Causes of vomiting in dogs and cats can be classified as gastrointestinal tract origin (such as obstruction, gastroenteritis, neoplasia, ulceration), and other causes such as kidney disease, poisons, parasitic infection, pancreatitis, liver disease and drug induced. The role of imaging vomiting animals is to determine whether it is gastrointestinal in origin.
If an animal is suspected of having vomiting associated with the gastrointestinal tract, the common approach is to start with radiography. At least two orthogonal views are needed. Occasionally 3 or 4 views are needed to make a diagnosis. Radiography can give us the overview of the abdomen. This is especially important and can easily detect foreign bodies of mineral and metallic opacity. It is somewhat challenging to detect foreign bodies of soft tissue opacity. Occasionally, we will not be able to detect opaque soft tissue foreign bodies due to silhouetting with the surrounding tissue. If a gastric foreign body is suspected, an additional lateral view is recommended. Gas normally accumulates in the pylorus on left lateral recumbency while fluid accumulates in the pylorus on right lateral recumbency. Detection of pyloric abnormalities such as foreign bodies or pyloric masses could be achieved by left lateral recumbency.
The important radiographic sign of foreign body obstruction is the distension of the gastrointestinal tract. The stomach is normally distended with gas and fluid secondary to gastric outflow obstruction. Commonly, the small intestine is gas dilated as a result of an obstruction in the lower gastrointestinal tract. The normal ratio of small intestinal external diameter to the height of L5 is ≤ 1.4. Dogs with a ratio of ≥ 2.4 have a higher probability of an obstruction.1 In cats, a small intestinal external diameter to the height of the cranial end plate of L2 ratio of > 2 is strongly suggestive of a small intestinal obstruction.2 An upper small intestinal obstruction will produce a shorter segment of abnormal gas-dilated small intestines. Sometimes it is difficult to differentiate chronic lower intestinal obstruction with functional ileus.
Linear foreign body obstruction normally leads to plication and a resultant abnormal pattern of round, tapered, short-tubular, crescent and comma-shaped gas. The plicated intestine may have normal small intestinal external diameter.
As a general rule, it is always difficult to determine the thickness of the wall of the gastrointestinal tract on radiography. Thus, radiography is not a sensitive modality to diagnose gastrointestinal wall thickness secondary to neoplasia.
Contrast upper gastrointestinal study is required to confirm wall thickness of the gastrointestinal tract and sometimes a diagnosis of ulcer could be made. Positive contrast upper gastrointestinal study is recommended for the evaluation of gastric emptying and gastrointestinal transit time.3 Negative-contrast pneumocolonography may be performed to differentiate small intestinal foreign body from feces in the colon.
The advantages of ultrasound include the ability to measure the thickness, to recognize the different layer and echogenicity of the wall of the gastrointestinal tract. It has been reported that ultrasound has a higher sensitivity than radiography to diagnose small intestinal foreign bodies.4 However, ultrasound is highly operator- and machine-dependent. The probability of missing gastrointestinal abnormalities for an inexperienced operator is high. The most common ultrasound feature is presence of hyperechoic shadowing foci in the small intestines. Ultrasonographic diagnosis of gastric foreign bodies is more challenging and less ideal due to the presence of gastric gas.
Increased thickness of the gastrointestinal tract alone without destruction of the 5 layered appearances is normally indicative of inflammatory disease, while loss of the wall layering has a higher probability of being a neoplasia than enteritis.5 For cats with T-cell lymphoma, thickening of the muscularis layer is the most commonly seen ultrasonographic feature.6
In conclusion, performing radiography prior to ultrasonography is recommended in the investigation of vomiting animals. This is to reduce the probability of not detecting gastric foreign body and preventing spending too much time in searching for the mineral or metallic foreign bodies readily seen on radiographs.
References
1. Finck C, d'Anjou MA, Alexander K, Specchi S, Beauchamp G. Radiographic diagnosis of mechanical obstruction in dogs based on relative small intestinal external diameters. Vet Radiol Ultrasound. 2014;55:472–479.
2. Adams WH, Sisterman LA, Klauer JM, Kirby BM, Lin TL. Association of intestinal disorders in cats with findings of abdominal radiography. J Am Vet Med Assoc. 2010;236:880–886.
3. Sharma A, Thompson MS, Scrivani PV, Dykes NL, Yeager AE, Freer SR, Erb HN. Comparison of radiography and ultrasonography for diagnosing small intestinal mechanical obstruction in vomiting dogs. Vet Radiol Ultrasound. 2011;52:248–255.
4. Brawner WR, Bartels JE. Contrast radiography of the digestive tract. Indications, techniques and complications. Vet Clin North Am Small Anim Pract. 1983;13:599–626.
5. Penninck D, Smyers B, Webster CRL, Rand W, Moore AS. Diagnostic value of ultrasonography in differentiating enteritis from intestinal neoplasia in dogs. Vet Radiol Ultrasound. 2003;44:570–575.
6. Zwingenberger AL, Marks SL, Baker TW, Moore PF. Ultrasonographic evaluation of muscularis propria in cats with diffuse small intestinal lymphoma or inflammatory bowel disease. J Vet Intern Med. 2010;24:289–292.