|Normal topography from ectatic stromal thickness.|
|Etched zone lines on the DigiForm lens improve ability to modify at exact location.|
Fitting contact lenses can be a challenge when it involves a patient with an irregular cornea, but the latest lenses and tools can prevent dropouts.
Defining irregular corneas seems to be an easy task. But is it? If the distortion is located outside the pupil area, visual acuity is unaffected. Keratoconus, pellucid marginal degeneration, keratoglobus, posterior keratoconus and post-refractive disorders collectively make up the family of non-inflammatory corneal ectatic diseases causing irregular corneas. Thinning disorders, including Terrien’s marginal degeneration, dellen and inflammatory melts, cause localized corneal irregularities but are not ectatic diseases. All of these disorders create a change of the corneal structure affected by the intraocular pressure within the globe to alter its shape.
Studies have indicated that the corneal epithelium can mold itself in the early stages of cornea irregularity to hide the underlying stroma that is becoming thin and irregular. The central cornea thickness is normally 535 microns. With the tear meniscus being 3 microns, epithelium 54 microns, Bowman’s 8-10 microns, stroma 480 microns, Descemet’s 3-13 microns, endothelium 10 microns, the redistribution of the corneal structures during corneal ectatic disease initially is undetected due to the thickening of the corneal epithelium masking the thinning of the corneal stroma. Topography seems normal, whereas the underlying cornea is abnormal. In those areas of thinner-than-average cornea stroma the epithelium becomes thicker than average. It is only when the protrusion of the cornea stroma becomes significant the epithelium has no alternative but to become thin from the increased sagittal height of the cornea.
Discussions in clinical literature cite the plasticity of the epithelium with ectatic corneas that a thicker-than-normal central cornea epithelium is an early warning sign of ectatic disease that is trying to neutralize the effect of inferior steepening. The epithelium seems to compensate or smooth out the irregularities of the cornea’s underlying layers. The outcome of this epithelium redistribution is a normal topography and near normal visual acuity. Measuring the epithelium separately from the stroma and exposing this abnormality will diagnose the need of corneal crosslinking before excessive corneal irregularity has a chance to reduce the quality of life.
Posterior keratoconus is another entity where the ectatic disease is a result of the irregularity on the posterior surface of the cornea whereas the anterior surface may be normal. Posterior keratoconus is a non-inflammatory thinning of the cornea, resulting in an anterior bulge of the posterior surface. The anterior surface remains regular and the irregularity is created on the posterior side of the cornea. Descemet’s membrane, particularly in the area of stromal thinning, demonstrates abnormal irregular thickness due to the structural stress.
The corneal structure no longer can support itself and begins to bend either in the anterior or posterior direction, causing the cornea to distort. This can be seen by measuring the difference between the superior versus inferior power difference known as the I-S value. A power difference greater than 1.4D is an indicator of corneal irregularity. The I”“S value is derived by calculating the difference between inferior and superior corneal curvature measurements at a defined set of five points above and below the horizontal meridian. Typically, the normal cornea curvature is very symmetrical. The inferior curvature map is a replication of the superior curvature map. The same can be said when comparing the nasal and temporal curvature map. When comparing the difference in corneal power between corresponding meridian points 180Â° apart for symmetry is known as Surface Asymmetry Index (SAI). If the SAI approaches zero, the cornea has perfect radially surface symmetry. When the deviation increases, it is an indication of corneal asymmetric and irregularity. Pachymetry is another indicator of ectatic disease. Normal corneas are significantly thicker peripherally than centrally (by approximately 50″“60 Âµm), and corneas that are not thicker peripherally suggest an ectatic disorder.
Using the above scenarios is valuable when prescribing specialized contact lenses for irregular corneas. Looking at the power range within the pupil is a good indicator of the type of contact lens that will produce a successful outcome. As the cornea irregularity progresses, the goal of contact lens therapy is to provide visual rehabilitation. Custom soft toric lenses, such as SpecialEyes 54, have been successful when the lens thickness is used to mask the corneal irregularity. Using the I-S and the SAI values to determine the amount of asymmetry will determine when these lenses are the best approach. When these values increase due to corneal asymmetry, hybrid lenses, corneal gas permeable lenses and gas permeable scleral values are necessary to mask the increasing irregular cornea.
Gas permeable corneal contact lenses, such as those from Rose K (from authorized manufacturers ABB Optical, Art Optical and Blanchard Contact Lens), have been the gold standard for irregular corneas. When the asymmetry increases, it becomes difficult to design a contact lens to float on tears when the cornea resembles a cliff. The best approach is to use a hybrid lens or scleral lens design that vaults over the irregular section of the cornea and lands on a more regular portion of the sclera. Sagittal height is another corneal parameter that will help determine the best contact lens approach. By looking at an elevation map, the rate of flattening as the cornea approaches the cornea (eccentricity) becomes apparent and will help in selecting the contact lens design.
Scleral contact lenses, such as DigiForm by TruForm Optics and Zenlens from Alden Optical, not only can solve the many optical and fitting problems of other contact lens modality failures, but they are also extremely helpful in dry eye patients with complications from Sjogren’s syndrome, graft versus host disease and other dry eye anomalies. Prescribing scleral lens designs are not without their own problems of handling, tight lens syndrome and discomfort. Many patients have to take their lenses out after a few hours to refresh the tears underneath the lens suffering from dry eye syndrome. Practitioners can communicate more effectively with fabricators with markings on the scleral lens to increase or decrease sagittal height relationships at exact locations. (See the etched zone lines on the DigiForm lens pictured below.)
Tools for prescribing contact lenses, such as topography, Oculus Pentacam, Ocular Response Analyzer from Reichert, Oculus Keratograph 5M, ocular coherence tomography, contact lens simulators and contact lens calculators, allow for significantly more accurate contact lens evaluations, which translates to superior lens design selections and ultimately successful contact lens outcomes. As additional tools become available, we can expect fewer dropouts by improving our ability to select lens designs from diagnostic data interpreting from these advanced ophthalmic devices.
Robert L. Davis, OD, FAAO, is in private practice in Oak Lawn, IL.
WHERE TO FIND IT:
ABB Optical Group
800.852.8089 | abboptical.com
Art Optical Contact Lens, Inc.
800.253.9364 | artoptical.com
Blanchard Contact Lens, Inc.
800.367.4009 | blanchardlab.com
716.686.4500 | reichert.com
866.404.1060 | specialeyesqc.com
TruForm Optics Inc.
800.792.1095 | tfoptics.com