USMLE – Investigation of Gastrointestinal Disease
A wide range of tests are available for the investigation of patients with gastrointestinal symptoms. These can be classified broadly into tests of structure, tests of infection and tests of function.
TESTS OF STRUCTURE: IMAGING
Plain radiographs of the abdomen show the distribution of gas within the small and large intestines and are useful in the diagnosis of intestinal obstruction where dilated loops of bowel and (in the erect position) fluid levels are seen. The outlines of soft tissues such as liver, spleen and kidneys are usually visible, and calcification of these organs as well as pancreas, blood vessels, lymph nodes and calculi may be detected. Abdominal radiographs do not help in cases of gastrointestinal bleeding. A chest radiograph shows the diaphragm, and erect films may detect subdiaphragmatic free air in cases of perforation. Unexpected pulmonary problems such as pleural effusions will also be revealed.
These provide more information than plain films. Barium sulphate is inert and provides good mucosal coating and excellent opacification. It can, however, solidify and impact proximal to an obstructive lesion. Water-soluble contrast is used to opacify bowel prior to abdominal computed tomography and in cases of suspected perforation but is less radio-opaque and is also irritant if aspirated into the lungs. Contrast studies are carried out under fluoroscopic control, which allows assessment of motility and correct patient positioning. The double contrast technique improves mucosal visualization by using gas to distend the barium coated intestinal surface.
Although the wall of the gut itself is not seen, barium studies are useful for detecting filling defects, which may be intraluminal (e.g. food or feces), intramural (e.g, carcinoma) or extramural (e.g, lymph nodes). Strictures, erosions, ulcers and motility disorders can all be detected.
Ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI)
These are increasingly used in the evaluation of intraabdominal disease. They are non-invasive and offer detailed images of the abdominal contents.
Fiberoptic endoscopy is used to examine the esophagus, stomach, duodenum and colon. Originally, light was passed down flexible quartz fibers and the reflected light passed back up to the investigator by thousands of bundles. In recent years video endoscopy has replaced fiberoptic endoscopes and the images are displayed on a color television monitor. Endoscopes have controls to allow steering of the tip and also possess channels for suction and insufflation of air and water. An increasing array of instruments can be passed down the endoscope to allow both diagnostic and therapeutic procedures.
Upper gastrointestinal endoscopy
After the patient has fasted for at least 4 hours, this is performed under light intravenous benzodiazepine sedation or using only local anaesthetic throat spray. With the patient in the left lateral position the entire esophagus (excluding pharynx), stomach and first two parts of duodenum can be seen.
Using a longer endoscope (enteroscope) it is possible to visualize a large portion of the small intestine. Enteroscopy is of special value in the assessment of obscure. recurrent gastrointestinal bleeding.
Sigmoidoscopy and colonoscopy
Sigmoidoscopy can be carried out either in the outpatient clinic using 20 cm rigid plastic sigmoidoscope or in the endoscopy suite using a 60 cm flexible instrument following a disposable enema for bowel preparation. When sigmoidoscopy is combined with proctoscopy, accurate detection of hemorrhoids, ulcerative colitis and distal colorectal neoplasia is possible. After full bowel cleansing it is possible to examine the entire colon and often the terminal ileum using a longer colonoscope.
Endoscopic retrograde cholangiopancreatography (ERCP) Using a side-viewing duodenoscope, it is possible to cannulate the main pancreatic duct and common bile duct. The procedure is valuable in defining the ampulla of Vater, biliary tree and pancreas. Its main uses include investigation of obstructive jaundice, biliary pain and suspected pancreatic disease, such as chronic pancreatitis and pancreatic cancer. Obstruction of the common bile duct by stones can be treated by stone extraction after sphincterotomy and strictures may be stented. The procedure is technically demanding and carries a significant risk of pancreatitis (3-5%), hemorrhage (4% after sphincterotomy) and perforation (1%).
Biopsy material obtained during endoscopy or percutaneously can provide useful information.
TESTS OF INFECTION
Stool cultures are essential in the investigation of diarrhea, especially when it is acute or bloody, to identify pathogenic organisms.
TESTS OF FUNCTION
A number of dynamic tests can be used to investigate aspects of gut function, including digestion, absorption, inflammation and epithelial permeability. In the assessment of suspected malabsorption, blood tests (full blood count, erythrocyte sedimentation rate (ESR), folate, B12, iron status, albumin, calcium and phosphate) are essential. Endoscopy with distal duodenal biopsy is also indicated in most cases.
Many diverse radiological, manometric and radioisotopic tests exist for investigation of gut motility but many are still research tests of limited value in daily clinical practice.
A careful barium swallow can give useful information about esophageal motility and video fluoroscopy, and is useful in suspected swallowing disorders. Esophageal manometry, often in conjunction with 24-hour pH measurements, is of value in diagnosing cases of refractory gastro-esophageal reflux, achalasia and other causes of noncardiac chest pain.
Delayed gastric emptying (gastroparesis) may be responsible for some cases of persistent nausea, vomiting, bloating or early satiety. Endoscopy and barium studies are often normal. Plotting a graph of the amount of radioisotope retained in the stomach against time is carried out after a test meal containing solids and liquids labelled with different isotopes.
Small intestinal transit
This is much more difficult to quantify and is seldom necessary in clinical practice. Barium follow-through examination can give a rough estimate by noting the time taken for contrast to reach the terminal ileum (normally 90 minutes or less). Orocecal transit can be assessed by the lactulose-hydrogen breath test. Lactulose is a disaccharide which normally reaches the colon intact: here, breakdown by colonic bacteria results in hydrogen production. The time at which this occurs, as measured in expired air, is a measure of oral-cecal transit.
Colonic and anorectal motility
A plain abdominal radiograph taken on day 5 after ingestion of different-shaped inert plastic pellets on days 1-3 gives an estimate of whole gut transit time. The test is useful in the evaluation of chronic constipation when the position of any retained pellets can be observed. and helps to differentiate cases of slow transit from those due to obstructed defecation. The mechanism of defecation and anorectal function can be assessed by a variety of tests such as anorectal manometry, electrophysiological tests and defecating proctography.
Many different radioisotope tests are used. In some, structural information is obtained. e.g. localization of a Meckel’s diverticulum or distribution of activity in inflammatory bowel disease. Others use radioisotopes for functional information, e.g. rates of gastric emptying, ability to reabsorb bile acids. Yet others are tests of infection and rely on the presence of bacteria to hydrolyze a radio-labelled test substance followed by detection of the radioisotope in expired air (e.g. urea breath test for H. pylori).