A pediatric condition, characterised by projectile vomitus and visible peristalsis. This is caused by hypertrophy of the pyloric sphincter which separates the stomach from the duodenum. Although it most commonly presents in a child of 4-6 weeks, some infants do present at birth.

Pathophysiologically similar to achalasia, this condition affects about 1 in 500 births, with males more commonly affected than females.

The typical story is of a baby, of about 6 weeks old, with a several day history of poor feeding, followed by episodes of projectile vomiting (literally ... it comes out like a fire hydrant) following feeding. The baby may be dehydrated as he/she will not have been able to consume any fluids for a few days. There is constipation and a failure to thrive.

On inspection of the baby's abdomen, peristaltic waves may be seen over the stomach. This can be accentuated by giving the baby a test feed. On palpation, the peristaltic waves should be felt and a mass (the muscle of the hypertrophied pyloric sphincter) to the right of the umbilicus may be palpable.

Blood tests will indicate the baby to have metabolic alkalosis and to be hypokalaemic. Ultrasound and upper GI x-ray studies are helpful in confirming the clinical diagnosis.

The treatment of choice is surgical - pyloromyotomy. This involves cutting the muscle layer of the sphincter in one direction and suturing it back perpendicular to the incision without cutting into the lumen of the sphincter; in so doing, the tightness is relieved without doing damage to the inside of the gastro-duodenal junction.

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