Acute vomiting in infants and children: SAQ

This infant has hypertrophic pyloric stenosis. The age of three weeks, the progressive projectile non-bilious vomiting in a hungry infant, the weight loss, the visible peristaltic wave, and the hypochloraemic hypokalaemic metabolic alkalosis on the gas together make the diagnosis, which is confirmed by an ultrasound showing a thickened, elongated pyloric muscle. ^[1]

Question 1 (10 marks)

Explain the diagnosis, the characteristic biochemical derangement, and how you would confirm it. (5 marks for the diagnosis and its clinical features; 5 marks for the biochemistry and confirmation.) ^[1]

The diagnosis is hypertrophic pyloric stenosis, which classically presents between two and eight weeks of age with progressively projectile, non-bilious vomiting in an infant who remains hungry, together with weight loss or poor weight gain. Supportive examination findings are a visible left-to-right peristaltic wave during a test feed and a palpable, olive-shaped pyloric mass in the right upper quadrant. The vomiting is never bilious, because the obstruction is proximal to the ampulla of Vater. ^[1]

The characteristic biochemical derangement is a hypochloraemic, hypokalaemic metabolic alkalosis. Persistent vomiting of gastric contents loses hydrogen and chloride, generating the alkalosis and the low chloride; potassium is lost in the vomit and further into the urine, and in the later dehydrated state the kidney conserves sodium and water at the expense of secreting hydrogen ions, producing the classic paradoxical aciduria. This infant's gas, with a pH of 7.52, a chloride of 88 mmol/L and a potassium of 3.0 mmol/L, fits this pattern exactly. ^[1]

Confirmation is by abdominal ultrasound, which demonstrates the thickened and elongated pyloric muscle and is the diagnostic investigation of choice. The essential accompanying blood tests are the venous gas and electrolytes, which both support the diagnosis and quantify the derangement that must be corrected before any operation. ^[2]