A hiatal hernia occurs when part of the stomach pushes upwards into the chest through an opening (referred to as a hiatus) in the diaphragm. The diaphragm is the sheet of muscle that separates the chest from the abdomen. The diaphragm is used in breathing and normally has a small opening that allows the esophagus to pass through when moving food to the stomach.
A hiatal hernia occurs when weakened muscle tissue allows the stomach to bulge through the diaphragm. Oftentimes, the reason for this is unclear. Some believe that pressure on your stomach or an injury to the area may contribute to the formation of hiatal hernia.
Persistent and intense pressure on the surrounding muscles, such as when coughing, vomiting, or straining during a bowel movement or straining while lifting heavy objects can also cause the stomach to push up through the hiatus.