Factors Affecting Repeatability of Peripheral Corneal Topography Measures

Abstract

Abstract: : Purpose: Repeatability of peripheral corneal topography measures is important in clinical practice to evaluate the progression of corneal distortions. Several studies addressed the repeatability at the corneal center, but there are not references on the consistency of peripheral measures, either within one session or between different sessions. Methods: Sixty eyes from thirty young adults (9 males, 21 females), aged 21 +/– 2,46 years, where evaluated with the Medmont E300 corneal topographer. Three readings were obtained in each of three different sessions at weekly intervals by an experienced observer. Nine locations on the central 3,5 mm where examined. While measuring, a second observer recorded incidences such as lack of fixation, need of repositioning on the chin–rest or tear instability. Results: No differences were observed for either the intrasession or intersession repeatabilities with gender or eye examined. At the center, each individual measurement within one session was not different from the mean value (mean diff.<0,03D; p>0,05; r²>0,99). The same happened between sessions (mean diff.<0,025D; p>0,05; r²>0,98). For peripheral measurements, all locations displayed significant differences between readings either intrasession or intersessions (p<0,05). Those differences were smaller than 0,01D when comparing each measurement against both the mean value within the same session (range 0,01 to 0,04D) and between different sessions (range 0,02 to 0,09D). Only differences for the most peripheral superior location were higher than 0,01D. Comparing single values against the mean of three within the same session, the maximum difference was found for the most peripheral superior measurement. Fixation stability did not play a significant effect on repeatability. Tear instability showed an effect, particularly at the most peripheral superior location. Head relocation was needed in 43% of the eyes due to facial physiognomy–related problems. Severe and moderate tear instability was present in 17% and 55% of the eyes, respectively. Conclusions: This study is of great importance for any clinical research involving corneal topography evaluation. Although not clinically significant even for the most stringent criteria, there was some evidence that consistency of peripheral topography measurements could be adversely affected by external parameters as tear film instability or the need of head repositioning on the chin–rest.

Keywords: topography • refractive surgery: corneal topography • contact lens