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Imaging in Hypertrophic Pyloric Stenosis

Practice Essentials

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. The presentation age of preterm infants  is defined less clearly. Studies suggest that preterm infants may present with HPS symptoms at a later chronological age than term infants.
 Stark et al reported a series of 2466 newborns with HPS, 208 (8.43%) of whom were premature. Chronological age at presentation was found generally to increase as gestational age decreased. Preterm birth was associated with longer interval from birth to presentation with HPS (median [IQR] days of 40 [30-56] vs. 33 [26–45] in full-term infants; P<0.001).<ref>2</ref> HPS occurs in infants usually in the first 2 months of life and is rare in infants 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.
 HPS occurs in 2-5 per 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. It is also associated with smoking, postnatal erythromycin administration
, and bottle feeding. Viral etiologies have also been postulated.
McAteer et al showed the association with bottle feeding to be greater in older and multiparous mothers.

Approximately 300 cases of adult-onset idiopathic hypertrophic pyloric stenosis have been reported. The most common clinical symptom is abdominal distention relieved by vomiting. In adults, upper gastrointestinal barium series and upper endoscopy are used to confirm the diagnosis and to rule out possible malignant disease.

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.

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 right 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,
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.

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.

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.
If the clinical suspicion for hypertrophic pyloric stenosis is moderate to high, ultrasonography is also recommended. (See the images below.)

Longitudinal ultrasonogram of the pylorus in a pat

Longitudinal ultrasonogram of the pylorus in a patient with surgically proven hypertrophic pyloric stenosis. Note the thickened, circular muscle, elongated pylorus, and narrowed pyloric channel.

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Transverse ultrasonographic image in a patient wit

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).

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Longitudinal ultrasonogram in a patient with hyper

Longitudinal ultrasonogram in a patient with hypertrophic pyloric stenosis demonstrates a redundant mucosa that creates the antral nipple sign.

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

Lateral view from an upper gastrointestinal study

Lateral view from an upper gastrointestinal study demonstrates the double-track sign.

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Upper gastrointestinal study from a child shows th

Upper gastrointestinal study from a child shows the string sign (see inset).

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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.

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.

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 exclude 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.

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