The presence or absence of bile in the emesis is useful diagnostic information. If the obstruction is proximal to the pylorus, the emesis will always be nonbilious. Bile-stained emesis implies that the obstruction is distal to the ampulla of Vater. Children with bilious emesis are presumed to have a surgical problem unless proven otherwise.

Projectile, nonbilious vomiting in a healthy infant is classic for HPS. The infant may have been vomiting for 1 to 2 weeks, but the emesis will become progressively more forceful and voluminous. Infants will typically be brought to medical attention between 4 and 8 weeks, although the diagnosis can be made outside of this window.

A dehydrated infant, as evidenced by a sunken fontanelle, with a palpable mass (described as an “olive”) in the right upper quadrant is classic for HPS. The “olive” represents a thickened and elongated pyloric muscle. However, if the infant is crying, the mass may not be palpable. Reverse peristaltic waves may also be seen in the upper abdomen. Physical examination of a crying infant can be quite challenging so it is important to ensure that the child is warm and comfortable. Bending the legs and offering a pacifier are both helpful. With the classic history and palpable “olive,” there is no need for additional studies. However, if a mass still cannot be appreciated, further work-up is warranted.

For decades researchers have investigated the etiology of HPS, with inconclusive results. Some speculate that there is a failure of muscle relaxation in the pylorus, while others argue it is caused by deregulation of enteric hormones. Histologic evidence suggests that there is abnormal innervation with immature ganglia in the circular muscle layer of the pylorus. Genetic and maternal factors as well as environmental factors such as bottle feeding, use of oral macrolide antibiotics, and infection have all been implicated in the development of the disease. While theories and evidence are widely varied, the true etiology of HPS remains to be elucidated.

Regardless of the etiology, there is universal consensus that HPS is characterized by hypertrophy and hyperplasia of the circular muscle layer of the pylorus. Muscle thickening causes the pyloric channel to become increasingly narrowed and elongated, eventually leading to gastric outlet obstruction. The smooth muscle of the stomach hypertrophies and dilates in response to vigorous peristalsis against an obstructed pylorus. As the stomach dilates and peristaltic contractions become stronger, the classic projectile vomiting of a large volume of gastric content occurs.

If there is no palpable “olive,” the gold-standard imaging modality is ultrasonography. Ultrasound measurements of pyloric channel length, muscle thickness, and diameter will diagnose HPS with a sensitivity and specificity close to 100 %. Most consider pyloric thickness greater than 3–4 mm and length greater than 15–16 mm diagnostic of HPS (Fig. 33.1). However, exact measurements vary based on the patient’s age and weight.

If ultrasound is equivocal, the diagnosis may be made with an upper gastrointestinal (UGI) contrast study. Typical findings include delayed gastric emptying, retrograde peristalsis in the stomach, and a string sign at the level of the pylorus. UGI studies may also be helpful in the setting of a negative ultrasound in order to assess for other pathology, particularly malrotation and gastroesophageal reflux.