Prescribing for Presbyopia
Simultaneous Versus Alternating Image CL Designs
BY DOUGLAS P. BENOIT, OD, FAAO
Simultaneous image designs come in a variety of configurations, including aspheric, concentric, and diffractive. All provide multiple images to the retina at all times. This is similar to what the visual system does automatically, until we hit presbyopia. Alternating image, or translating, lens designs provide two or three different power areas much like a spectacle lens. These designs are mainly GPs, although soft lens versions have been introduced.
Similarities and Differences
A diffractive design was available in the 1990s. It worked via a series of eschelets that diffracted the light entering the eye to create the visual effect. Today, we predominantly have aspheric and concentric design options. Both the aspheric and concentric designs come as center-distance or center-near configurations. In any of these designs, there is a change in power as you move from the center to the edge of the lens.
In theory, center-distance designs have a bias for better distance vision than near vision, and the converse could be said for the center-near designs. In reality, today’s versions of both lens designs provide pretty good vision at all viewing distances. Some manufacturers suggest using one of each design to enhance the visual outcome with their lens.
With most simultaneous image bifocal or multifocal contact lenses, except for some corneal GP designs, centration is a must. The optics must be positioned over the pupil for optimum effect. This is complicated because the pupil is not centered in the iris, and the visual axis is, therefore, not in the center of the pupil.
Fortunately, pupil size variation gives us a little leeway here. That means that we need to know the pupil size in average room light as well as in photopic conditions. It is also good to know the horizontal visible iris diameter, lid position relative to the limbus, lid tonicity, and corneal curvature. These data allow a more scientific approach to choosing the best lens design to fit a given patient.
There are soft and GP bifocal and multifocal lenses available. GP lenses are further divided into corneal and scleral designs. Soft and scleral GP designs give the visual system cues to enhance the image being viewed at a particular distance. Corneal GP multifocal lenses are mostly center-distance aspheric designs. Here, the power gradually becomes more plus as the edge of the lens is approached. Corneal GP lenses rely on subtle translation of the lens up when the patient looks down. With these, a lid-attachment fit is desired to allow for the translational movement.
Alternating image designs rely on translation as well. Here it is more critical, because the power is in defined areas of the lens. These designs need to center and be able to translate up on downgaze. For this reason, GP corneal lenses dominate this category. In many cases, lid attachment will provide enough translation to get the near segment before the pupil when patients look down to read.
In some situations, that is not sufficient, and a truncation on the lens bottom is required. The truncation creates a wider area that will allow the lower lid to hold the lens up on downgaze. The truncation also helps to prevent rotation of the lens, thereby aiding vision, particularly in toric designs.
With a little practice, it is possible to examine a patient and know immediately which lens design will have the greatest potential for success. CLS
Dr. Benoit is the senior optometrist with Concord Eye Center, a multi-subspecialty ophthalmology group in Concord, NH. He is a Diplomate of the American Academy of Optometry’s Section on Cornea, Contact Lenses and Refractive Technologies, currently the Special Advisor for the section. Dr. Benoit is a Distinguished Practitioner and Fellow in the National Academies of Practice-Optometry Section and is a Diplomate of the American Board of Optometry. He is also on the Advisory Board of the GPLI and is the 2016 GPLI Practitioner of the Year. Dr. Benoit is a consultant to, and clinical investigator for, Alcon.