Hiatus hernia in a child. Chest X-ray of a child with a hiatus hernia, showing an intrathoracic gastric air pouch next to the base of the left lung (right). A hernia is the protrusion of an organ or tissue through an abnormal opening. In this case part of the stomach is protruding into the chest cavity through the hole, known as the hiatus, that the oesophagus normally passes through. Hiatus hernias are extremely common and usually produce no symptoms. In severe cases they may cause gastro- oesophageal reflux, resulting in heartburn.