The esophagus connects to the stomach at an opening in the diaphragm called the hiatus. The lower esophageal sphincter (LES), which is normally at the same level as the diaphragm, keeps stomach contents (food, acid, and other digestive juices) from backing up (or refluxing) into the esophagus.

But when a sliding hiatal hernia is present, part of the stomach moves up through the hiatus and into the chest cavity. This pushes the lower esophageal sphincter (LES) up into the chest cavity away from the hiatus. Away from the hiatus, the LES loses the support that it needs from the diaphragm to stay closed. This raises the risk for symptoms of heartburn and gastroesophageal reflux disease.