- The story behind Corneal Ectasia -
Rob Davis OD FAAO
The corneal epithelium is the outer cover of Bowman’s membrane and the corneal stroma. The corneal epithelium gets its nutrients from the tears, which also reduce eyelid insult and external environment factors. The critical Meibomian gland lubrication plays an active role in the vitality of the corneal epithelium. This tear-epithelium relationship is poorly understood. Does the epithelium communicate its needs to the tears? Is the system passive or active, changing the tear constituents based on the changing environment? The average thickness of the normal corneal tear film is 7 microns; the corneal epithelium is 50 microns, Bowman’s membrane is 10 microns, the stroma is 485 microns, Descemet’s membrane is 10 microns and the endothelium is 5 microns thick.
Corneal (top) and epithelial thickness (bottom) map
Local Epithelial Thickness Difference
The epithelium is thinner in the superior region compared to the inferior. The eyelid blinks 300 to 1500 times an hour, chafing the superior surface during the blink. The eyelid generates greater frictional force applied to the superior cornea than to the inferior cornea. As the upper lid reaches its maximum speed about the time that it crosses the visual axis, the lid skims across the inferior part of the cornea. The temporal epithelium is also thinner than the nasal due to friction during lid closure; it is greater on the temporal side than on the nasal side because the outer canthus is higher than the inner canthus and the temporal portion of the lid is higher than the nasal side.
Helen Swarbrick was the first to observe corneal changes induced by orthokeratology, redistributing or remodeling the anterior epithelium. Our research facility has observed epithelium thickness variations because of the irregularity of the stroma beneath the epithelium, especially in ectatic diseases.
Corneal Ectasia and Epithelial Thickness
Corneal ectasia is a class of diseases that have a single hallmark characteristic: corneal thinning. The vast array of signs and symptoms are created by the changes within the corneal structure. Corneal topography that looks like pellucid marginal degeneration (PMD), keratoconus, Terrien’s marginal degeneration or keratoglobus without corneal thinning is not ectasia but corneal irregularity. Below is an example of a PMD look-alike corneal topography. But taking a closer look (pachymetry map below topography map), the thickness of this cornea ranges from 585 microns superiorly to 480 inferiorly after removing the epithelium thickness. Refracting this patient results in 20/20 monocular visual acuity because the epithelium smoothes out the corneal irregularity within the pupil zone.
PMD look-alike corneal topography (top) and epithelial thickness map (bottom)
Corneal topography only describes the shape of the cornea, but many practitioners use topography to diagnose ectatic disease. In ectatic disease, the corneal stroma can no longer support the corneal structure, and the tissue begins to bend, causing the shape of the cornea to distort. The epithelium will redistribute itself in an attempt to lend support to the cornea. The epithelium smoothes over the stromal irregularities by becoming thicker over the depressions and thinner over the elevations. Consequently, the epithelium camouflages the changes occurring in the stroma. In addition, in the early stages of ectatic disease, the distorted refractive error is neutralized by the epithelium. By measuring the epithelial thickness, clinicians can identify subclinical corneal ectasia. As the stroma continues to thin, the epithelium can no longer cover up the ectatic stroma, causing redistribution around the base of the ectasia and lending supportive strength to the corneal structure.
Corneal ectasia - corneal topography (top) and epithelial thickness map (bottom)
You Can’t Judge a Book by its Cover
In ectasia, the first corneal layer that begins to change is the stroma, which initiates a response from the epithelium to try to neutralize the changes occurring within the stromal layer. What type of communication exists between the two layers? As the stroma continues to thin, the epithelium can no longer support the stroma, which causes the cornea to increase in sagittal height. Visual acuity becomes distorted from the change in shape, and the topography begins to show irregularity in shape. The epithelium begins to redistribute itself, thickening around the base to try to strengthen the corneal structure. These observations resulted from viewing hundreds of thickness images and represent only a theoretical construct that needs validation from scientific-based investigations. Stay tuned – this may revolutionize the way we look at corneal topography and corneal ectasia.
Corneal (top) and epithelial thickness (bottom) map with epithelial redistribution
Dr. Robert L. Davis practices in Oak Lawn Illinois and is director of the Contact Lens Clinic at Davis EyeCare.
He is also co-founder of EyeVis, Eye and Vision Research Institute developing novel contact lens designs as well as forging the knowledge of anterior segment pathophysiology. He is an adjunct faculty member at Southern California College of Optometry - Marshall B. Ketchum University, University of Missouri in St. Louis, Illinois College of Optometry, Pennsylvania College of Optometry at Salus University and Pacific University.
Dr. Davis has been recognized as a Diplomate in the Corneal, Contact Lens and Refractive Technology section of the American Academy of Optometry and as well as an inductee in National Academy Practice in Optometry. He has held leadership roles with the American Optometric Association, American Academy of Optometry, National Academy of Practice and other professional organizations.
Dr. Davis has published and lectured on topics related to Contact Lenses, Myopia Control, and the management of eye disorders.