Tear fluid plasmin activity after excimer laser photorefractive keratectomy

Elevated tear fluid plasmin activity may correlate with delayed healing of corneal wounds. The present study was performed to establish the tear fluid plasmin activity after photorefractive keratoablation (PRK).

Tear fluid aspirated with microcapillaries was subjected to a fluorometric plasmin assay using the 7-amido-4-trifluoromethylcoumarin derivate of the tripeptide H-D-Val-Leu-Lys as substrate.

Tear fluid flow, plasmin activity, and flow-corrected plasmin excretion rate in tears (plasmin flux) were determined preoperatively and 1, 2, and 7 days after PRK. The preoperative tear fluid flow was 6.55 microliters/min (median; range, 1.8 to 21.8 microliters/min), plasmin activity was 1.29 IU/l (median; range, 0.6 to 6.9 IU/l), and the excretion of plasmin in tears was 11.7 microIU/min (median; range, 1.6 to 41.5 microIU). A statistically significant decrease in tear fluid plasmin activity was found during the follow-up period on the first (0.6 IU/l; range, 0.6 to 1.7 IU/l, P < 0.01) and second (0.65 IU/l; range, 0.6 to 1.49 IU/l, P < 0.01) postoperative days. On the other hand, significant elevation of both tear fluid flow and plasmin flux values occurred during the first two postoperative days. The median plasmin flux values on days 1, 2, and 7 were 57.35 microIU/min (range, 16 to 540 microIU/min, P < 0.01), 40.0 microIU/min (range, 13.3 to 222.8 microIU/min, P < 0.01), and 10.2 microIU/min (range, 2.2 to 90.7 microIU/min, P > 0.05), respectively.

The marked elevation of tear fluid flow coincided with the persistence of an epithelial defect. However, because of the acceleration of tear fluid flow, proteolytic activity due to plasmin (IU/l) actually decreases. Consequently, the increased excretion of plasmin in tears (plasmin flux) does not lead to highly elevated plasmin activity, which could inhibit wound healing. It seems to be a natural healing response because all corneas were epithelialized normally by or on day 3.