Applied Radiology

Dr. Hendrick is a Fellow in the Department of Radiology and Dr. Swischuk is Professor of Radiology and Pediatrics and Director of Pediatric Radiology, both at the University of Texas Medical Branch in Galveston, TX.

V omiting in infancy is a common problem and often is difficult to solve on clinical grounds alone. Usually the first therapeutic measure consists of changing the infant's formula, but many times multiple changes are required, and the infant continues to vomit. Without resolution these infants eventually are referred to a radiologist for evaluation, and in this regard, the following imaging choices are available: 1) an upper GI series, 2) ultrasound of the stomach, and 3) nuclear scintigraphy. However, before any of these studies are ordered, one should be aware of what kind of information each provides and when each should be used.

Vomiting

What causes vomiting? --There are many causes of vomiting, and while the majority are mechanical in origin and referable to the gastrointestinal tract, vomiting also can occur with certain metabolic diseases, neurologic diseases, and infections such as sepsis, pneumonia, and meningitis. Inflammatory diseases of the gastrointestinal tract also cause vomiting, and the most common problem is gastroenteritis, usually of a viral origin. In other cases vomiting can be due to obstructions secondary to problems such as intussusception, delayed midgut volvulus, and underlying congenital bands or stenoses, but overall, the most common cause of chronic vomiting in infancy is gastric in origin and centers around pylorospasm, pyloric stenosis, and gastritis. Infants with these conditions can be examined with a variety of imaging modalities, and exact choices and sequences may vary from institution to institution. In our institution, however, ultrasound of the stomach, and specifically the antrum, has proven to be the most useful initial, and usually only, study to be performed. Indeed, it is the mainstay of the investigation of these patients for it is the most direct way to identify the underlying problem.

If vomiting is present, does one need to demonstrate reflux? --Very often a radiologist will receive a request for an imaging study, usually an upper GI series, with the following inscription: "patient vomiting, spitting up, please rule out reflux". This, however, is a contradiction, for if the patient is vomiting, regurgitating, or spitting up, then the patient is refluxing. Therefore, there is no need to perform an upper GI series or any other imaging study to demonstrate that this is occurring. It is more important to determine why the patient is refluxing. In other words, is there a gastric outlet or duodenal obstruction, or is the problem simple chalasia with a normal antrum? The latter is diagnosed after exclusion of a gastric or duodenal obstruction, and in our institution ultrasound has proven most useful for accomplishing this task.

All of this is not to infer that other well known imaging studies should never be performed. On the contrary, nuclear scintigraphy (most sensitive), followed by the upper GI series both can identify occult reflux such as might occur with chronic aspiration, apneic spells, SIDS, etc.

What diseases may be encoun-tered? --For the most part, the following causes of vomiting in the infant patient may be encountered: 1) a normal antrum with chalasia, 2) pyloro-spasm, 3) pyloric stenosis, 4) gastritis, and 5) delayed congenital obstructions of the stomach and duodenum. Some of these conditions lend themselves to the use of one specific imaging modality over others, but overall, most initially can be evaluated, and usually accurately diagnosed, with ultrasound. Indeed, ultrasound exams which are now available with high resolution (7 to 10 MHz) linear transducers allow for the evaluation of the gastric antrum to the level of depiction of the individual layers of the gastrointestinal tract. At the same time, the radiologist is able to study the physiology of gastric emptying and, thus, ultrasound is an ideal study for the evaluation of chronic vomiting in infants.

Upper GI series

The upper GI series has the inherent problem of delivering ionizing radiation to the patient. This is not an insurmountable or deadly problem, though it is one that should be avoided, if possible. In the past, the upper GI series was the only study available and although it is quite good at demonstrating gastric outlet obstruction, delayed gastric emptying, and duodenal obstructions, the antral findings often were nonspecific. This commonly occurred with pylorospasm and pyloric stenosis but was less of a problem with gastritis. The upper GI series is much better at identifying problems such as the rare gastric diaphragm or the slightly more common duodenal diaphragm.

Ultrasound of the stomach

In our institution, as mentioned earlier, ultrasound has become the most commonly performed imaging procedure in the evaluation of infants with persistent vomiting. It yields accurate data as to the status of the antrum and also gives an accurate perception of gastric emptying. However, to be useful it must be performed with high resolution (7 to 10 MHz) linear transducers. When such transducers are used, not only is the antrum visualized, but so are all the layers of the gastric wall. Ultrasound also can demonstrate gastroesophageal reflux, though this should not be the focus of the study.

The diagnostic criteria for the ultrasonographic diagnosis of pyloric stenosis consist of a fixed, spastic, and elongated antropyloric canal associated with thickening of the circular antropyloric muscle. ¹ Initially, much attention was paid to the length and cross-sectional diameter of the pyloric canal. Indeed even volumetric measurements have been suggested, but most currently rely on measurement of the actual thickness of the pyloric muscle. There is no question that the pyloric canal is elongated in these patients, but the finding is so obvious that it is not worth pursuing in terms of specific measurement. Nonetheless, the diagnostic length of an elongated pyloric canal initially was said to be 1.7 cm or greater, but this measurement has dropped to around 1.2 cm. In reality, however, all of this is moot, for the diagnosis of pyloric stenosis, for the most part, is easily made with simple inspection. The configuration of the elongated pylorus, along with the thickened muscle, is so persistent that it leaves little doubt about the diagnosis (figure 1). Even measuring thickness of the muscle, if the observer is experienced, is not necessary. Measurements are of value only in borderline cases.

In terms of muscle thickness, 4 mm or greater initially was thought to be diagnostic of pyloric stenosis, though it subsequently was proven that 3 mm or greater is sufficient. ¹ Lesser degrees of thickening are not consistent with pyloric stenosis, and these patients should not be treated surgically. On the other hand, 2 to 3 mm of thickness should not be considered normal, and indeed, such thicknesses frequently can be seen with pylorospasm, with or without associated gastritis (figure 3). These patients have abnormal gastric emptying and will require treatment. For the most part this is accomplished medically with an antispasmodic agent such as metachlopromide or bentyl.

The classic findings of pyloric stenosis, as noted earlier, are relatively easy to identify (figure 1). The elongated pyloric canal with thickened muscle is rather characteristic. However, one or two pitfalls exist. The most common of these is echogenicity at the 6:00 and 12:00 cross/sectional positions which, when seen on a longitudinal section in the mid-sagittal plane, can cause the muscle to appear echogenic and almost invisible (figure 2). This echogenicity has been found to be due to reflections from the interfaces of the circular muscle fibers as they pass through the 6:00 and 12:00 positions. ² However, it also can result from a reverberation artifact. At any rate, once appreciated, it is of no real consequence.

The other major pitfall deals with posterior positioning of the pyloric canal. Characteristically in pyloric stenosis, the pyloric canal demonstrates some degree of posterior positioning and curvature. However, when the stomach is full of food or fluid, as is often the case in these infants, it can hide the posteriorly positioned pyloric canal, making it possible to miss the thickened muscle mass entirely. This was a problem in the early stages of ultrasound, but, with experience, it has now been resolved. Once these potential pitfalls are appreciated, the ultrasonographic diagnosis of pyloric stenosis, in experienced hands, approaches 100%. Indeed, from a practical standpoint it is almost foolhardy to endlessly try to palpate an olive in these patients if it is not palpable immediately. Ultrasound is so direct and definitive that it can save much time and effort.

Findings of pylorospasm are similar to those of pyloric stenosis, but while the pyloric canal initially may be elongated and fixed, it does not remain fixed permanently (figure 3). Eventually with pylorospasm the antropyloric canal opens and peristaltic activity passes through it. In many of these cases the pyloric muscle may be slightly thickened, but it is never thickened beyond 2 mm. If it is thickened to 3 mm or over, the patient should be diagnosed with pyloric stenosis rather than pylorospasm. ¹

If one adheres to these criteria, the diagnosis of pylorospasm is relatively straightforward and the findings easy to interpret. Pylorospasm is treated medically with the administration of antispasmodic drugs such as metachlopromide. This leads to relaxation of the antrum and normalization of gastric emptying. Most patients respond to this form of therapy, but in some cases, even if treated medically, progression to classic pyloric stenosis can be seen.

Gastritis also is relatively easily identified with ultrasound, and in infants it is more common than generally appreciated. ³ Most often, it is secondary to milk allergy. On ultrasound examination, the mucosa appears thickened and echogenic and measures well over the normal 2 to 3 mm ³ ; thickening may be circumferential or eccentric (figure 4).

Duodenal obstructions, such as duodenal bands with midgut volvulus (figure 5A), also can be identified with ultrasound. The main finding in these cases is a distended descending duodenum. ⁴ Similar findings are more vividly demonstrable with upper GI series, including spiraling of the small bowel along the twisted mesentery (figures 5B,5C).

In this regard, ultrasound has become valuable in demonstrating reversal of the normal relationship of the superior mesenteric artery and vein (figure 6). Although not foolproof, reversal of the position of these two vessels is highly suspicious and virtually diagnostic of malrotation. ^5,6 In addition, if volvulus is present, mesenteric fat and the mesentery itself, along with the mesenteric vessels, can be seen to form a concentric ring-like configuration leading to the so-called "whirlpool" sign (figure 6). ⁷ Ultrasonography also can demonstrate gastric diaphragms (figure 7A). However, many times these abnormalities are more vividly demonstrable with an ordinary upper GI series (figure 7B).

Nuclear scintigraphy

Nuclear scintigraphy has been found to be the least useful study for the evaluation of chronic vomiting. While it provides information as to rate of gastric emptying and the presence of reflux, it lacks specificity regarding the various antral problems encountered. In terms of gastric emptying, unless the study is performed properly (e.g., with the patient positioned on his right side or held upright) the findings can be misleading. If the study is performed with the patient on his back though in a semi-erect position, erroneous data may be obtained. Often in such cases the dilemma of the ultrasound study (or in former years, the upper GI series) demonstrating normal gastric emptying while the nuclear scintigraphy study suggests significant and prolonged delay in emptying occurs.

Nuclear scintigraphy has no specificity as far as the type of obstructive problem present in the antrum. Therefore, when it is performed as a first study and is abnormal, it is still absolutely necessary to perform another type of diagnostic study (usually ultrasound in our institution). As this is not in the least cost effective, nuclear scintigraphy is best used for the detection of occult reflux, as seen in patients with apneic spells, reactive airway disease with occult reflux, and SIDS.

Conclusion

Chronic vomiting in infancy is a common problem and is often difficult to solve clinically. With imaging, specifically the ultrasound study, a definitive diagnosis can be accomplished in virtually every case. This being the case, it is most cost effective to proceed to this study immediately and not perform the other studies instead. AR