Pediatrics
5-week-old with projectile vomiting and a palpable olive. Why you fix the electrolytes BEFORE you cut.
🎬 The Case
A 5-week-old boy. Four days of vomiting after every feed. He's hungry again immediately after. Something is wrong.
5-week-old male, born at 36 weeks. His parents brought him in because he vomits after every feed for the past 4 days. Not just spitting up. We're talking projectile vomiting that hits the wall. The milk is undigested. No bile. He's hungry again the moment it stops.
On exam: a firm, 2 cm olive-shaped mass in the epigastrium. You can see visible peristaltic waves moving across his abdomen left to right before each vomiting episode. Ultrasound shows an elongated, thickened pylorus.
What is the next best step?
⚙️ The Mechanism
Muscle hypertrophy you can feel with your fingers. That's rare in medicine. Appreciate it.
The The ring of muscle between the stomach and the small bowel. When it contracts, food passes. When it's too thick, nothing gets through.pyloric muscle starts hypertrophying in the first few weeks of life. By weeks 3-6, it's thick enough to almost completely block the passage of food from the stomach into the duodenum. The stomach fills, fills, fills, and then launches its contents outward.
Here is the key word: nonbilious. Bile comes from the common bile duct, which enters at the The first section of the small intestine, right after the stomach. Bile and pancreatic enzymes enter here via the ampulla of Vater.duodenum. The obstruction is at the pylorus, BEFORE bile can mix in. So what comes up is pure, undigested milk. If you ever see bilious vomiting in a newborn, your brain should immediately jump to something distal to the duodenum.
🦴 The Anatomy
Tap to compare. The ultrasound findings become obvious once you know what normal looks like.
Normal pylorus: thin muscle wall (<3mm), short channel (<15mm). Food passes without resistance. No olive. No drama.
Pyloric stenosis: the muscle wall is thickened (>3mm) and elongated (>15mm). The channel is nearly obliterated. Stomach contents have nowhere to go except back up, fast.
Ultrasound cross-section: the "donut" or "target" sign. Thick gray muscle ring surrounds a tiny lumen. Measure the wall (>3mm) and length (>15mm) to confirm. This is the diagnostic test of choice. No radiation. No barium.
📷 VISUAL ANCHORS
The imaging and anatomy boards expect you to recognize. Tap to expand.
🧪 The Chemistry
This baby has been vomiting HCl for days. Build the chain yourself.
The baby loses stomach acid every time he vomits. Build the full metabolic chain.
Tap a step to select it, then tap its slot above to place it.
👶 The Presentation
Every finding has a reason. Know the reason, own the board question.
| Finding | Why It Happens | Board Value |
|---|---|---|
| Projectile vomiting | Stomach contracts forcefully against obstruction, pressure builds, ejects contents | Nonbilious = tells you the obstruction is ABOVE bile entry (pre-duodenal) |
| Hungry right after vomiting | Child is actually starving. Nothing absorbed. Food never made it to intestines. | Distinguishes from other causes. A sick, infected child doesn't want to eat. |
| Olive-shaped mass | The hypertrophied pylorus is palpable as a firm, round, moveable mass in the RUQ/epigastrum | Pathognomonic when present. Not always felt in large babies or if stomach is distended. |
| Visible peristaltic waves | Stomach contracts, waves visible left to right across epigastrum as it tries to push food through | Watch for them by standing back and observing. Best seen during feeding. |
| Age 3-6 weeks | Muscle takes time to hypertrophy after birth | A 1-day-old cannot have pyloric stenosis. The muscle hasn't had time to grow. |
| Male, firstborn | Risk factor, not well-understood mechanistically | 4:1 male predominance. Firstborn child more commonly affected. |
🔍 The Differential
Four babies. One has pyloric stenosis. The clues will eliminate the others one at a time.
Each clue narrows it down. Click the baby that should be eliminated based on the current clue. Last one standing is your diagnosis.
🦹 The Villains
Tap each card. Know what makes it different. Only one has an olive and metabolic alkalosis.
🌲 The Algorithm
Step through the clinical decision. Tap each box to follow the logic.
A baby vomiting after every feed. Walk the tree. Each step narrows the answer.
⚠️ Board Traps
The question gives you a diagnosis and asks "next step." The answer is NOT pyloromyotomy. The answer is correct the metabolic derangement first. Pyloric stenosis is surgical, but it is NOT a surgical emergency. You have 24-48 hours to fix the labs before you cut. Anyone who takes this baby straight to the OR is putting them at risk for dangerous anesthetic complications.
The old way was a barium swallow (upper GI series, looking for the "string sign" where barium squeezes through a narrow pylorus). The modern standard is ultrasound. No radiation, no aspiration risk, no barium prep. If the question asks which test diagnoses pyloric stenosis, the answer is ultrasound unless a specific historical context forces otherwise.
This distinction decides your whole differential. Bilious vomiting in a neonate = surgical emergency until proven otherwise. Think malrotation with volvulus. Pyloric stenosis is always nonbilious because the block is proximal to where bile enters. If the vignette says bilious, you are not dealing with pyloric stenosis.
It's hypochloremic, hypokalemic, metabolic alkalosis. Not acidosis. Not hyperkalemia. The vomiting loses HCl, which raises pH (alkalosis). The kidney compensates by retaining H+ and dumping K+ (hypokalemia). The urine is paradoxically acidic in an alkalotic patient. If you can nail this chain, you will nail any question about vomiting-induced metabolic derangements forever.
🩸 The Fix
Never forget the order. The electrolytes come before the knife. Always.
CLINICAL REASONING
Infant with vomiting. Work through the key features.
🎯 Prove It
Don't kill them. Five questions per load, shuffled every visit.