Long, a sliding hiatal hernia happens when the upper portion of the stomach (the cardia) is displaced upwards and 'slides' through the opening between the abdominal and thoracic cavity. This type of hernia can appear and disappear because it can slide back and forth, so no, it's not seen all the time in a person who has one.
Most are not congenital, although I suppose it's possible. Many occur because the series of ligaments that hold the stomach and other organs in place, including those that position the diaphragm, can become stretched, lax, or frayed. One of these is the phrenoesophageal ligament which seems to play a big role in 'diaphragm placement.'
There is no % that can be given. If the hernia becomes large and causes a lot of problems that would include gastric reflux that cannot be controlled, that may be an indication for surgery.
well,most hiatal hernias do not need surgery,and there are simple things to do to help if it bugs you,,,try elevating the head of your bed several inches,this keeps the stomach pushing downward,,and not from pushing up into the hernia.
when it gives you pain,,try drinking a big glass of water,this often helps also.
as far as a sliding hernia,ive NEVER heard of this before.,,though i am not a doc,,me and my father both have one,and i kinda know more then most on this subject.
it basicly a spot somewhere on your diaphram where your stomach likes to push up into,,,as far as moving around on the diaphram,that dont make much since.
just watch,,i know you mentioned its small,but if you ever experience prolonged pressure,or pain in any way,have it checked quickly,as you could get what they call a strangulated hernia,where the stomach actualy gets up in the hernia,and gets stuck,and pinched in there,and this can cause that part of the stomach to loose blood and get infection,and cause all kinds of problems to you,,,very serious illness can result.