereas the cells in the crypts of Lieberkühn function as net secretors of electrolytes and water via the ATPase sodium/potassium pump. In these crypts, there is also a luminal bidirectional sodium/chloride channel, which is opened when there are higher levels of cyclic AMP and calcium ions. When these channels are open, there is a net movement of sodium, chloride and water into the lumen. Consequently, if there is a slight change in the flow across this channel, then secretion dramatically increases. Cholera toxin and E.coli cause an increase in the levels of cAMP, so driving chloride flow across the brush border into the lumen, and hence the net movement of water with it. This results in watery, secretory diarrhoea.

Colon

Whilst the majority of water and electrolyte absorption takes place in the small intestine, the colon is also important for the adequate reabsorption of fluid. It is often the adequacy of colonic function that determines whether or not the patient experiences diarrhoea. The maximal absorptive capacity of the adult large bowel is 2–3 L/day and, if the amount of fluid secreted from the small bowel exceeds this, then diarrhoea results.

The motility of the colon is produced via the haustrations, but these usually contract very slowly and 30 minutes can elapse between haustral contractions in adults.

Abnormalities diagnosed by histopathology