The cornea is the clear portion of the eye; it is covered by a layer of epithelium. This epithelium is much like our skin cells, but it usually isn’t pigmented and doesn’t have hair or blood vessels in it. In the cat, the epithelium is about 6-8 layers thick. Lining the inside of the cornea is one cell layer called the endothelium. These endothelial cells are “leaky” and allow fluid (“aqueous humor”) from inside of the eye to leak into the corneal tissue and then pumps it back into the front of the eye. This pumping mechanism keeps the cornea clear. The greatest portion of the thickness of the cornea (about 90%) is the “stroma” which is a collagen fibre matrix. The cornea in the cat is approximately 0.6mm thick (for comparison, a dime is approximately 1mm thick)
WHAT IS A SEQUESTRUM? Corneal sequestrum is a condition peculiar to the cat in which the corneal stroma dies and then degenerates. This condition is called necrosis. The dead stroma becomes pigmented and becomes irritating to the body causing pain and blood vessels to enter the stroma. There is often corneal edema (swelling) and white blood cell infiltration as a reaction to the necrotic tissue occurs.
WHAT CAUSES IT? The cause of corneal sequestrum is unknown. A few veterinary ophthalmologists suggest that it is caused primarily by abnormal development of the cornea. Others suggest that the dead tissue is secondary to a host of factors that include herpes virus, lack of tear production (“KCS”) or lack of an appropriate tear film (i.e. adequate amount of tears are produced but they do not adhere to the surface of the cornea), chronic corneal ulceration, and foreign objects in the eye. Additionally, some of these cats sleep with their eyes partially open (“lagophthalmos”), and the exposed portion of cornea dries out. Some affected cats have a subtle in rolling of the lower eyelid (“entropion”) which may irritate the cornea. The origin of the black/brown discoloration of the necrotic stroma is thought to be due to products in the tears, which are absorbed into the degenerative tissue.
WHICH CATS GET THIS CONDITION? Corneal sequestra are seen in cats of all ages except the newborn. Although sequestra most often occur in young adult cats, we have seen it in cats from 2 to 16 years of age. There is no difference in occurrence by sex of the cat, but there is a noticeable breed distribution:
The Persian, Himalayan, and Siamese are most often seen, but cases also occur with lesser frequency in the Exotic Shorthair and Domestic Shorthair breeds. This breed disposition may not be genetic, as these breeds share certain conformational features that may predispose the cat to corneal sequestration. These breeds have some degree of exophthalmos (protruding eyes), and thus are subject to additional risk of trauma.
WHAT WILL I SEE WHEN A SEQUESTRUM OCCURS? Most affected cats exhibit excessive blinking or squinting (“blepharospasm”), tearing, and prominence of the third eyelid. The sequestrum is a dark brown to black plaque which is oval or round and often raised above the corneal surface. People often describe it as a “scab” on the eye. Sequestra are usually seen in the centre of the cornea but may be off centre, too. Cats usually develop them on one eye, but it can occur in both eyes.
WHAT IS THE TREATMENT?The treatment of choice is surgical removal. Under general anesthetic, a procedure called a superficial keratectomy (“peeling” away of a thin layer of cornea along with the sequestrum) is performed. High-Frequency Ultrasound is performed to determine the depth of the lesion in the cornea. If the sequestrum is quite deep, other procedures may be needed. Corneoconjunctival transposition utilizes the cat’s own cornea and conjunctival tissue that are transposed over the damaged area after removal of the plaque. In some patients lamellar keratectomy is performed: once the plaque is removed a partial thickness donor cornea is transplanted into the bed of healthy donor tissue. Amniotic membrane grafting is the most advanced method. It brings stem cells to the cornea and decreases scarring.
WHAT WILL I NEED TO DO AT HOME?
- Keep the elizabethan collar on at all times (even at night). The stitches in the eye are very fine and can easily be damaged by a rubbing paw.
- Administer the medications at the appropriate times.
- When using eye drops, be sure to hold the lids open and raise the nose towards the ceiling so the medication is placed directly onto the eyeball. Wait 10 minutes between different drops so as not to flush out the previous medication before it has been absorbed. If you are required to use drops and ointment, use the ointment last as it is thick and forms a barrier, preventing any following drops from being absorbed.
Gently wipe away any discharge from the eye with a clean, moist Kleenex or washcloth.