Contact Lens Practice Pearls
Corneal GP Lenses Still Have Value for Irregular Corneas
BY GREGORY J. NIXON, OD, FAAO
The advent of new scleral lens designs in an array of diameters that are available in high-oxygen permeable materials has dramatically expanded their use in managing keratoconus, pellucid marginal degeneration (PMD), post-surgical fitting, and other irregular cornea conditions.
While scleral lens designs have a strong niche, there are still times when it is advantageous to address these challenging patients with a corneal GP lens.
Advantages and Disadvantages of Scleral Lenses
The main advantage of a scleral lens is the ability to completely vault over an irregular corneal surface and land on the normal conjunctival tissue to improve lens centration and stability. This is of particular benefit when a corneal GP is unable to maintain centration on a highly asymmetric corneal surface and on those that have more peripheral irregularities such as a large decentered cone or PMD.
While scleral lenses are often the preferred fitting option in these cases, they introduce a number of their own unique fitting challenges and post-fit complications. Namely, conjunctival impingement can hinder tear exchange, resulting in post-lens mucus and debris. This can lead to reduced vision from filming and fogging of the lens. But more importantly, this can generate hypoxic stress to the cornea, resulting in edema and neovascularization. Stimulation of these angiogenic factors is not conducive to maintaining long-term corneal health and is particularly troubling for increasing the risk of graft rejection in a post-penetrating keratoplasty eye.
Making a Case for Corneal GPs
When a corneal GP lens is able to achieve good centration, complications from tear exchange, posterior surface lens deposition, and hypoxia are significantly diminished compared to with a scleral lens. Therefore, when I approach a keratoconic lens fit, the first question I ask is: “Can I center a GP on this eye?” This is almost always dictated by the corneal topography that details the size, severity, and location of the cone. A small-to-moderate size cone with its apex within the central 6mm of the cornea has a good probability of being successfully fit with a corneal GP lens.
The goal is to achieve apical clearance or “feather touch” to minimize corneal agitation that might promote scarring. Contemporary corneal GP lenses have diameters that typically range from 9.2mm to 11.0mm and can have a variety of custom-designed spherical or aspherical posterior surfaces. Some lens manufacturers have unique proprietary designs, including quadrant-specific curvatures, that can be highly effective in conforming to the complex curvatures of irregular corneas. Regardless of which corneal GP design is used, avoiding direct contact with, or coverage of, sensitive limbal stem cells helps avoid some complications that can occur with scleral lenses.
The array of corneal GP, intralimbal, and scleral lenses available provide practitioners with a wealth of treatment options to aid complex contact lens fitting. Achieving adequate lens centration and stability is paramount to maximizing a successful fit for irregular corneas. While a scleral lens can often provide this centration, I would encourage the use of corneal GPs on eyes that have smaller and more central irregularities to minimize the long-term ocular health complications that can occur from bridging the limbus. CLS
Dr. Nixon is a professor of clinical optometry and director of extern programs at The Ohio State University College of Optometry. He is also in a group private practice in Westerville, Ohio. He is on the Allergan Academic Advisory Board, the B+L Advisory Board, the Alcon Glaucoma Advisory Board, and the Alcon Speakers Bureau. You can reach him at email@example.com.