Stenosis Pilorus Hipertrofi

01Pathophysiology

Hypertrophic pyloric stenosis (HPS) results from progressive hypertrophy and hyperplasia of the circular smooth muscle layer of the pylorus. The exact trigger remains unclear, but the net effect is a thickened pyloric channel that increasingly narrows the gastric outlet over the first weeks of life. This is why infants are born healthy and symptoms develop gradually between weeks 2 and 8, with a classic peak around week 3 to 5.

Because the obstruction sits at the pylorus (proximal to the ampulla of Vater), the vomitus contains only gastric contents: milk and gastric acid. This is why the vomiting is always nonbilious. The projectile nature reflects forceful gastric contractions attempting to push feeds past a near-complete mechanical obstruction. After vomiting, the stomach empties, and the infant feels hungry again, creating the characteristic "hungry vomiter" pattern. This detail is a classic discriminator on exam vignettes.

The metabolic consequences follow directly from the loss of gastric secretions. Each episode of vomiting removes hydrochloric acid (HCl), meaning the infant loses both hydrogen ions (${\text{H}}^{+}\backslash text\{H\}^+$) and chloride ions (${\text{Cl}}^{-}\backslash text\{Cl\}^-$). The kidneys attempt to compensate for the rising serum pH (alkalosis) by excreting bicarbonate (${\text{HCO}}_3^{-}\backslash text\{HCO\}\_3^-$), but as chloride depletion worsens, the kidneys cannot excrete bicarbonate without an accompanying anion. Instead, the kidneys retain ${\text{Na}}^{+}\backslash text\{Na\}^+$ by exchanging it for ${\text{K}}^{+}\backslash text\{K\}^+$ and ${\text{H}}^{+}\backslash text\{H\}^+$ in the distal tubule, resulting in paradoxical aciduria (acidic urine despite systemic alkalosis). The final electrolyte picture is the hallmark hypochloremic, hypokalemic metabolic alkalosis.

Dehydration compounds the problem. Volume depletion activates the renin-angiotensin-aldosterone system (RAAS), further driving potassium and hydrogen ion loss in exchange for sodium retention, which deepens the alkalosis and hypokalemia.

02Classification and Clinical Manifestation

HPS does not have formal disease subtypes or staging systems. However, clinical presentation can be organized by severity and progression:

A testable point: indirect (unconjugated) hyperbilirubinemia can appear in advanced cases due to caloric deprivation reducing hepatic glucuronyl transferase activity. This resolves after successful treatment and does not indicate liver disease.

03Diagnostic Workup

The workup logic is straightforward. When a 3 to 6 week-old infant presents with progressive, nonbilious, projectile vomiting and is eager to re-feed, HPS should be the leading diagnosis. The best initial test is abdominal ultrasound, which is also the most accurate test for this condition. Ultrasound is noninvasive, does not involve radiation, and has a sensitivity and a specificity both exceeding 95%. The key measurements to remember are the "pi-3-15" rule: pyloric muscle thickness $\ge 3\text{ mm}\backslash geq\; 3\backslash \; \backslash text\{mm\}$ and pyloric channel length $\ge 15\text{ mm}\backslash geq\; 15\backslash \; \backslash text\{mm\}$.

An upper GI series with barium is reserved for cases where the ultrasound is equivocal or when the clinical picture is atypical. On fluoroscopy, the classic finding is the "string sign", a thin stream of barium passing through the elongated, narrowed pyloric channel. While recognizable, this study involves radiation and contrast exposure, making it a second-line option.

Simultaneously, a basic metabolic panel should be obtained. The classic electrolyte pattern (low ${\text{Cl}}^{-}\backslash text\{Cl\}^-$, low ${\text{K}}^{+}\backslash text\{K\}^+$, elevated ${\text{HCO}}_3^{-}\backslash text\{HCO\}\_3^-$) not only supports the diagnosis but also dictates the timing of surgery. Surgery is only performed once electrolytes have been corrected. This metabolic panel therefore serves a dual diagnostic and preoperative planning purpose.

A plain abdominal radiograph is neither sensitive nor confirmatory for HPS. If obtained, it may show a dilated stomach with a paucity of distal gas, but this is nondiagnostic.

04Management and Treatment

Step 1: Fluid and electrolyte correction comes first. This is the most tested management principle in HPS. Despite being a surgically correctable condition, HPS is not a surgical emergency. Attempting surgery on an infant with uncorrected hypokalemic, hypochloremic metabolic alkalosis carries serious anesthetic risks, including cardiac arrhythmias and post-operative apnea. The initial resuscitation uses isotonic normal saline (0.9% NaCl) at 20 mL/kg boluses to restore intravascular volume. Once urine output is confirmed, potassium chloride (KCl) is added to maintenance fluids, typically at 20 to 40 mEq/L. The chloride in both the saline and the KCl is essential to correct the hypochloremia that sustains the metabolic alkalosis.

Step 2: Surgical correction after metabolic normalization. The definitive procedure is the Ramstedt pyloromyotomy. The surgeon makes a single longitudinal incision along the anterior surface of the pylorus, cutting through the serosa and the hypertrophied circular muscle layer, while carefully preserving the underlying mucosa and submucosa intact. This releases the constriction and permanently resolves the obstruction. If the mucosa is inadvertently perforated during the procedure, it must be repaired and the myotomy performed at a different site on the pylorus.

Step 3: Postoperative care. Feeding is typically restarted within hours of surgery. Parents should be counseled that mild vomiting may continue for 1 to 2 days post-operatively as the edema from surgery resolves. This does not require re-operation. Prognosis after pyloromyotomy is excellent, with nearly a 100% cure rate and essentially no recurrence.

Contraindications and special considerations: There is no role for medical (non-surgical) management as definitive treatment. Historically, IV atropine was studied as an alternative in settings where surgery was unavailable, but it is not standard of care. NPO status must be maintained preoperatively. Correct the metabolic abnormality before sedation or anesthesia.

05Differential Diagnosis and Distractors

The single most important discriminator on an exam question is the character of the vomitus. Nonbilious projectile vomiting in a 3 to 6 week-old points to pyloric stenosis. Bilious vomiting at any age is malrotation with volvulus until proven otherwise and demands urgent surgical evaluation.

06Traps and High-Yield Pearls

The most common way students lose points on HPS questions is by selecting surgery as the immediate next step in management. Vignettes are frequently written to present a clearly dehydrated infant with classic projectile vomiting and an olive-shaped mass, alongside lab values showing pronounced metabolic alkalosis and hypokalemia. The answer choices will include both "IV fluid resuscitation" and "Ramstedt pyloromyotomy." The trap is that students recognize the surgical diagnosis and jump straight to the operation. The correct answer is always to correct the electrolyte and fluid abnormalities first. Operating on an infant in metabolic alkalosis with hypokalemia risks fatal arrhythmias and respiratory depression under anesthesia. Remember: HPS is urgent but not emergent.

A second common trap involves confusing the character of the vomitus. If the vignette mentions bilious (green) vomiting, the answer is never pyloric stenosis regardless of the infant's age or other findings. Bilious vomiting in an infant is malrotation with volvulus until proven otherwise.

Third, do not confuse the ultrasound "target sign" of pyloric stenosis (thickened pyloric muscle in cross-section) with the ultrasound "target sign" of intussusception (telescoped bowel layers). The clinical context, patient age, and associated symptoms will differentiate them.

Finally, remember the erythromycin association. A vignette may describe an infant whose mother received erythromycin during late pregnancy or who received the drug directly in the first two weeks of life for pertussis prophylaxis. This macrolide acts as a motilin receptor agonist, promoting pyloric smooth muscle contraction and hypertrophy. This pharmacologic link is a favorite distractor-proof detail for test writers.

The core competency being tested in HPS questions is the ability to (1) recognize a classic surgical diagnosis in a pediatric patient, (2) understand that preoperative stabilization takes priority over definitive surgical repair, and (3) connect the pathophysiology of vomiting gastric acid to the resulting electrolyte and acid-base disturbance.