The cornea is the clear window at the front of the eye. Its main function is to transmit and bend light entering the eye so that it can be further focused by the lens onto the retina at the back of the eye. The normal horse cornea is about 1 to 1.5mm in thickness.

    Corneal ulcers are defects in the surface layer of cells of the cornea (epithelium) which can extend a variable depth into the underlying fibrous corneal stroma.  The deepest ulcers may extend down to the layers at the back of the cornea - referred to as a descemetocele.  In a very severe case the ulcer can perforate the cornea and threaten to blind the eye.

Causes of Corneal Ulcers

    Corneal ulcers usually result from injuries to the cornea.   The surface layer of epithelial cells normally provides a barrier against infection of the deeper layers.  If the epithelium is damaged and bacteria or fungal organisms are able to infect the deeper layers a more severe ulcer will result. In some species virus infection may initiate corneal ulceration.  Other causes of ulcers include a lack of tears (keratoconjunctivitis sicca), abnormal eyelid cilia/eyelid hair and inward rolling of the eyelid (entropion – often seen in foals).  In these situations eyelid hair rubs against the corneal surface.   Quite commonly in horses ulcers and corneal injuries may develop into deep corneal fungal infections.

    Most superficial corneal injuries which do not get infected with bacteria can heal in a day or so. These defects heal by superficial corneal epithelial cells adjacent to the ulcer sliding across to cover the ulcerated area. Occasionally a superficial ulcer may occur in which this mechanism is abnormal and then a superficial refractory or indolent ulcer may persist for weeks or months.

    If the ulcer becomes infected with bacteria it may get deeper very rapidly and if untreated may perforate the cornea. The eye may be lost if this happens and no treatment is provided.

Signs of Corneal Ulceration:

• Pain (the corneal surface has a very dense nerve supply and any injury to the corneal surface is VERY painful)
• Secondary inflammation inside the eye results in discomfort in response to bright light (photophobia).  This results from spasm of muscles in the eye
• Increased tearing is seen (tears spill onto the face). If the cornea is infected with bacteria the discharge from the eye may become thick and green or yellow in color.
• The horse reacts by squinting the eyelids shut (making examination difficult), retracting the eye into the orbit and protruding the third eyelid across the corneal surface. 
• The white part of the eye (conjunctiva and sclera) becomes red and inflamed
• The cornea may become a gray or white cloudy color
• A discrete defect where the superficial layer of cells is missing may be visible on the corneal surface. A definite cavity may be seen in the cornea as the ulcer deepens.
• If infected with bacteria the corneal layers may breakdown and become gel-like or mushy in appearance (so-called “melting ulcer”). The pupil in the affected eye is usually smaller than in the normal eye.

  

Diagnosis of Corneal Ulcers: 

    To effectively examine the eye of a painful horse requires sedation and local anesthesia to block the eyelid muscles so that the eye can be opened without exerting pressure on the eye (which would not be good in the case of a deep corneal ulcer since perforation could result).  The cornea is examined with magnification using a slit-lamp biomicroscope to assess the extent and depth of the ulcer and to look for any inciting cause (eyelid hairs for instance).  The edges of the ulcer are delineated and the depth into the cornea assessed by staining the cornea with fluorescein dye.

    Deeper ulcers need to have a swab taken for bacterial and fungal culture and sensitivity and often a delicate scraping is taken from the edge of the ulcer to examine under the microscope to see the types of cells present and to determine if bacteria or fungal organisms are present in the ulcerated corneal tissue.

    Corneal ulcers are usually initially treated medically with antibiotic drops or ointments applied to the eye and in most cases heal rapidly. Antifungal drugs are used in horses with fungal corneal ulcers. In some virus infections specific anti-viral drugs are needed. Control of pain with eye drops and oral medications is very important since these are very painful lesions.

    In horses with painful eyes it can be extremely difficult to apply medications to the eye.  It is usually impossible to tilt the horses head to one side to instill eye drops – compounded by the strong eyelid closure.  It is possible to injure the eye when applying ointments if the horse moves when the ointment tube is close to the eye.  Exerting pressure to open the eyelids when applying medication can cause perforation of a deep ulcer.

    For these reasons, in all but very small superficial ulcers, we usually hospitalize ulcer cases and install a sub-palpebral lavage (SPL) system to administer the medications.    The SPL makes it much easier and safer to ensure that medications reach the ulcer where they are needed.

    In cases where the ulcer becomes very deep in the cornea or if the cornea perforates surgical treatment is needed in most cases. Failure to quickly start medical therapy or recognize when surgery is needed is one of the most common reasons why an eye is lost when corneal ulcerations present.  Horses with deep ulcers should be referred to a veterinary ophthalmologist for evaluation and treatment.

Ophthalmologists treat severe ulcers with various microsurgical techniques including:

• Conjunctival grafts to bring a blood supply into the cornea to help healing and remodeling of the corneal defect
• Corneal grafts to strengthen the cornea over a deep ulcer or seal a corneal perforation and stop leakage and loss of the ocular contents
• Synthetic material patches which are sutured over the ulcer to encourage healing

In order to surgically repair a corneal laceration or ulcer it is essential that the surgeon has excellent magnification and lighting.  We use an operating microscope with co-axial lighting.  When operating on a structure as thin as the cornea we use very delicate microsurgical instruments and specialized suture materials rarely available in a general practice.

The prognosis for any corneal ulcer is always uncertain initially.  With vigorous medical and, when needed, surgical therapy even severe ulcers can be cured and vision preserved.