Measurement of the time course of optical quality and visual deterioration during tear break-up

Abstract

Purpose: To compare changes in optical quality and visual performance that accompany tear break-up (TBU) during blink suppression.

Methods: A three-channel optical system was developed that simultaneously measured refractive aberrations (Shack-Hartmann aberrometer), 20/40 letter contrast sensitivity (CS), and TBU (retroillumination, RI). Ten wearers of silicone hydrogel contact lenses were asked to keep one eye open for approximately 18 seconds, while CS, wavefront aberrations, and RI images were collected. The wavefront was reconstructed by zonal methods, and image quality was quantified with a series of metrics including RMS fit error. Novel metrics for quantifying TBU over the contact lens surface were developed by quantifying the contrast of the RI image and by using Fourier descriptors of the first Purkinje (PJ) image shape.

Results: There was a full range of TBU over the lens surface, with four subjects showing TBU across the corneal center and one subject with TBU in the inferior peripheral pupil. Among the four subjects with central corneal TBU, RMS fit error, RI contrast, and PJ Fourier descriptors showed high correlation with CS (r(2) range, 0.9187-0.9414, 0.6261-0.975, and 0.4917-0.8986, respectively). Some of the general optical-quality metrics such as blur strength, neural sharpness, and area of modulation transfer function (MTF) also showed that change correlated with CS loss.

Conclusions: Optical metrics of tear quality and retinal image quality are associated with the decline in vision that occurs with TBU. The evidence supports the hypothesis that blurry vision symptoms reported by contact lens wearers are caused by poor quality of the retinal image due to TBU.

Figure 1.

The optical quality system. The wavefront sensor and iris camera are components of the aberrometer. The IR LED and visual target are external, custom modifications. Solid lines: the path of rays for images conjugate to the retina. Dotted lines: the path of rays for images conjugate to the iris. Dashed line: the path of the laser probe (drawing is not to scale).

Figure 2.

Comparison of the modal and zonal wavefront reconstruction methods. (A–G) Images collected immediately after a blink; (H–N) images collected after 18 seconds of blink suppression. (A, H) The raw S-H images. (B, I) The modal reconstructed wavefront error maps; (C, G) the zonal reconstructed wavefront error maps; (D, K) the residual zonal wavefront after subtracting the map produced by modal fitting. The RMS of the residual wavefront is known as the RMS fit error, which is shown on the bottom right corner of (D) and (K). (E–G, L–N) The point spread function (PSF) generated from each corresponding wavefront.

Figure 3.

A time-series collected from one trial. Top row: four raw S-H images collected at 0, 4.4, 10.6, and 16.7 seconds during a blink-suppression trial. Rows 2 and 3: the corresponding zonal reconstructed WFE map and RI images, respectively. The RMS fit error of each wavefront (in microradians) is show on the bottom left corner of each image in the second row. The fourth row shows the simulated retinal image for a 20/40 letter E derived from the wavefront map shown in row 2. Bottom row: the simultaneously measured CS.

Figure 4.

(A, B) The changes in the RMSs optical quality metric with time; (C, D) the changes in the RMS fit error with time in subjects who experienced no tear break-up (A, C), and subjects with tear break-up (B, D). For each group, different symbols represent different subjects. (B, D, right) The WFE map and RI image observed immediately before blinking for the five subjects with tear break-up.

Figure 5.

Time-series for two subjects showing the RI and PJ changes during blink suppression. Four RI images collected at 0, 4.8 10.9, and 17.1 seconds after a blink are shown for subject 3 with tear break-up (top row) and subject 2 with no tear break-up (second row). Third and fourth rows: plots of the changes in RI image contrast and PJ Fourier descriptors during the same trials.

Figure 6.

Scatterplot showing the covariance of changes in RI contrast and PJ Fourier descriptors with changes in the wavefront RMS fit error metric. The numbers under each pair of symbols reflect the increasing time during which data were collected in the trial.

Figure 7.

The percentage of change from the beginning of the trial in CS of a 20/40 letter and 12 cyc/deg retinal image contrast are plotted as a function of time throughout a single trial for two subjects who experienced tear break-up (A, B). (C, D) Data replotted with the MTF data, delayed by 7.5 and 4.5 seconds. The percentage changes in CS and MTF are computed as MTF(t)/MTF(t₀) and CS(t)/CS(t₀)

Figure 8.

The decrease in optical quality is plotted as a function of the increase in contrast thresholds that accompanied tear break-up from a single trial. Three image-quality metrics are computed: B_ave (A), NS (B) and area of MTF (C). The change in contrast threshold is computed as log(CTt/CTt₀), and the changes in each optical quality metric is computed as the difference in each metric from t₀.

Figure 9.

Change in three tear optical quality metrics, RMS fit error, RI image contrast, and Fourier descriptor are plotted as a function of the change in contrast threshold in one subject in a single trial. The change in contrast threshold is computed as log(CTt/CTt₀). The changes in TOQMs are computed as (TOQMt − TOQMt₀)/TOQMt₀. Linear fits are shown with the R² values beside each.