| Central abdominal pain
Imaging Strategies Author: Charles Allison, MB ChB Editor: Ashley Davidoff MD The Common Vein Copyright 2006
BACKGROUND: The differential diagnosis of central abdominal pain is wide. Nociception from midgut structures (including the small bowel and proximal 2/3 of colon) typically present in the umbilical, or central, abdominal region. Possible etiologies of central abdominal pain include: Enteritis
Obstruction Ischemic bowel Meckel’s diverticulitis Appendicitis Intussusception Crohn’s disease Aortic aneurysm Imaging may not be necessary in classic presentations or for diagnoses such as viral enteritis. When it is felt imaging will be helpful, the choice of modality clearly hinges on the suspected diagnosis. WHAT STUDY? CT and plain radiography are the modalities most likely to be required. CT is the investigation of choice. It gives diagnostic information on the widest spectrum of pathologies, and is the preferred modality for detecting many of them, including appendicitis, ischemic bowel, Meckel’s diverticulitis and bowel obstruction. Intravenous contrast is required, but oral contrast is not necessary in the case of suspected complete obstruction, where free fluid in the bowel provides adequate ‘endogenous’ contrast; indeed, oral contrast administration in such cases may obscure the cause of the obstruction and interfere with enhancement of the bowel mucosa. Plain abdominal radiographs are helpful when taken in the upright and supine positions. The upright position aids reading of the film, allowing identification of free air (collects under diaphragm in upright position) and air-fluid levels within the bowel which may be indicative of ileus or obstruction. Enteroclysis, where a catheter is introduced into the small bowel and contrast introduced, may increase the diagnostic potential of plain films (and CT). Double contrast with barium and methylcellulose is typically used. Small bowel follow-through may be appropriate for suspected small bowel obstruction; in such cases it aids management 68-100% of the time. It is also useful in suspected Crohn’s disease, where it may help identify area of stricture or ulceration. Ultrasound is generally inferior to CT in patients presenting with central abdominal pain, though it is preferred over CT in the pediatric population, in whom the more common causes of abdominal pain (volvulus, intussusception) can be readily identified. MRI provides excellent cross sectional imaging, providing superior images of the soft tissues and organs. It is not a first line imaging modality for central abdominal pain but may be a reasonable choice if plain films and CT are relatively contraindicated, such as in pregnancy. Some diagnoses are not aided by imaging, such as gastroenteritis. WHY? CT scanning is very fast and provides information on the full spectrum of aortic, pancreatic and bowel pathologies. CT has a high sensitivity (94%) and specificity (95%) for diagnosing appendicitis; may show wall thickening, strictures, fistulas and ulceration from Crohn’s disease; localizes aortic aneurysm leaks and detects small bowel obstruction (SBO) in around 90% of cases. In cases of intermittent or subacute SBO, enteroclysis can be used to improve diagnostic accuracy. The uniform distension produced by the methylcellulose allows better assessment of the small bowel in terms of dilatation and mucosal wall abnormalities. Abdominal CT does involve a relatively large dose of radiation; approximately 500 times that of a plain chest radiograph, the equivalent of about 3.3 years of normal background radiation. Plain radiography is significantly cheaper and involves less radiation than CT. Upright films may be uncomfortable or impossible to obtain in patients with acute pain, but allow for more ready diagnosis of obstruction or perforation. It lacks the breadth of diagnostic possibilities that make CT the first choice modality. Enteroclysis can also be used with plain films or CT, allowing better assessment of small bowel distension, strictures, polyps or neoplasms. The uniformity of bowel distension (and hence quality of images) and lack of time required for contrast to pass the pylorus make enteroclysis superior to small bowel follow through. The insertion of the catheter, though minimally invasive, may be uncomfortable to the patient, however. Small bowel follow through or barium enema can be used to evaluate for small bowel obstruction, ileitis (as in yersinia infection) and inflammatory bowel disease. They are rarely used in the acute setting, when CT imaging is preferred. This is because a SBFT study may take over four hours to complete, as passage of contrast relies on endogenous gut motility. Barium enema is uncomfortable to the patient and involves much movement to aid retrograde passage of the contrast, which may be difficult for elderly, acutely unwell patients. Ultrasound is non-invasive with no radiation burden. It is more acceptable to the pediatric population who may be reluctant or unable to comply with the CT examination. Children typically have less tissue for the ultrasound waves to transverse before reaching the abdominal cavity and as such may provide better images. Intussusception and midgut volvulus can be readily identified; SBO can be diagnosed with a sensitivity of around 90%. It is excellent for diagnosis and measurement of aortic aneurysm in adults. Experience of ultrasound for evaluation of suspected small bowel obstruction in adults is somewhat limited in the US. MRI provides equivalent diagnostic accuracy to CT in most pathologies. It is less validated than CT in evaluation of bowel obstruction and not recommended unless the patient is pregnant. Though it does not carry the radiation burden of CT, it is more expensive, time consuming and may be more geographically limited and less acceptable to the patient. WHEN? Imaging should be performed at time of presentation if the patient is in acute pain to expedite diagnosis / management. HOW TO ORDER? It is important to reference the symptom or the sign as the clinical indication and not the diagnosis. ie Patient with “central abdominal pain” is acceptable, whereas R/O obstruction is not acceptable. Optimal ordering would list symptoms/signs as well as the putative diagnosis being investigated: “Central abdominal pain, vomiting r/o small bowel obstruction”. This allows the radiologist to suggest alternative imaging strategies if a suboptimal approach has been ordered. PATIENT PREPARATION CT preparation depends on the use of contrast. For renal stones no contrast is required and the patient may be scanned immediately. Administration of oral contrast requires drinking or NG administration of 30cc gastrograffin diluted in 900cc water to ensure it adequately coats the length of the bowel. Intravenous contrast is given in the scanner and may be timed to highlight the arterial or venous supply to a specific organ. The patient’s allergies, medications and renal function need to be identified before hand as the contrast is iodine based and carries risk of precipitating renal failure in at risk patients (diabetic, on metformin etc). The study itself takes about 1 minute once the patient is on the table. The patient should be fasting for 4 hours before an ultrasound examination; the procedure takes about 30 minutes. Patients undergoing MRI should be warned that they will be required to lie still in a confined tunnel-like space for up to 30 minutes. The machine is very noisy and ear protection will be provided. They will have a call button to abort the procedure if a problem should arise. CLINICAL RED FLAGS:
Onset of central severe abdominal pain followed by persistent vomiting and complete constipation indicates a complete obstruction. This may require urgent surgical management of prevent necrosis of the affected segment of bowel and may be the first presentation of an underlying malignancy.
REFERNCES:
ACR Approriateness Crtieria http://www.acr.org/s_acr/bin.asp?CID=1207&DID=11770&DOC=FILE.PDF http://www.acr.org/s_acr/bin.asp?CID=1207&DID=11758&DOC=FILE.PDF
Enteroclysis – What is it and why do we do it? – Radiographic Imaging CEU Source |