Illustration showing a narrowing (stenosis) of the pylorus. The pylorus is the opening from the stomach to the duodenum (the first part of the small intestine). This narrowing has left the pylorus unable to open. Over time, this causes the muscles to thicken. As a result, food is unable to move from the stomach to the duodenum. Pyloric stenosis is a condition that mostly affects babies, and resulting in projectile vomiting. Atropine can be used to treat the condition, but surgery is often needed in a procedure known as a pyloromyotomy.