Laboratory Studies
Infants with severe vomiting can develop profound hypochloremia and hypokalemia. The classic biochemical abnormality in hypertrophic pyloric stenosis (HPS) is hypochloremic, hypokalemic metabolic alkalosis.
Imaging Studies
Ultrasonography has become the criterion standard imaging technique for diagnosing hypertrophic pyloric stenosis. It is reliable, highly sensitive, highly specific, and easily performed. An experienced ultrasonographer increases the test's predictive value. Necessary measurements include pyloric muscle thickness and pyloric channel length. Muscle wall thickness 3 mm or greater and pyloric channel length 14 mm or greater are considered abnormal in infants younger than 30 days.
Barium upper GI (UGI) study is an effective means of diagnosing hypertrophic pyloric stenosis when ultrasonography is not diagnostic. It should demonstrate an elongated pylorus with antral indentation from the hypertrophied muscle. The UGI may demonstrate the "double track" sign when thin tracks of barium are compressed between thickened pyloric mucosa or the "shoulder" sign when barium collects in the dilated prepyloric antrum. After UGI barium study, irrigating and removing any residual barium from the stomach is advisable to avoid aspiration.
Although UGI endoscopy would demonstrate pyloric obstruction, physicians would find it difficult to differentiate accurately between hypertrophic pyloric stenosis and pylorospasm. Endoscopy is reserved for patients with atypical clinical signs when ultrasonography and UGI studies are nondiagnostic. Endoscopic dilatation has rarely been used as a method of treatment. This treatment is not standard for hypertrophic pyloric stenosis; endoscopy should be used rarely, if ever.
