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Module 34 |
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Module 34: |
Soft Contact Lenses: |
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Part 3 | ||
| Fitting Astigmatism and Fitting Presbyopia | |||
| Table of Contents
(with bookmarks)
Soft lenses for the astigmatic patient Does the patient really need astigmatic correction? Toric soft lenses vs hard gas permeable lenses How much astigmatism correction does the patient need?
Over-plusing the contact lens power
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Soft Lenses for the astigmatic patient
Does the patient really need astigmatic correction?
Not all low astigmats (<1.25 D) require astigmatic correction. Not all of them want astigmatic correction, particularly if the dominant eye does not have significant astigmatism. If in doubt, trial fit with spherical soft lenses and over-refract. Show the patient the difference between astigmatic correction and correction with the spherical equivalent. Toric lenses are more expensive to replace, although the cost differential has come down significantly in recent years. Toric lenses generally are not as comfortable as spherical lenses. The difference in vision may not outweigh these other factors. The trial contact lens that you use may make a difference in this determination. Thicker, lower water content soft lenses tend to "mask" low amounts of astigmatism better than thin lenses.
Toric soft lenses vs. hard gas permeable lenses
To be able to offer the astigmatic patient the best lens for his/her eyes, we need an understanding of the options in terms of lens design and the advantages and disadvantages of each type.
The patient with significant astigmatism (generally > .5D) can be fit with toric soft contact lenses or hard gas permeable (HGP) contact lenses. Although HGP lenses can efficiently fit almost any degree of astigmatism, many astigmats less than 2.50 D are fit with toric soft lenses because of the initial comfort factor.
Toric soft contact lenses correct astigmatism by providing a cylinder correction in the lens itself. The correction is similar to a glasses correction. For this to work, the lens must not rotate significantly on the cornea. |
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Rotational stability is accomplished with a variety of lens designs, including weighting the bottom of the lens (prism ballast, top image) and/or flattening the bottom edge of the lens so that it rests against the lower lid (truncation, bottom image).
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| The conventional HGP lens corrects astigmatism with a "tear lens". This rigid, spherical lens is fit to one curve of the cornea (picture on the left). Because of astigmatism, the lens does not fit the other curve perfectly, but the space between the lens and cornea is filled in with tear fluid (picture on the right), which acts as a refracting surface to correct the astigmatic error. The tear lens is represented by the black area in the picture on the right. |
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For the conventional HGP design, lens rotation does not affect the astigmatic correction. The conventional HGP lens works well only for low to moderate amounts of astigmatism. HGP lenses can also be designed to fit the cornea exactly for higher amounts of astigmatism (bitoric design). A bitoric HGP design is more complicated and more costly.
The toric soft contact lens must maintain little or no lens rotation on the eye for maximum acuity. The regular HGP lens does not have this problem. For this reason, it is best to determine the patient's cylinder power and axis "sensitivity" before a lens type is recommended. If the patient notices a decrease in vision with a small axis rotation, she may be better off with an HGP lens fitting.
At the completion of the manifest refraction, have the patient view a line on the chart one line above the line of best corrected visual acuity (BCVA). Ask the patient to let you know when she notices the letters blurring. Slowly rotate the cylinder axis away from the refracted axis. Stop when the patient notices blurring. The axis sensitivity is the degree of rotation from the refracted axis. For example: suppose the refracted axis is 90 and the patient reports blurring at axis 100, then the axis sensitivity would be 10 degrees. Usually, the greater the cylinder power is, the more sensitive the patient is to axis rotation.
If the patient has an axis sensitivity of 5 degrees, and the best fitting soft toric lens frequently rotates 10 degrees off axis, then the patient may be better off wearing HGP lenses.
How much astigmatic correction does the patient need?
A similar procedure can be followed for cylinder power. The cylinder power is reduced by .50 D while simultaneously changing the sphere .25 D to keep the spherical equivalent. For example: change a +1.50 D cylinder power to +1.00 D, and at the same time change the sphere power .25 D in the plus direction. Again, the patient reports when blurring occurs. The distance traveled on the cylinder power wheel is the measure of cylinder power sensitivity. For example: suppose the refracted cylinder power is 1.5 D. The patient reports blurring at 1.00 D. The cylinder power sensitivity is thus .5 D.
Most patients do well with an automatic .5 D reduction in cylinder power in the SCL prescription. Depending upon the cylinder power sensitivity, a greater reduction may be possible.
A diagnostic or "trial" set of soft toric contact lenses (torics) should be used. Some practitioners have one or two favorite lens designs, and they fit these from inventory. Evaluating a lens with the exact parameters needed decreases the "chair time" needed to fit torics. However, many manufacturers have liberal return policies that allow you to fit from a limited trial set and return an ordered lens that may not be optimum when worn by the patient.
Toric lens parameters vary from brand to brand. Variables include the following:
As you can imagine, fitting torics form inventory requires a lot of lenses. To help you sort things out, there are publications that organize all this information for you (e.g. Tyler's Quarterly). Obviously, you will be spared some frustration by beginning your fitting with a brand that includes the parameters that you need for a particular patient.
To begin your toric lens fitting, you will need the manifest refraction and the keratometry readings. Toric lenses usually have a large diameter to aid stability, so measuring the patients cornea is usually not necessary.
Just as with spherical soft lenses, the glasses lens power (or manifest refraction) will need to be adjusted for vertex power. Toric vertex adjustment is a little tricky because you may have to adjust both the sphere power and the cylinder power. Let's look at an example:
MR = -8.00+2.00X180
First of all, we convert to contact lens language (minus cyl.).
MR = -6.00-2.00X90
From the optics modules, you may (hopefully) remember that we are really dealing with two different lenses. On an axis cross, the powers convert to:
If we look up each of these powers on a vertex conversion chart, -6.00 D at a vertex distance of 12mm converts to -5.62 D at zero vertex distance, and -8.00 D converts to -7.25 D. Therefore, our CL Rx will be -5.62-1.63X90. You can also use the optics calculator to arrive a the same answer. Our ideal toric trial lens power would be -5.50-1.50X90. Remember that you want to start with a cylinder power that is less than the refracted power. Many manufacturers offer -1.25 and -1.75 cylinder powers and do not offer a -1.50 cylinder power. If this is the case, you would want to start with the -1.25 cylinder power.
Just as with a spherical contact lens fitting, you will follow the manufacturers recommendation for the starting base curve. Most lenses come in "medium", "steep", and "flat" base curves.
Example:
Av. K reading Base Curve
< 43.00 9.1 (flat) 43.00 - 45.00 8.7 (medium) > 45.00 8.4 (steep)
The same basic evaluation criteria used with spherical soft lenses (Modules 32 and 33) also applies to toric lenses.
For best vision, the axis of the contact lens cylinder correction should line up with the axis as determined by the manifest refraction. All is well if the contact lens rotates to the correct position after insertion, and it should stay in the correct orientation throughout the day. This, of course, is not always the case.
All toric soft lenses have some type of markings that assist in the evaluation of lens rotation. It might be a single mark at the 6 o'clock position, marks at 3 and 9 o'clock, three marks at 5, 6 and 7 o'clock, or some other marking scheme. The marks are viewed with a slit lamp to determine the degree of rotation of the lens. The rotation of the lens should not be evaluated until the lens has had time to stabilize on the eye. Ten to 15 minutes after insertion is usually sufficient time for stabilization.
If a toric lens consistently rotates to the same position on the eye, then the prescription can be adjusted to accommodate the rotation. For this adjustment, the "LARS" system is used. LARS stands for "left-add, right-subtract". If the lens is rotating to the right, then the degree of rotation is subtracted from the refractive axis. If the lens is rotating to the left, then the degree of rotation is added to the refractive axis.
Let's look at an example:
This patient is wearing a toric soft lens (light blue color over a blue iris) with a 6 o'clock rotational marker and two marks 15 degrees to either side. Ideally, after the lens has stabilized, the center marker should be at the 6 o'clock position as you look at the lens with a slit lamp.
Let's suppose that our lens stabilizes to a position with the lens rotated somewhat counterclockwise, like this:
The arrow marks the 6 o'clock position, so our lens has not rotated enough for the left 15 degree marker to align with the 6 o'clock position. So, we will have to estimate the degree of rotation. It looks like it has rotated about 2/3 of the way toward the marker. We estimate that it has rotated 10 degrees.
Let us say that the patient's glasses prescription is -2.00-1.25x170, and this is the power of the lens that we have inserted. Our lens has rotated to our right. The LARS rule is: left add, right subtract. Therefore, we will subtract 10 degrees from 170 to get a contact lens axis of 160. This means that we will order the patient a new lens with the axis at 160. Theoretically, the new lens should behave the same way, rotating 10 degrees to the right after stabilization. If so, the cylinder axis will be in the correct position for good vision, at axis 170.
As with a spherical soft lens, the toric lens should center well and exhibit some movement, at least when pushed with the lower lid. A loose fitting toric lens will generally not be a successful fit, as the excessive movement contributes to lens rotation and blurry vision.
Once an acceptable fit has been achieved, with minimal rotation after being allowed enough time to stabilize, an over-refraction should be performed. Perform the refraction as you would with an eye not wearing a contact lens. In other words, perform the normal refractometry routine, checking sphere, cylinder axis, and cylinder power. Record the results. To arrive at the final contact lens prescription, place the trial contact lens power specifications (adjusted for rotation) into a trial frame. Then place the over-refraction results into the same trial frame along with the other lenses. Now place the over-loaded trial frame onto a lensometer and read the final contact lens power Rx. This is the contact lens power that you will order, along with the other specifications of the particular trial lens that was used. If the results of the over-refraction were minimal, you can dispense with this final exercise and simply order the specifications of the trial lens as adjusted, if necessary, for lens rotation.
The insertion, care, and handling of toric soft lenses are very similar to that of standard soft lenses. The patient need not pay particular attention to the orientation of the insertion, as the lenses should rotate to the proper position in a short period of time. The patient should, however, pay particular attention to inserting the lenses so that they are not inside-out. This is usually not a problem as most toric lenses give an obvious "taco test".
The patient should be seen back in a week or two for a review of the fit. Many soft toric contact lens vision and comfort complaints originate from toric lens dehydration. This may become apparent in the return visit, but may not show up until much latter. If the symptoms are relieved by regular use of re-wetting drops, then the patient will need to make this a part of his routine.
If the patient complains of intermittent blurring, then the lens may be too loose, allowing for excessive rotation upon blinking. A steeper base curve may be necessary.
If the patient complains of constant blurring, the lens may be constantly misaligned, or the prescription may be wrong. If the fit seems adequate (not too steep or flat), a predictable axis misalignment can be again adjusted with the LARS principle. The prescription can be rechecked with over-refraction if necessary. When evaluating vision complaints, always keep in mind that the patient may have switched the lenses. If all else fails, the fit can be started from scratch before soft lenses are abandoned for RGP lenses. Some would argue, why put up with this non-sense? Simply fit RGP lenses in the first place! To be fair, many patients are successfully fit with soft toric lenses the first time.
Presbyopia is the term for difficulty focusing at near due to the aging lens of the eye. Symptoms usually first occur between the ages of 40 and 50, with most patients needing some degree of near compensation by age 45. Near compensation means more plus power, which brings the near point closer. For more information on accommodation and near correction, see Module 4.
There is no ideal contact lens correction for presbyopia. The presbyopic contact lens wearer must compromise, therefore, motivation is a key factor in successfully fitting the presbyope with contact lenses. Before modifying the contact lenses for near vision, be sure that your patient understands that readers can be worn over distance powered contact lenses. This is not always obvious to them, and it is the most simple solution. Of course, not everyone wants "simple", some do not want glasses, even for near vision.
Over-plusing the contact lens power
Helping the new presbyope who already wears contact lenses may be as simple as over-plusing the correction in both eyes. Over-plusing means changing a +1.50 correction to +2.00 or changing a -3.00 correction to -2.50. Hyperopes usually tolerate this better than myopes. The idea is to over-refract binocularly in the plus direction until the distance vision is unacceptable, and then back off a little until it is acceptable. If overplusing is not acceptable, then monovision may be tried.
Monovision is a technique that corrects only one eye for near vision, while leaving the other eye for distance vision. Not everyone can tolerate having one eye blurry all the time. The success rate is enhanced by keeping the dominant eye the distance eye, and by minimizing the power difference between the two eyes.
There have been many methods described for determining the dominant eye, and some of them don't work very well. Try first asking the patient which eye is dominant, Many patients already know what you are talking about and can give you the information. My personal experience has been that the vast majority of right handed people are also right eye dominant. My experience has also been that many left handed people are also right eye dominant. There are some people who seems not to have a dominant eye.
If the patient is not aware of eye dominance, there are two methods that may work to determine dominance:
The camera method: Keep an old camera handy and have the patient take a picture. Whichever eye is used to look through the viewfinder is the dominant eye.
Look through the hole method: The patient extends both arms and holds the hands together so that a hole is formed through which the patient views a distant object. You cover one eye at a time and ask if the object can be seen. Whichever eye it is that can see the object is the dominant eye.
Keep in mind that most people are right eye dominant, including almost everyone who is right handed. But, this is not always the case. That's why we check.
When adjusting the contact lens power for monovision, the distance eye is given as much plus power (or as little minus power) as the patient will tolerate before complaining about blurry distance vision. This narrows the power difference between the two eyes and improves the intermediate and near vision. The near eye is given the minimum additional plus power to do the job. Keep in mind that, because of the vertex distance change, less plus power will be needed than what is given in the glasses bifocal correction. In other words, a +0.75 add with contact lenses is equivalent to a +1.00 add power in a glasses bifocal. When in doubt, give less power and have the patient try it for a few days.
Examples:
A 45 year old right eye dominant presbyope with a distance contact lens correction of -3.00 OD and -3.00 OS might end up with a monovision contact lens correction of:
OD -2.75 OS -2.25
Our patient tolerates a .25 reduction in the minus correction for the right eye, which focuses at distance. He can read well enough with a +0.75 D "add" for his left eye, which is the reading eye.
A 60 year old right eye dominant presbyope with a distance contact lens correction of +1.50 OD and +1.50 OS might end up with a monovision contact lens correction of:
OD +2.00 OS +3.25
This hyperope can tolerate a .5 D increase in the correction for the right eye, which is the distance eye. He can read well enough with a +1.75 D "add" for his left eye, which is the reading eye.
From the above you might surmise that the 45 year old will have the greater chance of success because there is less power difference between the two eyes. If your patient does not tolerate monovision, bifocal contact lenses may be considered.
Bifocal contact lenses offer the possibility of success (compromised) for some of your contact lens patients who do not want to wear glasses for anything. The downside for the fitter is increased "chair time". The downside for the patient is increased expense.
The are several design types of bifocal contact lenses. The simultaneous view bifocal design uses concentric circles of different powers that project focused light from two different distances simultaneously on the retina. The aspheric bifocal design is a type of progressive power design. The translating or alternating bifocal design is similar to the bifocal in a pair of glasses. The optics of these designs are complex and are beyond the scope of this module. They are mentioned in order to familiarize you with the vocabulary.
The fitting techniques and characteristics of bifocal contact lenses are very specific to the brand. Proper alignment on the cornea is usually critical, and pupil size plays a role. Many practitioners prefer to begin with a "modified monovision", meaning one eye is fit with a conventional contact lens and the other eye is fit with a bifocal contact lens.
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