Pannus is an immune mediated corneal inflammatory disease. When the eye is without oxyen for a long period of time (termed hypoxia), such as with contact lens wearers, the tissues of the cornea will send molecules out to the surrounding areas that stimulate growth of new vessels towards the hypoxic area. These new vessels usually begin growing in the lower, peripheral part of the cornea and grow towards the center. The vessels are very thin and fragile, and therefore break very easily, so the vessels will lay down a fibrovascular foundation to assist them in their growth.  This process will appear like a opaque colored patch with overlaying blood vessels.

This is a very common finding and is benign if found early and the underlying cause for the hypoxia is corrected for. The vessels will regress but the underlying fibrous fondation sometimes will not. Pannus is not limited to occur only on the cornea. It is a common occurrance in the joints when rheumatoid arthritis is present. The chonic inflammation caused by the arthritis provides an environment that leads to the pathophysiology just described.