Pyloric Stenosis

• Pyloric smooth muscle hypertrophy and hyperplasia: The fundamental pathological process involves selective hypertrophy (increased cell size) and hyperplasia (increased cell number) of the circular and, to a lesser extent, longitudinal muscle fibers of the pylorus. This results in a firm, olive-shaped pyloric muscle mass (typically 2–4 cm in diameter) that mechanistically obstructs the pyloric channel. The mucosa and submucosa remain normal. The etiology of this muscle overgrowth remains incompletely understood but likely involves dysregulation of growth factor signaling pathways, particularly involving hepatocyte growth factor (HGF), epidermal growth factor (EGF), and their respective receptors. Abnormalities in nitric oxide synthase and impaired nitric oxide-mediated smooth muscle relaxation have also been implicated, contributing to both muscle overgrowth and pylorospasm.

• Obstruction of gastric emptying and gastric outlet mechanics: The hypertrophied pyloric muscle physically narrows the pyloric channel, progressively impeding antral peristalsis and gastric emptying. This mechanical obstruction initially allows some gastric contents to pass but becomes increasingly restrictive as the infant feeds, leading to symptoms that typically appear after 2–3 weeks of age. The gastric wall responds by developing increased peristaltic force in attempts to overcome the obstruction, eventually resulting in secondary gastric wall changes including edema and further muscularization of the antrum.

• Metabolic and electrolyte consequences: Recurrent, projectile vomiting of gastric contents (which contains hydrochloric acid, potassium, and sodium) leads to severe hypochloremic, hypokalemic metabolic alkalosis. The loss of gastric hydrochloric acid (HCl) removes hydrogen and chloride ions, raising serum bicarbonate concentration and blood pH. Simultaneously, volume depletion activates the renin-angiotensin-aldosterone system, promoting renal sodium reabsorption but also driving urinary potassium wasting. The kidneys attempt to compensate for metabolic alkalosis by reducing bicarbonate reabsorption and conserving hydrogen ions, but the aldosterone-mediated potassium loss paradoxically perpetuates the alkalosis because hypokalemia causes the proximal tubule to increase ammonia production and augments distal tubular hydrogen ion secretion. This creates a vicious cycle of worsening alkalosis and hypokalemia.

• Genetic and familial predisposition: Pyloric stenosis demonstrates clear familial clustering, with concordance rates in monozygotic twins approaching 100% and significantly elevated risk in first-degree relatives of affected infants. If the mother was affected, offspring risk is approximately 15%; if the father was affected, risk is approximately 5%. This male-to-female transmission difference may reflect incomplete penetrance and variable expression in females. Multiple candidate genes have been identified (including neuronal nitric oxide synthase mutations and polymorphisms in EGF and hepatocyte growth factor pathways), but no single Mendelian inheritance pattern has been definitively established, suggesting multifactorial inheritance with environmental influences.

• Age and developmental factors: The condition manifests between 2 and 8 weeks of age, with peak incidence at 3–5 weeks. This critical developmental window suggests that pyloric stenosis results from aberrant maturation of pyloric smooth muscle during infancy rather than a congenital structural malformation. The progressive nature and late presentation after birth (infants are typically born with normal pyloric function) indicate that postnatal factors modulating muscle growth are paramount.

• Environmental and perinatal risk factors: Prematurity, exposure to certain medications in utero or postnatally (notably macrolide antibiotics such as erythromycin, which can induce pyloric stenosis when used for gastroesophageal reflux in young infants), male sex, first-born status, and ethnicity (Northern European, Caucasian higher prevalence; Asian, African descent lower prevalence) all increase risk. Maternal factors including advanced maternal age and primigravidity have been associated with increased risk in offspring.

• Projectile vomiting: This is the cardinal symptom and results directly from forceful gastric peristalsis against an obstructed pylorus. The vomiting is characteristically described as "projectile" because gastric contractions forcefully expel gastric contents across a distance rather than simple regurgitation. Vomiting typically occurs shortly after feeding and is non-bilious (because the obstruction is proximal to the pylorus, preventing bile reflux). The infant appears hungry immediately after vomiting and eagerly accepts another feeding, distinguishing this from gastroesophageal reflux or food aversion.

• Progressive dehydration and poor weight gain: Recurrent vomiting causes loss of fluids and electrolytes, manifesting as decreased skin turgor, dry mucous membranes, sunken anterior fontanelle (in young infants), and decreased urine output. Affected infants fail to gain weight and may lose weight despite apparently adequate feeding. The infant develops characteristic signs of dehydration including sunken eyes, lethargy, and decreased activity.

• Visible gastric peristaltic waves: During examination, particularly if performed shortly after feeding or during observation of the infant feeding, characteristic left-to-right peristaltic waves may be observed crossing the epigastrium as the stomach attempts to empty against the obstructed pylorus. These waves represent visible contractions of the hypertrophied gastric musculature.

• "Olive" mass palpation: The pathognomonic physical finding is a firm, smooth, olive-shaped mass palpable in the epigastrium just to the right of midline, at the level of the inferior liver edge. This represents the hypertrophied pyloric muscle. The mass is best felt with gentle, deep palpation during relaxation of the infant's abdominal wall musculature, sometimes after vomiting has emptied the stomach. Successful palpation of the olive mass occurs in approximately 60–80% of experienced examiners but may be difficult in tense, crying infants or those with significant abdominal distention.

• Abdominal distention and visible peristalsis: The stomach may become markedly dilated from retained food and secretions, causing noticeable epigastric fullness or visible distention. Prominent gastric peristaltic waves may traverse the abdomen visibly.

• Dehydration-related signs: As the condition progresses, infants develop signs of moderate to severe dehydration including weight loss (up to 10% of birth weight or more), lethargy, weak cry, and ultimately signs of hypovolemic shock (tachycardia, weak pulses, delayed capillary refill) if surgical treatment is delayed.

• Clinical diagnosis supported by imaging: While the clinical presentation (projectile vomiting in a 3–5-week-old male infant) is often highly suggestive, abdominal ultrasonography is the gold standard imaging modality and should be performed to confirm the diagnosis before surgical intervention. Ultrasound demonstrates a pyloric muscle thickness greater than 3–4 mm (some sources use 2.5–3 mm as the cutoff) and a pyloric channel length exceeding 14–16 mm. The pyloric muscle mass appears as a hypoechoic (dark) rounded or oval structure at the level of the gastroduodenal junction, often with visible peristaltic waves within the retained gastric contents behind it. Sensitivity and specificity of ultrasound approach 95–100% when performed by experienced operators.

• Laboratory findings reflecting metabolic derangements: Serum electrolyte analysis reveals hypochloremia (chloride <90 mEq/L), hypokalemia (potassium <3.5 mEq/L), and elevated serum bicarbonate (>30 mEq/L), consistent with hypochloremic, hypokalemic metabolic alkalosis. Blood gas analysis shows elevated pH (typically >7.50), elevated HCO3− (typically >35 mEq/L), and a base excess significantly elevated (often +15 or greater). Blood urea nitrogen (BUN) may be disproportionately elevated relative to creatinine, reflecting prerenal azotemia from dehydration. These laboratory abnormalities are not specific to pyloric stenosis but reflect the metabolic consequences of vomiting and dehydration.

• Gastric imaging with barium studies: Although now largely superseded by ultrasound, upper gastrointestinal barium studies historically demonstrated a "string sign" (a thin, elongated pyloric channel narrowed by the hypertrophied pyloric muscle), a distended stomach with retained food, and delayed gastric emptying. The antrum may appear diminished in caliber. Barium studies are now reserved for cases in which ultrasound is inconclusive or unavailable.

• Diagnostic criteria: The diagnosis of pyloric stenosis is confirmed by demonstration of pyloric muscle thickness >3 mm (or >2.5 mm in some protocols) and pyloric channel length >14 mm on ultrasonography in an infant with clinical signs of gastric outlet obstruction (projectile non-bilious vomiting, failure to gain weight, dehydration).

• Differential diagnosis considerations: Gastroesophageal reflux disease (GERD) presents with vomiting but typically in older infants and without projectile quality; the infant does not appear acutely ill or severely dehydrated. Malrotation with intermittent volvulus can present with bilious vomiting (suggesting small bowel involvement) and may cause shock. Atresia (esophageal, duodenal, or jejunal) presents immediately at birth or within hours with vomiting and abdominal distention. Food allergy or milk protein intolerance typically presents with mucous or bloody diarrhea in addition to vomiting. Gastroenteritis presents with vomiting and diarrhea simultaneously. Central nervous system abnormalities may cause vomiting but typically with other neurological findings. Increased intracranial pressure (from hydrocephalus, for example) may cause projectile vomiting but is typically accompanied by other signs of neurological dysfunction.

• Preoperative fluid and electrolyte resuscitation (first-line stabilization): Before surgical intervention, aggressive intravenous fluid resuscitation is essential to correct dehydration, hypochloremia, and hypokalemia. Normal saline (0.9% NaCl) is the initial fluid of choice because it replaces both sodium and chloride losses. However, given that hypochloremic metabolic alkalosis is the characteristic disturbance, solutions containing increased chloride concentration are preferred; many institutions use solutions combining normal saline with added potassium chloride (typically 10–20 mEq/L of KCl, adjusted based on serial serum potassium measurements). The goal is to restore intracellular and extracellular potassium stores, which may require 24–48 hours or longer of careful fluid management. Potassium supplementation should begin only after documenting urine output (to avoid hyperkalemia if the infant is oliguric), and serum electrolytes should be monitored frequently (every 4–8 hours initially) during resuscitation. Fluid administration rate is typically 10–20 mL/kg/hour of isotonic crystalloid, adjusted for ongoing losses (from continued vomiting) and clinical response (urine output, vital signs, weight, clinical perfusion). A bolus of 20 mL/kg of normal saline may be given over 15–30 minutes for signs of hypovolemic shock. Nasogastric decompression with a 10–12 French tube on intermittent or continuous suction helps prevent further vomiting and fluid loss while allowing assessment of gastric output.

• Pyloromyotomy (definitive surgical treatment): Once the infant is metabolically stable and well-hydrated (typically assessed by normalization or near-normalization of serum electrolytes, adequate urine output, and improved clinical perfusion), Ramstedt pyloromyotomy is performed. This procedure involves a small supraumbilical or periumbilical transverse incision (open approach) or laparoscopic approach. The pyloric muscle is divided along its length down to but not through the pyloric mucosa, dividing the muscle fibers without entering the gastric lumen. The procedure typically takes 15–30 minutes and is curative in >99% of cases. Laparoscopic approaches are increasingly used and offer faster recovery and less postoperative pain, though open pyloromyotomy remains the gold standard in many centers due to shorter operative time and equivalent outcomes.

• Postoperative feeding protocol: Postoperatively, infants are typically allowed nothing by mouth for 4–6 hours, followed by gradual advancement of feeding. Most protocols allow clear liquids (such as sterile water or electrolyte solutions) at 2–3 hours postoperatively in small volumes (5–10 mL per feeding every 2–3 hours), progressing to half-strength formula and then full-strength formula over 24–48 hours as tolerated. Early feeding protocols have been shown to be safe and reduce postoperative hospital length of stay. Some residual vomiting may occur immediately postoperatively but typically resolves within 48 hours as the pylorus becomes less edematous.

• Treatment of complications: If postoperative hemorrhage occurs (rare, <1% of cases), fluid resuscitation and blood products are administered; exploration is rarely necessary. Perforation of the pyloric mucosa (1–2% of cases) requires closure with fine absorbable sutures in a single or double layer and possibly coverage with omentum. Wound infection (in open procedures) is managed with antibiotics and local care.

• Pharmacological therapy (rarely used): Although atropine or other anticholinergic agents may transiently decrease pyloric muscle contractions and have been used historically to manage mild cases, they are not effective long-term solutions and have fallen out of favor. Similarly, domperidone (a dopamine antagonist and prokinetic agent) is not effective in treating established pyloric stenosis, though it has been explored for prevention in at-risk infants. Pyloromyotomy remains the only definitive treatment.

• Hypochloremic, hypokalemic metabolic alkalosis (acute complication during presentation): This is the most common metabolic consequence of recurrent vomiting and results from loss of gastric HCl and renal potassium wasting. Severe alkalosis (pH >7.65, HCO3− >45 mEq/L) can cause irritability, seizures, tetany (from associated hypocalcemia and hypomagnesemia), and cardiac arrhythmias. Management consists of fluid resuscitation with chloride-rich solutions and careful potassium supplementation to correct the underlying deficits.

• Hypovolemic shock and acute kidney injury (life-threatening acute complication): Severe dehydration from prolonged vomiting can progress to hypovolemic shock with hypotension, poor perfusion, oliguria, and metabolic acidosis superimposed on the metabolic alkalosis (creating a mixed acid-base disorder). Acute kidney injury may develop from prerenal azotemia if fluid resuscitation is delayed. Management includes aggressive intravenous fluid resuscitation, vasopressor support if hypotension persists despite fluid administration, and careful monitoring of renal function and urine output.

• Gastric perforation during surgery: Although rare (<2% of cases), inadvertent perforation of the pyloric mucosa during pyloromyotomy requires recognition and repair intraoperatively. If unrecognized, postoperative peritonitis may develop. Intraoperative recognition and primary closure dramatically reduce morbidity.

• Dehydration and failure to thrive: Untreated pyloric stenosis results in progressive dehydration, electrolyte depletion, malnutrition, and failure to gain weight or actual weight loss. With prompt diagnosis and surgical treatment, this is rapidly reversed, and infants resume normal growth trajectories.

• Aspiration pneumonia: Recurrent projectile vomiting, particularly in lethargic or dehydrated infants, carries risk of aspiration of gastric contents into the lungs, leading to aspiration pneumonia. This risk is minimized by prompt nasogastric decompression and careful fluid resuscitation.

The stem that gives it away

• Demographics + timing: a firstborn male, 3–6 weeks old, previously feeding well, now with non-bilious projectile vomiting who is hungry immediately after vomiting ("hungry vomiter"). Bilious emesis argues against pyloric stenosis because the obstruction is proximal to the ampulla of Vater.
• Physical exam buzzwords: palpable "olive" in the right epigastrium and visible left-to-right gastric peristaltic waves. A palpable olive in a classic stem is enough to proceed without contrast studies, but ultrasound is still routinely obtained.

The single best next step

• Abdominal ultrasound is the initial imaging study of choice (consistent with ACR Appropriateness Criteria for vomiting in infants) — no radiation, direct measurement of muscle thickness and channel length.
• Correct the fluids and electrolytes first — surgery is never the immediate answer. Pyloric stenosis is a medical emergency and a surgical urgency; operating on an alkalotic infant risks postoperative apnea, because alkalosis blunts the central respiratory drive. If a question offers "take to OR now" versus "IV normal saline with KCl after urine output," choose resuscitation.

The metabolic association examiners love

• **Hypochloremic, hypokalemic metabolic alkalosis with *paradoxical aciduria***: volume depletion drives aldosterone-mediated Na⁺ reabsorption; with chloride unavailable, the distal nephron secretes H⁺ (and K⁺) instead, so the urine turns acidic despite systemic alkalemia. Urine chloride is low.
• Macrolides: erythromycin (and azithromycin) exposure in the first weeks of life — or via breast milk — is associated with hypertrophic pyloric stenosis; this drove changes in neonatal pertussis prophylaxis counseling.

Distractors to avoid

• Bilious vomiting → think malrotation with midgut volvulus; the next step there is an upper GI series, not ultrasound.
• **Vomiting from birth with a *double bubble*** → duodenal atresia, associated with trisomy 21 — not a 4-week-old.
• GERD infants are happy spitters who gain weight and are not dehydrated or alkalotic.
• Post-pyloromyotomy emesis for a day or two is expected edema, not failed surgery.