Normally the colon contains air and stool. Both of which expand the inside of the colon. A collapsed colon means that there is almost no air or stool in the large intestine.

When a doctor sees a collapsed colon on an X-ray or CT scan, the main concern is an obstruction in the small bowel (upper intestine). If a patient with an X-ray showing a collapsed colon was having abdominal pain, a bowel obstruction would be the likely diagnosis. With this, the patient usually feels crampy, with waves of pain that can be very intense.

The doctor would also look for other signs on the pictures. The most important sign is small bowel filled with a lot of air. This would almost surely confirm the diagnosis of a bowel obstruction.

Here’s what happens. When there is an obstruction in the upper intestine, both the upper and lower intestines sense that something is stuck inside and needs to be moved along. So they keep contracting.  The intestines don't “know” that nothing can move beyond the blockage.

As a result of pushing against the blockage, the part of the intestine that is in front of the obstruction will begin to widen. At the same time, the lower intestine keeps contracting as well. This pushes out all the contents below the blockage. Once everything in the colon is out, the colon will collapse. The person will stop passing any stool or gas from the rectum.