Dyscoria is a distortion or irregular development of the pupil of the eye that leads to vision problems as the pupil may not dilate and expand correctly in changing light environments. In horses dyscoria is most commonly noted in breeds that are part of the gaited horses bred in the Rocky Mountain areas of the United States. These breeds include the Rocky Mountain Pleasure Horse, Kentucky Mountain and crosses between these breeds. This condition can also be found in dogs and humans.
Most horses with dyscoria also have an eye condition known as ASD or anterior segment dysgenesis. This condition causes an irregular formation of the front parts of the eye that is genetically linked but often not serious. It is not progressive and does not become more pronounced as the horse matures, in fact many horses have such mild symptoms of the condition that it is not known unless an ophthalmologic examination is completed by a vet. Diagnosis of dyscoria is done by applying drops of mydriatic drug that causes normal pupils to dilate. The pupil with dyscoria will not dilate normally and will confirm diagnosis of the condition.
Often in cases of dyscoria the pupil will not be shaped uniformly and either the top or bottom of the normally smooth and rounded pupil will be flat or even convex in appearance. The pupil may appear to have protrusions or bumps from the iris into the pupil. In addition the cornea or outer layer of the eye may appear to bulge or protrude, further causing the appearance of the misshapen pupil. In some cases the pupil with dyscoria will look elongated and flattened, rather like the pupil of a cat's eye turned sideways.
In all horses with ASD and dyscoria the condition is congenital or is present at birth. Since it is an inherited condition there is no way to correct the problem once present, but proper care of the eye and carefully monitoring the horse to ensure that he or she has enough vision to be ridden or pastured safely is key.
Horses with dyscoria and ASD usually have the condition bilaterally or in both eyes. In some horse the pupils are more misshapen on one side of the head than the other, so vision may be only limited in one eye, not both. In this case the horse will usually adjust to the different vision fields and will naturally compensate by the time he or she is a yearling.
The most common colorations in which ASD and dyscoria are noted are the chocolate browns with flaxen or white manes and tails, also known as the silver dapple. This genetic combination that creates the wonderful coloration is believed to be responsible for the eye conditions seen in these horses. While not as well known for the problem the Belgium Draft Horse and even Shetland Ponies and Miniature Horses with the silver dapple coloration have been found to have dyscoria to varying degrees.