Imaging in Hypertrophic Pyloric Stenosis 

Hypertrophic pyloric stenosis (HPS) is commonly encountered in pediatric practice. The typical infant presents with nonbilious projectile vomiting and dehydration (with hypochloremic metabolic alkalosis) if the diagnosis is delayed. Premature infants tend to present at 3-6 weeks from birth—not at 3-6 weeks from the due date—and these infants may have borderline normal muscle thickness, because they are comparatively smaller. Hypertrophic pyloric stenosis is rarely seen in children older than 6 months.

This condition accounts for one third of nonbilious vomiting occurrences in infants and is the most common reason for laparotomy before age 1 year. A striking male preponderance is seen, with a male-to-female ratio of 4-6:1. In addition, pyloric stenosis and esophageal atresia may coexist.^{[1, 2]}

Anatomy

In hypertrophic pyloric stenosis, the circular muscle layer becomes thickened, which narrows the pyloric channel and elongates the pylorus. During this process, the mucosa becomes redundant and may appear hypertrophic. With elongation and thickening of the muscle, the pylorus deviates upward toward the gallbladder, which serves as a marker, because in hypertrophic pyloric stenosis, the pylorus can be seen adjacent to the gallbladder and anteromedial to the left kidney. The thickened pylorus narrows the pyloric channel, resulting in gastric outlet obstruction, gastric distention, and retrograde peristalsis in the stomach.

Clinical details

In the past and with experience,^{[3, 4]} the pyloric olive, which represents the thickened and elongated pylorus, was said to be felt by surgeons in up to 80% of patients. Review of the more recent radiologic and surgical literature indicates that the olive currently is felt much less frequently (23% of the time in one reported case series).

The low rate of positive palpation for the pyloric olive may be the result of several factors. Patients present at an earlier age when the olive is smaller; with earlier presentation, the incidences of dehydration, metabolic alkalosis, weight loss, and failure to thrive as manifestations of HPS decrease dramatically.^[5] Consequently, infants who present at a younger age are better nourished such that abdominal wall fat may obscure palpation of the mass. In addition, the skill of palpation may become lost as more medical school graduates come to rely heavily on ultrasonography for diagnosis.^[6]

Preferred examination

The preferred diagnostic test for hypertrophic pyloric stenosis is a contentious topic, with a wealth of articles that discuss the cost-effectiveness and the changing face of this disease.^{[7, 8, 9]}

The first and most important step in patient workup of suspected hypertrophic pyloric stenosis is a thorough physical examination. If the pyloric olive is felt (see Clinical details above), the patient may proceed directly to the operating room without imaging.^[4] However, many surgeons are uncomfortable with this protocol because a false-positive physical examination then leads to a negative laparotomy. Therefore, ultrasonography is recommended, because its sensitivity and specificity are close to 100% for this disease.^{[10, 11]} If the clinical suspicion for hypertrophic pyloric stenosis is moderate to high, ultrasonography is also recommended. See the images below.

If the vomiting infant is outside the usual age range for hypertrophic pyloric stenosis or if the clinical suspicion is low, an upper gastrointestinal (UGI) study is recommended, because this study more effectively rules out other problems, such as malrotation and gastroesophageal reflux.^[9] See the following images.

Some investigators have reported that a UGI study is the most cost-effective study^[8] (more than ultrasonography) in the vomiting infant because a negative ultrasonogram often leads to a UGI study to rule out other diagnoses that a focused ultrasonographic evaluation does not detect.^[9] A second test, such as ultrasonography, rarely follows a negative UGI study for hypertrophic pyloric stenosis.^{[8, 9]}

In experienced hands, ultrasonography is the preferred modality in the workup of any vomiting infant. The technique includes feeding glucose water to the baby, which often improves visualization of the pylorus and, in the case of a negative study, allows continuous observation of the gastroesophageal junction to diagnose reflux. The radiologist's skill and clinical suspicion ultimately determine which test is appropriate.

Limitations of technique

Ultrasonography has high sensitivity, specificity, and accuracy in the diagnosis of hypertrophic pyloric stenosis. However, errors in diagnosis do occur and relate to false negatives and false positives

False negatives may result from operator inexperience, in which the pylorus may not be identified. Another cause may be from distended formula and a gas-filled stomach. These cause the pylorus to fold backward on itself such that it may remain hidden behind the stomach. The overdistended antrum may be mistaken for the pylorus; in such cases and in any infant whose pylorus is not visualized on ultrasonogram, place a nasogastric tube and withdraw the gastric secretions.

Muscle thickness increases with patient size, and borderline measurements are seen early in the disease and with premature infants, which may also cause false-negative findings. Some clinicians advocate observation and then repeating the ultrasonographic study in 2-3 days to confirm the diagnosis if the patient is stable and is not dehydrated.

False positives may result from pylorospasm, a dynamic process that changes over time. The normal pylorus opens at least once every 15 minutes. The thickened muscle and elongated pylorus should be fixed. In addition, the postoperative appearance of the pylorus can lead to false-positive findings. Symptoms may take time to clear and, therefore, so do the abnormalities on ultrasonography. This modality may show hypertrophic pyloric stenosis (thickened muscle thickness) for up to 12 weeks following pyloromyotomy. In these cases, a UGI study may provide more information than ultrasonography to rule out incomplete myotomy.

Differential diagnosis and other problems to be considered

Midgut volvulus is part of the differential diagnosis; however, other conditions to be considered include malrotation, with or without midgut volvulus; antral polyps; gastric duplication; focal foveolar hyperplasia; and pylorospasm.

Special concerns

The following are issues that may arise in a child with hypertrophic pyloric stenosis:

• Failure to choose the best test, which is dictated by patient history, physical examination, and the surgeon's level of suspicion.
• Failure to choose the best radiologic investigation for the vomiting infant. If imaging is requested despite a child having a palpable pyloric olive, confirm hypertrophic pyloric stenosis by ultrasonography.
• If the clinical history suggests hypertrophic pyloric stenosis and the child is stable, perform ultrasonography to diagnose or rule out this condition. If the ultrasonographic findings are negative, perform a UGI study to confirm or rule out other pathology.
• Ultrasonography, although reliable for diagnosing hypertrophic pyloric stenosis, may miss malrotation, which is the most serious cause of vomiting in infants. These children require a UGI examination. Thus, if concern exists about malrotation, with or without volvulus (no olive is felt; patient is sick), a UGI study is necessary.

Abdominal radiographs may show a fluid-filled or air-distended stomach, suggesting the presence of gastric outlet obstruction. A markedly dilated stomach with exaggerated incisura (caterpillar sign) may be seen, which represents increased gastric peristalsis in these patients (see the image below). If the patient has recently vomited or has a nasogastric tube in place, the stomach is decompressed and the radiographic findings are normal.

A UGI study was once considered the test of choice for hypertrophic pyloric stenosis. Findings on UGI studies include the following:

• Delayed gastric emptying (if severe, this may prevent barium from passing into the pylorus and severely limit the study)
• Cephalic orientation of the pylorus
• Shouldering (ie, filling defect at the antrum created by prolapse of the hypertrophic muscle)
• Mushroom or umbrella sign (ie, thickened muscle that indents the duodenal bulb; the name refers to the impression made by the hypertrophic pylorus on the duodenum)
• Double-track sign (ie, redundant mucosa in the narrowed pyloric lumen, which results in separation of the barium column into 2 channels) (see the image below) Lateral view from an upper gastrointestinal study demonstrates the double-track sign.
• String sign (ie, barium passing through the narrowed channel, creating a single, markedly attenuated, and elongated track) (see the following image) Upper gastrointestinal study from a child shows the string sign (see inset).
• Pyloric teat (ie, outpouching created by distortion of the lesser curve by the hypertrophied muscle)
• Retained secretions and retrograde peristalsis

Degree of confidence

Plain film radiography provides a low degree of confidence in making the diagnosis of or in ruling out hypertrophic pyloric stenosis. A UGI study has high sensitivity (>90%) and low specificity.

False positives/negatives

High intestinal obstruction can be seen with midgut volvulus, duodenal obstruction (from stenosis, duodenal web, annular pancreas), gastric outlet obstruction caused by focal foveolar hyperplasia, and eosinophilic gastroenteritis, among others. False-negative radiographs can be seen in a child who has recently vomited.

Ultrasonography is important in the diagnosis of hypertrophic pyloric stenosis and has likely contributed to the changing face of the disease, because this modality results in earlier diagnosis and treatment owing to the accessibility and accuracy of ultrasonography.^[5] This modality is the method of choice for both the diagnosis and exclusion of hypertrophic pyloric stenosis, because ultrasonography has a sensitivity and specificity of approximately 100%.^{[10, 11]} Thus, ultrasonography is also recommended in patients whose disease is clinically suspected but in whom the pyloric olive cannot be felt.^{[4, 10, 12, 13, 14, 15]}

In a study by Leaphart et al, ultrasonography confirms hypertrophic pyloric stenosis when the pyloric muscle thickness (MT) is greater than 4 mm and the pyloric channel length (CL) is greater than 15 mm.^[16] The investigators studied the diagnostic criteria for this disease in newborns younger than 21 days from 2000 to 2006 and found that ultrasonographic measurement of hypertrophic pyloric stenosis was significantly decreased in younger patients (MT, 3.7 +/- 0.65 mm; CL, 16.9 +/- 2.8 mm) versus older newborns (MT, 4.6 +/- 0.82 mm; CL, 18.2 +/- 3.4 mm). Of important note, the mean ultrasonographic measurement for young newborns with hypertrophic pyloric stenosis typically fell within the range currently defined as normal or borderline.^[16] A linear relationship existed between pyloric MT and CL and patient age, suggesting that 3.5 mm MT be considered the cutoff in younger patients.^[16]

Technique

Ultrasonography is performed with a 7.5- to 13.5-MHz linear transducer in the supine child. Transverse images at the epigastrium identify the pylorus to the left of the gallbladder and anteromedial to the right kidney (see the image below). A distended stomach, however, displaces and distorts the pylorus and may require the placement of a nasogastric tube to withdraw the stomach's contents. A gastric aspirate of more than 5 mL in a baby who has been without oral intake (NPO) for several hours indicates gastric outlet obstruction. Right posterior oblique positioning and scanning from a posterior approach may help to improve visualization of the pylorus.^[17]

Ultrasonographic signs of hypertrophic pyloric stenosis, originally described in 1977^[18] and further defined, are as follows:

• An MT (serosa to mucosa) greater than 3 mm (a correlation between MT and the patient's age exists; the most reliable ultrasonographic sign is an MT greater than 3 mm. Because this measurement can be increased falsely with off-axis imaging, attention to technique is important.)
• Target sign on transverse images of the pylorus (see the following image)Transverse ultrasonographic image in a patient with proven hypertrophic pyloric stenosis demonstrates the target sign and heterogeneous echo texture of the muscular layer (the pylorus is deep to the anechoic gallbladder).
• Pyloric channel length greater than 17 mm
• Pyloric thickness (serosa to serosa) of 15 mm or greater
• Failure of the channel to open during a minimum of 15 minutes of scanning
• Retrograde or hyperperistaltic contractions
• Antral nipple sign^[19] (ie, a prolapse of redundant mucosa into the antrum, which creates a pseudomass) (see the image below) Longitudinal ultrasonogram in a patient with hypertrophic pyloric stenosis demonstrates a redundant mucosa that creates the antral nipple sign.
• Double-track sign on ultrasonography^{[20, 21]} (ie, redundant mucosa in the narrowed lumen, which creates 2 mucosal outlines) (see the following image) Lateral view from an upper gastrointestinal study demonstrates the double-track sign.
• Other findings include reversible portal venous gas; nonuniform echogenicity of the pyloric muscle (see the image below)Transverse ultrasonographic image in a patient with proven hypertrophic pyloric stenosis demonstrates the target sign and heterogeneous echo texture of the muscular layer (the pylorus is deep to the anechoic gallbladder).

Degree of confidence

A positive hypertrophic pyloric stenosis finding by ultrasonography almost always indicates this condition. A negative examination can be false in a patient who is seen early in the disease or in a younger patient whose MT is less than 3 mm.

False positives/negatives

The diagnostic accuracy of ultrasonography for hypertrophic pyloric stenosis is high. Sensitivity and specificity approach 100%.^{[10, 11]} Possible sources of false negatives (see Limitations of techniques in the Introduction section) are an overdistended stomach (the pylorus is hidden behind the antrum), failure to identify the pylorus (eg, operator inexperience in performing ultrasonography for evaluation of this condition), and a small infant or early presentation. Another possible source for a false-positive finding is pylorospasm (typically transient).

Although a false-negative clinical diagnosis causes diagnostic delay, a false-positive diagnosis results in a negative laparotomy. Therefore, imaging has become more important in the diagnosis of hypertrophic pyloric stenosis.

Nuclear medicine scanning is not routinely used for hypertrophic pyloric stenosis; however, possible findings include delayed gastric emptying.

Degree of confidence

The degree of confidence is poor.

False positives/negatives

Nuclear medicine scanning has a high sensitivity but low specificity for hypertrophic pyloric stenosis.