Managing the Three D's (Part 2)

BY EDWARD S. BENNETT, O.D., M.S. ED.
MAR. 1996

Last month, I discussed how to manage the dryness and discomfort occasionally associated with RGPs. This month, I'll focus on decreased vision.

INITIAL DECREASE IN VISION

Poor surface wettability, decentration or flexure can cause an initial decrease in vision.

Suspect poor surface wettability if the tear film beads up on the lens surface immediately after lens insertion.This can be caused by a heat buildup during manufacturing, inadequate surface polish, residual pitch polish left on the surface, or use of hand cream containing lanolin by doctors, staff members or lab technicians prior to handling the lens.

To avoid poor wettability, clean, then soak lenses in the recommended wetting/conditioning solution for 24 hours. If the problem persists, use a laboratory cleaner (i.e., Boston Laboratory Cleaner) or solvent (i.e., Fluorosolve), then condition the lens in the palm of your hand with the wetting/conditioning solution. If results are still not satisfactory, polishing the surface should solve the problem. If you encounter poor surface wettability regularly, be sure you're using a reputable laboratory authorized to manufacture your RGP materials of choice.

An RGP lens can decenter for many reasons including a decentered corneal apex, unusual corneal topography, tight or loose lid tension or a thick design. Not only can decentration cause reduced or fluctuating vision, but corneal curvature distortion and lens adherence may occur due to lens-to-cornea misalignment.

If the lens decenters inferiorly, verify the center thickness to ensure that it is equal to the desired center thickness for the material. So-called "thin lens" designs are becoming more popular and the resulting decrease in mass can prevent the lens from dropping inferiorly.

Likewise, a uniform edge thickness is important. Use a minus lenticular or similar edge design for all plus and low minus (typically less than 1.50D) powers, and a plus lenticular design for all high minus (typically greater than -5.00D) powers. I recommend a lid attachment design with an anterior positioned apex, such as the Fluorocon 9.5mm design from PBH.

A superiorly decentered lens, especially if accompanied by little or no movement, can result in superior flattening and inferior steepening (i.e., a "pseudo-keratoconic" corneal topography). Widen or flatten the peripheral curves, decrease the optical zone diameter, or increase the center thickness. A laterally decentered lens is often the result of either against-the-rule corneal astigmatism or a decentered corneal apex. Use an aspheric design or one with a larger diameter or steeper base curve.

If you note poor initial visual acuity and the problem is not poor surface wettability or decentration, then suspect flexure. Typically this occurs when a steep base curve radius lens is fit on a moderate-to-high astigmat. It's easily diagnosed via poor visual acuity accompanied by a sphero-cylindrical overrefraction, cylindrical over-keratometry readings, and a spherical base curve radius. Select a flatter base curve radius. If this doesn't solve the problem, increase center thickness or decrease optical zone diameter.

ACQUIRED DECREASE IN VA

Acquired decrease in vision is often the result of acquired poor wettability, warpage or power change.

Acquired poor wettability can result from poor tear quality, inadequate blinking, surface scratches, use of lanolin hand creams, or poor compliance. If compliance is a problem, reinforce proper lens cleaning and handling. Remind patients to clean their lenses every night and to supplement this with weekly enzymatic cleaning and frequent use of rewetting drops to remove loosely adherent debris. Tell them to avoid using hand creams and soaps that contain lanolin prior to handling RGP lenses.

Warpage, which you can confirm via the radiuscope, is usually the result of aggressive digital cleaning and is more likely to occur with the softer, more flexible, high Dk lens materials. Instruct patients to clean their lenses gently in the palm of the hand. It may be necessary to change to a lower Dk material.

Aggressive cleaning with an abrasive cleaner may increase minus power as well. Decreased center thickness often accompanies the power change and patients usually don't complain until the change is one diopter or greater. Verify power and center thickness, order new lenses and educate the patient about proper cleaning methods. CLS

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis, and is executive director of the RGP Lens Institute.