SIGNS AND SYMPTOMS
Unlike bacterial corneal ulcer patients, marginal sterile corneal ulcer patients present with only mild conjunctival injection, little to no conjunctival chemosis, ocular irritation and normal vision. One or more marginal subepithelial corneal infiltrates are common in many conditions, and usually do not warrant much concern. Accompanying sequelae may include mild iritis, folds in Descemet's membrane if there is substantial corneal edema, and posterior synechiae in chronic cases.
Patients with Staphylococcal hypersensitivity reactions may present without symptoms. Here, scattered multiple areas of sub-epithelial and anterior stromal infiltrates, with or without epithelial defects, line the limbal area (mostly inferiorly). The entity is usually bilateral. One distinct characteristic is the notable clear zone that lies between the areas of infiltrate and the limbus.
Frequently the patient will provide an ocular history of having a dry eye and a systemic history of rheumatoid arthritis or other collagen vascular diseases. Other known associations are vernal keratitis, vitamin A-deficiency and contact lens solution reaction. The principle differential diagnoses include infectious corneal ulcer, marginal sub-epithelial infiltrates secondary to contact lens wear and Mooren's marginal corneal ulcer.
Sterile infiltrates usually represent a low-grade immune response to bacterial exotoxins. For bacterial infection or inflammation to occur, the microorganism must be able to adhere to the corneal surface. Staphylococcus aureus, Streptococcus pneumoniae and Pseudomonas aeruginosa are significantly more adherent than other organisms, possibly accounting for their more frequent involvement in corneal disease.. These organisms are more adherent to the cornea and are tightly adherent to themselves, providing a resistance to phagocytosis by host inflammatory cells.
Staph. aureus is recognized as one of the common opportunistic ocular pathogens. The organism is a gram-positive non-encapsulated coccus capable of producing a variety of exotoxins and enzymes. In addition to its ability to infect the central cornea, it is a leading cause of sterile marginal keratitis.
Powerful exotoxins released by bacteria colonizing the eyelid margin induce peripheral corneal destruction through antigen-antibody reactions. Polymorphonuclear leukocytes and fibroblasts, which migrate to the area to help fight exotoxins, produce collagenase and proteoglycanase enzymes that often produce additional damage.
The treatment strategy for marginal sterile keratitis is two-fold: (1) control and eradicate the microorganism and (2) control and eliminate the destructive elements and sequelae of inflammation. Eyelid scrubs BID/TID using commercially available lid scrubs or baby shampoo will begin the process of cleansing the lid margins.
To fully eradicate dense colonies of lid margin bacteria, prescribe a topical aminoglycoside (gentamicin, tobramycin) or fluoroquinolone (ciprofloxacin, norfloxacin, ofloxacin) QID. The antibiotics kill the bacteria and also mechanically wash organisms and their toxins away from the eyelid margin. If the patient complains of discomfort, prescribe a cycloplegic. Rx a topical steroid based on the severity of the condition.
Since the corneal tissue is free from infection and its damage originates from the secondary effects of inflammation, the most expeditious treatment is both topical antibiotics and topical anti-inflammatories. Many practitioners are apprehensive about prescribing topical steroids in the face of corneal epithelial compromise, although this does not usually pose a serious threat. Nevertheless, if you prefer a more conservative approach, begin with antibiotic treatment alone for 24 to 48 hours, then judiciously add a topical steroid thereafter and monitor for IOP rise.
Today, there is debate regarding the need to culture before starting treatment. Because culture results are not available for 24 to 48 hours, many believe it is only necessary when a condition fails to respond to the prescribed therapy. We advocate treating immediately and then culturing if the condition does not improve or worsens within the first 48 to 72 hours.