A diaphragmatic hernia is a protrusion of the upper part of the stomach through the diaphragm, the dome-shaped muscle that separates the two thoracic and abdominal cavities. This means that part of the stomach is abnormally protruding and into the thoracic cavity. Most cases of diaphragmatic hernia are mild and asymptomatic, but in severe cases, diaphragmatic hernias can be uncomfortable, painful, and can lead to reflux.