Corneal Ulcers – An Overview

Introduction

Corneal ulcers are defined as any disruption of, at minimum, the outermost layer of the cornea. The cornea is the clear, dome-shaped layer at the front of the eye, immediately behind the eyelids that acts similarly to the windshield of your car [FIG 2].

It functions to protect the inner structures of the eye while allowing light to pass through. Corneal ulcers are a common and potentially serious eye condition that can occur in both dogs and cats. All ulcers are painful and require immediate attention by your veterinarian.

What are the Different Kinds of Corneal Ulcers?

There are many kinds of corneal ulcers that are classified by depth [FIG3] – superficial, stromal, or deep – and by cause – simple or complicated. Simple ulcers [FIG 4] are those that are not infected. These ulcers can range in depth from superficial to stromal. Superficial simple ulcers typically heal quickly and with minimal scarring. Stromal ulcers often require more vigilant monitoring and may cause scarring. Complicated ulcers [FIG 5] are those that are infected by bacteria or fungus and can range in depth from superficial to very deep.

Infected ulcers often require frequent application of targeted antibiotic or antifungal medication and often result in some scarring. Ulcers that develop because of trauma, like a claw injury, if small can sometimes be healed without surgery however, if full thickness, long, or if iris tissue protrudes though the cornea [FIG 6], surgical repair is often required. Indolent ulcers [FIG 7] are a unique class of corneal ulcer. Indolent ulcers nearly always occur in older animals and may be initiated spontaneously or from trauma. These ulcers, while superficial and rarely infected, can be very slow to heal and require multiple rechecks by your veterinarian who will try to promote healing by stimulating blood vessels to grow into the cornea [FIG 8].

Causes of corneal ulcers in dogs and cats can include trauma to the eye, such as scratches or foreign objects or they can develop because of dry eye syndrome or underlying diseases such as corneal dystrophy or entropion. Some breeds, especially those with very short noses (brachycephalics) are at increased risk for ulcer formation due to the over-exposure of their eyes and reduced eyelid closure for corneal protection.

How Do I Know if My Dog Has a Corneal Ulcer?

Signs of a corneal ulcer most commonly include sudden and persistent squinting [FIG 9], tearing [FIG 10], redness, and rubbing, but rarely, some pets don’t seem to show any signs of irritation. Persistent squinting is an obvious sign of irritation that should never be ignored. In more severe cases, the eye may appear cloudy or have divot present in the surface of the cornea. In addition, the pupil may be smaller on the affected side (anisocoria) and/or there may be clouding of the eye [FIG 11] depending on the severity or duration of the ulcer.

If a corneal ulcer has occurred traumatically such as from a penetrating object/claw, or if a very deep ulcer has progressed to rupture, fluid may leak from inside the eye. This fluid is not the same fluid as tears. In addition, there may be blood or pus in the eye [FIG 12] and/or the iris may protrude through the cornea [FIG 13]. In most cases, surgery will be needed to repair the cornea when fluid or iris tissue is seen to be escaping through the cornea.

Diagnosis

During a routine eye examination, corneas are stained with fluorescein to look for the presence of an ulcer quickly and painlessly [FIG 14]. Fluorescein stain is an orange-yellow water-soluble dye that temporarily fluoresces green when exposed to blue light.

As fluorescein stain can only stick to areas of the cornea not covered by epithelium, the protective outermost layer of the cornea [FIG 2], the ulcer is highlighted for easy identification. To identify very small ulcers or to determine the depth of the ulcer, magnification is required. When examining the stained cornea, your veterinarian will be able to determine the size, shape, and depth of the ulcer, the mostly likely duration and cause of the ulcer, and the likelihood of current infection.

Knowing as much as possible about the ulcer will allow your veterinarian to provide the best prognosis for healing, and to initiate appropriate therapy to prevent or control infection. Consultation with a veterinary ophthalmologist may be required if the ulcer is noted to be very deep, infected, or if the cornea has perforated.

Corneal perforation is confirmed if a clear rivulet of fluid, noted to originate and egress from the ulcerated site, is seen to pass through the sheet of fluorescein stain present on the surface of the cornea. This is called a positive Seidel’s test. When perforated and the corneal defect is large, surgery is often required to restore the integrity of the cornea.

Treatment of Corneal Ulcers

Most ulcers healed within 7 days after starting treatment however, when ulcers take longer to heal, three things need to be considered; is it Infected, is an Inciting cause still present, or is it Indolent? These are the three “I’s”.

The initial therapeutic plan is developed after determining the depth and underlying cause of the ulcer. Unless an obvious cause is identified, most treatment will involve antibiotic medication (usually a broad-spectrum antibiotic that is applied directly to the eye), oral anti-inflammatory medication, oral pain control, and sometimes an oral antibiotic. In addition, it is critical that the patient be prevented from harming their eye further by ensuring that they always wear an E-collar (cone).

With increased depth of the ulcer, the medications applied become thinner (ie liquid eye drops vs ointment) and the frequency of application is often greater. When an ulcer is identified to be deep and infected, the frequency of eye drops can be a high as every 30-to-60 minutes for several days at a time!

When an ulcer does not heal in the expected 5-to-7-day time frame, treatment is redirected. Redirection is based on suspicion for infection, presence of an inciting cause, or likelihood of indolence. A corneal ulcer that initially appeared simple, can change in colour to appear yellow-grey and soft.

This change occurs more commonly in the flat-faced (brachycephalic) breeds. These visual hallmarks of infection can be confirmed by collecting swabs for laboratory studies that include bacterial culture and sensitivity, as well as cytology. When confirmed, the therapy is changed to better target the infection.

This may be in the form of additional antibiotics, anti-fungal medication, drops that help to stop corneal melting, and additional pain control.
To look for an inciting cause, an ophthalmologist will use specialized equipment to examine the eye with magnification. In young dogs, common culprits for persistent ulcers include ectopic cilia [FIG 15], entropion [FIG 16], cat claw injuries, and plant-based foreign bodies.

Persistent ulceration in older dogs may include any of the above causes listed above, but often occur secondary to indolence. Indolent ulcers, while superficial and rarely infected, can be very slow to heal and require multiple rechecks by your veterinarian who will try to promote healing by stimulating blood vessels to grow into the cornea.

With age, the ulcerated outermost layer of the cornea fails to attach to the underlying deeper layer of the cornea. Removal of the loose epithelial layer of the cornea and disruption of the underlying corneal tissue by your veterinarian will help new epithelial tissue grow, stick to, and seal the cornea. During the healing process, blood vessels are often noted to grow into the cornea. This is a positive sign and should not alarm you if noted.

In severe cases of corneal ulceration, surgery may be required to restore the integrity of the cornea. Typically, surgery will be recommended when the ulcer is more than 50% deep with no signs of healing, the depth of the ulcer is increasing and rupture is imminent, or the cornea has already perforated.

There are many surgical methods available to repair the cornea and nearly all will need to be performed by a veterinary ophthalmologist. While technically routine to perform, successful surgical procedures require sophisticated magnification and microsurgical instrumentation that is uncommonly available to general practice veterinarians. The method chosen by your veterinary ophthalmologist will be the one best suited to your pets needs.

If more than one appropriate surgical option is available for your pet, your ophthalmologist will be able to discuss the differences – most of which differ by degree of scarring and cost. Surgical options can include primary corneal stitches [FIG 17], placement of a conjunctival graft [FIG 18], corneo-conjunctival transposition [FIG 19], placement of a biological scaffold [FIG 20], penetrating keratoplasty, or application of cyanoacrylate glue.

Prevention of Corneal Ulcers

Preventing corneal ulcers in dogs and cats involves protecting their eyes from potential external sources of trauma, such as sharp objects or chemicals, as well as from self harm associated with irritation by underlying disease or poor conformation.

When an underlying disease or condition known to increase the risk of ulcer formation is present (eg dry eye, entropion), control or resolution of that disease or condition is strongly recommended. Prompt attention to the clinical signs associated with corneal ulcers will help to mitigate the depth and duration of corneal ulcers.

Review

Corneal ulcers, painful disruptions of the eye’s protective corneal layer, can affect all animals. They can be caused by trauma, underlying diseases, or breed-specific predispositions, especially in brachycephalic breeds. Recognizing symptoms like persistent squinting, redness, and tearing is vital for prompt veterinary intervention.

Diagnosis typically involves fluorescein staining and magnified examination, with treatments ranging from antibiotics to, in severe cases, specialized surgeries. Addressing potential underlying conditions and protecting pets form eye trauma, are instrumental in preventing corneal ulcers.