Scale and spatial distribution of aberrations associated with tear breakup.

PURPOSE: We examined the spatial correlation between tear breakup (TBU) and the associated optical anomalies on multiple spatial scales. METHODS: Five subjects refrained from blinking while the time course and patterns of TBU were sequentially observed using fluorescein, retroillumination, and Shack-Hartmann (SH) aberrometry. Wavefront error maps were developed using Zernike polynomials, as well as local zonal analysis of measured wavefront slopes. The difference between these maps reveals the presence of very high-order aberrations missed by standard modal fitting methods. Size of SH spots was also quantified to estimate optical perturbations on a microscopic scale. The spatial correlation between TBU and optical aberrations was also computed. RESULTS: Degradation of the tear film increased wavefront aberrations over all spatial scales measured. Consistent with tear thinning, blink suppression induced an irregular pattern of phase advances in regions of TBU. SH spot size also increased in regions of TBU, which indicates the presence of optical aberrations on a scale smaller than individual lenslets. CONCLUSIONS: The optical signature of TBU caused by blink suppression is a combination of wavefront aberrations on macroscopic and microscopic scales due to non-uniform tear film thinning and possible exposure of a rough epithelial surface. Localized optical defects correspond temporally and spatially with TBU revealed by fluorescein and retroillumination. In addition to gross wavefront aberrations, scatter develops in areas of TBU that will further contribute to image degradation and visual disturbances after TBU.

Kinetic analysis of topographical parameters and interference pattern of tear lipid layer in normal subjects.

PURPOSE: To investigate the post-blink changes of the topographical regularity indices, the corneal wavefront aberrations and the tear lipid layer's interference pattern, and analyze the relationship between these parameters' changes. METHODS: Sequential topographical measurements were recorded on 23 normal participants. The surface regularity index (SRI), surface asymmetry index (SAI), corneal total higher order aberrations (HOA) root mean square (RMS), Coma RMS and Spherical RMS were calculated from the topographical measurement. The incidence of the type 1 (with an initial decrease after blink) alteration trend, and the times to the initial minimum were determined in every parameter. The movement of the tear lipid layer's interference pattern was recorded, and the spreading time of the lipid layer was measured subjectively by three independent observers. RESULTS: The incidence of the type 1 SRI trend (82%) was the most common among those measured. The average time to the type 1 SRI trends' minimum was 4.05 ± 1.46 s. There was no correlation between the time to the minimum of type 1 trends of the topographical indices, the HOA, Coma and Spherical RMS. The mean lipid layer spreading time was 5.09 ± 1.90 s. We found weak but significant correlation between the times to the minimum of type 1 SRI trends and the lipid spreading time (r = 0.540, p = 0.021).The minimum of the SRI index occurred before the stoppage of superficial lipid layer's motion. CONCLUSION: Relatively long lipid layer spreading time was observed suggesting that the tear film build-up process did not finish completely at the end of the initial rapid tear film alteration phase. The detected initial decrease of the SRI after blink related to the post-blink tear film motion. This initial pattern is characteristic of the majority of the normal population in spite of the post-blink alteration pattern of the topographical SAI or corneal aberrations. The SRI seems to be the most useful indirect topographical parameter of the tear film dynamics.