Manuscript Review by Kathy Dumbleton

Corneal epithelial homeostasis following daily and overnight contactlens wear
Ladage,P.M. Yumamoto,K. Li,L. Ren,D.H. Petroll,W.M. Jester,J.V.Cavanagh,H.D. Contact Lens and Anterior Eye 2002/00/00//;25(1):11-21.

This important paper summarizes the results from a series ofhuman and rabbit studies which were conducted to investigatethe effect of contact lens wear on corneal epithelial homeostasis.The authors introduce the subject by explaining the role of thecorneal epithelium in defence against infection. Epithelial renewalis vital to the maintenance of corneal health. Cell proliferation,migration and exfoliation are the three crucial mechanisms involvedin this process.

Two experimental and one control lens types were worn in thehuman studies reported; the experimental lenses were a hyperDk/t silicone hydrogel (SH) lens (balafilcon A, Dk/t = 110) anda hyper Dk/t RGP lens (tisilfilcon A, Dk/t = 97); the controlsoft lens was etafilcon A (Dk/t = 32.5). Lenses were worn oneither a daily wear (DW) or extended wear (EW, 6 nights or 30nights) basis in order to assess the effects of DW and EW oncorneal epithelial thickness, cell surface size, exfoliationrates andPseudomonas aeruginosa(PA) binding to exfoliated cells.

In these studies the authors reported that RGP lens wear hadmore of an impact on epithelial thickness and enlargement ofsurface cells than soft lens wear. EW was found to result inepithelial thinning and surface cell enlargement, with relativelymore thinning occurring following low Dk soft lens wear thanSH lens wear. Epithelial thickness did recover partially withlonger term RGP EW. Exfoliation of epithelial cells decreasedwith EW to a similar degree regardless of oxygen transmissibility.While PA binding increased with both soft lens types regardlessof wearing schedule (DW or EW), only EW with RGP lenses resultedin a slight increase in PA numbers. A recovery in epithelialexfoliation and a return to lower levels of PA binding were reportedover time. The length of EW with SH lenses (6 versus 30 nights)was found to have no effect on the epithelial homeostasis assessedin these studies.

In addition to the materials worn in the human studies, twoRGP materials were worn in the rabbit studies, a low Dk/t material(DK/t = 10) and a medium Dk/t material (Tolofocon A, Dk/t = 43).Rabbits wore a lens in one eye only for a period of 24 hoursor had one eyelid sutured for 24 hours during these studies.All rabbits were sacrificed after treatment and the results fromthe experimental eyes were compared with the contralateral controleyes. A series of experiments were conducted on the rabbit eyesto investigate the life cycle of corneal epithelial cells fromthe proliferation of basal cells, their vertical migration, theprogressive signs of apoptosis and exfoliation of the cells.

In these studies, short term contact lens wear with all materialsor eyelid closure resulted in a significant suppression of centralepithelial basal cell proliferation. Cell proliferation at thelimbus was similarly suppressed following RGP lens wear, butremained unaffected by soft contact lens wear or eyelid suturing.RGP contact lens wear also resulted in a delay in the cell differentiationrate and upward migration of the epithelial cells. Normal apoptosisand exfoliation was suppressed by eyelid suturing and with alllens types worn by the rabbits regardless of oxygen transmissibility.

These results support the hypothesis that corneal epithelialhomeostasis resulting in exfoliation is an apoptopic regulatedprocess. The studies also confirm that epithelial surface cellsmigrate centripetally and die principally in the central cornea.This may be attributed to the strong sweeping action of the eyelidcreating high shearing forces at the corneal apex resulting inincreased apoptosis in this region. This theory is further supportedby the results from the rabbit suturing experiments when theaction of blinking was eliminated.

Contact lens wear and eyelid suturing were shown to suppressapoptosis and in the central cornea to allow a concomitant increasein surface epithelial cell size. Despite these restraints tonormal cell turnover, epithelial thickness is known to decreasewith overnight contact lens wear, in all probability as a resultof the effect of decreasing cell proliferation in the basal layersbeing greater than the diminished cell loss at the surface ofthe epithelium. Results from these studies indicate that thedepression of cell proliferation also appears to be both oxygenand lens dependent.

In summary, the authors conclude that while epithelial homeostasisappears to be suppressed by contact lens wear in general, theeffects appear to be diminished to some extent with hyper Dk/tlens materials (RGP and SH). Further research is however requiredto gain a deeper understanding of the mechanisms involved andchronic and adaptive effects which may occur.