Tear pH after instillation of buffer in vivo

The pH of the conjunctival fluid was measured with a micro-electrode at the middle of the inferior conjunctival fornix of 160 eyes (86 subjects). After instillation of buffer of pH 5 (in 22 eyes pH 9) the linear course of pH changes was followed until the starting value had been regained. The coefficient of regression K was computed (pH-units/min.). In normal eyes the K-value declines with increasing age from 1.0 to 0.5, corresponding to a return to the starting value after 2 to 5 minutes. The K-value is reduced significantly in keratoconjunctivitis sicca (K = 0.21, pemphigoid (0.36), during local anaesthesia (0.38 +/- 0.03 against 1.04 +/- 0.10 prior to 0.4% oxibuprocain), in the presence of a cotton plug in the inferior fornix, and in wearers of soft contact lenses. The K-value is normal in cases of lacrimal occlusion (owing to overflow), in exophthalmos and endothelial dystrophy, following corneal grafting, in conjunctivitis and keratitis, and in wearers of hard contact lenses. In cadaveric eyes we find incomplete, irregular neutralization.

Tear dilution is the most important factor in the elimination of buffered eye drops in vivo, compared with the proper buffering capacities of the tears and the tissue.