prescribing for presbyopia
Custom Toric Multifocals—Is It Finally Their Time?
BY CRAIG W. NORMAN, FCLSA
Toric soft lenses are a staple of today's contact lens practice, having long ago lost their characterization as a “special” lens design. Fitting torics is simply an expected skill for practitioners who fit contact lenses, with scores of options available in many materials, prescriptions, designs, and for all replacement cycles.
The last frontier for managing regular astigmats is presbyopia. In this regard, we are truly approaching the time in which we have many varied soft lens options that we can count on working effectively.
The article “Contact Lenses 2008” (January 2009 issue) by Jason Nichols, OD, MPH, PhD, FAAO, noted that soft toric lenses now represent about 22 percent of fits in the United States. Many of these wearers are now or soon will be presbyopic. While some major soft lens manufacturers have addressed this issue with stock parameter multifocal and bifocal torics, the offerings remain limited, especially for higher amounts of astigmatism or axes that require less than 5-degree increments.
Custom torics are available in many parameter options. Some designs offer as many as three or four base curves, while others such as the Intelliwave Multifocal Toric (Art Optical Contact Lens) can be made to order in any base curve. Most designs offer a wide range of sphere, cylinder, and add powers. Diameters usually range from 13.5mm to 15.0mm, but made to order is also available.
Designs in this category are usually back-surface torics with the add power incorporated on the front surface. All are currently prism-ballasted. While most custom soft toric multifocals range from six-month to annual replacement, some such as the C-Vue Advanced Toric Multifocal (Unilens Vision, Inc.) are available for monthly replacement.
Initial Lens Selection
Trial fitting is usually not necessary; rather, custom toric multifocals are either calculated by a nomogram or by supplying the laboratory with key patient information. Use the K readings as a reference point to determine the base curve and spectacle prescription, converted to minus cylinder form. It's important to compensate for vertex distance if either the sphere or cylinder power is greater than ±4.00D. Use the same axis and add power as in the spectacle prescription. You may incorporate an additional +0.50D for the tear layer effects caused by these larger, thicker, less-flexing lenses. Lens diameter is based on the horizontal visible iris diameter, which is great for smaller and larger corneas.
Most toric multifocal designs are center-near with some being aspheric, so they may offer various zone diameters to take patient pupil size into account. Providing pupil size to the manufacturer will help in determining what is best for a particular patient.
Methafilcon A (55 percent water), hioxifilcon B (49 percent water) and acofilcon B (49 percent water) are the most commonly available materials for these designs, although some others are also used. While these serve most ocular requirements, we're looking forward to silicone hydrogel toric multifocals, which may be available as soon as early 2010.
Go Beyond Stock Parameters
Stock parameter soft torics for presbyopia will remain the “goto” lens for most patients, but when stock parameters will not meet your patients' needs consider a custom-made toric multifocal. You'll be surprised at how easy they are to fit and how successful they can be. CLS
Craig Norman is director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute. He is also a consultant to B&L.