Contact lens management

Four Strategies For Keratoconus

Make your next keratoconus fitting a rewarding experience.

GREGORY W. DENAEYER O.D.

Restoring visual function to a keratoconic eye with a contact lens can be a life changing event for the patient and a rewarding experience for the practitioner. However, designing a lens that provides optimal vision, fits well, and is comfortable can be a challenging endeavor. This article will highlight four strategies that you can use to successfully fit a keratoconus patient.

Specialty soft contact lenses

Most practitioners equate keratoconus fitting with gas-permeable (GP) lenses, which inherently mask corneal irregularity. However, remember that keratoconus encompasses a broad range of expression. Patients that have mild keratoconus, with steep keratometry readings in the mid-to-high 40D range, may be successful with soft contact lenses. Numerous companies now manufacture specialty contact lenses, which include several design features that can increase fitting success for those patients with keratoconus. First, these lenses are available with base curves in the 6-7mm range to align the steep area, and a relatively flatter paracentral curve in the 8mm range to match the high eccentricity of a keratoconus eye.

Secondly, the center thickness of the lens can be increased, sometimes up to 3mm, in order to help mask corneal irregularity. Finally, custom amounts of toricity can be added to the power to maximize acuity. I recommend fitting and over-refracting with trial lenses.

Bi-aspheric GPs

Two common problems with fitting keratoconus corneas with corneal GP lenses are central bearing and lens decentration. Bearing of a GP lens may induce keratitis and there is some evidence that suggests this can lead to an increased incidence of scar formation.^1-3 A lens that is excessively decentered on an asymmetric steep cornea will provide less-than-optimal vision and be uncomfortable to wear.

Bi-aspheric corneal GP lenses address these issues. As the name implies, these GP contact lenses have aspheric back and front surfaces. The back aspheric surface helps to align the relatively high eccentricity of the keratoconus cornea. Bi-aspheric GP lenses diameters are typically 9.6mm or larger in order to help distribute the weight of the lens. The front aspheric reduces spherical aberrations, as well as aberrations secondary to a lens that decenters. All this adds up to improved comfort and vision for the patient, and improved fitting success for you.

Piggyback

Piggybacking a corneal GP lens on a soft lens is an under utilized modality when fitting keratoconus corneas (figure 1). Silicone hydrogels and GP materials have hyper transmissibility values that significantly reduce the risk of hypoxic related complications.

Figure 1. Piggybacking contact lenses is an underused modality.

A soft lens placed underneath a corneal GP lens can help you and your keratoconic patient in three ways. The soft lens may help to mask some of the irregularity, which can improve the fit of the GP lens. The soft lens acts as a bumper to the rigid lens, which improves a patient's comfort. Keratoconus patients often drop out of GP lenses due to discomfort. Lastly, the soft contact lens helps protect the cornea from bearing and friction from the GP lens, which will reduce the risk of GP induced chronic keratitis and possible secondary scar formation.

Fit the patient in a low modulus, low power, silicone hydrogel soft contact lens. Silicone hydrogels have a relatively high Dk that will reduce the risk of hypoxia, and the low modulus will more easily conform to the steep irregular cornea. The effective power of the soft contact lens will be less when used underneath a GP lens secondary to a refractive index change. The soft lens should have adequate corneal coverage and slight movement with blinking. A steeper lens may be needed if you observe excessive movement or edge fluting.

Take over keratometry or topography measurements with the soft lens in place in order to start the GP fitting. Your approach and the fitting characteristics of your keratoconus corneal GP of choice should generally be the same as fitting without piggybacking.

Scleral contact lenses

Scleral GP contact lenses have become popular because they are often able to fit severe corneal cases when other designs failed. With a range of 13mm to over 20mm in diameter, scleral lenses rest on the relatively normal scleral conjunctiva and completely vault over the corneal surface (figure 2). This allows for good centration and avoids corneal bearing with subsequent keratitis and comfort issues.

Figure 2. Scleral GP contact lenses are often able to fit corneas with severe keratoconus.

In addition, these lenses are meant to semi-seal to the eye, so no movement should be observed upon blinking. Tear exchange occurs through peripheral lens flexure. Scleral lenses that are larger than 17mm have to be filled with saline before insertion to avoid air bubbles. Since the fitting characteristics of these lenses are unique and new to most practitioners, attend a hands-on workshop in order to properly become familiar with these lenses. Fitting with trial lenses is required. Also, requesting advice from the manufacture's fitting consultants to help improve success.

Incorporating these strategies into your armamentarium will help improve the fit, comfort, and vision for these clinically challenging patients. OM

1. Zadnik, K. Barr, JT. Comparison of flat and steep rigid contact lens fitting methods in keratoconus Optom Vis Sci. 2005 Dec;82(12):1014–21.
2. Barr, JT. Wilson, BS. Estimation of the incidence and factors predictive of corneal scarring in the CLEK study Cornea. 2006 Jan;25(l):16–25.
3. Barr, JT. Zadnik, K. Factors associated with corneal scarring in the CLEK study Cornea. 2000 Jul;19(4):501–7.