A hernia is the protrusion of an organ outside of the body cavity in which it normally lies.
A hiatus is a gap, in this case specifically a 'gap' in your diaphragm. This gap is called the esophageal hiatus, and it allows the esophagus (the tube food goes down when you swallow) to pass from your thoracic cavity (lung area) to your abdomen (stomach area).
Thus, a hiatal hernia is a condition in which a segment of the stomach is pulled up through the diaphragm. This occurs in about 15% of the population, and generally goes unnoticed. However, in a few people it can cause serious problems, generally difficulties with swallowing or GERD (gastroesophageal reflux disease). Hiatal hernias come in two types:
Sliding hiatal hernias:
Every time you swallow, your esophagus shortens. Normally your stomach stays put, but in some individuals it can be pulled up slightly through the esophageal hiatus. In these cases, it will generally slide back down into position once the swallow is completed.
This type of hernia is frequently associated with GERD, as it will interfere with the functioning of the lower esophageal sphincter and the gastroesophageal flap valve, which act as valves to keep acid from moving up the esophagus. When a hiatal hernia causes noticeable symptoms, it is most often a sliding hernia, and the symptoms are GERD-like; heartburn, acid reflux, regurgitation, and nausea. It is unclear if a severe sliding hiatal hernia alone is sufficient to cause GERD, or if other factors must also be present.
Para-esophageal hiatal hernias
In this case, a bulge of stomach sticks out though the esophageal hiatus beside the esophagus. This bulging pocket of stomach tissue does not slide up and down, and is not affected by swallowing.
These hernias can press against the esophagus and prevent food from entering the stomach, or may become ulcerated or infected if stomach acid or food is caught in the pocket. Fortunately, large para-esophageal hernias are not common.
It is generally believed that the most common cause for both types of hiatal hernias is an abnormally large esophageal hiatus. In some cases hiatal hernias can be found in very young children, presumably present from birth, but generally the risk of a hernia increases with age, as soft tissue looses elasticity and tone as you age.
Hiatal hernias are usually identified when a patient complains of upper abdominal pain or GERD; a doctor will preform an upper gastrointestinal x-ray with a barium swallow (AKA a barium esophagram) or endoscopy (AKA esophagastroduodenoscopy). Large para-esophageal hernias may require surgery to stick everything back in place and tighten the esophageal hiatus. Sliding hiatal hernias rarely require surgery; as the primary effect of theses hernias is GERD, the doctor will usually simply treat the symptoms of GERD. Severe cases of either type of hiatal hernia may warrant a fundoplication to help fix the lower esophageal sphincter in place.