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Horner syndrome is a pattern of abnomalities involving one eye and the surrounding area. It is caused by faulty nerve supply (the sympathetic nerves) to the affected side of the face. The pupil on the affected side is smaller than the other pupil (miosis), the affected upper eyelid droops (ptosis), there is flushing of the skin as well as decreased sweating of the affected side, and the affected iris is lighter than the iris of the unaffected eye. The result can be rather striking: one blue eye, one brown eye. (Different colored eyes are called "heterochromia iridium") This phenomenon reflects the fact that the development of proper eye color is dependent upon proper nervous stimulation in the first two years of life.
Horner syndrome may be genetic, inherited as an autosomal dominant trait, that is, from one parent or the other. Other causes involve some sort of disruption of the nerve pathways due to birth trauma, chest surgery, certain tumors, and CMV (cytomegalovirus) infection. Injury to the nerve pathways after the age of two does not result in decreased iris pigmentation.