Hypertrophic pyloric stenosis (HPS) is commonly encountered in pediatric practice. The typical infant presents with nonbilious projectile vomiting and dehydration (with hypochloremic hypokalemic 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. [1, 2]
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. [3, 4]
HPS occurs in 2-5 in 1000 infants. While the etiology remains elusive, a multivariate cause including genetic and environmental exposures is accepted. It has a predilection for male infants, first-born infants, and those born to younger mothers. The male-to-female ratio for HPS is 4-6:1. HPS occurs in infants usually in the first 2 months of life and is rare in infants older than 6 months.
It is also associated with smoking, postnatal erythromycin administration, and bottle feeding. Viral etiologies have also been postulated.  Recently, McAteer et al showed the association with bottle feeding to be greater in older and multiparous mothers.  HPS is also seen in association with genetic and chromosomal disorders.
The earliest case of pyloric stenosis described in medical literature dates to 1717. However, it was not until Harald Hirschsprung's presentation of 2 cases in 1887 that widespread reporting of cases began. Markowitz aptly reviews the history of HPS noting its unique course of diagnosis and cure before imaging and yet the definitive etiology remains unknown. 
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 right kidney. The thickened pylorus narrows the pyloric channel, resulting in gastric outlet obstruction, gastric distention, and retrograde peristalsis in the stomach.
In the past and with experience, [8, 9] 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.  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. 
A peristaltic wave through the stomach may also be observed during visual inspection of the abdomen. 
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. [13, 14, 15]
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.  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. [16, 17, 18] 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 excludes out other problems, such as malrotation and gastroesophageal reflux.  See the following images.
Some investigators have reported that a UGI study is the most cost-effective study  (more than ultrasonography) in the vomiting infant because a negative ultrasonogram often leads to a UGI study to evaluate other diagnoses that a focused ultrasonographic evaluation does not detect.  A second test, such as ultrasonography, rarely follows a negative UGI study for hypertrophic pyloric stenosis. [14, 15]
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 fluid and 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, one should 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 authors report an increase in muscle thickness with fluid resuscitation.  Observation and repeat ultrasound examination in 2-3 days can confirm the diagnosis if the patient is stable.
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 in the evaluation of incomplete myotomy.
Differential diagnosis and other problems to be considered
Midgut volvulus is part of the differential diagnosis for high intestinal obstruction; however, other conditions to be considered include malrotation, with or without midgut volvulus; antral polyps; gastric duplication; focal foveolar hyperplasia; and pylorospasm.
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 exclude this condition. If the ultrasonographic findings are negative, perform a UGI study to confirm or exclude 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). This peristaltic wave may be seen on visual inspection of the abdomen.  If the patient has recently vomited or has a nasogastric tube in place, the stomach is decompressed and the radiographic findings are normal.
An 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
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.
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.  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%. [16, 17, 20, 19] Thus, ultrasonography is also recommended in patients whose disease is clinically suspected but in whom the pyloric olive cannot be felt. [9, 16, 21, 22, 23, 24]
In a study by Leaphart et al, ultrasonography confirmed hypertrophic pyloric stenosis when the pyloric muscle thickness (MT) was greater than 4 mm and the pyloric channel length (CL) was greater than 15 mm.  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.  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. 
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. 
Ultrasonographic signs of hypertrophic pyloric stenosis, originally described in 1977  and further defined, are as follows:
A 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
Other findings include reversible portal venous gas; nonuniform echogenicity of the pyloric muscle
Degree of confidence
A positive hypertrophic pyloric stenosis finding by ultrasonography almost always indicates this condition is present. 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.
Many articles since Teele and Smith’s original description of the ultrasound diagnosis of HPS have justified measurement criteria based on each group’s specificity, sensitivity, and positive predictive value. Values reported in the literature range from MT of 2.5 mm to 4 mm or thicker and CL 14 mm up to 19 mm. The risk of a false-positive test may result in a negative laparotomy. Forman concludes that an MT of 4 mm and CL of 16-20 mm may decrease sensitivity for HPS, but these thresholds avoid a negative laparotomy.  The authors comment that a patient with a normal ultrasound, based on these criteria, and persistent symptoms may be reevaluated. Of note, as discussed above, younger infants may have relatively smaller measurements of the pylorus in the setting of HPS.
The diagnostic accuracy of ultrasonography for hypertrophic pyloric stenosis is high. Sensitivity and specificity approach 100%. [16, 17] 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.
Iqbal et al showed that age and weight correlate positively with ultrasound measurement of the pylorus in pyloric stenosis and that it did not affect the diagnostic criteria. In their series of 318 ultrasound studies, there was 100% sensitivity and specificity for HPS when either the pyloric channel thickness or length of 3 mm and 15 mm, respectively, were met. Further, they demonstrated a negative correlation between pyloric measurement, age, and weight when the pylorus was normal.  If a question remains, follow-up ultrasound can be performed in a few days to reassess muscle thickness.
Another possible source for false-positive findings is pylorospasm (typically transient). In this scenario, one should wait up to 15 minutes and remeasure the muscle to avoid mistaking muscle spasm for hypertrophy.
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. The degree of confidence is poor. Nuclear medicine scanning has a high sensitivity but low specificity for hypertrophic pyloric stenosis.