EXAMINATION OF THE HUMAN CORNEA WITH THE CONFOCAL MICROSCOPE AFTER EXCIMER-LASER PHOTOREFRACTIVE KERATECTOMY

Background In photorefractive keratectomy (PRK) procedures, a variable superficial central corneal scar formation (''haze'') can be observed following removal of corneal stromal tissue. Today, a near to normal slit lamp finding is observed one year postoperatively in most patient s. We employed the slit scanning confocal microscope to study the corn eal morphology years after PRK. Methods We selected 5 patients, who ha d been subjected to unilateral photorefractive surgery 1-3 years earli er and who had no corneal haze upon slit lamp examination. As controls we investigated the non - treated corneas of these patients, 5 health y controls and 5 contact lens wearers. The confocal microscopic invest igation was performed with 25x, 40x and 50x water immersion objectives . The video signal was synconized with the slit scan and stored on S-V HS video tape. By reviewing the videos in the single frame mode, all c orneal layers could be qualitatively evaluated. Results Some minor abn ormalities were observed in the epithelium of all PRK - treated eyes. In the epithelial basal cell layer some round structures of about the size of a cell with high reflectivity were observed. These changes wer e only occasionally found in contact lens wearers, but not in non trea ted or normal control eyes. Bowman's layer was absent in the PRK treat ed eyes, instead, a fine layer of collagen tis sue of increased reflec tivity was found. The subepithelial corneal nerve plexus was normal in all non - treated eyes, whereas in the PRK - treated corneas nerve sh ape and branching pattern were changed to quite an extent. In the ante rior stroma the keratocyte nucleus patterns indicated an increased cel l density and irregular spacing, whereas a normal keratocyte pattern w as found in the deeper stromal layers. A significant finding was the o bservation of rod and needle shaped highly reflective structures, whic h were limited to the area of the excimer laser keratectomy with a pre dominance in the anterior stroma. These longitudinal structures themse lves consisted of linearly arranged highly reflective granules, which sometimes also were found as isolated dots within keratocyte processes . In long term contact lens wearers a comparable granule type. however with a singular and scattered arrangement, was variably found in all corneal regions and layers. In normal controls none of these findings were present. In contact lens wearers and PRK patients with a contact lens history, the corneal endothelium showed some degree of polymegath isms but no other specific findings. Discussion Up to now, refractive surgery with the excimer laser has been reported to elicit no other st romal changes but a mild fibroblast activation with subsequent scar ti ssue formation. In clinically clear corneas after PRK, we have describ ed a new type of stromal deposit observed 1-3 years after surgery. As acute wound healing responses might have been exspected to have passed at this point, this highly reflective stromal deposit can be assumed to consist of linear keratocyte processes filled with some highly refl ective (degenerative?) matter as well as a corresponding exracellular stromal deposit arranged parallel to the stromal collagen bundles. Pos sibly, these stromal deposits represent the result of an inflammatory or degenerative stromal response resulting in the formation of stromal lipofuscein deposits. Visual acuity was not impaired in the patients investigated in this study. As these stromal deposits appear to be per sisting years after surgery and possibly are irreversible in nature. a long term effect on the corneal physiology and function should carefu lly be monitored.