This question piqued my interest for the following reason: people always forget about the #1 cause. (If you're wondering what on earth I'm talking about, the X-ray looks something like this.)
And the #1 cause is, of course, a laparotomy (or laparoscopy). Since the abdomen here is opened (by the surgeons), air enters, landing up under the diaphragm on an upright X-ray film. If you don't consider this, you can get quite a fright looking at a patient's X-ray after surgery!
In a similar vein, trauma (e.g. gunshot wounds to the abdomen) can be a common cause, depending on the local crime statistics! The abdominal cavity is under a slight negative pressure relative to that of the atmosphere, and so blasting (or slicing) a hole in the barrier between the two (the abdominal wall) will allow air inside.
Amongst the non-traumatic causes, the top three to remember are:
- #1. Perforated duodenal ulcer (70% of above category)
- #2. Perforated gastric ulcer (10%)
- #3. Perforated colonic diverticulum (5%)
That's a nice functional approach, I think, without too much to memorise. Think surgery and trauma, then think of the non-traumatic 'top three'. The full list fills books, but this only makes it less helpful, not more.Note that unless the patient is upright when the X-ray is taken, you'll never be able to see free air in the abdomen. In the upright position, the air sits between the diaphragm and the liver on the left and the stomach and other organs on the right. It therefore creates a conspicuous black space where before all was white. When an X-ray is taken with a patient lying down, no such good fortune occurs - the air is between the abdominal wall and the rest of the abdominal contents, and since the X-ray is still taken from front on, the air no longer announces its presence so helpfully.