Traditionally, veterinarians have relied on barium liquid as the contrast agent of choice for gastrointestinal studies. Recently, the limitations of barium liquid have led to the development of barium-impregnated solid radiopaque markers for the diagnosis of motility disorders and bowel obstructions. Most of the research on radiopaque markers in the veterinary literature refers to one type of marker: barium-impregnated polyethylene spheres (BIPS; Med-ID, Grand Rapids). These spheres have been specifically validated for use in dogs and cats and are the only radiopaque markers with which there is extensive clinical experience. Information on the administration of BIPS, the interpretation of BIPS studies and frequently asked questions about these radiopaque markers can be found at www.bipscapsules.com.
Barium-Impregnated Polyethylene Spheres
Each diagnostic set of barium-impregnated polyethylene spheres consists of ten 5 mm diameter and thirty 1.5 mm diameter radiopaque spheres contained in either one large capsule or four small capsules. The primary function of the 5 mm spheres is to detect gastrointestinal tract obstructions and that of the 1.5 mm spheres is to quantitatively assess the gastric emptying rate and intestinal transit time of food.
The method of administration of the radiopaque markers varies depending on the clinical situation. Specifically, clinicians must decide if they should administer the spheres on an empty stomach, with Prescription Diet d/d or i/d (Hills Pet Products, Topeka), or with Prescription Diet r/d (Hills Pet Products, Topeka). As a general rule studies are performed on an empty stomach in acute presentations when the primary goal is to rule out physical obstructions of the bowel. Studies are performed with d/d or i/d in chronic presentations when the aim is to detect out partial obstructions and motility disorders. The markers are fed with r/d to investigate the prognosis and cause of chronic constipation.
The best time to take radiographs during a radiopaque marker study depends on the reason for the study, the clinical state of the patient, whether the spheres were given with food and logistical concerns such as time of presentation and the availability of staff. Studies are commonly performed overnight. The only time it is not appropriate to take radiographs during a radiopaque marker study is when the reference range is so wide as to prevent differentiation of normal from abnormal passage (e.g., at four hours). Two radiographic views must always be taken.
Interpretation of radiopaque marker studies takes into account the survey radiographic information, the time at which the radiograph was taken, the method of administration and the radiographic pattern. Common patterns include: retention of spheres in the stomach; delayed orocolic transit with bunching of the spheres; delayed orocolic transit without bunching of the spheres; normal passage of the spheres (e.g., within the reference ranges). Like all new diagnostic techniques, correct interpretation of radiopaque marker studies takes a little practice. However, in saying this, it is important for clinicians not to lose sight of the very simple fact that if the large markers have entered the colon, an obstruction of the bowel is highly unlikely. If you cannot identify the colon on your radiographs, perform an air enema (20 mL/kg) and repeat the radiograph. The principal indications and contraindications for radiopaque marker studies are listed in Tables 1 and 2.
Comparison of Radiopaque Marker Studies To Other Diagnostic Methods
Radiopaque marker studies do not replace any currently available diagnostic modalities. Rather, they are a valuable alternative approach. Compared to barium sulfate solutions, radiopaque markers have a number of advantages. They increase the likelihood of detection of obstructions by those who experience difficulty interpreting barium-follow-through studies. They are more sensitive for the detection of motility disorders because they provide quantitative information on gastric emptying and intestinal transit times and they mimic the transit of food not liquid (the transit of liquid is rarely disrupted as much as food by motility disorders). Their administration is more convenient: no stomach tubing is required and radiographs do not need to be made at set intervals. They are less likely to be aspirated and are less likely to cause peritonitis if the bowel is perforated. They do not obscure abdominal detail on the radiographs reducing the need for survey films prior to the contrast procedure.
Compared to barium sulfate solutions, radiopaque markers have the following disadvantages. They do not provide as much information about mucosal detail as can be obtained by a single or double contrast gastrogram (indicated in absence of endoscopy to help diagnose disorders such as gastric ulcers). They do not outline the luminal borders of the GI tract and so are less valuable for differentiating dilated or displaced organs (e.g., megaesophagus or GDV) or detecting gastric foreign bodies. Studies can take longer as the solid markers move somewhat more slowly through the GI tract than liquid.
In contrast to ultrasound, a radiopaque marker study provides more functional information on gastric emptying rate and intestinal transit time, but less morphologic information about the abdomen. Radiopaque marker studies complement the diagnostic spectrum of endoscopy because they allow diagnosis of motility disorders and obstructive diseases of the lower small bowel (such as annular adenocarcinoma), neither of which can be diagnosed by current endoscopic techniques. In contrast to scintigraphy, radiopaque marker studies offer a more practical method of quantifying emptying and transit for clinical situations. However, scintigraphy allows greater accuracy and the potential to assess the passage of specifically tagged nutrients.
Key point: If large BIPS enter the colon it is highly unlikely the patient has a gastrointestinal obstruction.
Conclusion
Radiopaque markers are a valuable addition to the diagnostic tools of the practicing veterinarian. The technique does not replace more traditional liquid barium contrast studies but does offer advantages in certain situations. Radiopaque marker studies offer veterinarians in private practice the opportunity to investigate motility disorders, previously largely the purvey of those in tertiary institutions. In my experience, their use offers an expedient method of diagnosing obstructions of the bowel and the technique complements commonly used diagnostic methods such as endoscopy.
TABLE 1: Indications for radiopaque marker studies
Ruling out obstructions of the GI tract in dogs and cats with acute-subacute vomiting
Ruling out linear foreign bodies in cats.
Screening patients with chronic vomiting or diarrhea for partial obstructions of the bowel (e.g., from neoplasia) and for motility disorders.
Investigating dogs with chronic or recurrent bloating for delayed gastric emptying.
Investigating cats and dogs with inexplicable anorexia for “low” partial obstructions.
Investigating cause and prognosis of animals with recurrent idiopathic constipation.
TABLE 2: Contraindications for radiopaque marker studies
Patients with shock or peracute signs.
Patients too weak to swallow.
Patients with “coffee ground” vomitus or melena.
References
1. Hall JA, Willer RL, Seim HB, et al. Gastric emptying of nondigestible radiopaque markers after circumcostal gastropexy in clinically normal dogs and dogs with gastric dilatation-volvulus. Am J Vet Res 53:1961-1965, 1992.
2. Fucci V, Pechman RD, Hedlund CS, Venugopalan CS. Large bowel transit times using radiopaque markers in normal cats. J Am Anim Hosp Assoc 31: 473-477, 1995.
3. Allan FJ, Guilford WG, Robertson ID. Gastric emptying of solid radiopaque markers in healthy dogs. Vet Radiol Ultrasound 37:336-344, 1996.
4. Sparkes AH, Papasouliotis K, Barr FJ, Gruffydd-Jones TJ. Reference ranges for gastrointestinal transit of barium-impregnated polyethylene spheres in healthy cats. J Small Anim Pract 38: 340-343, 1997.
5. Chandler ML, Guilford WG. Radiopaque markers to evaluate gastric emptying and small intestinal transit time in healthy cats. J Vet Int Med 11:361-364,1997.
6. Guilford WG, Lawoko CRO, Allan FJ. Accuracy of localizing radiopaque markers by abdominal radiography and correlation between their gastric emptying rate and that of a canned food in dogs. Am J Vet Res 58:1359-1363, 1997.
7. Bruce SJ, Guilford WG, Hedderley DI, McCauley M. Development of reference intervals for the large intestinal transit of radiopaque markers in dogs. Vet Radiol Ultrasound 40:3-8, 1999.