The eyelids are a physical barrier for the eye and are instrumental in replenishing the tear film over the cornea. In this role the eyelids have close contact with the ocular surface, sweeping over it an average of 12 times a minute during waking hours. The forces exerted by the eyelids can influence the regularity of the corneal surface and the eye's optics.
It is known that corneal changes occur from eyelid abnormalities such as chalazia, hemangiomas, ptosis and ectropian. The increased or altered pressure on the cornea from these eyelid conditions primarily alters corneal astigmatism. The pressure from healthy eyelids when the palpebral aperture is narrowed also causes corneal distortions. This occurs with downward gaze when both the upper and lower eyelids move closer to the centre of the cornea. Videokeratoscopes have allowed detailed investigation of the cornea after reading, showing bands of wave-like corneal distortion that run parallel to the eyelid margin. These changes are most easily observed in the tangential power topography maps, which are the most sensitive to localised corneal changes.
While the localised change appears to correspond primarily to the eyelid margin, there is a significant refractive change over the pupil diameter. At the Contact Lens and Visual Optics Laboratory, for visual tasks of 15 minutes duration we have measured corneal changes up to +0.30 / -0.24 x 88 (for 20° downward gaze) and up to +0.87 / - 0.75 x 90 (for a larger downward gaze angle of 40°). While these were the maximum changes observed, it does demonstrate that refractive assessment may need to be qualified in terms of prior visual tasks.
Eyelid-induced corneal changes have been observed after as little as one minute of horizontal eye movements, with their magnitude and regression being approximately proportional to the length of time spent reading. A number of factors have been found to influence the magnitude of eyelid-induced corneal changes including the magnitude of the downward gaze angle and the type of visual task (reading, steady fixation, microscopy and computer work). Rigid contact lens wear during reading has been found to substantially diminish effect of eyelid pressure, while soft contact lenses masked some of eyelid force to the cornea.
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| Figure 1: Tangential power topography maps for a subject after reading for 15 minutes in 40° downward gaze. Pre-task map (top left), post-task map (bottom left), difference (post-task minus pre-task) map (top right) and vertical cross-section of difference map (bottom right). |
While it is known that the eyelids have close contact with the corneal surface, the area of contact between the two surfaces is unknown. The ‘lid-wiper’, an area of the marginal conjunctiva, is thought to be responsible for the distribution of the tear film layer during blinking. A portion of the ‘lid-wiper’ is known as Marx’s line. This narrow line of squamous cells extends along the entire length of the upper and lower eyelids and can be visualised by staining with lissamine green vital dye. The squamous cell phenotype suggests it is a tissue subject to mechanical contact and so may be responsible for the eyelid-induced corneal changes observed after reading.
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| Figure 2: Eyelid anatomy schematic (left) and the upper eyelid everted showing Marx’s line stained with lissamine green (right). |
A condition termed ‘lid-wiper epitheliopathy’ has been coined to describe irregular staining with fluorescein and rose-bengal adjacent to Marx’s line. Some association has been observed between the presence of this staining and the severity of dry-eye in both contact lens wearers and non-contact lens wearers. It has been suggested that this staining may help diagnostically when patients present with dry eye symptoms, but lack other clinical signs. It is thought to occur due to a compromised surface increasing friction between the cornea and eyelids.
A factor that may be critical to both the magnitude of eyelid-induced corneal changes and to lid-wiper staining is eyelid pressure. There have been previous attempts to quantify eyelid pressure, however the reliability of the results is unclear. An eyelid pressure measurement technique has been developed in our laboratory using thin pressure sensors mounted on a contact lens. The quantification of eyelid pressure will enable better understanding of corneal changes observed after reading and the presence of upper eyelid marginal staining associated with dry eye and contact lens wear. Other applications include eyelid reconstructive surgery and the design of soft and rigid contact lenses as the eyelids are critical to the stability and movement of a contact lens on the eye.
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| Figure 3: Front (right panel) and side (left panel) photos of eyelid pressure measurement using a pressure sensor mounted on a specially designed rigid contact lens with plastic support beam. |
Read More:
- Shaw AJ, Collins MJ, Davis BA, Carney LG (2008). Corneal refractive changes due to short-term eyelid pressure in downward gaze
. Journal of Cataract and Refractive Surgery 34(9): 1546-1553. - Shaw AJ, Collins MJ, Davis BA, Carney LG (2009). Eyelid pressure: inferences from corneal topographic changes . Cornea 28(2): 181-188.