The cornea is the transparent dome-shaped layer covering the front of the eye including the iris, pupil, and anterior chamber. It protects the eye from dust, germs, and other debris, as well as reshaping and focusing light rays onto the retina. It is supplied with a rich supply of nerves.
The cornea is composed of five layers, the epithelium, Bowman’s membrane, stroma (the thickest layer of the cornea), Descemet’s membrane and endothelium. The cornea is bathed in tears to keep it nourished and prevent it from drying out.
The outer layer of the cornea can be damaged due to scratches, particularly in younger cats, this is known as a corneal abrasion. Cells quickly repair this damage. Erosion of the cornea, typically involving more than one layer is known as a corneal ulcer or ulcerative keratitis and can occur as a result of injury or trauma (such as rubbing an itchy eye), infection (such as fungal infection, herpes or calicivirus), chemicals, damage due to structural abnormalities such as entropion, dry eye (lack of tear production) and foreign body. Secondary bacterial infection can occur once the cornea has become damaged, further complicating the problem.
What are the symptoms of a corneal ulcer?
Pain. Due to the rich supply of nerves, the deeper the injury, the greater the pain.
Sensitivity to light.
Excessive tear production (epiphora).
Eyelid spasms (blepharospasm).
Cloudiness of the cornea.
How is a corneal ulcer diagnosed?
Your veterinarian will perform a physical examination, paying careful attention to the eye. Large ulcers may be visible to the naked eye. But smaller ones may need to be seen using a fluorescein stain, which is an orange dye which is placed in the eye, staining any parts green which have corneal erosion.
Determining the underlying cause should be attempted also. If your cat is experiencing flu-like symptoms such as a runny nose, sneezing, fever, then a presumptive diagnosis of “cat flu” (herpes or calicivirus) can be made. Scratches will show up on the fluorescein stain. A Schirmer tear test may be necessary to check for dry eye. Samples may be taken and tested for bacterial or fungal infection.
How is a corneal ulcer treated?
Treatment is aimed at addressing the underlying cause and helping the cornea heal while controlling inflammation, pain and preventing secondary infection. This may include drops to stimulate the production of tears, supportive care and/or anti-virals, anti-fungal medications, surgery to correct eyelid abnormalities.
Atropine is a medication which causes the pupil to dilate and helps relieve pain as spasms of the ciliary muscle (which controls the dilation and constriction of the pupil) cause pain in the already damaged cornea. Unfortunately, as the pupil is dilated, the eye becomes more sensitive to light, therefore your cat should be provided with a darkened room he can use if necessary.
Antibiotics to treat or prevent bacterial infection.
More serious corneal ulcers will necessitate the eye be protected while the ulcer heals. This usually involves suturing the eyelid shut for several days.
If your cat is continuing to paw at the eye, an Elizabethan collar may be used.
As the ulcer heals, you may notice red streaks in the eye (known as neovascularization), this is blood vessels which have formed to help with repair of the ulcer. They will eventually go away.