Tear Turnover (TT) and corneal epithelial permeability (P_dc)

Purpose: TT is a measure of the affects of tear secretion, evaporation and drainage on the tear film. P_dc is a measure of the integrity of intracellular junctions and cellular membranes. TT and P_dc were determined in a group of normal subjects and in patients with keratoconjunctivitis sicca. Methods: TT and P_dc were determined simultaneous using a novel fluorophotometric method for a group of 13 normal subjects and 13 patients with keratoconjunctivitis sicca (KCS). Disodium fluorescein (DSF) was instilled into one eye and carboxy fluorescein (CF) into the fellow eye of each subject. Results: Mean TT rates for normal subjects were 21.27 ±9.15 and 21.78 ±8.88% for DSF and CF respectively. Mean P_dc values in normal subjects were 0.1868 ±0.855 and 0.1710 ±0.2509 nm/sec for DSF and CF respectively. Mean TT rates for the KCS group were 13.15 ±6.28 and 11.78 ±10.61%/min for the DSF and CF groups respectfully. Mean P_dc values in normal subjects were 0.7385 ±0.5429 and 0.4996 ±0.7590 nm/sec for DSF and CF respectively. TT rates were not correlated with age or with P_dc values. P_dc was found to increase with age and with increased corneal punctate staining. Conclusions: The mean P_dc value was 3-4 higher and the TT 38-46% lower in KCS subjects compared to normal subjects. Because individuals with KCS have a low TT and a high P_dc, they are doubly at risk for the therapeutic and toxic effects of substances introduced into their tearfilm.