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A. J. Shaw, M. J. Collins, B. Davis, L. G. Carney; Eyelid Pressure: Inferences From Corneal Topography Changes. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1026. doi: https://doi.org/.
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The cornea is the principal optical element of the eye so the regularity of its surface topography is critical for visual optics. However the cornea is known to be susceptible to forces exerted by the eyelids. These corneal changes are bands of ‘wave-like’ change that are parallel to the position of the eyelid margin. There is little known about eyelid pressure on the cornea which is dependent on eyelid force and the contact area. By analysing the depth and width of corneal topography changes after various downward gaze tasks, inferences could be drawn about upper and lower eyelid pressure.
Corneal topography changes due to eyelid pressure were measured using the Medmont E300 Corneal Topographer (Medmont Pty. Ltd. Victoria, Australia), for eighteen subjects aged between 18 and 29 years. Four conditions were considered, consisting of two downward gaze angles (20° and 40°) and two visual tasks (reading and staring). The amplitude and width of the ‘wave-like’ changes were analysed for each of the four conditions and for both upper and lower eyelids. Anterior eye digital photography was used to determine the position of the eyelids in downward gaze and the width of Marx's line.
For each condition the average peak-to-valley amplitudes of corneal change were between 1.4 and 2.4 µm. For the upper eyelid, the downward gaze angle magnitude had a significant impact on the peak-to-valley amplitude (p<0.001), with corneal changes after the 40° tasks being 25% greater than after the 20° tasks. The topographical changes showed a characteristic ‘wave-like’ pattern, with an outer peak, a valley and an inner peak (closer to corneal centre). The upper eyelid produced a larger outer peak compared to the inner peak (p<0.001). The corneal changes after the 40° downward gaze tasks were greater for the lower eyelid than for the upper eyelid (p<0.01). The amplitude of corneal change produced by the upper eyelid was associated with the width of Marx's line (R2=0.32, p<0.05).
Analysis of the eyelid-induced corneal topography changes gives insight into the eyelid pressure in different situations. The upper eyelid seems to exert greater pressure on the cornea in larger downward gaze angles. The asymmetrical surface shape (outer versus inner peaks) suggests that the upper eyelid is angled when in contact with the cornea. In 40° downward gaze, it can be inferred that the lower eyelid exerts greater pressure on the cornea than the upper eyelid. There was some evidence that Marx's line is the site of frictional contact between the eyelids and the cornea.
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