Article Date: 11/1/2005

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contact lenses

New Options for Difficult-to-Please Patients
Take a look at new soft toric and multifocal contact lens tools and designs.

Contact lens correction for the proverbial "birthday change" and the dreaded "stigmia" has been the elusive holy grail for eyecare practitioners. Previous options typically resulted in less than optimum visual acuity and unacceptable compromise between distance and near vision. However, advances in lens design, the availability of extensive diagnostic fitting sets and the ability to refine lens power by utilizing computerized tools have bridged the gap to success.  

It all starts with a proper fit

Successful fitting of both toric and multifocal soft contact lenses begins with just that: successful "fitting." Often times we immediately focus on the selection of power and cylinder axis, but our initial thought should be choosing the fitting parameters. With the complex optics of these modalities, performance is best with the correct overall diameter and base curve.

The ideal lens will center over the pupil, cover the limbus 360°, overlap at least 1.0mm - 1.5mm onto the conjunctiva, and move freely, but not excessively, with each blink. A lens that is too loose will result in variable visual acuity, and too tight of a fit will cause the lens to vault and distort the optics.

You can use keratometry or retinoscopy over the lens to assess the fitting relationship in addition to what you see in the slit-lamp. If a lens is too tight, you'll see a scissors-reflex with over-retinoscopy and the mires of the keratometer will be difficult to super-impose. After allowing a diagnostic lens to equilibrate, suspect an inappropriate design or fitting relationship if visual acuity is unacceptable and can't be improved with a spherical or sphero-cylindrical over-refraction. Potential visual acuity also reveals information about the fitting relationship.

For the soft-torics tool box

A recent advancement in soft toric prescription has been the introduction of computerized tools for calculating obliquely crossed cylinder effects and resultant power. Prior to the availability of these calculators, lens power parameters were primarily determined with the "LARS" (Left add, right subtract) method of adjusting for lens rotation. The limitations of rotation only calculations (LARS alone) are significant, in that LARS will not reveal underlying:

• errors in refraction

• vertex calculation errors

• possible cylinder masking with thicker/stiffer ballasted lenses, or

• lens draping effects, secondary to the underlying corneal topography.

In other words, a lot of unknown power variables may be present. The simple soft contact lens over-refraction will pick up virtually all of these variables and, when calculated properly, the visual results for patients can be excellent.

Several cross-cylinder calculators are available via the Web, for the Palm or PC as well as the old standby hand-held versions. The tool that we have found most useful and are teaching our students to use for soft toric lens orders is ToriTrack (CooperVision).

The advantage of this calculator is the simple fact that it does not require on-eye estimation of lens rotation, which can be a significant source of error. The program is able to determine the resultant lens power to order based on three known variables: baseline manifest refraction, contact lens power and sphero-cyl over-refraction. The calculator also aids in selecting appropriate empirical lens parameters based on corneal diameter, apical radius of curvature and refractive error. Base curve is selected based on overall sagittal height of the cornea. Lens power is suggested to compensate for tear lens effects that occur with certain thicker ballasted-lens designs.

You can use ToriTrack to determine final lens power after performing a refraction over any soft toric lens design. However, keep in mind that the optical calculations will only work if the fit is appropriate and the lens is stable on the eye. If a toric lens rotates on the eye and either LARS or a computerized tool is used to optically compensate for this misalignment, the next lens must rotate the same amount and in the same direction as the initial lens. Otherwise, you are chasing a moving target. In instances of rotational instability, try a different base curve, overall diameter or change design.

Making multifocals work

All current soft multifocal lens designs operate on the principle of "simultaneous vision." In simple terms, the optics for distance, intermediate and near vision are simultaneously projected through the pupil. The brain learns to selectively attend to the information it needs at a given time. Try the "screen door" analogy to explain the concept to the patient. You can choose to either focus on the individual squares that make up the screening material or ignore it and look past to observe what is beyond the screen. It's crucial to help the patient develop realistic expectations for this type of contact lens.

The advantage of multifocals is that they provide a range of visual acuity that works for both the computer monitor that's arms length away, and for reading at a normal 16-inch working distance. Although visual acuity may not be equivalent to that provided in single-vision or translating designs, the goal is to find a soft multifocal contact lens option that works for 75%-80% of a patient's visual demands.

It's critical to determine how the patient uses his eyes throughout the entire wear schedule. Also, explain that although multifocal contact lenses will reduce his dependency on spectacles, you may not be able to eliminate them altogether. For those problem-solving visual demands that don't encompass the majority of the day, readers worn over the contact lenses may be the best option. They will help patients read very small print, especially in dim-lighting situations, and/or provide better visual comfort when they're reading for prolonged periods.

For the patient with precise near-visual acuity demands, over-spectacles may be necessary for distance when the patient drives.    

Pearls for success

Step one in lens parameter selection is to consult the manufacturer's recommended fitting guide. These are invaluable resources that have been developed as a result of extensive clinical trials and patient experience with a particular design. However, there are a few pearls that you can apply to the prescription of most contemporary multifocal contact lenses.

K Sphere power. For the low astigmat, rather than assume a patient will accept an exact equivalent sphere correction or ignore the cylinder altogether, determine the "best sphere" refraction. This is simple to accom- plish. Remove all cylinder from the phoropter, and repeat the binocular balance and final refraction sequence to an endpoint that provides the best visual acuity with only a spherical correction. This technique is also useful for prescribing soft toric lenses that are available in limited cylinder power options. Just perform a "best sphere" refraction with the available cylinder power in the phoropter.

K Add power. Think in terms of least possible add. An increase in add power will typically also affect distance visual acuity. In some cases unequal add powers may provide the best balance for distance, intermediate and near vision.

K Dominant eye. Determine the dominant or sighting eye, and don't be afraid to rely on a modified bifocal technique to problem solve. I like to use the +2.00D swinging lens test rather than the "hole-in-the-hand" technique. Have the patient view the distance Snellen chart through his or her full distance correction binocularly. Hold a +2.00D loose trial lens in front of the right eye and then the left. The eye that experiences the most visual disturbance with the fogging lens in front of it is the dominant eye.

K Over-refraction. Over-refract with loose lenses and have the patient view binocularly. For every change that improves distance vision, check to see how near is affected and vice-versa. The patient should make the ultimate decision as to whether the change is an acceptable compromise based on their lifestyle and visual comfort.

If the over-refraction reveals that greater than a 0.50D change is needed to improve visual acuity, change the add power rather than the distance power. In other words, increase the add power if possible if the near over-refraction is +0.75D or greater; decrease add power when the distance over-refraction is greater than -0.50D.

Now apply it to a new option

The new Proclear Multifocal (CooperVision) blends the balanced progressive technology of the Frequency Multifocal with the biomimetic, dehydration resistant Proclear material. The design consists of a "D" and "N" lens, which include a series of spherical and aspherical zones. The "D" lens, typically placed on the dominant eye, has a 2.3mm-wide spherical central distance zone, surrounded by an aspheric ring for intermediate range and finally a peripheral spherical near zone.

The "N" lens has a slightly smaller 1.7-mm central zone offers the most plus power for near, surrounded by an aspheric ring for intermediate range. That in turn is surrounded by the spherical peripheral ring for distance vision optics. Although you'll prescribe one lens with center near optics and one with center distance optics, both lenses provide correction for all distances.

Additionally, the larger optical zone of the "D" lens is advantageous for distance tasks in dim lighting, such as night driving. The fitting guide and above-mentioned general pearls direct trial lens selection. Begin by determining best sphere and dominant eye for "D" and "N" lens specifications. The lens is available in four add powers (+1.00D, +1.50D, +2.00D and +2.50D), but remember to lean towards the least amount of add possible. So if the patient requires a +1.75D add for spectacles, begin with the +1.50 D add in the contact lenses.

Allow the lenses to equilibrate and assess binocular visual acuity at distance and near. Adaptation is similar to prescribing PAL spectacles, so limit changes on the first day. If the fit and binocular visual acuity are acceptable, wait to refine the prescription until after a one-week trial.

Refining the power

If the best sphere refraction indicates that a patient has the potential to see 20/20 with spectacles the visual acuity expec- tations should be as follows:

Binocular Visual Acuity

Distance OU   20/20

Near OU   20/20

If it is not acceptable then refer to the expectations for monocular acuity to determine which lens should be modified.

Monocular Visual Acuity

Distance Vision

"D" Lens Eye   20/20

"N" Lens Eye   20/40 or
   better

Near Vision

"D" Lens Eye   20/40 or
   better

"N" Lens Eye   20/2

Mixing things up   

You can tailor the prescription to individual needs. If altering sphere or add power does not consistently meet a particular patient's near demands, consider prescribing two "N" lenses for occupational needs or vice versa for more precise distance needs. Patients can use over-spectacles for night driving over two "N" lenses or the patient can change to a "D" lens on the dominant eye after work. For the previously emmetropic, early presbyopes, prescribe an "N" lens on the non-dominant eye only.

The Soflens Multifocal (Bausch & Lomb) has been on the market for several years now and will soon be available in the balafilcon (Purevision) silicone hydrogel material. This design is a center-near aspheric with a low add version for early presbyopes as well as a high-add design with an extra plus-powered anterior aspheric cap. The lens is also available in two base curves to allow an optimal fitting relationship on nearly every cornea.

Initial parameter selection once again begins with determining the best distance sphere refraction and dominant eye. Select the add design based on the baseline near refraction. Start with the low Add OU for early presbyopes who require less than +1.50D add. For all others simply select a high Add OU. Refine the distance prescription by adding -0.25D or -0.50D to the dominant eye if necessary and focus on the non-dominant eye for problem-solving, intermediate or near vision. Consider unequal add powers if the over-refraction reveals more than a 0.50D change at any distance.

Finally, do not forget the importance of a proper fit. So, for vague complaints of inconsistent visual acuity that either varies with blinking or becomes worse towards the end of the day, change the base curve. The Soflens Multi-focal can also be successfully prescribed for the previously emmetropic new presbyope. Depending upon the visual demands of the patient, again consider initially prescribing a lens on the non-dominant eye only.

What about the astigmatic presbyope?

Probably just as daunting as the previously emmetropic presbyopes are the significantly astigmatic presbyopes seeking contact lens correction. If the astigmatism is primarily corneal, GP lenses still offer the most stable optics and the ability to correct the astigmatic refractive error. However, most soft contact lens wearers prefer to seek hydrogel options. Two new designs are meeting those needs. The UltraVue 2000T (CooperVision) and the Progressive Toric (CIBA Vision) are toric multifocal lenses that combine the best of both technologies.

A good rule of thumb is to focus on the astigmatic optics for distance first and then refine the multifocal prescription. Use the same troubleshooting pearls previously described. These include modifying final lens power from the use of a sphero-cylinder over-refraction and ToriTrack, and then refining add power based on the patient's experience during the trial period in his or her environment.

The effort pays off

Fortunately, we now have a multitude of contact lens possibilities for both the astigmat and the presbyope. Every practitioner who fits contact lenses can attest that these patients appreciate the opportunity to try different options and typically refer their friends, family members and co-workers. OM

New Silicone Hydrogel Toric Contact Lens
Cristina M. Schnider, O.D., M.B.A.
Director, Professional Education Vistakon

The new Acuvue Advance for Astigmatism (Vistakon) is the first silicone hydrogel daily wear contact lens for individuals with astigmatism. The lens utilizes an accelerated stabilization design that works with the eyelids to balance the lens in place when the eye is open and quickly re-align the lens if it rotates out of position, providing wearers with all-day vision and comfort.

The design makes the fitting process for astigmatic patients easier by reducing chair time. Because it was designed to use the eyelid to stabilize the lens (allowing it to settle into place within one minute and prevent rotation), practitioners can go from "zero to fitting in 60 seconds." This is a marked advantage over the traditional toric lens fitting process, which required waiting up to 15 minutes for the lens to settle before assessing its fit.



Optometric Management, Issue: November 2005