Corneal swelling response to contact lenses worn under extended wear conditions.

Corneal thickness changes were monitored in ten subjects (1) over a 36-hour period without contact lens wear and (2) during 7 days continuous wear of three types of hydrogel contact lenses of different back vertex powers. Measured on awakening, the mean overnight central corneal edema without lenses was 3.0% ± 1.2%. With the lenses used in this study, the mean overnight corneal swelling over the week ranged from 9.7% to 15.1%. Following sleep with lenses, the cornea showed evidence of gross stromal edema (striae and folds in the posterior cornea) that reduced or disappeared during the day depending on the level of edema finally reached. The mean contact lens-induced edema 12 hours after eye opening varied from 1.6% to 5.8% for the various lenses. Considerable individual variations were noticed in the corneal edema response under the conditions of sleep, sleep with lenses, and open eye lens wear. Lenses of higher minus power (greater peripheral lens thickness) but the same center thickness produced greater central corneal edema, indicating that with hydrogel lenses of the same material, average thickness over a central zone of the lens determines swelling response. The levels of edema recorded with the high minus power (—9.00 diopters [D]) low (38.6%) and medium (45%) water content lenses, an average of 13.9% overnight and 5.5% during the day, were cause for concern as obvious signs of stromal edema (striae) persisted with these lenses throughout the study. No decrease in the level of overnight edema (adaptation) was observed during the week of lens wear for any lens. The mean corneal deswelling for the group of subjects over the 12 hours following sleep with these lenses was found to be both limited and reasonably consistent for all lens types (8.2% ± 1.1%). Consequently the level of daytime corneal edema was determined primarily by the overnight swelling. We suggest that the desirable maximum overnight swelling with extended wear contact lenses is 8% as this level of edema allows the cornea to regain normal thickness during the day. Invest Ophthalmol Vis Sci 24:218-226, 1983