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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