Infantile hypertrophic pyloric stenosis (IHPS) is a condition seen in infants, wherein the muscles in the pylorus (lower part of the stomach) become enlarged enough to prevent the food from entering into the intestine. This results in projectile vomiting, where the food is ejected forcefully from the mouth, sometimes over a distance of several feet. Characteristically, no bile is present in the food thrown up. Since little or almost no food reaches the intestine, stools are smaller and fewer, and the infant fails to gain weight. In more advanced cases of the disease, fluid and salt abnormalities with hypochloremic metabolic alkalosis may also set in. No definite etiology has been proved for IHPS. Most theories indicate that the condition may be due to abnormal muscle innervations, immature ganglion cells, decreased nitric-oxide stimulation of muscle fibers, and/or abnormal levels of gastrin.
IHPS is seen most commonly between the age of 3-6 weeks. Around 2-3 per 1000 infants suffer from this condition and the disease has an incidence rate of about 1 in every 500 live births. In fact, IHPS accounts for a third of all non-bilious vomiting occurrences in infants, and is the most common reason for laprotomy before one-year of age. Diagnosis depends upon the triad of projectile vomiting, visible peristalsis, and a palpable pyloric tumor or radiographic evidence of "string sign". Ultrasound analysis is the preferred mode of diagnosis. Treatment involves pyloromyotomy, a surgical procedure involving cutting through the thickened muscles of the pylorus. This is a relatively simple surgery, and patients are usually sent home 48 hours post-surgery.
A definite genetic basis has been attributed to IHPS, considering that an increased incidence is observed in families in which a sibling or parent has had the disease. However, the pattern of inheritance is debatable. On the one hand, a sex-modified model of multifactorial inheritance has been proposed considering the fact that the disease affects males four times more than females. On the other hand, pedigrees consistent with autosomal dominant mode of inheritance have also been reported. The nitric oxide synthase 1 (NOS1) gene on chromosome 12 has been implicated as a candidate gene. It has been proposed that patients may have a defect in production of nitric oxide, a chemical that relaxes the pylorus muscle.