contact lens primer
Creating a Better Locus of Focus
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
Some aspheric soft contact lenses are available for correcting presbyopia. Others minimize the vision symptoms created by spherical aberration. Optical surfaces with higher degrees of eccentricity (rate of flattening) result in larger effective adds for aspheric multifocals. Surfaces with low degrees of eccentricity are used to decrease spherical aberration. To paraphrase Christopher Snyder, OD, spherical aberration reducing aspheric soft lenses enhance vision by creating "a better locus of focus."
Aspheric multifocal soft contact lenses have been available for the past two decades. Unlike many rigid aspheric multifocal designs, soft multifocals don't suffer from extreme lens decentration or from potential reduced tear exchange because of the necessity of prescribing steep lens-to-cornea fitting relationships.
Aspheric multifocal lens designs tend to work best for emerging presbyopes. The more mature presbyope requiring a 2.00D or greater add power for near tasks often benefits from wearing a modified monovision design if you prescribe aspheric multifocal lenses.
You can achieve modified monovision by prescribing a single vision distance or near lens in one eye and either an intermediate/near or distance/intermediate aspheric soft lens in the fellow eye. Or you may prescribe an aspheric soft multifocal lens for both eyes in which one lens optimizes distance vision and the other lens optimizes near vision.
Aspheric Options Abound
The soft aspheric multifocal designs currently available differ in effective add power, lens design and replacement schedules. Replacement schedules range from daily disposable to annual replacement. The near portion of most aspheric soft multifocals is located centrally. However, a few designs feature a central distance zone, some for use in modified monovision systems and some for binocular use.
The CIBA Vision Focus Progressive contact lens is a concentric design with a 2mm aspheric near zone in the lens center surrounded by a spherical distance power zone. A progressive add of up to 3.00D may be achieved, according to the manufacturer. Most other center-near aspheric multifocals (such as Sunsoft Additions and Unilens Softsite Multifocal) are designed with the asphericity more continuous within the optic zone.
Tips for Success
Lens centration is critical for successfully fitting most soft multifocal designs. Loose spectacle trial lenses or lens flipper bars are helpful in fine-tuning distance and near powers. Avoid zealously prescribing excessive minus power to address your patients' distance vision complaints or prescribing excessive plus power to alleviate near symptoms.
Chasing powers at one distance will often compromise the vision at other viewing distances. Unless you're prescribing modified monovision, be wary of changing the power indicated by the vertexed manifest refraction by more than 0.50D.
Show Patients the Rewards
Aspheric single vision soft contact lenses are prescribed to reduce spherical aberration. Even though the over-refraction cylinder might not be different from that obtained with a spherical soft lens in place, many patients experience enhanced vision through aspheric lens designs.
Aspheric soft contact lenses designed to reduce spherical aberration are generally prescribed to improve vision for patients who have less than 1.00D of residual astigmatism. If the resulting vision through the aspheric soft lens isn't acceptable, prescribe a soft toric or rigid contact lens.
Dr. Edrington is a professor and is in the contact lens service at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.