To understand corneal sequestrum , we first need knowledge of the anatomy of the cornea (clear portion of the eye). Covering the outside of the cornea is 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 cell layers thick. Lining the inside of the cornea is one cell layer called endothelium. These endothelial cells are 'leaky' and allow fluid from the inside of the eye (aqueous humor) 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 fiber matrix. The cornea in the cat is approximately 0.6 mm thick. (For comparison, a dime is approximately 1 mm thick.) 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 and white blood cell infiltration as a 'foreign body' reaction to the necrotic tissue occurs. Various names have been given to this lesion since it was first described in 1965. These include corneal mummification, the 'isolated black lesion', corneal nigrum, keratitis nigrum, focal degeneration of the cornea, corneal necrosis, primary necrotizing keratitis and corneal sequestration or sequestrum. The cause of the corneal sequestrum is unknown. A few veterinary ophthalmologists suggest primarily corneal abnormal development. Other ophthalmologists suggest that the dead tissue is secondary to a host of factors which cause corneal irritation. Factors such as herpesvirus, lack of tear production (keratoconjunctivitis sicca or KCS), chronic corneal ulceration and foreign objects in the eye have all been suggested as causes of corneal sequestrum. Additionally, some of these cats sleep with their eyes open (lagophthalmos) , and the tear film is not spread adequately over the central cornea which results in drying. Some affected cats have a subtle inrolling of the lower eyelid (entropion) which may irritate the cornea. The origin of the black discoloration of the necrotic stroma is thought to be products in the tears which adsorbs into the degenerative tissue.
Corneal sequestra are seen in cats of all ages except the newborn Although sequestra (plural of sequestrum) most often occurs 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:
Incidence of Corneal Sequestrum by Breed
| Himalayan | 35% |
| Persian | 35% |
| Siamese | 15% |
| Domestic | 10% |
| Abyssinian | 5% |
The Persian, Siamese and Himalayan are most often seen, but cases also occur with lesser frequency in the Exotic Shorthair and Domestic Shorthair breeds. This breed predisposition 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 lagophthalmos (their eyes protrude), and thus are subject to additional risk of trauma.
The usual clinical presentation is a cat with a history of chronic eye problems. Most affected cats exhibit blepharospasm (excessive blinking or squinting), 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. Often clients will describe it as 'a scab on the eye'. Application of fluorescein dye to the cornea often reveals dye retention around the edge of the lesion, but the sequestrum itself does not stain. Sequestra are usually axial (in the center of the cornea), but may be paraxial (off-center). Cats usually develop sequestra on one eye, but bilateral cases occur. The cornea around the sequestrum will be vascularized superficially to varying degrees.
An area of corneal sequestrum surrounded by vascularization and ulceration (arrow).
Some cats are present with a faint brown or amber opacity confined to the stroma. Such a patient may not show signs of discomfort. These are thought to be early lesions, lesions following sloughing or surgical removal and subsequent healing, and re-epithelialization of superficial sequestra. These faint sequestra will usually progress, become more dense, acquire distinct borders, separate from the adjacent and underlying stroma and begin the process of ulceration and irritation with loss of the overlying corneal epithelium. The lesion will become larger, darker and deeper. Corneal perforation and globe rupture can and does occur. Some ophthalmologists classify the sequestrum which is darker as a Type I sequestrum, while the lighter brown lesion which is covered by an intact but degenerate epithelium as a Type II lesion. Other ophthalmologists feel that they are both different stages of the same disease process.
The treatment of corneal sequestrum in the cat varies considerably between veterinary ophthalmologists. Few adopt a conservative approach and allow the sequestrum to slough while treating with antibiotics. Extrusion may take months to years. Waiting is sensible when the lesion is loose and begins to slough. Treatment and ocular pain may be prolonged. The second treatment is surgical excision. The main advantage to surgical excision of sequestra is a significant shortening of the course of the disease (during which the patient is very uncomfortable) and the prevention of the globe rupturing. Superficial sequestra may be treated by superficial keratectomy. The cornea is incised and the lesion removed and the cornea is given support while it heals. Some superficial sequestra may be supported postoperatively with a soft contact lens and third eyelid flap. Sequestra which require deeper dissection may require the placement of a conjunctival or corneal graft for support. Post surgical medical management involves topical antibiotics, possibly anti-viral medications and atropine. An elizabethan collar to prevent self trauma is mandatory. Once the cornea has re-epithelialized, some surgeons recommend the use of topical antiinflammatory medications to reduce fibroplasia and subsequent scarring. Caution is required however, as some cats with sequestrum are positive for feline herpesvirus, and topical steroid may incite reactivation of latent virus. A polymerase chain reaction (PCR) test for feline herpesvirus is recommended for cats with corneal sequestrum.
Dr. Hacker wishes to thank Dr. Michael Zigler for much of the information contained herein.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
Copyright © 1998 * Animal Eye Specialists, El Cerrito, CA. * All Rights Reserved
Return to Home Page |
Top of Page |
Who are we? |
Directions to our clinic |
Handout Index |