“Cherry eye” refers to a distinct veterinary condition commonly seen in dogs, where a reddish mass suddenly appears in the corner of the eye. Although humans can experience various forms of ocular swelling, the specific biological mechanism that causes “cherry eye” does not exist in human anatomy. This difference is rooted in the evolutionary structure of our eyelids and tear production systems.
Defining Cherry Eye in Animals
“Cherry eye” is the common name for nictitans gland prolapse, which is the prolapse of the gland of the third eyelid. This condition occurs in animals, such as dogs and cats, that possess a functional third eyelid, also known as the nictitating membrane. This protective, thin membrane is located in the inner corner of the animal’s eye and houses a specialized tear-producing gland.
The prolapse occurs when the connective tissue anchoring the gland weakens, allowing it to flip out of its normal position. Once exposed, the tissue swells and becomes irritated, appearing as a smooth, pink or red globe resembling a cherry pit. This gland produces a significant portion of the animal’s tears, so its displacement can lead to chronic dryness and irritation if left untreated.
The Human Anatomical Difference
Humans cannot develop cherry eye because we lack the functional anatomical structure where the condition originates. Unlike dogs, human eyes do not have a full, mobile third eyelid containing a major tear gland. The remnant of this structure in humans is the plica semilunaris, a small, non-functional fold of tissue near the inner corner of the eye.
Our primary tear production is managed by the main lacrimal gland, located in the upper outer portion of the eye socket, and accessory glands within the conjunctiva. Since the specific gland responsible for cherry eye does not exist as a separate, major tear-producing unit in the human medial canthus, the mechanism for nictitans gland prolapse is absent. The associated gland is too small and non-functional to prolapse in the characteristic manner seen in canines.
Related Human Eye Conditions
While humans do not get cherry eye, they can experience conditions involving the lacrimal system that result in visible swelling or protrusion near the eye. One such condition is dacryocystitis, an infection and inflammation of the lacrimal sac. This causes a painful, red swelling in the inner corner (medial canthus) of the lower eyelid. This location is visually similar to where cherry eye appears in dogs, but the cause is a blocked tear duct, not a prolapsed gland.
Another condition is dacryoadenitis, the inflammation of the main lacrimal gland. This presents as swelling and tenderness in the upper, outer portion of the eyelid, the opposite side of the eye from a cherry eye. In older adults, the main lacrimal gland can weaken and descend, a condition known as lacrimal gland prolapse, which appears as a mass or fullness in the outer upper eyelid. These conditions involve swelling of tear-related structures but are distinct from the specific mechanism defining canine cherry eye.