To understand corneal sequestration, it helps to have some knowledge of feline corneal anatomy. The cornea is the (normally) clear surface of the eye. The outside layer is called the epithelium. It is somewhat similar to skin, though it is normally free of pigment, hair, or blood vessels for the sake of vision. If the epithelium is missing, by definition there is a corneal ulcer. These are usually painful because nerve endings are quite dense in the superficial cornea. The bulk of the cornea is comprised of stroma. The total cat cornea is less than 1 mm thick. For comparison, a dime is 1 mm thick.
Corneal sequestration (or formation of a corneal sequestrum) is a condition that only affects cats. In this condition, the stroma (the supporting tissue or matrix of the cornea) degenerates and turns a brown to black color. The color change is due to absorption of tear pigments into the dead corneal tissue. A sequestrum will look like a brown to black "plaque" or "scab" on the eye. It may be elevated from the rest of the eye surface. There will be variable degrees of cloudiness surrounding the sequestrum. The body tends to treat this area as a foreign substance and will attempt to slough the affected area. Unfortunately, this process can take months to years if the eye is left to heal on its own. In the meantime, the cat is usually painful (or at least irritated) and vision declines. The surface layer or epithelium of the cornea cannot adhere to a sequestrum, thus the eye will have a chronically open wound if a sequestrum is present. This causes pain and puts the eye at risk for infection.
There are several causes for a corneal sequestrum and they all have the common theme of chronic corneal irritation. The main causes are herpetic infection of the eye, injured cornea with aberrant healing, low tear production (usually associated with herpes), poor tear quality, inability to completely or normally close the eyelids over the eye, insufficient frequency of blinking, or entropion (inward rolling of the eyelid, commonly due to herpesviral infections). Some breeds are predisposed to this condition, including Persians and Himalayans.
The problems with untreated sequestra are chronic ocular pain, decreased vision, and potential for either infection or rupture of the eye. Remember that pain can be difficult to evaluate in cats. Their instinct is to hide pain. The vast majority of cats seem to feel better once a sequestrum is removed.
In most cases, surgical removal of a sequestrum is the ideal course of treatment. This generally elicits the quickest recovery and elimination of pain. Rarely, medical therapy and monitoring will be recommended if the sequestrum is loose and about to be spontaneously extruded. There is some risk for recurrence of the sequestrum at the same location. To reduce the risk for recurrence and to promote healing, a graft is commonly placed over the surgical site.
Medical therapy will be required until the eye has healed from surgery and some recheck examinations will be required. You should find that the path to recovery is much quicker with the surgical approach. The cat cornea tends to heal very well after this type of a surgery. The appearance of the eye will improve greatly over the first 6 months after surgery, with a concurrent improvement of vision.
Please let Dr. Jones, Dr. Zirofksy, Dr. Sandberg or a member of our team know if you have more questions or concerns.
Large corneal sequestrum (the black spot you see on this eye).
Small corneal sequestrum in the central cornea.