Bilateral congenital cystic eye with intraocular tissue differentiation in a horse.

Congenital cystic eye (CCE) is a rare congenital ocular disease, which has been suggested to be caused by problems with optic vesicle invagination. This is characterized by the formation of intraorbital cystic lesions lined by undifferentiated neuroepithelium. However, cases involving ocular structures with varying degrees of differentiation have also been reported as CCE in humans and animals. In horses, CCE have only been reported as a case presenting as neuroepithelial-lined cysts without the formation of ocular structures. This paper describes large bilateral intraorbital cysts in a 1-day-old horse. The histopathological findings revealed that the cysts were covered by neuroepithelium with formation of the lens, ciliary body, and optic nerve, and it was diagnosed as CCE. This report provides further variations of CCE in horses.