Corneal ulcers are defects of the ‘cornea’; the clear ‘window’ of the eye. In this article, ophthalmology expert Claudia Busse explains about corneal ulcers in dogs and cats.
What is the cornea?
Figure 1: The anatomy of the eye
The cornea is a very special tissue that is completely transparent. In contrast to the skin, it lacks pigment and even blood vessels to maintain its transparency. The cornea is very rich with nerves, making it a very sensitive tissue. This is the reason why even small particles such as dust on the surface of the eye can be so uncomfortable.
The cornea is made of three layers:
- a very thin outer layer (epithelium)
- the thick middle layer (stroma)
- a very thin inner layer (endothelium)
All three layers are important for the cornea to work. The outer layer or ‘epithelium’ can be thought of as a layer of cling film that forms the surface of the cornea and protects it from infections. It works as a shield to the eye. The thick middle layer or ‘stroma’ is what gives the cornea its strength and stability.
What are corneal ulcers?
Corneal ulcers are classified by the depth, depending on the layers they affect.
- If only the epithelium is missing this is classified as a ‘superficial ulcer’ (or surface defect only).
- If the ulcer reaches into the stroma it is classified as a ‘deep ulcer’.
Figure 2: Left eye of a patient with a deep corneal ulcers. Note the indentation in the cornea and the surrounding swelling making the cornea appear blue. This patient required surgery.
While superficial or surface ulcers are uncomfortable and present a risk of infection to the eye; the eye is not at the risk of bursting unless additional problems occur. However when the ulcer gets deep the eye becomes weak and can even perforate. Deep ulcers lead to visible indentations on the surface of the eye and can be accompanied by inflammation inside the eye (Figures 2 & 3).
Figure 3: Left eye of a patient with a deep ulcer due to a scratch. Note the deep indentation in the cornea and the pus inside the eye indicating severe inflammation.
Signs of corneal ulcers usually include eye pain (squinting, tearing, depressed behaviour) and ocular discharge, which can be watery or purulent. Sometimes a lesion may already be visible on the surface of the eye. In this case it is particularly important to seek help from your veterinary surgeon as soon as possible.
How is a corneal ulcer diagnosed?
Figure 4: Right eye of a patient with a superficial corneal ulcer due to hair rubbing on the surface of the eye. Fluorescein dye is highlighting the defect.
To diagnose a corneal ulcer your vet may use a special dye that highlights any defects of the surface layer by staining the underlying tissue green (Figure 4). This test is called the fluorescein test.
Why do corneal ulcers occur?
When the presence of an ulcer has been confirmed, it is important to try and find a reason for it. Most ulcers occur due to an initial trauma. This is more likely to happen in dogs and cats with very prominent eyes (also called ‘brachycephalic’ animals), for example in Pugs and Pekingese dogs or Persian cats. In cats the flare up of a Feline Herpes Virus infection is also a common cause for the development of a corneal ulcer. Many conditions can increase the risk of corneal ulcers. Reduced tear production is a common contributing factor, but other conditions such as an incomplete blink, in-rolling of the eyelid (also called ‘entropion’) or eye lid tumours may contribute to the occurrence, but even more so may interfere with the healing process.
How are corneal ulcers treated?
To treat an ulcer it is essential that the underlying cause is identified and if possible corrected. This will stop the ulcer from getting worse and allow the eye to heal as quickly as possible. The treatment plan will usually include eye drops to treat or prevent infection but may include other medication depending on the cause and severity of the ulcer. Painkillers and/or antibitotics by mouth may also be necessary.
Do any corneal ulcers require an operation?
Figure 5: Same patient as in figure 2, approximately 2 months after the surgery to repair the ulcer. Healthy cornea and conjunctiva were moved towards the defect and are now visible as a scar. The graft is likely to clear a bit more over time.
If an ulcer is deep or the cornea is even ruptured, surgery is required to save the eye. Different techniques are available, but all of them place healthy tissue into the defect to stabilise the cornea. Very small suture material, as thin as a human hair, is used to repair the cornea and an operating microscope should be used to handle the small and very fine structures of the eye.
In most patients the healthy tissue is taken from the same eye from an area adjacent to the corneal ulcer.
- The pink tissue next to the cornea (the conjunctiva) can be used for that to place a ‘conjunctival pedicle graft’ into the defect.
- More commonly, healthy corneal tissue attached to conjunctiva is used as it provides more strength to the wall of the eye. This is called a ‘corneoconjunctival transposition’ (Figure 5).
- Corneal grafts are also possible but rely on the often limited availability of donor corneal tissue. Grafting surgeries are very successful in saving eyes, but can lead to scaring of the cornea leaving it less transparent in areas.
How can corneal ulcers be prevented?
Corneal ulcers are best prevented and if they are present they should be treated as soon as possible to stop them from getting bigger and deeper.
Particularly in patients with prominent eyes, regular eye examinations should be performed to detect weaknesses in the corneal health. Indications of that may include white or brown marks on the surface of an otherwise comfortable eye or sticky discharge that continues to recur. Any painful eye should be presented to a veterinary surgeon as soon as possible.
Eyes may be cleaned with tap water that ideally should be boiled and cooled down again, using a lint-free towel. This should however not replace or delay the visit to a veterinary surgeon, as many ulcers require medication to achieve fast healing and prevent ill effects to the transparency of the cornea and therefore the sight of the dog or cat. Particularly; if a deep indentation or bulging tissue is noted on the surface, the eye should not be manipulated to prevent any additional damage.