Hypertrophic pyloric stenosis is a disorder in which the lumen of the pylorus is obstructed due to the hypertrophy of the pyloric muscle leading to partial or complete gastric outlet obstruction. It is predominantly a condition seen in infants but it has been reported in adults as well. It is characterized classically by hypochloremic, hypokalemic metabolic alkalosis.
Hypertrophic pyloric stenosis (HPS) is typically a disease seen in infants with a reported incidence of 0.1 to 0.8% but it has also been reported in about 300 adults    . HPS in infants has higher incidence amongst first-born males and is encountered more frequently in Caucasian babies . Maternal history of HPS is a reported risk factor . Infantile HPS can be associated with syndromes like Turner's, tracheoesophageal fistula, esophageal atresia, trisomy 18 .
HPS is classified as primary and secondary.
Primary HPS is also called idiopathic HPS (IHPS) and is seen in infants, while it is rare in adults probably because they are asymptomatic and remain undiagnosed. The exact etiology of IHPS is unknown. It is characterized by pyloric muscle hypertrophy and hyperplasia. IHPS in infants typically develops in the first to the third month after birth with symptoms of non-bilious projectile vomiting immediately following feeds. As the disorder progresses, the infant does not gain weight and develops dehydration. There is visible gastric peristalsis. An olive-shaped mass, due to the hypertrophied pylorus, can be palpated in the epigastrium or right upper abdominal quadrant. Features of associated syndromes like Turner's may also be present. Adult onset IHPS presents in middle-aged men with epigastric pain, early satiety, nausea, and vomiting following meals  . Unlike infants, an upper abdominal mass is rarely palpable in adult IHPS.
Secondary HPS occurs secondarily with the healing of duodenal or gastric ulcers, hypertrophic gastritis, malignant tumors, gastrointestinal stromal tumors, bezoars, vagal hyperactivity, and postoperative extrinsic adhesions   . It is associated with replacement of the pyloric muscle fibers by fibrous tissue and there is minimal or no hypertrophy of the pyloric muscle.
The diagnosis of IHPS can be suspected in an infant presenting with typical manifestations of non-bilious projectile vomiting after feeds and weight loss but they are not often seen in the initial course of the disease. A careful history of onset and progression of the manifestations is essential before proceeding with the physical examination which may reveal visible peristalsis, and a palpable olive-shaped enlarged pyloric mass  . Hypochloremic, hypokalemic metabolic alkalosis is the classical result on serum biochemistry testing. Ultrasonography is the preferred method of confirming the diagnosis in infants with endoscopy being reserved for infants with atypical clinical presentation.
Clinical diagnosis of adult HPS is difficult and upper gastrointestinal (UGI) endoscopy is necessary to exclude gastric outlet obstruction. A fixed, severely narrowed pylorus with a smooth border described as "cervix sign" is typically seen on endoscopy . Video capsule endoscopy is a newer modality which has been used to detect adult HPS and differentiate it from idiopathic gastroparesis .
A plain X-ray of the abdomen is not helpful and may only show a distended stomach with mildly distended intestines in HPS. Fluoroscopic UGI series with barium (barium meal) will demonstrate delay in gastric emptying, peristaltic waves (caterpillar sign), an elongated pylorus with a thin lumen (string sign) which appears duplicated due to puckering of the mucosa (double-track sign), indentation of the antrum by the pylorus (shoulder sign) or base of the duodenal bulb (mushroom sign) and beak shaped pyloric entrance (beak sign). In adult HPS, UGI series findings include delayed gastric emptying due to pyloric stenosis, a narrow, elongated pyloric canal, and a mushroom sign.
Ultrasound is preferred to the UGI series in infants as it avoids radiation and can visualize the pyloric muscle. Various sonographic signs have been described such as the antral nipple sign, cervix sign, and target sign. However, sonography is not useful in adult IHPS, although some do advocate it . Computed tomographic (CT) scans help to exclude secondary causes of HPS.