prescribing for presbyopia
Try This Toric Soft Bifocal Lens
BY DAVID W. HANSEN, OD, FAAO
When your astigmatic presbyopic patients want soft lenses,be a "can do" practitioner. When you successfully design lenses for these patients, they appreciate it and usually provide future referrals.
Soft Bifocals vs. RGP Designs
The significant difference between rigid gas permeable and soft presbyopic designs is the technique that provides translation from distance activities to near focusing. RGPs must move in front of the pupillary aperture in order for the patient to see different distances. Soft lenses don't move excessively, so you must design presbyopic lenses with different "zone" sizes to create the same accommodation needs for all distances. Precisely measure the pupils to determine zone size for each eye.
Horizon Progressive Toric
Westcon Contact Lens Company introduced the first toric soft bifocal seven years ago. The earlier aspheric designs covered the entire optical zone, giving acceptable intermediate and near visual acuity but unpredictable distance vision. To alleviate the problems, Westcon combined aspherical and spherical curves on the anterior surface of their progressive soft lens, while keeping the posterior surface spherical or toric. The aspheric curve is cut only in the center of the front surface to 2.0mm on the dominant eye and 3.0mm on the non-dominant eye. The spherical remainder of the front optical zone provides a large distance power area. The two different center zone sizes enhance each visual range, providing all visual ranges in both eyes a true bifocal design.
Figure 1. Westcon Horizon Bifocal and Progressive
are basically the same design with the only exception in the add zone. Dots at 3 and 9 o'clock determine lens orientation on
Horizon Bifocal Spherical Or Toric Design
This is a concentric near center add design with a toric posterior curve. Thin zones or double slab- off stabilize the lens axis and provide centration. The basic concentric design has been available for years, but it is well known that the optics are easy to distort in polishing this complex surface. Zones can range from 1.8mm up to 4.0mm. Make the zones smaller on the dominant eye. However, if the patient has a strong need for distance; the zones can be manufactured to compensate for these visual demands. Eye dominance (or controlling eye) tests are equally important in the pre-diagnostic regimen.
Designing the Lens
Normal-size pupils (3.0mm to 4.5mm) require a near center zone size of 2.5mm for the dominant eye and 3.0mm for the non-dominant eye. On smaller pupils, particularly with hyperopes, start with 2.0mm (dominant) and 2.5mm (non-dominant). For larger pupils, use a 3.0mm for the dominant and 3.5mm for the non-dominant eye.
Troubleshooting requires listening to the patient! If the patient complains of diplopia, ghosting or 3-D effect at near, make the near OZ 0.5mm larger on both eyes. If the patient complains of diplopia, ghosting or 3-D effect for distance, then make the near zone OZ 0.5mm smaller on both eyes. If the distance correction is good and the patient requires more plus power at near, increase the add no more than +0.75D. If you need more than +1.00D, increase the near zone size 0.5 mm and do not increase the power.
Take that bold step and prescribe a soft toric bifocal contact lens. As Mikey says, "Try it! You'll like it!"
Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.