Article Date: 7/1/2005

contact lens management
Meeting the Keratoconus Challenge

New contact lens alternatives for keratoconus patients.

This eye was post PKP and fit with an intra-limbal design, resulting in poor centration and comfort.

This same eye was fit with a Jupiter Mini-Scleral. Note the improved centration, as well as the fenestration at eight o'clock.

Keratoconus is something that we are familiar with and see routinely in our offices. It can be very challenging to fit these patients successfully, but by the same token, very rewarding.

The typical therapeutic sequence for the treatment of these patients is to correct them with spectacles until they can no longer provide satisfactory vision. Soft/toric contact lenses are sometimes used next but usually with limited success, because soft lenses tend simply to drape over the cornea, transferring the corneal irregularities to the anterior surface of the soft lens. Eyecare practitioners then turn to GP lenses; in fact 75% of diagnosed keratoconus patients are best corrected with GP lenses. They have several therapeutic advantages over the other non-surgical options.

Evaluating the GP option

They allow for a uniform refracting surface and eliminate most astigmatic error; resulting in increased visual acuity. They can, however, be difficult to fit. Patients have to "build-up" their wearing time. The lenses may cause punc

tate keratopathy and epithelial defects which can ultimately lead to further corneal scarring. Standard curve GP lenses are often implemented first and used until the keratoconus progresses so far that standard curves are no longer able to provide an adequate fit.

Keratoconus-design GP lenses are ultimately used. These are by far the most successful non-surgical means of treating keratoconus. These GP lenses have a high Dk value and steep central curves to avoid excess bearing on the apex of the cone. The peripheral curves then flatten disproportionately to avoid tight lens syndrome.

The first generation hybrid lens was the piggyback lens design. This was simply a rigid lens fitted over a hydrogel lens. Although the soft lens improved comfort and wearing time while the GP improved vision, persistent decentration, handling difficulties, and expense prevented this from being a widely- used treatment.

Newer hybrid lens designs fused a GP lens to a soft lens skirt. Again the GP center provided better vision and the soft skirt increased comfort and wearing time for the patient. The fusing of the GP and the soft lens in one overcame the centration problems encountered in earlier designs. Although still widely used today, these lenses present some problems as well. In addition to poor movement and limited tear exchange, the GP center has a relatively low Dk of 14, further starving an already sick cornea of oxygen. Problems with breakage and tearing of the RGP/soft lens junction limit the average life of this lens to six months to a year. An additional disadvantage is the expense:

These lenses often cost twice that of a keratoconus design GP lens.

 


The soft lens/GP lens interface of a hybrid lens.

Breakage of these lenses both at the GP center and at the soft lens/GP junction limit durability.
Rizzuti's sign is appreciated when a slit lamp beam is focused on the nasal aspect of the limbus. When viewed temporally, the apex of the cone is illuminated.

The alternative scene

New, alternative lenses are the latest option for this population of previously underserved patients. Keratoconus-design hydrogel lenses have recently come on the market. Innovations In Sight has created a lens called the Super Nova HydroKone, and it was recently put to use in our clinic. It's a glycerol methacrylate lens with 59% water content and a Dk of 21.0. It uses a steep aspheric central curve, flatter para-central curve, with a center thickness of 0.35mm (low minus) to provide stability.

A base curve range of 4.1-9.3mm and total diameters of 12.0-17.0mm are available in powers from +50.00 to -75.00. A wonderful feature of these lenses is that once the trial lens is fit, a sphero-cylindrical over refraction is performed and a lens is made-to-order, including the residual cylinder.

Innovations In Sight has another lens to treat these patients — the Jupiter Mini-Scleral GP. A new spin on an old idea, the Jupiter lens is a large-diameter (14.4mm - 18.6mm) GP lens made of Boston XO or Equalens 2 with a 100 Dk and 85 Dk values respectively. It vaults the entire cornea, creating a large tear lens between the cornea and the posterior surface of the lens to neutralize surface corneal irregularities. The lens actually rests outside the limbus on the less sensitive sclera and can use a fenestration for added tear exchange. It's a very interesting alternative for keratoconic patients as well as patients suffering from other corneal irregularities.

Quarter Lambda Technologies is developing a new generation hybrid lens, the SynergEyes KC, which holds a great deal of promise in overcoming problems encountered with earlier generations of this type of lens. It has a central GP made of Paragon HDS (100 Dk) and a surrounding soft skirt (31% water). It's been reported that the soft lens-GP interface is more than 10 times stronger than previous lenses of similar design. Additionally, the lens is slated to be less expensive than previous hybrid lenses. Another exciting aspect of is the use of wavefront-guided lens design, offering a made-to-order lens with rotation stabilized by proprietary thin zones.

Provide more answers

We've used all of these lenses in our clinic with mixed success. None of them will provide increased comfort or improved vision for all specialty contact lens patients. However, they do offer new options in treatment modalities for these challenging patients. While GP lenses currently remain the primary modality to treat keratoconus, we have a host of alternative lenses at our disposal for GP-intolerant patients, as well as exciting new lenses on the horizon.

References available on request.

DR. HOLT IS IN GROUP PRACTICE AT THE MOUNT OGDEN EYE CENTER AND BOUNTIFUL HILLS EYE CENTER IN UTAH. HE COMPLETED A RESIDENCY AT THE UNIVERSITY OF UTAH MORAN EYE CENTER AND VETERANS HOSPITAL.



Optometric Management, Issue: July 2005