Introduction

Hypertrophic pyloric stenosis (HPS) is a common gastrointestinal cause of gastric outlet obstruction infants and is one of the most common surgical conditions of infancy. ¹ It is caused by the idiopathic diffuse hypertrophy and hyperplasia of the circular muscle fibres of the pylorus with the proximal extension into the gastric antrum resulting in construction and obstruction of the gastric outlet. In response to outflow obstruction and vigorous peristalsis, stomach musculature becomes uniformly hypertrophied and dilated.

Epidemiology

Pyloric stenosis has an incidence of 2 to 4 per 1000 live births in Western population² and it appears to be less common in infants in African and Asian populations. It is four to five times more common in males.³ The cause of pyloric stenosis is unknown. Genetic, familial, gender and ethnic origin can influence the incidence rates of HPS. Offspring of parents with this condition have a higher risk of developing HPS and in many series first-born males are more frequently than the other siblings.⁴

Clinical presentation

Pyloric stenosis is not present at birth. In the early phase, there may just be regurgitation or occasional non-projectile vomiting. The onset rarely occurs before the first week of life and it commonly present at 2–8 weeks of age, however, it is reported in premature babies prior to this corrected age. The peak usually occurs at four weeks and it is seldom delayed beyond the second to third month. Within a variable time after the onset of symptoms, the vomiting becomes more projectile and it generally occurs during or soon after feeding. However, at times, it may occur up to several hours later. The frequency of vomiting varies, in some infants vomiting occurs after each feed while in others it may be somewhat intermittent.

Shortly after vomiting, the infant is often hungry and will take another feed immediately. The vomitus is non-bilious but due to the frequency and force of vomiting, it may occasionally become blood tinged. The amount of stool may be very small and infrequent depending on the amount of food that reaches the intestine. The degree of dehydration, lethargy and metabolic derangement depends on the time interval between the onset of symptoms and presentation. In some late presentations the infant may be severely malnourished, dehydrated or even collapsed.

It is important to note that it may take some time for the clinical features, metabolic disturbance and ultrasound findings to become established and one should not exclude HPS if symptoms persist / recrudesce after a relatively normal examination of an infant presenting very early. A planned review should features evolve is appropriate. Likewise, it is important to consider HPS in a previously benign ‘posseting’ or ‘refluxy’ baby, whose regurgitation of milk has ‘changed’ in intensity or frequency rather than attributing this to ‘posseting getting worse’. In this situation, the clue to the possibility of HPS is the change in the nature of the previous ‘vomiting’.

Examination findings

With established HPS, the infant fails to thrive due to calorie losses, becomes dehydrated due to fluid losses and appears ‘hungry’ unless significant dehydration or alkalosis has started to affect the infant’s activity level. The infant usually appears ‘bright and active’ unless significantly dehydrated, compared to the infant with a urinary tract infection who may be constitutionally unwell.

On physical examination, gastric distension or visible peristaltic waves may be seen moving from the left upper abdomen toward the epigastrium, and right side in some cases.⁵ The palpable finding of a firm, mobile and non-tender ovoid mass (‘olive’) either to the right of the epigastrium or in the midline, deep to right rectus muscle and under the liver edge is diagnostic. This finding of a palpable mass requires much patience as the success of such a finding is dependent on an empty stomach and a relaxed anterior abdominal wall in a non-crying settled infant. If the stomach is significantly distended during palpation, aspiration of gastric contents using a nasogastric tube may be helpful to increase the likelihood of feeling the hypertrophied muscle. Also, palpation during a test feed may allow a previously non-palpable hypertrophied pylorus to be felt during peristaltic contractions. The best position for palpation is on the infant’s left side. The inability to palpate an olive-shaped mass does not exclude the diagnosis of HPS and often an ultrasound is needed to clarify the diagnosis.⁶

With extensive and protracted vomiting, metabolic derangement will occur. Vomiting of gastric contents leads to depletion of sodium, potassium and hydrochloric acid, which results in the characteristic finding of hypokalaemic, hypochloraemic metabolic alkalosis.⁷ The kidneys conserve sodium at the expense of hydrogen ions, resulting in a paradoxical aciduria. With the increasing degree of dehydration, renal potassium losses are accelerated in an attempt to retain sodium and fluid.