Corpora nigra cysts

The corpora nigra (orange arrows) is a proliferation of normal iris tissue at the top and the bottom of the pupil in the horse. The corpora nigra is thought to act as a sun shield and to protect from UV radiation while the horse is grazing. Occasionally the corpora nigra can develop a cyst which is a round, fluid filled structure at the edge of the pupil. These cysts can be small or they can get quite large and in some cases interfere with vision. This can impact day to day work especially in bright light conditions. When horses are outside on a bright, sunny day the pupil gets smaller. A cyst at the edge of the pupil can cover this small opening obstructing vision. This can lead to behavior and training issues.

Treatment of a corpora nigra cyst is usually not required but if needed, non-invasive treatment can easily be performed standing in sedated horses. A transcorneal diode laser is used to ablate the cyst wall. Post operatively the horse is placed on a topical anti-inflammatory ointment and an oral anti-inflammatory for a short period of time. This treatment is very successful and is most often a one time treatment, however any horse can develop a new cyst.
If your horse has a corpora nigra cyst please reach out to Dr. Tolar for an evaluation to see if they are a candidate for laser treatment.