Case Based Pediatrics For Medical Students and Residents
Department of Pediatrics, University of Hawaii John A. Burns School of Medicine
Chapter X.8. Hypertrophic Pyloric Stenosis
Kevin H. Higashigawa, MD

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This is a 3 week old male infant who presents to the emergency department with a chief complaint of vomiting x 3-4 days. His mother states that the vomiting has gotten progressively worse and now seems to "shoot out of his mouth." The emesis always occurs after feeding, sometimes vomiting the entire volume of his feed. The vomitus is non-bilious and non-bloody. After vomiting, the infant remains hungry and is still eager to feed. He is exclusively bottle fed with formula. There is no history of fever, URI symptoms, or diarrhea. He is less active than normal. He is making fewer wet diapers and less stool than usual. There is no history of trauma or recent travel. There are no ill contacts.

Exam: VS T 37.0, P 170, R 50, BP 80/50, O2 saturation 99% on RA. Length is 54 cm (50th percentile) and weight is 3.6 kg (25th percentile; previously 50th) and head circumference is 37 cm (50th). He is a well-developed, well-nourished male in no distress. His skin is normal. HEENT exam is normal. His neck is supple. Heart auscultation reveals tachycardia and a regular rhythm. Lungs are clear. His abdomen is slightly distended with active bowel sounds. No hepatosplenomegaly is noted. Attempting to palpate an olive mass is inconclusive. He has no inguinal hernias. Genitalia are normal. Extremities are normal. Color, perfusion, and capillary refill are good. Neurologic examination is normal.

CBC is unremarkable. Electrolytes: Na 131, K 3.2, Cl 95, bicarb 30. An IV fluid infusion is started. An abdominal series shows no obstruction, but the stomach is dilated. An ultrasound study confirms the diagnosis of pyloric stenosis. The patient undergoes a pyloromyotomy and recovers without complications.

Hypertrophic pyloric stenosis (HPS) is a common cause of GI obstruction in the young infant. HPS occurs in approximately 3 of every 1000 live births in the United States (1). Males are four times more likely to develop HPS than females. A familial pattern exists, although HPS does not follow classic Mendelian genetics. The risk for developing HPS is about 7% if the father was previously affected and about 10-20% if the mother was affected (1,2).

The manifestations of GI obstruction do not typically occur until about the 2nd to the 6th week of life (1). HPS presents after several weeks of life because the pylorus is normal at birth and hypertrophies as time progresses (2). The hallmark of gastric outlet obstruction is non-bilious vomiting. The vomiting occurs immediately after feeding and varies in intensity, depending upon the degree of stenosis present. Eventually, the vomiting increases in severity to become projectile and will typically involve the entire volume of the feed. Asking parents if the emesis is "projectile" is not very useful since, in the eyes of parents, ALL vomit "projects". Thus, a more discriminating question to ask, is to stand an arm's length from a wall and ask them if the emesis will hit the wall from that distance. If the answer is "yes", then it is projectile vomiting. After regurgitation, the infant will remain hungry and want to feed again. Approximately 8% of patients will have some degree of hematemesis related to gastritis or esophagitis (3).

The exact etiology of HPS is unclear. One theory proposes the lack of pyloric inhibitory innervation leading to reduced levels of nitric oxide, a smooth muscle relaxant. As a result, the pylorus experiences unopposed contraction following muscarinic stimulation (4). Elevated levels of prostaglandins have also been implicated owing to the increased incidence of pyloric stenosis in infants who have received PGE to maintain a patent ductus arteriosus. HPS has also been associated with other GI anomalies, such as tracheoesophageal fistula, pyloric atresia, antral webs, gastric duplications, and gastric volvulus. Furthermore, HPS has been linked to other disease states, such as eosinophilic gastroenteritis, epidermolysis bullosa, trisomy 18, and Turner Syndrome (1).

On physical exam, the infant may exhibit poor weight gain or even weight loss. Marasmus, however, or severe protein-calorie malnutrition, is rarely seen today. Jaundice may be seen in approximately 5% of infants (1). After feeding, a wave of gastric peristalsis may be seen traversing the abdomen from left to right, representing intense contractions against an obstruction. Abdominal distention may be a late finding, as is usually the case with proximal GI obstructions. The hypertrophied pylorus may be palpable. The pylorus is firm, mobile, and olive-shaped. It is located in the right upper quadrant of the abdomen, beneath the liver edge (1). It is best palpated from the left side while the infant is feeding since the abdominal muscles are relaxed. A palpable "olive" is pathognomonic of HPS (2), but it is very hard to feel in practice (requires experience to appreciate this accurately).

The diagnostic test of choice is the ultrasound, which has approximately 90% sensitivity (1). Criteria for diagnosis include an elongated pyloric channel (longer than 16 mm), an enlarged pyloric diameter (greater than 14 mm), and a thickened muscle wall (greater than 3.5 mm) (3). If an ultrasound is non-diagnostic but clinical suspicion remains high, an upper GI series may be helpful. Characteristic signs of HPS include an elongated pyloric channel with a "shoulder sign" (representing the hypertrophied pylorus bulging into the antrum) and streaks of barium flowing through the stenosed channel, producing either a single "string sign" or a "double track sign" (if there are parallel streaks) (1).

The "classic" laboratory finding in HPS is a hypochloremic, hypokalemic metabolic alkalosis. Repeated vomiting results in a loss of HCl, causing the hypochloremic metabolic alkalosis. The patient is likely dehydrated from repeated GI loss and poor oral intake. A metabolic acidosis (lactic acidosis) may result with severe dehydration. Due to more expedient diagnosis and treatment, however, more than 90% of patients with HPS do not typically present with any metabolic disturbance (5). Levels of glucuronyltransferase can be decreased in a small percentage of infants, as the liver is deprived of substrate from poor caloric intake, leading to an indirect hyperbilirubinemia (2).

The differential diagnosis of HPS is extensive. Vomiting in infants under 1 month of age is more likely due to a serious cause (often one requiring surgical intervention). Vomiting in older infants is more often secondary to gastroenteritis, but serious etiologies occur which may be difficult to diagnose.

Management of HPS initially consists of fluid replacement and management of electrolyte abnormalities to stabilize the patient. In the 1960s, HPS was treated medically with oral atropine, and surgery was reserved for sicker infants (4). However, due to the improvements in surgical technique and associated lower mortality and morbidity rates, as well as the rapidity of the resolution of symptoms, pyloromyotomy is now the treatment of choice. Currently, the mortality rate for pyloromyotomy is between 0 and 0.5% and can now be performed laparoscopically (1). HPS is not a surgical emergency. The infant should be fluid resuscitated prior to surgery and the patient's alkalosis should be resolved (bicarbonate level < 30 mEq/dL) to minimize anesthetic risk (3). Post-operative vomiting may occur secondary to edema of the pylorus at the incision site. In cases of incomplete pyloromyotomy, endoscopic balloon dilation has been successful. Of interest is that other countries may not treat HPS routinely with surgery, but rather IV fluid maintenance until the condition resolves with time in most instances. Pyloromyotomy is reserved for those infants who fail to improve.


1. What is the "classic" presentation of HPS?

2. How is HPS diagnosed?

3. What is the "classic" laboratory finding in HPS?

4. What is the initial step in management?

5. Which of the following sets of electrolytes could be seen with HPS (Na, K, Cl, bicarb):
. . . . . a. 130, 2.7, 90, 28
. . . . . b. 130, 5.8, 94, 22
. . . . . c. 130, 3.9, 98, 17
. . . . . d. 148, 4.1, 108, 13

Related x-rays

Yamamoto LG. Gastric Dilatation in a 2-Week Old. In: Yamamoto LG, Inaba AS, DiMauro R (eds). Radiology Cases In Pediatric Emergency Medicine, 1996, volume 5, case 17. Available online at:


1. Wyllie R. Chapter 329 - Pyloric Stenosis and Other Congenital Anomalies of the Stomach. In: Behrman RE, Kliegman RM, Jenson HB (eds). Nelson Textbook of Pediatrics, 16th edition. 2000, Philadelphia: W. B. Saunders Company, pp. 1130-1131.

2. Hostetler MA. Chapter 165 - Gastrointestinal Disorders. In: Marx JA, Hockberger RS, Walls RM, Adams J, Barkin RM, et al (eds). Rosen's Emergency Medicine: Concepts and Clinical Practice, 5th edition. 2002, St. Louis: Mosby, Inc, p. 2298.

3. Engum SA, Grosfeld JL. Chapter 67 - Pediatric Surgery. In: Townsend Jr. CM, Beauchamp RD, Evers BM, Mattox KL (eds). Sabiston Textbook of Surgery, 16th edition. 2001, Philadelphia: W. B. Saunders Company, pp. 1484-1485.

4. Rudolph CD. Medical Treatment of Idiopathic Hypertrophic Pyloric Stenosis: Should We Marinate or Slice the "Olive"? J Pediatr Gastroenterol Nutr 1996;23(4):399-401.

5. Papadakis K, Chen EA, Luks FI, Lessin MS, Wesselhoeft CW, DeLuca FG. Changing Presentation of Pyloric Stenosis. Am J Emerg Med 1999;17(1):69.

Answers to questions

1. A 3 to 4 week old male infant who presents with progressively severe, non-bilious vomiting, which may be projectile. The vomiting occurs immediately after feeding, after which the infant is still hungry and wants to feed again. On physical exam, the infant may display signs of dehydration. Visible waves of peristalsis may be seen and an "olive" may be palpable.

2. A palpable "olive" is pathognomonic but is very difficult to determine with certainty. If the pylorus cannot be palpated, ultrasound is diagnostic with 90% sensitivity.

3. The "classic" laboratory finding is a hypochloremic, hypokalemic metabolic alkalosis. However, due to more expedient diagnosis, this metabolic abnormality is seen in less than 10% of patients.

4. The initial step in management involves fluid resuscitation and correction of any metabolic abnormalities. HPS is not a surgical emergency, and any fluid deficits or alkalosis should be corrected prior to surgery to decrease surgical/anesthetic risks.

5. Electrolyte patterns are not pathognomonic for pyloric stenosis. The correct answers are a and c. Pattern "a" is a classic early vomiting picture, often seen with HPS. Pattern "c" is a picture of vomiting resulting in dehydration and lactic acidosis. This can also be seen later in the clinical course of HPS as dehydration worsens. Pattern "b" is typical of adrenal crisis (low Na, high K). Pattern "d" is typical of hypernatremic dehydration.

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