The thickness of the normal cornea is relatively constant, varying only a few percentage points throughout the day because of changes in evaporation from the surface. Maintenance of corneal thickness depends on the pump and barrier functions of the epithelium and endothelium, the latter being mainly responsible for corneal dehydration. Our results are in line with those of Liu and Pflugfelder,
25 who studied aqueous tear-deficiency dry eye, and with the findings of Guzey et al.,
26 who studied trachomatous dry eye. They both found central corneal thickness to be decreased. However, the methods used are different. We used CMTF, which has been shown to be a very accurate and precise technique, comparable to pachymetry. In addition, it allows corneal sublayer pachymetry, which is not possible with other in vivo techniques.
27 Liu and Pflugfelder
25 hypothesized that increased production of inflammatory and catabolic cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-1 explains the corneal thinning. Moreover, an abnormally rough ocular surface contributing to increased shear force may mechanically affect corneal thickness. Our findings demonstrate, however, that the thicknesses of the epithelium and Bowman’s layer were normal. Consequently, the thinner cornea in SS must result from the thinning of the stroma. If shear forces play a significant role in thinning, then thinning of the epithelial cell layer would be expected. The present findings may support the idea of an ongoing inflammatory process leading to stromal thinning. Phenomena such as apoptosis
28 29 and increased stromal proteolysis (Selzer M, Afonso A, Monroy D, Lokeshwar B, Pflugfelder SC, ARVO Abstract 3072, 1998)
30 have been attributed to SS.
In the SS group, the apparent IOP was lower than in the control group. The central corneal thickness influences the results of Goldmann applanation tonometry, which may explain our finding. According to a recent meta-analysis by Doughty and Zaman,
31 in eyes with chronic diseases, a 10% difference in central corneal thickness results in 2.5 mm Hg difference in IOP. We noted a 6% (31.5 μm) decrease in the central corneal thickness and 2.8 mm Hg lower mean IOP.