Richard Baker, O.D.
Just as presbyopes are the most underserved group of the contact lens-wearing population, so are gas permeable (GP) multifocal and bifocal designs the most underutilized types of contact lenses.
Yet, GP designs give you the opportunity to improve the presbyopes quality of life by providing crisp vision at all distances without spectacles, or without the loss of stereopsis and vision quality of monovision. A recent survey of diplomates of the Cornea and Contact Lens Section of the American Academy of Optometry shows a recent trend in these practitioners preferences of bifocal contact lenses over monovision.1
Many O.D.s opt for what they think is an easier solutionmonovision, single-vision contact lenses and reading glasses, or spectacles. They think that GP bifocal and multifocal designs are too complicated. Its not uncommon for a patient to say she has never heard of bifocal contact lenses when a doctor presents this option.
GP bifocals, however, are a good primary management option for most presbyopes interested in contact lenses. These designs are not complicated to fit and the learning curve may take as few as three fits of any specific lens design. The 50 members of the RGP Lens Institute Advisory Committee said as much when they were asked how many fits it took them to become comfortable with any particular GP bifocal lens design.
Also, newer lens designs provide better optical quality than older designs, and the availability of higher add powers in aspheric multifocal designs make GP multifocal or bifocal lenses viable options for many presbyopes. Here, well review the steps in fitting presbyopes in GP bifocal and multifocal designs, and which design best meets an individuals needs. To get started, youll need at least one set each of simultaneous vision and translating bifocal trial lenses.
Evaluating for Fit: Anatomical Considerations Bifocal/Multifocal Resources These resources are available through the RGP Lens Institute, the educational division of the Contact Lens Manufacturers Association (CLMA), at www.rgpli.org.
Anatomy of the Fit
Evaluate these six anatomical features when sizing up a presbyope for GP lenses.
1. Lid position.
2. Corneal diameter.
3. Pupil size and dynamics.
4. Fissure width.
5. Lower lid pupil edge.
6. Location/amount of astigmatism.
Consider these anatomical features when evaluating an individual for GP bifocal or multifocal lens wear:
Lid anatomy and position. GP translating designs can either be segmented (i.e., crescent or executive) or annular, both of which are prism-ballasted. Multifocal designs can either be aspheric (i.e., progressive) or annular in designboth of which are thin and not prism-ballasted. Theyre called simultaneous vision designs because all three viewing zones are often in front of the pupil at the same time.
The patients lid anatomy and position are important factors. Measure and record the lid-to-limbus relationship, the lid tonicity, and the palpebral fissure size. Avoid a translating design if the lower lid is positioned 1mm below the lower limbus in straight-ahead gaze. The lens will not shift sufficiently with downward gaze, and the bifocal segment will position in front of the pupil. Likewise, a very small palpebral fissure may make it impossible for you to position the near segment line at or below the lower pupil margin as desired. It will, instead, position too high and interfere with distance vision.
Lid tonicity is important as well. As a person ages, the lids can become more flaccid. This makes lens translation more challenging. When fitting GP bifocal lenses, have the patient view inferiorally; the lens should shift superiorally with most of the near zone translating, or shifting, in front of the pupil. If this does not occur, the lids may be too loose for translating bifocals. If it occurs partially or intermittently, increasing the lens edge lift may enhance translation. Try selecting a flatter base curve (i.e., 0.50D flatter) or flatter peripheral curve radius.
A translating bifocal should fit slightly flatter than K so it falls quickly and sits at or near the lower lid. A lid that picks up a lens more than 1mm with the blink may result in intermittent blurred vision at distance because it may be pulling the segment excessively into the papillary zone. This can happen if the patients lids are too tight. In these situations, increase the prism ballast (again by 0.50D) to allow for a quicker release after blinking.
If the lower lid position is upswept, you may need to offset the truncation 1015 to align with the lid. Finally, if the lens rotates excessively (typically nasally), select a flatter base curve. This will allow the upper lid to exert more effect instead of allowing the lens to fall and position near the lower lid. The opposite is true with a thin lens such as an aspheric design. If the lid picks up and moves the lens excessively with the blink, select a base curve radius 0.50D steeper. This often results in better centration and less lag on the blink.
Pupil size. Take two pupil size measurements. For the first, hold the PD ruler across the pupil under normal illumination. Then, record the pupil size under scotopic conditions. Pupil size is particularly important when determining whether an aspheric design is appropriate.
A pupil diameter less than 5mm under mesopic conditions indicates that a translating design would work because an aspheric lens can interfere with distance vision in low-light conditions such as night driving. Likewise with a small pupil, an aspheric designsuch as a segmented designmust shift some or translate with downward gaze so that the pupil views through the paracentral to mid-peripheral part of the lens (assuming that this lens is a distance-center design.)
When fitting a translating design, which is recommended for a large pupil, your should evaluate the seg position in relation to the pupil in straight-ahead gaze. Position the lens at or slightly inferior to the lower pupil margin.
Tear film evaluation. Tear volume decreases over time, so you must assess a patients tear quantity and quality. To achieve a successful fit, the tear film break-up-time should be more than five seconds upon repeat measurement. Measure tear volume, too. Zone Quick (Allergan/Menicon) or Tear Volume Test (CIBA Vision) are two tests for this. A wetting value of >10mm often predicts success.
You must also rule out blepharitis and meibomian gland dysfunction before fitting a patient with bifocal contact lenses. Such conditions must be resolved before you can reevaluate the tear film for GP contact lens wear.
Corneal topography. This is not mandatory for GP bifocal and multifocal fitting, but it is beneficial. If the peak of the cornea positions nasally, temporally, super- iorally or centrally, you can use a simultaneous vision design. But, excessive decentration rules out a simultaneous design because blur at distance will likely be a problem. If the lens is decentered, you will need to monitor the patient closely.
Aspheric multifocal contact lenses fit 0.75D to 2.00D steeper than K for low-eccentricity designs and 2.50D to 5.00D steeper than K for high-eccentricity ones. They should fit fairly tight (about a 1mm lag with the blink) and show good centration. However, a tight-fitting, superiorally decentering lens-to-cornea relationship may cause corneal distortion over time if left unmonitored.
RGP Bifocal Fitting, Evaluation and Problem-Solving video and CD-ROM.
Clinical Management Guide on fitting and troubleshooting spherical, presbyopic, keratoconus and high toric lenses.
A laminated bifocal pocket card.
On-line symposia second Tuesday of each month, 9-10:30 p.m. ET.
Patient brochures.
A Matter of Powers
A GP bifocal or multifocal enhances a patients corrected vision best when you thoroughly evaluate these visual factors:
Refractive power. The most successful GP bifocal and multifocal fits achieve ametropia of 1.00D or more. Weve found that individuals accustomed to wearing spectacles or contact lenses for distance correction are more likely to be motivated to wear GP lenses for at least eight hours a day. If the refractive cylinder and corneal toricity are similar, an aspheric lens design should succeed. If there is significant residual astigmatism, choose a translating design with front-surface cylinder capability.
Add power. An aspheric lens design usually provides patients in a low add power (up to about +1.50D) satisfactory vision at all distances. This is especially true with wearers of single-vision GP lenses. Once they become presbyopic, they can switch to a design like their old lens in terms of thickness and fit. Properly fitted, you can incorporate any add power into a translating design. Thus, its the design of choice for patients with add powers of +1.75D or greater.
With the exception of a few translating trifocal designs, most aspheric lenses do not provide intermediate correction. Patients with intermediate prescriptions may be disappointed or need glasses for intermediate viewing with a translating bifocal. The use of uneven adds (i.e., +2.25D in one eye, +1.50D in the other) is an option that will work in some cases.
When a translating bifocal wont work for a patient requiring a high add, several options are available. In recent years, several high-add aspheric designs have been introduced with up to +2.50D in add power. These lenses improve upon previous designs by incorporating a front-surface plus, which allows for higher add powers. Or, try prescribing a modified bifocal lens. That is, slightly over-plus in one eye (typically the non-dominant eye) to optimize the near and intermediate powers while achieving satisfactory distance vision. The other eye achieves good distance and intermediate vision, and satisfactory near vision.
Assessing the Needs Simultaneous vs. Translating Bifocals: The Indications These conditions or patient needs may dictate your choice of presbyopic GP contact lens design.
To achieve a truly successful fit, you must gather and analyze information on how and where the patient will be using his or her vision. Heres how to evaluate vocational and avocational needs:
Present the options. The best time to first mention the bifocal GP option is before a patient reaches presbyopia. Explain the benefits and drawbacks of single-vision, monovision and bifocal contact lenses. Mention the bifocal contact lens option last. Although soft bifocal lenses are an option, emphasize the vision benefits at all distances of GP designs and the visual freedom patients can have at work, social functions, etc. (see box above).
Also, provide the patient with realistic expectations. He or she may want perfect vision at all distances, which is probably unrealistic. Explain that GP bifocals are different from spectacles. They are dynamic pieces of plastic that move on the eye, so vision may not be as crisp or as stable, which makes achieving a good fit critical. A lens exchange or two may be necessary. However, a motivated patient will succeedespecially one who switches from soft contacts to GPs.
These individuals are typically concerned about initial comfort. Explain that both aspheric multifocals and translating designs should move little with the blink. The patient should not be as aware of the lens as she would with a spherical GP design. A topical anesthetic may aid the initial fitting.
Simultaneous vision: Low lower lid; small fissure; loose lids; corneal apex lateral or superior; no residual cylinder; low add power; frequent computer use; participation in sports.
Translating bifocal: large pupil size; corneal apex inferior; significant residual cylinder; high add power; critical distance vision requirement.
Determine the needs. Ask the patient what are her primary visual needs. For example, if she uses a computer 30% or more in a day for work or leisure, an aspheric lens design may enhance intermediate vision. You can actually put the patient in front of a computer to determine her working distance. If critical distance vision is important, choose a translating design.
Also ask patients about their work environment. Excessive wind or dust, poor air quality or prolonged computer use makes contact lens wearers vulnerable to dryness, so frequent application of rewetting drops will be necessary. And, many individuals do not stop playing sports when they become presbyopic. A GP multifocal will fit fairly tightly, minimizing dislocation and loss, while providing satisfactory distance vision.
Simulate the visual environment. Trial lenses for over-refracting can help you do this. Using +0.25D and +0.50D flipper bars can help finalize the prescription. In fact a 0.25D change can have a significant subjective effect on vision. When the lenses have settled and a the patient has a tentative prescription, have the patient walk around the office and outside, look through a magazine and simulate other daily activities. With a translating bifocal, it may be a good idea to have the patient get into her car, look in the mirrors and straight ahead. Also, make sure the patient realizes that when she smiles, the seg will push up and this could blur distance vision while driving.
Other means of gaining proficiency. Youll need several diagnostic fitting sets when fitting the presbyope. At minimum youll need one simultaneous vision and one translating set, although two different sets in each category are optimum. An invaluable resource is any GP contact lens laboratory.
Presbyopes are a relatively untapped group eager to try and use a device that will give them visual freedom and good vision. Todays GP bifocal and multifocal contact lens designs are not especially difficult to fit, and the benefits to you and your practice can be rewarding.
Dr. Bennett is associate professor, co-chief of the Contact Lens Section and director of student services at University of Missouri-St. Louis School of Optometry. He is also executive director of the RGP Lens Institute.
1. Westin E, Wick B, Harrist R. Factors influencing success of monovision contact lens fitting: Survey of contact lens diplomates. Optometry 2000;71(12):757-763.
