Third Eyelid in Dogs: Why It's Showing and When to Worry

Your dog's third eyelid (the nictitating membrane) is a thin, pale pink fold in the inner corner of each eye. It sweeps across the eye to protect it, and a gland inside it makes a large share of the tears.^1,2 It's normal to see a little of it when your dog is sleepy or just waking up.³ It isn't normal for it to stay up while your dog is awake. A pink lump at the inner corner is usually cherry eye; a third eyelid that covers part of the eye can point to eye pain, Horner's syndrome, dehydration or another problem, and needs a vet check.^4,5

What the Third Eyelid Does

The third eyelid sits under the lower lid in the inner corner of the eye. It has a T-shaped piece of cartilage that keeps it stiff, and it moves across the eye to wipe and protect the surface.^1,6 The gland at its base is a major tear gland. Estimates of its share vary: the American College of Veterinary Ophthalmologists has put it at roughly a third to two-thirds of the tears, and more recently at 40 to 60% of the tear film; VCA says up to half of the watery part.^7,2,1

When Seeing It Is Normal

• When sleepy or asleep: the eyes roll back slightly, so the third eyelid shows. It should disappear once your dog is fully awake.³
• Under sedation or anesthesia, for the same reason.
• Uneven pigment: if one third eyelid is dark and the other pale, the pale one can look raised. This is harmless.⁴

Causes of a Third Eyelid Showing

Horner's Syndrome in Dogs

Horner's syndrome happens when the nerve supply to the eye is interrupted somewhere along its path from the brain, down the neck and chest, and back up through the middle ear. It usually affects one side and starts suddenly, with four classic signs: a small pupil, a drooping upper eyelid, a sunken eye and a raised third eyelid.^9,10,12

• Causes: middle or inner ear disease, neck or chest injuries (including bite wounds and pulling hard on a collar), tumors, disc disease, nerve damage from surgery, and many cases with no cause found.^9,10
• No cause found in about half of dogs: a review in the Canadian Veterinary Journal says idiopathic cases make up roughly half of canine presentations.¹⁰
• Golden Retrievers are especially prone to the idiopathic form. One review reports an incidence of 2.6% in Golden Retrievers compared with 0.03% in other breeds.^10,12
• Diagnosis: eye, ear and neurologic exams, X-rays and sometimes CT or MRI, plus an eye-drop test that helps locate where the nerve is affected.^9,10
• Outlook: treatment targets the cause. Idiopathic cases usually clear up on their own over several weeks to about 4 months.^9,10

Cherry Eye and Why the Gland Should Be Saved

Cherry eye is a painless pink swelling that can usually wait for a prompt regular appointment, though repair is easier when it's done early.⁸ The standard treatment is surgery to put the gland back in place, not remove it. Removing the gland leads to dry eye later in life in 30 to 40% of dogs, according to the Merck Veterinary Manual, and the ACVO says a prolapsed gland should never be removed.^8,2 Even after successful repositioning, some dogs still develop dry eye later, so regular checks matter.⁸ See cherry eye in dogs and dry eye in dogs.

Treatment

• Treatment depends entirely on the cause, so a diagnosis comes first.⁴
• Cherry eye: surgery to anchor the gland or tuck it into a pocket.^8,7
• Scrolled cartilage: surgery to remove the bent part of the cartilage.⁶
• Painful eye problems: treating the ulcer, glaucoma or inflammation lets the third eyelid settle back.⁴
• Tumors: biopsy, then removal and checks for spread.¹¹
• Horner's syndrome: treat any underlying disease; idiopathic cases are usually monitored.⁹
• Don't use human eye drops. They rarely help and can make diagnosis harder.⁴

For other eye problems, see dog eye problems, corneal ulcers and conjunctivitis.

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