Fat stranding about the appendix means that there is usually edema or inflammatory change involving the appendix. This does not however diagnose appendicitis because there can be other causes of fat stranding about the appendix and adjacent structures. Stranding can be seen throughout the body for different reasons.
Appendicitis is diagnosed when you have an abnormal appearing appendix surrounded by fat stranding or inflammatory change. Usually the appendix will be dilated, fluid filled, have a thickened wall in addition to fat stranding in appendicitis. This will often be accompanied by a history which supports appendicitis.
Stranding around the appendix is seen when you have inflammatory bowel disease such as Crohn’s involving the bowel in the region of the appendix. In these cases, you will have thickening of the bowel in this region with stranding surrounding the bowel and appendix. The appendix can also be thickened from the inflammatory change involving the bowel. Usually the abnormal stranding on CT is not centered on the appendix but is more generalized in the region.
Stranding around the appendix can be seen when there is diverticulitis of the cecum. In these cases, the stranding will involve a diverticulum or out pouching of the cecum accompanied by stranding. Since the appendix arises from the cecum, the inflammatory change can involve the appendix. In these cases the appendix will usually be normal.
Inflamed cecum from other causes can also lead to stranding around the cecum and appendix. There can be reduced blood flow and thickened bowel wall when the blood flow is reduced. This is called ischemic bowel. The stranding can extend to involve the appendix. Colitis or inflamed colon can also occur because of infectious cases and lead to stranding about the appendix on CT.
I sometimes see more generalized stranding about the appendix when there is extra fluid or inflammatory changes that extend from the pelvis. In these cases, the appendix is often normal. The process is clearly not centered on the appendix. Often the clinical history will not be suspicious for appendicitis.
Stranding around the appendix is therefore not specific for appendicitis. Careful evaluation of the images on CT is needed to come up with the best diagnosis. Appendicitis has to be accompanied by an abnormal appendix in addition to stranding in most cases. Often an alternative explanation can be found for stranding around the appendix when it is normal. The clinical history also plays an important role in the diagnosis.