Endoscopic anatomy of the gastrointestinal tract

Oesophagogastroduodenoscopy

Oesophagogastroduodenoscopy (OGD) is used to investigate the oesophagus, stomach and duodenum. A gastroscope, which measures 9.9 mm in diameter and 100 cm in length, is most commonly used for the procedure. An accessory channel is available to place instruments, including biopsy forceps, clips and bipolar cautery probes, down through the scope.

The oesophagus is typically described as having three segments: cervical, thoracic and abdominal. It extends from the upper oesophageal sphincter at approximately 15 cm from the incisors to the lower oesophageal sphincter at 30–40 cm from the incisors ( Fig. 67.1 ). The gastro-oesophageal junction is referred to as the Z line and can be identified as the transition from the paler, pearly-coloured oesophageal mucosa (squamous mucosa) to the pinker gastric mucosa (columnar epithelium) ( Fig. 67.2 ). This landmark can sometimes be challenging to identify: for example, in patients with a hiatus hernia or a Barrett's oesophagus. Other features that aid in identifying the gastro-oesophageal junction include the junction between the distal end of the oesophageal longitudinal vessels and the proximal end of the gastric longitudinal folds.

The stomach is divided into five sections extending from the cardia at the gastro-oesophageal junction to the fundus, body, antrum and the pylorus, which terminates at the duodenum. The cardia, fundus and body are usually characterized by prominent folds, or rugae, whereas the lining of the antrum is relatively flat ( Fig. 67.3 ). When a patient is lying in the left lateral decubitus position, the gastric greater curvature is located at the bottom of the screen and the lesser curvature is located at the top of the screen : this orientation is essential when reporting the location of gastric lesions. The pylorus is the short, circular channel that then leads into the first part of the duodenum, often referred to as the duodenal bulb. In most patients, the gastroscope may routinely be advanced into the second or third portion of the duodenum.

Common indications for OGD include upper gastrointestinal bleeding, dysphagia, early satiety, dyspepsia and reflux. OGD can be both diagnostic (for example, obtaining biopsies) and therapeutic (for example, endoscopic haemostasis for gastrointestinal bleeding). The risks associated with OGD include sedation-associated cardiopulmonary complications, bleeding, infection and perforation. During OGD, insufflation of CO ₂ allows for visualization; photographs are taken routinely from specific locations throughout the oesophagus, stomach and duodenum in order to document a patient's anatomy and pathology.

Tips and Anatomical Hazards

• Barrett's oesophagus is often suspected endoscopically on the presence of pinkish gastric mucosa above the Z line, and is confirmed histologically by the replacement of the normal squamous oesophageal mucosa by specialized intestinal columnar epithelium (intestinal metaplasia). This lesion is a precursor to the development of malignancy and warrants ongoing endoscopic surveillance.

• The diagnostic accuracy for detecting Helicobacter pylori infection is improved when biopsies are obtained from both the gastric antrum and the body.

Push enteroscopy

Push enteroscopy allows access to distal duodenal or proximal jejunal lesions that are beyond the reach of a standard gastroscope. For this procedure, an endoscopist may use a paediatric colonoscope, which measures 9.7 mm in width and 160 cm in length; the extra length, when compared to a standard gastroscope, enables access to more distal regions of the lumen of the small intestine. This procedure is often performed for the investigation of iron deficiency anaemia or occult gastrointestinal bleeding. Another indication for push enter­oscopy is access to more proximal small bowel lesions (tumours or angiodysplastic lesions) that have been localized by capsule endoscopy or another imaging modality. Small bowel tumours may be biopsied, and angiodysplastic lesions can be readily treated using argon plasma coagulation, through the accessory channel of the endoscope. The risks associated with the procedure are similar to those of OGD.

Capsule endoscopy

For capsule endoscopy, a patient swallows a small capsule that takes high-definition images throughout the gastrointestinal tract ( Fig. 67.4 ). Common indications for the procedure include obscure gastrointestinal bleeding, iron deficiency anaemia or evaluation of small bowel Crohn's disease. Capsule endoscopy is especially helpful when routine endoscopic evaluation with gastroscopy and colonoscopy is not diagnostic and there is a suspected small bowel source of haemorrhage. Full bowel preparation is suggested prior to administering a capsule to improve visibility. Capsule endoscopy is only a diagnostic test; once a lesion has been identified, another procedure is required for either biopsy or therapy (see ‘Deep enteroscopy’). Notably, studies have shown that CT enterography is superior to capsule endoscopy for the diagnosis of small bowel tumours, whereas capsule endoscopy is superior to CT enterography for the evaluation of small bowel angiodysplastic lesions. The use of capsule endoscopy is contraindicated in the setting of a known small bowel stricture because the capsule could become lodged and surgery might be required for its removal. The major risks associated with capsule endoscopy are bowel obstruction and incomplete small bowel examination.