When presented with an apparently simple (non-infected) corneal ulcer which fails to improve within 5-7 days, complicating factors must be considered, including:
Entropion is a frequent primary cause of chronic corneal ulceration in young dogs. This is an inherited condition in many breeds, including:
- Chow Chow
- Shar Pei
- Bull Mastiff
- Golden Retriever
- Labrador Retriever
- American and English Cocker spaniels
- Many other sporting breeds
Corneal or conjunctival foreign body — in some cases these may be extremely small and require slit lamp biomicroscopy for identification.
Aberrant eyelashes — rarely do distichia (extra lashes originating from the meibomian gland openings in the eyelid margin) cause ulceration although they may cause tearing. In contrast, ectopic cilia protruding thr
ough the eyelid conjunctiva are in more direct contact with the cornea and often cause ulceration and pain. Both occur more commonly in younger animals.
Lagophthalmos (failure to blink completely) — this is seen most often in breeds with prominent eyes (Pekingese, Pug, Boston terrier) but may also occur as a sequela to corneal scarring causing reduced corneal sensation.
Note: rarely is rubbing at the eye alone responsible for delayed healing.
Tear Film Abnormalities
Deficiency in aqueous production is the most common and is detectable by the Schirmer tear test; values less than 15 mm/min are suspicious, less than 10 mm/min in an adult dog is significant. In addition, deficiencies in the mucous or lipid layers may also cause delayed corneal healing, particularly in the central cornea within the palpebral fissure. Since topical atropine can reduce tear function, its use is contraindicated in superficial corneal ulcers.
Although many corneal ulcers are caused by trauma, they may also be the result of several syndromes of corneal degeneration. Most of these occur more frequently in older dogs and some have an inherited predisposition. These conditions fall into two major categories: abnormal deposits within the cornea, and defective cell adhesion.
These may occur unilaterally or bilaterally and area often spontaneous although they may be associated with hypothyroidism. They appear as single or multiple granular corneal opacities which are usually self-limiting and do not interfere with vision. When initially observed, conservative management with observation only is recommended for 1-2 months. If there is evidence of progression during that time, topical chelating solutions (2% EDTA) can be applied to bind calcium and thus, slow the progression of the opacity. In rare cases, usually in dogs over 12 years old, affected eyes may become suddenly painful with ulceration occurring as a result of a “foreign body” type of response to the abnormal material in the cornea. Once ulceration is observed, surgery (superficial keratectomy) is indicated to remove the deposit. These ulcers are often non-painful and may progress to perforation.