Alyra J. Shaw

Eyelid pressure and contact with the ocular surface.

Purpose. To investigate static upper eyelid pressure and contact with the ocular surface in a group of young adult subjects. Methods. Static upper eyelid pressure was measured for 11 subjects using a piezoresistive pressure sensor attached to a rigid contact lens. Measures of eyelid pressure were derived from an active pressure cell (1.14-mm square) beneath the central upper eyelid margin. To investigate the contact region between the upper eyelid and the ocular surface, the authors used pressure-sensitive paper and the lissamine-green staining of Marxs line. These measures, combined with the pressure sensor readings, were used to derive estimates of eyelid pressure. Results. The mean contact width between the eyelids and the ocular surface estimated using pressure-sensitive paper was 0.60 +/- 0.16 mm, whereas the mean width of Marxs line was 0.09 +/- 0.02 mm. The mean central upper eyelid pressure was calculated to be 3.8 +/- 0.7 mm Hg (assuming that the whole pressure cell was loaded), 8.0 +/- 3.4 mm Hg (derived using the pressure-sensitive paper imprint widths), and 55 +/- 26 mm Hg (based on contact widths equivalent to Marxs line). Conclusions. The pressure-sensitive paper measurements suggested that a band of the eyelid margin, significantly larger than the anatomic zone of the eyelid margin known as Marxs line, had primary contact with the ocular surface. Using these measurements as the contact between the eyelid margin and the ocular surface, the authors believe that the mean pressure of 8.0 +/- 3.4 mm Hg is the most reliable estimate of static upper eyelid pressure.

Eyelid pressure: inferences from corneal topographic changes.

Purpose: It is known that eyelid pressure can influence the corneal surface. However, the distribution of eyelid pressure and the eyelid contact area and the biomechanics of the changes are unknown. Although these factors are difficult to directly measure, analysis of eyelid-induced corneal topographic changes and eyelid morphometry enables some inferences to be drawn. Methods: Eighteen subjects, aged between 19 and 29 years, with normal ocular health were recruited. Corneal topographic changes were measured after 4 conditions consisting of 2 downward gaze angles (20 and 40 degrees) and 2 types of visual tasks (reading and steady fixation). Digital photography recorded the width of Marx line, the assumed region of primary eyelid contact with the cornea. Results: Significantly larger corneal changes were found after the 40-degree downward gaze conditions compared with 20-degree conditions because of the upper eyelid contact (P < 0.001). For the 40-degree downward gaze tasks, the lower eyelid changes were greater than those because of the upper eyelid (P < 0.01). The upper eyelid Marx line width was associated with the amplitude of corneal change (R2 = 0.32, P < 0.05). Conclusions: Analysis of the corneal topographic changes gives insight into the pressure applied by the upper and lower eyelids in different situations. These include greater upper eyelid pressure with increasing downward gaze and greater lower eyelid pressure compared with the upper eyelid in 40-degree downward gaze. There was some evidence that supports Marx line as the primary site of contact between the eyelid margins and the cornea.

Corneal refractive changes due to short-term eyelid pressure in downward gaze.

PURPOSE: To assess corneal refractive changes after 15‐minute visual tasks and their association with eyelid morphology. SETTING: Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. METHODS: Eighteen young subjects with normal ocular health were recruited. Corneal topography was measured with a videokeratoscope before and after 4 conditions consisting of 2 downward gaze angles (20 degrees and 40 degrees) and 2 types of visual tasks (reading and steady fixation). Anterior eye photography in downward gaze was used to determine the eyelid angle, tilt, and position with respect to the cornea. RESULTS: Corneal refractive power changed significantly after the 15‐minute downward gaze tasks. The largest mean corneal spherocylindrical change was +0.33 −0.30 × 84 after reading in the 40‐degree downward gaze (4.0 mm corneal diameter). The refractive changes were significantly larger after the 40‐degree tasks than after the 20‐degree tasks (P<.001). The changes in refractive root‐mean‐square error were significant for all conditions, except the 20‐degree steady fixation task, with 4.0 and 6.0 mm analysis diameters (P<.05). Significant correlations were found between some aspects of eyelid morphometry and corneal refractive change. CONCLUSIONS: The pressure of the eyelids on the cornea in short‐term downward gaze resulted in optically and clinically relevant corneal changes. Correlation between the refractive corneal changes and eyelid parameters suggests that the angle, shape, and position of the eyelids influence the nature of the corneal changes. When high accuracy is required, refraction should be qualified by the visual tasks undertaken before assessment.

Using optical coherence tomography to assess corneoscleral morphology after soft contact lens wear.

Purpose. To evaluate the use of optical coherence tomography (OCT) for assessing the effect of different soft contact lenses on corneoscleral morphology. Methods. Ten subjects had anterior segment OCT B-scans taken in the morning and again after 6 h of soft contact lens wear. For each subject, three different contact lenses were used in the right eye on non-consecutive days, including a hydrogel sphere, a silicone hydrogel sphere, and a silicone hydrogel toric. After image registration and layer segmentation, analyses were performed of the first hyper-reflective layer (HRL), the epithelial basement membrane (EBL), and the epithelial thickness (HRL to EBL). A root mean square difference (RMSD) of the layer profiles and the thickness change between the morning and afternoon measurements were used to assess the effect of the contact lens on the corneoscleral morphology. Results. The soft contact lenses had a statistically significant effect on the morphology of the anterior segment layers (p < 0.001). The average amounts of change for the three lenses (average RMSD values) for the corneal region were lower (3.93 ± 1.95 &mgr;m for the HRL and 4.02 ± 2.14 &mgr;m for the EBL) than those measured in the limbal/scleral region (11.24 ± 6.21 &mgr;m for the HRL and 12.61 ± 6.42 &mgr;m for the EBL). Similarly, averaged across the three lenses, the RMSD in epithelial thickness was lower in the cornea (2.84 ± 0.84 &mgr;m) than the limbal/scleral (5.47 ± 1.71 &mgr;m) region. Post hoc analysis showed that ocular surface changes were significantly smaller with the silicone hydrogel sphere lens than both the silicone hydrogel toric (p < 0.005) and hydrogel sphere (p < 0.02) for the combined HRL and EBL data. Conclusions. In this preliminary study, we have shown that soft contact lenses can produce small but significant changes in the morphology of the limbal/scleral region and that OCT technology is useful in assessing these changes. The clinical significance of these changes is yet to be determined.

A Technique to Measure Eyelid Pressure Using Piezoresistive Sensors

In this paper, novel procedures were developed using a thin (0.17 mm) tactile piezoresistive pressure sensor mounted on a rigid contact lens to measure upper eyelid pressure. A hydrostatic calibration system was constructed, and the influence of conditioning (prestressing), drift (continued increasing response with a static load), and temperature variations on the response of the sensor were examined. To optimally position the sensor-contact lens combination under the upper eyelid margin, an in vivo measurement apparatus was constructed. Calibration gave a linear relationship between raw sensor output and actual pressure units for loads between 1 and 10 mmHg (R 2 = 0.96 ). Conditioning the sensor prior to use regulated the measurement response, and sensor output stabilized about 10 s after loading. While sensor output drifts slightly over several hours, it was not significant beyond the measurement time of 1 min used for eyelid pressure. The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. Eyelid pressure readings were observed when the upper eyelid was placed on the sensor, and removed during a recording. When the eyelid pressure was increased by pulling the lids tighter against the eye, the readings from the sensor significantly increased.