prescribing for presbyopia

Have You Forgotten the Decentered deCarle RGP Bifocal?

BY DAVID W. HANSEN, OD
July 1999

The correction of presbyopia with contact lenses challenges a practitioner to make his first fit a successful one. But not all first attempts will be winners, so persistence is a virtue.

The Decentered deCarle Design

In 1959, John deCarle, M.D., reported on an annular corneal contact lens that created two power zones by grinding different curvatures on the posterior optical zones with the distance zone curvature being steeper than that of the near zone. The center distance zone is made small enough so that the pupil is partially covered by it, allowing a portion of the near peripheral zone to remain on the pupillary area at all times. DeCarle was the first to describe this "bi-vision" effect.

Side view of the deCarle bifocal.

Diagram front view of the deCarle bifocal.

Photo of the deCarle lens centered on eye.

This decentered annular concept produces a relatively thin lens with usually no more than 1.5 prism base down to ballast the system. Fit slightly larger for stabilization, the distance zone is decentered superiorly to provide usable distance acuity, while the near zone is large to accommodate a wider reading zone. Rotation is not usually a problem with this lens design. Front toric and bitoric options are available to reduce or eliminate residual astigmatism, and the thinner ballasting system produces a thinner lens with greater comfort.

Fitting This Lens

The deCarle lens is usually fit on the flattest "K" meridian, but can be fit steeper for astigmatic corneas. You can fit a wide range of corneal curvature readings by adjusting the base curve to accommodate for translation.

The lens diameter is determined by the interpalpebral aperture size. Patients with larger interpalpebral fissures (>9.00mm) are good candidates for this design. However, patients with smaller palpebral apertures can also wear the design if the diameter is reduced. The usual horizontal/vertical size is 9.4mm/ 9.1mm, which allows for a 0.3mm truncation that helps reduce rotation and gives the lens good stability. The optical zone is usually 8.0mm for this diameter.

Lens Movement

Since this lens is decentered in the front of the pupil, minimal movement is preferred. The wider, deeper bifocal segment makes it ideal for patients with deskwork demands and for those who use computers. To adjust add power to accommodate the intermediate distance and still allow for reading, don't prescribe the "full" add power.

With bifocal lenses, selecting the best design to meet the needs of the patient in the least amount of chair time wins the fit.

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