Introduction
Background
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. HPS accounts for one third of nonbilious vomiting occurrences in infants and is the most common reason for laparotomy before age 1 year.
Despite its status as a common disease, the cause of HPS is unclear; however, a definite genetic component exists because up to a 5-fold increase in HPS incidence is observed in families in which a sibling or parent has had the disease. Pyloric stenosis and esophageal atresia may coexist.1
Ultrasonography (US) is important in the diagnosis of HPS and has likely contributed to the changing face of the disease, because this modality results in earlier diagnosis and treatment.
Pathophysiology
Full-thickness biopsies in patients with HPS demonstrate both hypertrophy and hyperplasia of the circular muscle layer of the pylorus. No clear pathophysiologic sequence or etiology has been described; however, proposed theories include, among others, abnormal circular muscle innervation, immature ganglion cells, decreased nitric-oxide stimulation of muscle fibers, immature ganglion cells, and abnormal gastrin levels.
Interestingly, pyloromyotomy works by 2 mechanisms: initially, the pyloric channel widens as a result of incising the muscle; afterward, the procedure secondarily induces regression of muscle hypertrophy and, over time, the disease itself.
Frequency
United States
HPS is seen in 2-5 infants per 1000 in North America and accounts for 30% of all patients who present with nonbilious vomiting before age 1 year. The onset of vomiting may occur as early as the first week of life and as late as age 5 months. A striking male preponderance is seen, with a male-to-female ratio of 4-6:1.
International
HPS is more common in the white population; the incidence in the Asian, Indian, and black populations is one third to one fifth lower than the incidence in whites.
Mortality/Morbidity
Deaths resulting from HPS are rare. Morbidity is linked directly to the duration and frequency of the patient's vomiting. Protracted vomiting causes decreased intake and increased loss of essential nutrients and electrolytes, which then results in dehydration and metabolic alkalosis. In a 25-year review of 901 infants with surgically proven HPS, Hulka et al describe a trend toward shorter duration of symptoms, earlier presentation, and a 50% reduction in hypochloremic metabolic alkalosis and dehydration.2 This improving trend is, in part, a result of the accessibility and accuracy of US in the diagnosis of HPS.
Race
HPS is more common in whites and is seen less commonly in black infants or infants of Asian descent.
Sex
Males are significantly more affected than females at a rate of 4-6:1.
Age
HPS is most commonly seen in infants aged 3-6 weeks. The thickness of the circular muscle in HPS parallels the patients' age; those presenting at an older age have a muscle thickness (MT) greater than those who present in the usual age range. 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 MT because they are comparatively smaller. A large epidemiologic study reported that the interval between birth and the onset of symptoms in preterm infants was longer than that in full-term infants.3 HPS is rarely seen in children older than 6 months.
Anatomy
In HPS, 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 HPS, 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.
Presentation
HPS is the most common indication for laparotomy in infants and accounts for 30% of patients who present with nonbilious vomiting. Parents may describe the vomiting as occurring with every feeding or intermittent vomiting that may be projectile; the vomiting worsens over time. Bilious vomiting is rare. Workup of the vomiting child first requires a detailed history, followed by a complete physical examination.
In the past and with experience,4,5 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.2 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 US for diagnosis.6
Preferred Examination
The preferred diagnostic test for HPS 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 HPS is a thorough physical examination. If the pyloric olive is felt, the patient may proceed directly to the operating room without imaging.5 However, many surgeons are uncomfortable with this protocol because a false-positive physical examination then leads to a negative laparotomy. Therefore, US is recommended because its sensitivity and specificity are close to 100% for this disease.10,11 If the clinical suspicion for HPS is moderate to high, US is also recommended.
If the vomiting infant is outside the usual age range for HPS or if the clinical suspicion is low, an upper gastrointestinal (UGI) study is recommended because it more effectively rules out other problems such as malrotation and gastroesophageal reflux.9
Some investigators have reported that a UGI study is the most cost-effective study8 (more than US) in the vomiting infant because a negative US often leads to a UGI study to rule out other diagnoses that a focused US evaluation does not detect.9 A second test, such as US, rarely follows a negative UGI study for HPS.8,9 In experienced hands, US 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 Techniques
US has high sensitivity, specificity, and accuracy in the diagnosis of HPS. However, errors in diagnosis do occur and relate to the following:
False negatives
Operator inexperience: The pylorus may not be identified.
Distended formula 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 US, place a nasogastric tube and withdraw the gastric secretions.MT increases with patient size. Borderline measurements are seen early in the disease and with premature infants. Some clinicians advocate observation and then repeating the US study in 2-3 days to confirm the diagnosis if the patient is stable and is not dehydrated.
False positives
Pylorospasm: The normal pylorus opens at least once every 15 minutes. Pylorospasm is a dynamic process that changes over time. The thickened muscle and elongated pylorus should be fixed.
Postoperative appearance: Symptoms may take time to clear and, therefore, so do the abnormalities on US. US may show HPS (thickened MT) for up to 12 weeks following pyloromyotomy. In these cases, a UGI study may provide more information than US to rule out incomplete myotomy.
Differential Diagnoses
Other Problems to Be Considered
Malrotation, with or without midgut volvulus
Antral polyp
Gastric duplication
Focal foveolar hyperplasia
Pylorospasm
More on Hypertrophic Pyloric Stenosis |
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References
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Further Reading
Keywords
idiopathic hypertrophic pyloric stenosis, infantile hypertrophic pyloric stenosis, pyloric stenosis, HPS, IHPS, projectile vomiting, nonbilious infantile projectile vomiting
