Keratoconus Management with Ellip-See-Con-K Lenses
The ESC-K design is a posterior aspheric, conic section, Gas Permeable contact lens. The aspheric back surface flattens in all meridians from center to edge. The progressive flattening of the ESC-K back surface minimizes localized bearing points resulting in improved alignment and comfort. This improved lens to cornea relationship provides a more uniform tear film and effecient masking of corneal astigmatism. ESC-K has proven to be a successful problem solver design for Keratoconus, corneal transplants, or other irregular corneal topographies.
Fitting Keratoconus patients is extremely challenging for even the expert contact lens specialist. Each case is different and can become a costly learning experience, if you don't have the proper diagnostic tools. Fitting philosophies will vary from patient to patient based on each particular corneal topography. Traditional keratometry readings can serve as a reference point but are usually of little value in determining final contact lens parameters. The keratometer reads a very small central section of the cornea. The cone is usually inferior and generally much steeper than the central and superior sections of the cornea. Corneal topographers provide a more complete picture of the corneal surface but will not eliminate the need for a diagnostic fitting. A trial lens on the cornea is the only way to finalaize specific contact lens parameters. Keratoconus lenses must be designed to accomodate both the steep conical area and the flatter superior and peripheral areas of the cornea. This will require muliple curves of spherical or aspheric values to create the proper fitting relationship. A conic section aspheric design, such as ESC-K will provide a posterior lens surface that progressively flattens from apex to edge.
Fitting ESC-K GP Lenses
This is only a suggested started point. Traditional keratometry readings can serve as a reference but are usually of little value in determining the final base curve selection. The keratometer measures approximately 3mm of the central cornea. The cone is normally inferior and generally much steeper than the central and superior sections of the cornea. A successful fit must be based on your observations of the fluorescein pattern, lens movement, position and visual accuity. A gross observation of your trial lens should be made to evaluate generalposition and movement. Low riding lenses are not uncommon on cone patients and should not be rejected based on position alone. If a lens is generally centered over the pupil and lens movement is not excessive, you are now at a point where a more critical evaluation should be accomplished with a Burton or Slit lamp. Acceptable lens position and movement would maintain the visual axis within the optical zone. Decentration or movement resulting in an interruption in vision, would not be acceptable.
1. Select a base curve .50 diopters flatter than the average "K"
Example: 45.00 / 50.00 Keratometer Readings
47.00 (7.18) Base Curve
A trial fitting is essential for your Keratoconus patient. If your patient is not wearing a GP lens, you will have to rely on Topography of Keratometry findings to select an intial trial lens.
2. Observe Central fluorescein pattern. Minimal clearance to light touch at the apex of the cones should be the objective. (See Figure 1)
Fluorescein should be used to closely evaluate the fitting relationship of your trial lens. An acceptable fluorescein pattern would show minimal clearance to light touch at the apex of the cone. A three point touch would be acceptable, light touch at the apex and mid-periphery.
Adjacent to the cone, fluorescein will generally form a surrounding band of pooling. This pooling should begin to thin as the lens surface comes into closer alignment at the mid-cornea. 360 degress of peripheral edge lift should be present with no seal-off points. Apex touch should be minimized.
3. A flat fitting lens could display any or all of the following symptoms. (See Figure 2)
A. Harsh or broad area of apical touch
B. Excessive peripheral stand-off or pooling
C. Excessive movement or displacement with blink
If your trial lens demonstrates any of the above symptoms, begin steepening the base curve in 0.50 diopters steps as required. Diameter and peripheral curve changes will also influence lens lift.
Controlled Edge Lift
The Conform-A-Spheric will help the lens fitter control edge lift and tear exchange. There are three edge lift options available, CAS 121, 141, 161. The CAS 121 is the steepest of the 3 and will reduce edge lift and tear exchange. The CAS 141 is a mid range appropriate for the average fit. The CAS 161 is the flattest of the 3 and will increase edge lift and tear exchange.
No pronounced junctions
or pressure points.
Once an acceptable fit is achieved, over refract to determine the final lens power. Residual cylinder may be present, but all attempts should be made to avoid complex toric designs. Whenever possible, your best spherical over-refraction should be used to determine the final lens power.
4. A steep fitting trial lens could display any or all of the following symptoms. (See Figure 3)
A. Excessive apical clearance or central pooling.
B Trapped bubble adjacent to the cone.
C. Mid-peripheral seal-off.
D. Lack of lens movement.
Base curve flattening in 0.50 diopter steps will be your most effective change to loosen a tight fitting lens. A diameter or optic zone reduction, as well as a flatter peripheral curve, could be used to loosen a tight fitting lens.