What is corneal sequestrum?
Feline corneal sequestrum (FCS) is a common eye disease characterised by round or oval brown or black plaques (spots) on the cornea which are made up of necrotic (dead) corneal tissue. FCS is also known as feline corneal necrosis, corneal mummiﬁcation, focal degeneration, corneal nigrum, keratitis nigra, primary necrotizing keratitis and chronic ulcerative keratitis.
The cornea is the transparent layer which covers the front of the eye and refracts (bends) light onto the lens, it is made up of five layers:
- Bowman’s layer
- Descemet’s membrane
Image courtesy National Eye Institute, Flickr
Examination reveals necrosis of the epithelium, superficial stroma and in some cases the deep stroma, with inflammatory cells (giant cells, macrophages, lymphocytes and plasma cells) and blood vessels in underlying and adjacent healthy corneal tissue.
Cats of any age and breed can be affected, however, there is a higher incidence in Persian, Himalayan and American (modern) Burmese which may have a genetic basis or due to their prominent globes which increase the risk of exposure keratitis and lagophthalmia (inability to close the eyes).
The exact etiology of corneal sequestrum is still not known, however, chronic trauma, prolonged irritation and ulceration to the cornea often precede corneal sequestrum. Common causes include dry eye, trichiasis (inward growing eyelashes which rub on the cornea), entropion (inward turning eyelid), feline herpesvirus and eyelid abnormalities which impact on the cat’s ability to blink.
Irregular, brown to black plaques which are predominantly located in or near the centre of the normally clear cornea. The plaque may be dark brown with an ill-defined border and only slightly protrudes or a black, dry plaque with well-defined borders which rises above the level of the cornea.
Most cases are unilateral (affecting one eye), but in some cases can be bilateral (affecting both eyes). Bilateral sequestrum is more common in the breeds mentioned above, or cats with eyelid abnormalities. The sequestrum is usually in proximity to the eyelid abnormality. Blood vessels may grow towards the corneal sequestrum to reject the tissue and heal the defect.
- Blepharospasm (involuntary spasms of the eyelid muscles)
- Protruding third eyelid
- Eye discharge
- Opacity (cloudiness) of the cornea
- Odema (swelling) around the perimeter of the plaque
- Lacrimation (watery eyes)
- Photophobia (sensitivity to light)
- Superficial vascularisation (formation of blood vessels)
- Chronic ocular pain (cats often mask signs of pain, common signs of pain include lethargy and loss of appetite)
Plaques can remain for months or years and can cause secondary conjunctivitis or eye infections. The dead corneal tissue can slough off or act as a foreign body, which invokes an immune response, both of which can cause the eyeball to rupture.
Note: Iris melanosis is another common ocular condition in which brown spots develop in the eye. This condition is benign, but should always be evaluated and closely monitored by a veterinarian as in some cases it can progress to melanoma a cancer of the pigment-producing cells known as melanocytes.
Most cases can be diagnosed based on appearance, however, a thorough ophthalmic exam is essential to determine the underlying cause and rule out other potential eye diseases including cancer.
The veterinarian will perform several tests to evaluate the cat’s vision and determine a possible underlying cause.
- Menace response: The veterinarian covers one eye and quickly moves the hand towards the eye to evaluate the blink reflex.
- Schirmer tear test: This test involves placing a filter strip under the bottom eyelid to measure the amount of tears.
- Fluorescein stain test: Fluorescein is an orange dye which is placed in the eye to evaluate the cornea for ulcers, however, the stain is often not retained in cats with corneal sequestrum.
- Rose bengal stain: This sodium salt stain binds with devitalised (dead) tissue which makes the area more apparent.
- Slit-lamp biomicroscopy: An ocular examination using a slit-lamp, a microscope with a bright light which allows the veterinarian to examine the structures of the eye and will provide information on the depth of the sequestrum.
Identify and correct the underlying cause where possible, this may include surgery to correct eyelid abnormalities or cryotherapy to destroy hair follicles to prevent eyelashes rubbing on the cornea.
- Medical management for corneal sequestrum where the cat is not in pain and the sequestrum is superficial may include broad-spectrum antibiotics, ocular lubricants and painkillers and a third eyelid flap to protect the cornea.
- Ocular antivirals (famciclovir) for cats with feline herpesvirus for one to two weeks to evaluate if medical therapy will resolve the sequestrum.
- Keratectomy (surgical removal of the diseased cornea) under anesthesia. Surgery is usually performed by a veterinary ophthalmologist (a veterinarian who specialises in eye disorders) with microsurgical knives. This surgery is extremely delicate as the cat’s cornea is only 0.6 mm thick. Once the corneal sequestrum has been excised the eyelids are stitched shut (tarsorrhaphy) while the area heals.
In some cases, if there is a recurrence, it may be necessary to perform a conjunctival pedicle flap or a clear corneal-conjunctival transposition.
- The conjunctival pedicle flap involves dissection of a thin tongue of conjunctiva off the sclera (the white part of the eye) and sutured directly onto the cornea.
- Clear corneal-conjunctival transposition involves taking a section of normal cornea adjacent to the area where the diseased cornea was removed, and is transposed over the keratectomy site and sutured down.
- Long term artificial tears will be necessary to prevent a recurrence of the sequestrum.