prescribing for presbyopia

Mapping the Way to Successful Bifocal RGP Selection

BY DAVID W. HANSEN, OD
MARCH 1998

When presented with the challenge of fitting a presbyopic patient with contact lenses, do you have a game plan to help you decide which lens type will best suit the patient's needs? I've found that corneal topography can streamline the diagnostic multifocal RGP selection process.

Seeing the Whole Picture

Keratometry measurements monitor only the central cornea and are a poor way of selecting specialty designs such as multifocals. Videokeratography helps you: determine the general shape of the surface; find specific curvature of the central region with respect to the peripheral corneal surface; locate the apex, or the greatest curvature of the surface; locate the zenith, or the highest point of the topography; determine the shape factor, or the mathematical formula for corneal eccentricity; assess astigmatism and corneal irregularity using the corneal irregularity measure (CIM); and simulate lens fit without placing a lens on the eye.

Key Points of Interest

When studying a corneal map, it's important to understand where the apex and zenith are located. Finding the power of the cornea at the pupillary axis, which may or may not be the steepest portion, will give you the information needed to select the base curve of the initial bifocal lens.

Next, determine if astigmatism will influence the design you choose. Determine whether the astigmatism is against-the-rule or with-the-rule, and assess the axis position. Investigate the direction and magnitude of corneal shape change based on the shape factor and the CIM. The shape factor may be helpful in selecting an aspheric (simultaneous) design.

Selecting the Design

Clinically, most RGP bifocal lenses will position toward the apex and zenith. Therefore, the contact lens will probably gravitate toward the steepest zone on the cornea. If this area is above the pupillary axis, use either an aspheric or multiple aspheric design. If the steepest portion and zenith are located centrally on the cornea near the pupillary axis, then consider prescribing either a translating, simultaneous or aspheric simultaneous design (Fig. 1). Choose a translating design when the apex is located below the pupillary opening (Fig. 2).

FIG. 1: Cornea acceptable for aspheric RGP bifocal or translating bifocal. FIG. 2: Steepest portion of cornea below center. Fit an RGP translating bifocal.

Many topographers have contact lens modules that can simulate the fluorescein pattern, which is especially helpful when fitting translating designs. It may be more difficult to predict the positioning and fit of aspheric multifocal designs. In this case, there's no substitute for the actual RGP lens on the eye which usually is affected by other factors such as eyelids, lacrimation and peripheral conjunctival artifacts.

Dr. Hansen, a diplomate and fellow of the AAO, is in private practice in Des Moines, Iowa.