prescribing for presbyopia

RGP Multifocal Fitting and Pupil Size

BY DAVID W. HANSEN, OD
MARCH 1999

Most experts agree that pupil size is extremely important for optimum success in fitting multifocal contact lenses. So what's the best way to measure this dynamic hole? To answer this question, we'll review the design of aspheric RGP multifocal lenses. The posterior curve is often a constantly changing surface from the center of the lens to the peripheral edge. Knowing the type (eccentricity value) of the posterior curve affects the fit of the multifocal and the resultant visual acuity. This is why so much emphasis has been given to the centration of aspheric multifocals.

Pupil Measurement

It's helpful to take two measurements of the pupillary aperture before determining the diagnostic multifocal contact lens. You can take the first measurement with a PD ruler with hemisphere markings and normal room illumination. Once you measure the pupils under mesopic conditions, turn down the lights to obtain scotopic readings. Obtain the second mesopic measurement with the biomicroscope reticle using a small vertical beam on the limbus. If both measurements aren't similar, re-measure.

Pupil Size in Design Selection

Most aspheric multifocal designs have a predetermined posterior curve specified by the manufacturer to accommodate the distance and near prescriptions. In order for the patient to utilize this "add power," it's imperative that their pupil size is taken into consideration. If the patient's pupil is too small to reach the area of the multifocal add, they will never obtain the full add effect. If the pupil is too large with the same geometric design, they will experience blur for distance and intermediate areas. This is why pupil sizes are important when designing and ordering aspheric multifocal lenses.

It is visually meaningless for a manufacturer to supply an aspheric design which offers a +3.00D add at 3.5mm from the center (7.0mm zone) if the patient's pupil is only 3.5mm in diameter in normal room illumination because their eye will never translate into the near zone. Remember that all RGP bifocal and multifocal contact lenses need to translate to achieve optimal visual acuity at distance, intermediate and near.

Other options required to achieve maximum visual acuity for distance and near include a decreased distance zone size, a "plus add effect" on the peripheral front surface or translating bifocal design. Many manufacturers can change the size of the zones to accommodate pupil dynamics (Fig.1). Making the distance zone area 0.5mm smaller than the mesopic pupil size is ideal and allows for optimal distance viewing while still moving into the near zone. Not all laboratories require pupillary measurements to order aspheric multifocals because they have precalculated zone sizes for their designs.

Patients need time to adapt to a new modality. For some, it takes days or weeks to adjust to visual acuity changes, especially those with different illumination levels. Modifications to contact lenses can assist patients who are not able to adjust or who need smaller or larger central zones. Patient education is the key to success. They must realize that bifocal performance may vary with changes in illumination.