Referring to FIG. 1, a corneal abrasion is an injury to the surface epithelium 6 of the cornea 5 that is superficial enough not to involve the basement membrane, i.e., Bowman's layer 10. It is typically due to mechanical trauma, but may also be associated with degenerative changes in the cornea and chronic edema. The defect is seen as superficial on a slit lamp and takes up fluorescence, shining green upon illumination with cobalt blue light. Treatment for uncomplicated abrasions is usually prophylactic antibiotics with pressure patching over 24 hours to decrease the mechanical movement of the blinking lid, which disrupt the epithelial cell growth to heal the defect.
Commonly abrasions heal without complications. However, a significant subset of patients manifest delayed corneal wound healing due to underlying conditions including, but not limited to: tear film deficiency (dry eyes), hypoesthesia (contact lens wearers, history of ocular herpes), scars, and diabetes mellitus.
Many patients exhibit manifestations of recurrent corneal erosions following a primary corneal abrasion. Typically, recurrent corneal erosion occurs in many clinical conditions that affect the corneal surface, such as previous trauma, corneal dystrophies or degenerations, but may also occur spontaneously. Recurrent corneal erosions affect the outermost layer of corneal cells, i.e., the epithelium 6. They occur when the basal layer of epithelial cells adhere poorly to the cornea, causing them to slough off easily. Upon awakening, patients often experience severe pain, blurred vision, and light sensitivity when the eyelid pulls the loosened epithelial cells off the cornea. After the cornea heals, the problem commonly recurs, as the name implies.
The epithelium 5 adjacent to an abrasion or erosion expands via mitosis to fill in the defect. Lesions that are purely epithelial can heal quickly and completely without scarring. Lesions that extend below the Bowman membrane 10 (12 microns thick) are more likely to leave a permanent scar. The epithelial healing process begins when basal epithelial cells undergo mitosis, producing new cells that occupy fresh wounds. Basal cells adhere the epithelium 6 to the stroma 7 in two ways: they secrete the basement membrane, and they contain hemidesmosomes, which are essentially linchpins that protrude through the posterior surface of basal cells and into the stroma 7; each is held in place by an anchoring fibril. Any disruption to basal cell production makes the eye more prone to recurrent erosion.
Recurrent corneal erosion (RCE) syndrome is a condition that is characterized by a disturbance at the level of the corneal epithelial basement membrane i.e., Bowman's layer 10, resulting in defective adhesions and recurrent breakdowns of the epithelium 6. Management of RCE syndrome is usually aimed at regenerating or repairing the epithelial basement membrane to restore the adhesion between the epithelium 6 and the anterior stroma 7. Recurrent corneal erosions occur because there is a defect in the epithelial basement membrane and in hemidesmosome formation, resulting in epithelial loss, microcysts, and bullae formation. They occur after injury, because of improper or inadequate healing of the basement membrane, either because the basal epithelial cells fail to produce proper basement membrane complexes to attach to the Bowman layer 10 and stroma 7 or because of faulty basement membrane adherence. EM studies have shown that during recurrent corneal erosion episodes there is separation of the anchoring system at the level of the epithelial cell membrane or below the level of the anchoring plaques. Normal and degenerate polymorphonuclear leucocytes (PMNs) were found within and between the epithelial cells and within the anchoring layer, i.e. Bowman's layer 10. The degenerate PMNs may secrete metalloproteinases that cleave the Bowman layer 10 below the anchoring system.
Medical management of RCE is aimed at regenerating or repairing the epithelial basement membrane to restore the adhesion between the epithelium 6 and the anterior stroma 7. Typically, treatment is required to promote healing and to relieve the painful symptoms. The healing rate for an abrasion due to RCE syndrome is generally slower than the healing rate for a similar abrasion caused by primary trauma.
Patients with RCE typically respond to topical lubrication therapy, debridement of the epithelium and basement membrane and bandage soft contact lenses. However, many patients may continue to have painful recurrent erosions despite all of these measures. Management includes: placement of a pressure patch over the eyelids for 1 or 2 days and an antibiotic ointment. Sometimes these measures must be followed for several months after resolution of the episode through the use of lubricating and dessicating ointments at bedtime to control symptoms.
In some cases of multiple recurrent erosions, soft contact lenses can be helpful. Bandage lens treatment, if used for this indication, must be continued for up to 8-26 weeks to facilitate repair of the corneal epithelial basement membrane. However, persistent use of soft contact lens increases the risk of infectious corneal disease and is generally deferred until other treatments prove to be ineffective. In some cases, the recurrence of mild corneal erosions may be prevented with the use of hypertonic sodium chloride drops 2%-5% several times during the day and sodium chloride ointment 5% at bedtime.
In resistant cases of RCE, more invasive or surgical intervention is often required. Mechanical debridement, depending on the size of the defect, can be performed, in which a diamond burr is used to “polish” Bowman's membrane 10 after mechanical debridement. The goal of surgical management is superficial debridement to remove the abnormal epithelium 6 and basement membrane, thereby leaving a smooth substrate of the Bowman's layer 10. The adjacent normal epithelium 6 may then be able to resurface in this area, allowing formation of competent attachment complexes and resulting in cessation of erosive symptoms with a reduced frequency of recurrences. In some instances, patients experience reduction of vision and/or recurrent erosions from the ongoing deposition of an abnormal basement membrane and fibrillar collagenous material between the epithelium 6 and Bowman's layer 10.
Laser-based techniques have also been utilized to treat recurrent erosions. These include: excimer laser phototherapeutic keratectomy, excimer laser photoablation, Nd:YAG laser treatment, superficial phototherapeutic keratectomy. These techniques attempt to remove enough of the superficial Bowman layer 10 to permit formation of a new basement membrane with adhesion structures. Surgical superficial keratectomy and penetrating and lamellar keratoplasty have also been performed on rare patients with severe recurrent disease. These technique, although shown to be efficacious, are extremely expensive and not available to the typical ophthalmic practitioner and are rarely used.