Pellucid marginal corneal degeneration: evaluation of the corneal surface and contact lens fitting

AIM: To quantify corneal irregularities, to describe the fitting with contact lenses, and to answer the question whether or not contact lenses with a special back surface design could improve visual acuity in patients with pellucid marginal corneal degeneration (PMCD). METHODS: 13 eyes were fitted with contact lenses with a special back surface. Videokeratographic data were assessed. The patients were followed up for an average period of 22.2 months. Lens tolerance and corrected visual acuity were evaluated. RESULTS: The mean eccentricity did not exceed 0.7 in all patients. Either the superior or the inferior eccentricity, or both, were negative in all patients. Using Fourier analysis all PMCD subjects showed an increased irregular astigmatism of the anterior cornea. Using Zernike coefficients seven eyes (53.8%) had a higher order aberration root mean square error (HOA RMS error) out of the normal range. The visual acuity with contact lenses improved in all eyes with an average increase of 2.7 lines (maximum eight lines). No serious complications were observed. CONCLUSIONS: Quantitative evaluation of videokeratographic data may help to diagnose PMCD and to distinguish PMCD from other ectatic corneal diseases. Contact lenses with a special back surface design can improve visual acuity and lens tolerance

Regression of corneal topography changes following reading

Purpose Reading causes the corneal shape to change. We studied the time course of recovery of corneal topography changes following various periods of reading. Methods Six young subjects with normal ocular health were recruited for the study. Experiments were conducted early in the morning on four separate days and the subjects were instructed to perform no significant reading tasks prior to the experiment. At each session, baseline corneal shape was measured by taking six videokeratographs and averaging the maps. The subject then read a novel for 10, 30, 60 or 120 mins. Following the reading task, videokeratoscopy was measured at 17 pre–defined time intervals up to 3 hrs following the reading task. Six videokeratographs were acquired at each of these time points during the recovery phase and later averaged. During the recovery period the subjects were not permitted to perform any intensive reading or near work. Analysis took the form of comparison of the corneal topography pre–reading versus post–reading during the recovery phase. Results All six subjects showed statistically significant changes in the cornea immediately following all four reading conditions (10 to 120 mins). In the corneal region corrsponding to the position of the upper lid margin, the group average maximum change in local refractive power was 0.94 D (+/–0.44) for the 120 min reading condition, 0.64 D (+/–0.31) for the 60 min condition, 0.60 D (+/–0.28) for the 30 min condition and 0.44 D (+/–0.42) for the 10 min reading condition. A one–way repeated measures ANOVA showed that the regression of corneal power over time was highly significant for all conditions (p=0.001). Plotting the regression of corneal power following reading showed a rapid recovery of shape following all 4 reading conditions during the initial 10 mins, with a subsequent slow recovery phase. As a generalization of the regression trends, we found that the topography was almost completely recovered to pre–reading shape after as many minutes as the subject had previously been reading. For example, 120 mins of reading required approximately 120 mins for full corneal shape recovery. Conclusions Corneal topography and optics change following reading and the magnitude of the changes are related to the length of time spent reading. The regression of these changes have an initial rapid phase of approximately 10 mins followed by a slower recovery phase