What is the appropriate diagnostic work‑up for a term infant aged 2–6 weeks presenting with progressive non‑bilious projectile vomiting, weight loss, and a palpable olive‑shaped mass in the right upper quadrant?

Diagnosis of Infantile Hypertrophic Pyloric Stenosis

Abdominal ultrasound is the definitive diagnostic test for infantile hypertrophic pyloric stenosis (IHPS) in a term infant aged 2–6 weeks presenting with progressive non-bilious projectile vomiting, weight loss, and a palpable olive-shaped mass. 1

Clinical Presentation

The classic presentation includes:

• Progressive projectile non-bilious vomiting beginning typically between 3–6 weeks of age 1, 2
• Palpable "olive-shaped" mass in the right upper quadrant, though this finding has become less common with earlier diagnosis 3, 4
• Visible peristaltic waves across the abdomen after feeding 2
• Weight loss or failure to thrive as the obstruction progresses 5, 2
• Dehydration with associated metabolic alkalosis (hypochloremic, hypokalemic) in advanced cases 3

Important caveat: The frequency of palpable olive-shaped masses has decreased significantly over time due to earlier diagnosis with ultrasound, and may be present in fewer cases than historically reported 3. Do not wait for this physical finding to pursue imaging.

Diagnostic Algorithm

Step 1: Initial Clinical Assessment

Document the following key features:

• Vomitus characteristics: Confirm non-bilious nature (bilious vomiting would indicate malrotation/volvulus requiring immediate upper GI series) 1, 6
• Age at symptom onset: Typically 3–6 weeks, though can present earlier or later 1, 3
• Feeding pattern: Progressive worsening after feeds 2, 7
• Hydration status: Assess for sunken fontanelle, dry mucous membranes, decreased urine output, skin turgor 1
• Weight trajectory: Document weight loss or poor weight gain 5, 2

Step 2: Imaging—Abdominal Ultrasound

Ultrasound is the first-line and definitive imaging modality for suspected IHPS 1. The American College of Radiology recommends initiating abdominal ultrasound to evaluate non-bilious projectile vomiting for hypertrophic pyloric stenosis in infants 2 weeks to 3 months of age 1.

Ultrasound diagnostic criteria:

• Pyloric muscle thickness ≥3–4 mm 3
• Pyloric channel length ≥15–17 mm 3
• Visualization of hypertrophied pyloric muscle creating gastric outlet obstruction 7

Key advantage: Ultrasound has become the gold standard because it allows earlier diagnosis before the development of severe dehydration, metabolic derangements, and the classic palpable olive 3, 4.

Step 3: Laboratory Assessment (If Indicated)

Obtain laboratory studies only if there are signs of dehydration or metabolic disturbance to guide preoperative resuscitation 7:

• Serum electrolytes: Look for hypochloremic, hypokalemic metabolic alkalosis 3
• Blood urea nitrogen and creatinine: Assess degree of dehydration 3
• Venous blood gas: Document metabolic alkalosis severity 3

Modern practice note: With earlier diagnosis, severe metabolic derangements have become less common 3. However, preoperative correction of electrolyte abnormalities and dehydration is essential before surgical intervention 7.

What NOT to Do

Avoid Plain Abdominal Radiography as Primary Diagnostic Tool

Plain radiographs have limited diagnostic value for IHPS 1. While they may show a distended stomach with minimal distal gas, they cannot confirm the diagnosis and should not delay ultrasound 3.

Do Not Perform Upper GI Series for Typical IHPS

Upper GI series is not indicated when ultrasound can establish the diagnosis 8. Reserve upper GI series for:

• Bilious vomiting (to rule out malrotation with volvulus) 1, 6
• Atypical presentations where anatomic abnormalities need exclusion 8

Critical pitfall: While IHPS classically presents with non-bilious vomiting, rare atypical cases with bilious vomiting have been reported due to earlier diagnosis 9. If bilious vomiting is present, proceed immediately to upper GI series to exclude malrotation before considering IHPS 1, 6.

Do Not Confuse with Gastroesophageal Reflux

Distinguishing IHPS from benign GER:

• GER presents with intermittent regurgitation from birth, not progressive projectile vomiting starting at 3–6 weeks 8, 1
• GER infants typically have normal weight gain, whereas IHPS causes weight loss 5, 6
• Projectile, forceful vomiting is characteristic of IHPS, not GER 1, 2

Imaging is not necessary for uncomplicated GER with normal weight gain 8, 1.

Preoperative Management

Once IHPS is confirmed by ultrasound:

1. Stop oral intake and place nasogastric tube for gastric decompression if needed 6
2. Initiate IV fluid resuscitation with correction of electrolyte abnormalities (particularly chloride and potassium) and metabolic alkalosis before surgery 7, 3
3. Obtain surgical consultation for pyloromyotomy, which is the definitive treatment 2, 7, 4

Optimal outcome depends on adequate preoperative preparation, not emergency surgery 7. Surgery should be performed once the infant is adequately hydrated and electrolyte abnormalities are corrected 7.

Red Flags Requiring Alternative Diagnosis

• Bilious vomiting: Indicates malrotation with volvulus until proven otherwise—proceed immediately to upper GI series 1, 6
• Bloody vomitus or stools: Suggests intussusception, GI bleeding, or other serious pathology 5, 6
• Abdominal distension or severe tenderness: Points toward obstruction or other surgical abdomen 5, 6
• Fever with toxic appearance: Consider sepsis, meningitis, or other systemic infection 5, 6
• Neurological signs: Evaluate for increased intracranial pressure from CNS pathology 5