Partial prolapse of the antero-medial corpus adiposum in the horse.

The research article discusses the anatomical feature known as the “third eyelid” in horses, how a portion of the fat enclosing this feature can prolapse and cause symptoms similar to ‘cherry eye’, a condition commonly found in dogs and cats.

Composition and Function of the Third Eyelid

• The third eyelid consists of a large fold of the conjunctiva called the membrana nictitans, originating from the lower conjunctival fornix.
• It is reinforced internally by a T-shaped curved hyaline cartilage plate.
• A large fat pad, part of the corpus adiposum outside the periorbital fascia, lodges the thickest part of this cartilage on the side of the globe.
• The forward movement of the fat pad, caused by the globe retracting into the orbit, enables the third eyelid to move across the cornea, whilst blinking.

Nictitans Gland

• Located within the fat pad is the nictitans gland which creates the tear film in front of the cornea.
• This gland is supported by robust vascular septa that separates its individual lobes.
• The gland is covered by the same connective tissue as the cartilage, and fat from the corpus adiposum penetrates this gland, stabilizing the assembly of cartilage, gland, and fat.

Data on Prolapse

• Occasionally, a prolapse of the nictitans gland occurs in dogs and cats, a condition referred to as ‘cherry eye’.
• The prolapse is usually due to a weakness in the connective tissue attachment to the cartilage, causing the distal glandular tissue to push between the free margin of the third eyelid and the ventromedial corneoscleral tunic.
• This prolapse can lead to blockage of venous and lymphatic drainage from the gland, resulting in a hyperemic appearance and edematous swelling.

Treatment and Relevance to Horses

• The recommended treatments for ‘cherry eye’ are to either suture the gland back into its natural position or perform an excision if the former is not feasible.
• This study found that, unlike in dogs and cats, horses experienced a prolapse of the corpus adiposum, not the nictitans gland, leading to symptoms similar to ‘cherry eye’.