April 21, 2019
What is the cornea?
The cornea is the transparent membrane that makes up the front of the eyeball. The cornea is about one half millimeter in thickness and composed of four layers. The most superficial or outermost layer is the ‘epithelium’. Below the epithelium is the ‘stroma’, which is the thickest and toughest layer. Below the stroma is a thin elastic layer called ‘Descemet’s membrane’, and the very thin inside lining of the cornea is called the ‘endothelium’.
An erosion of a few layers of the epithelium is called a ‘corneal erosion’ If the damage extends into the stroma, it is called a ‘corneal ulcer’. If the ulcer extends to the level of Descemet’s membrane, it is called a ‘descemetocoele’. If the ulcer perforates, the liquid inside the eyeball leaks out, the eye collapses, and irreparable damage frequently occurs.
What causes a corneal ulcer?
A corneal ulcer is a non-specific sign akin to vomiting or diarrhea. Adequate management implies investigation into etiology and management of primary conditions. Potential causes include:
- trauma from cat scratches, tree branches,
- chemical burns (from shampoos for example)
- microbial infections
- tear film abnormalities
Some breeds of dogs, such as Boxers, have genetic corneal abnormalities predisposing them to slow healing or indolent ulcers, discussed in detail as a separate topic.
Corneal ulcers are painful. Shutting the eye or excessive blinking or spasm of the eyelids and a watery discharge are common signs. The pet may avoid bright light.
A corneal ulcer is diagnosed by examination and use of ‘fluorescein stain’, which adheres to ulcerated areas. Special lights will make the stain ‘fluoresce’. If the ulcer is very deep, samples may be taken for culture and cytology.
The appropriate treatment for a corneal ulcer depends on how deep it is. A superficial ulcer will generally heal without problems in three to five days. An antibiotic drop or ointment may be used during this time to prevent any bacterial infection. Sometimes, devitalized cells at the edges of the ulcer require debridement to facilitate healing. A soft contact lens acts as a bandage is advantageous in certain situations.
Deeper ulcers require more aggressive treatment. The cornea normally lacks blood supply and healing occurs by migration of blood vessels from the limbus. Surgical procedures for deeper ulcers are aimed at supporting and protecting the eye and bringing the blood supply to the corneal ulcer. This may be achieved by temporarily stitching the ‘third eyelid’ across the eye or stitching the eyelids together. Once the ulcer is healed, the stitches are removed and the pet can use the eye again.
Very deep ulcer/perforations
Very deep ulcers require a conjunctival graft, in which a synthetic biomaterial or a. strip of the conjunctiva, or a frozen corneal homologous graft is sutured permanently into the ulcer, to fill the defect and bring in blood vessels. With time, these grafts heal with vascularized scars that gradually become more transparent with time. Along with surgical procedures, intensive treatment with both topical and systemic medication may be needed to treat infection and inflammation of the cornea and internal parts of the eye.
Corneal ulcers can progress quickly from superficial to deep ulcers. It is very important to have any eye problems checked early to on to avoid complications. Some eye medications can make ulcers worse, so do not apply any drops or ointments unless advised to do so by the veterinarian. The vet will need to monitor the ulcer regularly to ensure healing is progressing. Blood vessels that have migrated into the cornea during the healing process will remain afterwards. These gradually regress, but sometimes drops or ointments will be recommended to help the process along.