Corneal topography affected by external factors

External factors can make data acquisition difficult during corneal topography examinations, according to a study published in the March issue of Eye & Contact Lens.

The study, led by Jose Manuel Gonzalez-Meijome, MD set out to analyse the factors affecting data acquisition during corneal topography examination with the Medmont E-300 videokeratoscope and to provide strategies to minimize their effects.

Sixty eyes from 30 young adults were examined, whilst a second observer recorded incidences that had the potential to affect data acquisition. These factors were correlated with difficulty of measurements as judged by the practitioner who performed the examination. Measurements of axial curvature were also analysed to evaluate the variability expressed as intrasession and intersession coefficient of variation and the standard error of the mean (SEM).

The results of the study indicated that the level of difficulty was generally low, with 70% of eyes being easy or very easy to measure. Corneal topography measurements were difficult in 27% of the remaining eyes or very difficult (3%). Only fixation instability (p<0.001) and the need for head repositioning (p=0.024) were associated with a higher level of difficulty. External factors were also shown to affect the variability of measurements at certain corneal locations, particularly in the vertical meridian when related to tear instability and in the horizontal meridian when related to repositioning on the chin rest owing to physiognomy interferences with the keratoscope cone.

The study concluded that the level of subjective difficulty found during videokeratoscopy examination is correlated strongly with fixation instability.