Definition And Epidemiology

Hypertrophic pyloric stenosis is an acquired condition characterized by progressive idiopathic hypertrophy of the pyloric circular muscle.
This hypertrophy causes narrowing and elongation of the pyloric channel, leading to gastric outlet obstruction.
The condition is not present at birth and develops entirely postnatally.

Factor	Clinical Details
Incidence	Occurs in 2 to 5 per 1000 live births.
Demographics	Demonstrates a strong Caucasian and Northern European predominance.
Sex And Birth Order	Males are affected 4 to 6 times more frequently than females, with firstborn males being the most commonly affected group.
Genetics	Exhibits high heritability (87%) and is linked to NOS1 gene variants. Offspring of affected mothers carry the highest risk.
Environmental Triggers	Associated with formula bottle-feeding, maternal smoking, and exposure to macrolide antibiotics (such as erythromycin) during the first two weeks of life.

Pathophysiology And Metabolic Derangements

A deficiency in nitric oxide occurs due to reduced neuronal nitric oxide synthase (NOS), which prevents normal smooth muscle relaxation.
This lack of relaxation triggers compensatory hyperperistalsis, which subsequently leads to muscle hypertrophy.
Histological examination reveals degeneration of intramuscular ganglion cells within Auerbach plexus.
Hormonally, formula feeding increases gastrin levels and decreases vasoactive intestinal peptide (VIP), further promoting pyloric contraction.

The classic metabolic derangement is a hypochloremic, hypokalemic metabolic alkalosis.
Massive loss of gastric hydrochloric acid triggers the alkalosis, prompting the kidneys to compensate by retaining hydrogen ions at the expense of potassium and sodium.
Paradoxical aciduria develops in severe, prolonged dehydration because the kidneys preferentially reabsorb sodium in exchange for hydrogen to maintain intravascular volume, yielding acidic urine despite the systemic alkalosis.

Clinical onset typically occurs between 3 to 6 weeks of age and is exceptionally rare beyond 6 months.
Infants present with non-bilious, projectile emesis immediately following feeds, which progressively worsens over time.
Despite the vomiting, the infant remains ravenously hungry post-emesis.
Additional signs include small starvation stools and clinical markers of dehydration, such as weight loss, lethargy, and decreased urine output.
Icteropyloric syndrome, characterized by unconjugated hyperbilirubinemia, is seen in approximately 5% of cases and resolves post-pyloromyotomy.

Visible gastric peristaltic waves may be observed moving from left to right across the epigastrium during or immediately after feeding.
A palpable firm, olive-shaped mass (the pyloric olive) is classically felt in the mid-epigastrium or right upper quadrant, particularly during a test feed or post-emesis.

Modality	Diagnostic Criteria And Findings
Ultrasound	Represents the test of choice with near 100% sensitivity and specificity. Diagnostic measurements include a muscle wall thickness >3-4 mm, pyloric channel length >14-16 mm, and a total pyloric diameter >12-14 mm.
Upper GI Contrast	Reserved for equivocal ultrasound cases or to rule out malrotation. Characteristic findings include the string sign (elongated, narrow channel), shoulder sign (hypertrophied muscle bulging into the antrum), mushroom sign, and double tract sign.

Gastroesophageal reflux disease, which typically presents with effortless regurgitation rather than projectile vomiting.
Malrotation with volvulus, which strictly presents with bilious emesis and requires emergent surgery.
Systemic infections or metabolic errors, which often present with concurrent fever, diarrhea, or lethargy.

The presentation is a medical emergency requiring urgent fluid resuscitation, not an immediate surgical emergency.
Maintain the infant strictly nil per os (NPO) and insert a nasogastric or orogastric tube for gastric decompression.
Initiate intravenous fluid therapy with a Normal Saline bolus, followed by maintenance fluids consisting of 5% Dextrose with 0.45% Normal Saline.
Add potassium chloride to the intravenous fluids only after adequate urine output is established.
Surgical intervention is absolutely contraindicated until the alkalosis resolves, as general anesthesia during severe alkalosis depresses the respiratory drive and risks post-operative apnea.
Target a serum chloride level above the mid-90s mEq/L prior to proceeding with surgery.

The definitive surgical procedure is the Ramstedt pyloromyotomy, which involves a longitudinal incision and blunt splitting of the hypertrophied circular muscle down to, but sparing, the submucosal layer.
Surgery can be performed via open approaches, laparoscopically, or through emerging endoscopic techniques like gastric peroral endoscopic myotomy.
Conservative medical therapy utilizing intravenous followed by oral atropine sulfate acts as a muscle relaxant but is reserved strictly for patients unfit for general anesthesia, as it carries a lower success rate and mandates a longer hospital stay.

Potential complications include incomplete myotomy, unnoticed intraoperative mucosal perforation, post-operative vomiting (which is common and usually self-limiting), and post-operative apnea secondary to inadequately corrected metabolic alkalosis.