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Corneal disease (Proceedings)

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Apr 01, 2009

This article will discuss common corneal conditions and recommended management for dogs and cats.

Indolent ulcers are a very common condition seen in dogs. There are multiple names for the condition, including "Boxer Ulcer", SCCED, Refractory ulcer, and recurrent epithelial erosion. This condition is seen almost exclusively in middle aged to older dogs (> 6 years old). They often occur spontaneously with no known trauma. The apparent discomfort level can be greatly variable, from intense blepharospasm to minimal signs of pain. Typical clinical signs include mild corneal cloudiness +/- corneal vascularization. Fluorescein stain will reveal a superficial ulcer with characteristic non adherent epithelial cells adjacent to the ulcer. The vascular response can be aggressive or non-existent.

Histopathology of affected corneas reveals a loss of normal basement membrane and the presence of a characteristic acellular hyalinized zone on the superficial stroma. The basement membrane in non-affected areas of the same cornea is normal, suggesting that this is not a basement membrane dystrophy as was previously thought. The acellular hyalinized zone likely interferes with the formation of normal adhesion complexes between the epithelial basal cells and the superficial stroma.

Various medications have been used to help encourage healing of these ulcers. Topical antibiotics are the mainstay of treatment, but are often overused. Indolent ulcers are rarely infected, thus antibiotic therapy is prophylactic. Topical antibiotics with minimal toxic effect on growing epithelial cells are used at a frequency of once to twice daily until healing has occurred. Tobramycin is a common choice. Serum is an effective treatment for stromal, melting ulcers, but does not seem to have significant benefit for indolent ulcers. EDTA, PSGAGs, chondroitin sulfate, cyanoacrylate (tissue glue), Substance P, IGF, and MMP inhibitors have all been used with varying success, often in combination with other procedures.

Surgical procedures are generally recommended for the treatment of indolent ulcers. Simple manual debridement is the most common procedure performed. Either a sterile cotton swab or a blade can be used, but I feel a blade is more effective. The epithelium should be debrided until it no longer is easily removed. This procedure can be repeated multiple times until healing occurs or done prior to more aggressive techniques. Linear Grid Keratotomy and Multifocal Superficial Punctate Keratotomy are both variations of a procedure known as anterior stromal micropuncture. These are effective at facilitating healing and are very low risk procedures. There are multiple theories as to why these procedures work. The disruption of the acellular hyalinized zone may allow healing epithelial cells a gap for adhesion complex formation. It has also been suggested that they increase the production of various cytokines and growth factors which improve the healing response. These procedures leave minimal corneal scarring when performed correctly and can be performed without general anesthesia. Laser keratoplasty and thermokeratoplasty involve the creation of multiple anterior stromal "burns" for the purpose of disrupting the acellular zone on the ulcer surface, much like the LGK. I reserve these for corneas with significant corneal edema or for ulcers not responsive to multiple micropuncture attempts. Superficial keratectomy involves the surgical excision of the anterior stroma in the affected area. This is a very effective technique at healing the ulcer, however it is more expensive and technically difficult than the previous techniques. I have never had to resort to this surgery to heal an indolent ulcer.

Stromal ulcers include any corneal defect that, through the process of proteolysis, has eroded through the stroma. These are considerably more serious in terms of the health of the eye and have the potential to result in loss of the eye if not treated appropriately. Proteolytic enzymes from both bacterial toxins as well as bystander effect from WBCs and fibroblasts are responsible for the progression. Aggressive treatment is necessary.

Deep ulcers should be evaluated based on size, depth, location, corneal consistency, and the location of corneal vascularization. Surgery is an option for any ulcer greater than 50% depth, but non-surgical mehods can be effective for many deep ulcers if the conditions are right. Work-up of stromal ulcers should include a culture and sensitivity as well as corneal cytology to help determine the causative agent. Bacteria agents are the most common offenders (esp. P. aeruginosa and B-hemolytic Streptococcus), but fungal keratitis has been reported in dogs and cats. Antibacterial therapy is ideally tailored to the culture and sensitivity results, but this is not always practical. A gram stain can be performed to give immediate direction as to the type of antibiotic that should be used. For most deep ulcers, especially ones with a malacic appearance, the antibiotic should be given frequently initially (q 2-4 hours) to saturate the cornea. After several days, I reduce the frequency to QID until the ulcer is sufficiently healed.