Anyone who has ever experienced having an eyelash trapped under a contact lens or a grain of sand blown into their eye quickly discovers that the cornea is loaded with pain receptors. A corneal ulcer – a scratch or scrape involving the cornea – is a relatively common, potentially vision-threatening disease of the cornea in cats.
Cornea 101 – the basics
The cornea is the clear membrane that covers the surface of the eyeball. Anatomically, it is composed of several layers. The outer surface layer is called the epithelium. Just beneath the epithelium is the stroma. The innermost layer is called Descemet’s membrane.
“All cat breeds are potentially at risk of developing corneal ulcers”, warns Dr. Chris Pirie, a board-certified veterinary ophthalmologist at the Cummings School of Veterinary Medicine at Tufts University. However, cats in general have pretty refined defense mechanisms to prevent damage to their corneas. They have vibrissae (specialized “whiskers” above their upper eyelids) that can detect objects that approach their eyes, allowing them to take evasive action. They also have a well-developed blink response. Attached to the back of the eyeball is a muscle called the retractor bulbi muscle. When this muscle contracts, the eyeball is pulled back into the socket. This retraction of the eye allows the nictitating membrane – sometimes called the “third eyelid” – to elevate, protecting the cornea. Despite these sophisticated mechanisms, cats will occasionally suffer trauma to the cornea, and an erosion occurs on the corneal surface. If the erosion goes through the entire epithelium into the stroma, this erosion is called a corneal ulcer. If the ulcer goes deep into the stroma all the way down to Descemet’s membrane, the condition is called a descemetocele (pronounced “dess-a-met-a-seal”). If the ulcer goes deeper, through Descemet’s membrane, the fluid inside the eyeball flows out and the eye collapses.
The latin root for cornea is kerat. When the cornea becomes inflamed, the medical term for this is keratitis. If an ulcer is the cause of the inflammation, the condition is called ulcerative keratitis.
“Corneal ulcers in cats are typically secondary to a primary ocular disorder” says Dr. Pirie. “The most common of these would include feline herpes virus.” Trauma is another common cause of corneal ulcers in cats. Although a scratch from another cat is a frequent cause of corneal damage, there are more innocent causes of trauma, such as rubbing the eye on the carpet, or an unexpected interaction with a plant or tree branch. Chemical burns, such as that caused by shampoo can cause corneal ulcers if the shampoo gets into the eye. Decreased tear production – a condition called keratoconjunctivitis sicca or “dry eye” – can predispose cats to corneal ulcers. This condition is more common in dogs than cats. “Conformation plays a significant role” adds Dr. Pirie. “Breeds at risk, based on conformation alone, would include any of the brachycephalic (flat-faced) breeds, for example Persians and Himalayans. These breeds often have macroblepharon (wide eyelid openings) and lagophthalmos (inability to completely close the eyelids), both of which contribute to premature drying of the cornea and disruption of the normal tear film.”
Eyelash and eyelid disorders can also lead to corneal trauma. Eyelash disorders such as distichia (extra eyelashes) or ectopic cilia (misdirected eyelashes) can lead to corneal trauma as the abnormal lashes rub against the cornea. Fortunately, eyelash disorders are rare in cats. Eyelid disorders are more common. Entropion is a condition in which the eyelid rolls inward, causing hair near the eyelid margin to contact the cornea. As the hair continually rubs against the cornea, an ulcer can form.
Julie is an 18 year-old domestic shorthaired cat, one of six cats owned by Mary Baysinger. In 2007, Julie was diagnosed with intestinal lymphoma. Julie responded well to chemotherapy and is doing well nearly 3 years after her diagnosis. However, Julie lost almost 3 pounds early in the course of her illness, causing her eyes to become sunken. When her eyes retracted into their sockets, she developed bilateral entropion – both lower eyelids rolled inward, toward the eyeball. Poor Julie developed corneal ulcers on both eyes. At age 18 and with her cancer in remission, Ms. Baysinger was reluctant to have Julie undergo general anesthesia and corrective surgery. “Julie has done so well, despite her age and her intestinal cancer. If something happened to her while under anesthesia, I don’t think I’d forgive myself”, said Ms. Baysinger. Fortunately, she has been able to successfully manage Julie’s ulcers with frequent applications of antibiotic eye ointments that form a protective barrier between the hairs and the cornea. “These eye ointments have made her much more comfortable. If she were younger and healthier, I’d do the surgery right away. For now, she seems happy and comfortable”, she adds.
How can you tell?
Corneal ulcers are painful, and most cats with corneal ulcers will exhibit some signs of discomfort, such as tearing, rubbing the eye, and keeping the affected eye partly or completely closed. To prove that an ulcer is the cause of the discomfort, a fluorescein stain is usually performed. To perform this test, a drop of a fluorescent orange-colored liquid is applied to the cornea. If the cornea is intact, the dye washes smoothly over the corneal surface. If an erosion or ulcer is present, however, the dye will adhere to the exposed stroma and can be easily detected using a black light.
Treatment of corneal ulcers varies, depending on the depth and severity of the ulcer. Antibiotic drops or ointment is applied to the cornea several times a day, to prevent an infection from occurring, and superficial ulcers typically heal in 3 to 5 days. Irritation of the cornea often leads to spasm of a muscle inside the eye called the ciliary muscle. When this muscle spasms, it causes pain for the cat. Atropine drops or ointment, applied to the affected eye, causes paralysis of the ciliary muscle and reducing pain and discomfort. Atropine will cause the pupil to dilate widely, making the affected eye very sensitive to light and causing squinting, especially in bright light. “If herpes is suspected, antiviral medicine is warranted” notes Dr. Pirie. Ulcers caused by feline herpesvirus-1 take longer to heal than superficial ulcers caused by trauma. Cats who rub at their eye a lot may need to be fitted with an Elizabethan collar to prevent further trauma.
Occasionally, a superficial ulcer is slow to heal because the edges of the ulcer contain dead cells. The dead cells prevent a new layer of epithelium from growing across the ulcer and filling in the defect. In these cases, removing the dead cells from the edges of the ulcer with a sterile cotton swab may be necessary to start the healing process. After a few (5 to 7) days of treatment, the fluorescein stain test is performed again. If the cornea does not take up any stain, it is considered to be healed.
Deep ulcers and descemetoceles that are at risk for perforating require more aggressive therapy, such as applying a soft contact lens to the affected cornea, or some type of surgical technique designed to cover the ulcer or descemetocele . A common surgical procedure is a conjunctival graft. In this procedure, a small piece of tissue adjacent to the cornea is sutured over the ulcer. This allows blood vessels to deliver nutrients, antibodies, and infection-fighting cells to the damaged cornea, as well as providing mechanical support, in a similar fashion to how a skin graft promotes healing of a severe burn. Another common procedure is partial tarsorrhaphy (pronounced tar-sor-a-fee), in which the eyelids are temporarily sutured together. The partial closure of the lids protects the cornea. Once the cornea has healed, the tarsorrhaphy is reversed so that the cat can use the eye again. If special techniques, diagnostic equipment or surgical skill is required, your veterinarian may suggest referral to a board-certified veterinary ophthalmologist.
Ulcerative keratitis is a potentially vision-threatening condition. Prompt diagnosis and aggressive treatment is required to preserve vision and keep cats comfortable.
Sidebar – administering eye medications
Administering eye medication
“We often suggest owners try to have the animal learn a positive association (i.e. receiving a treat) with the application of a medication. However, this is often more effective with dogs”, says Dr. Chris Pirie, a veterinary ophthalmologist at Tufts. “Regarding the application of the medication (i.e. drops), we generally suggest the owner rest their right hand (if right-handed) on top of the animals head, while holding the bottle using their thumb and forefinger. The palm and pinky finger are concurrently used to elevate the upper eyelid to allow application of the drop from above. During all of this, the left hand is used to stabilize the animals head and the owner’s body (animal sitting on their lap) to prevent the animal from backing up and getting away. It is important to note that the hand applying the medication is resting on the animals head, as if the animal moves, so does the dropper , which reduces the risk of the dropper tip hitting the globe and causing further injury.”
Further info: Corneal Ulcers in Animals