Ординатура / Офтальмология / Английские материалы / Contact Lenses in Ophthalmic Practice_Mannis, Zadnik. Coral-Ghanem, Kara-Jose_2003

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visual demands and priorities, and promotes the selection of a modality that best meets the patient’s needs.

The amount of add power needed by the patient is important in selecting an appropriate modality. Thus, it is useful to divide presbyopia into three stages: early, moderate, and advanced (Table 12.1).

2. What is monovision?

Monovision is the correction of one eye for distance vision and the other eye for near tasks. This method of presbyopic correction is extremely flexible and can be used with spherical or toric hydrogel lenses or rigid gas permeable lenses. The technique can even be modified to incorporate multifocal correction in one eye and single vision correction in the other. Most patients tolerate monovision reasonably well. Success rates of 70% to 80% have been reported.12 The patient’s success with monovision ultimately depends on his or her ability to suppress a central, out-of-focus image in one eye while maintaining peripheral fusion. Many patients seem to have less difficulty suppressing a blurred image from one eye than adjusting to the superimposed images created in each eye by simultaneous vision lenses.

3.How do you fit a patient with monovision contact lenses?

1.Determine the patient’s balanced distance correction and an appropriate add power.

2.Determine the patient’s dominant eye.

3.Fully correct the distance refractive error in the patient’s dominant eye. Choose a lens that adequately corrects near vision for the nondominant eye.

4.Adjust the power in 0.25-D increments to maximize visual performance.

Monovision is easily demonstrated. Simply hold up a loose plus lens over the nondominant eye, and allow the patient to alternately look at

Table 12.1. Options for correction of presbyopia with contact lenses

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distant and near objects. Initially, the patient may be most comfortable if the imbalance between the two eyes is minimized. Even if the refraction indicates that a patient needs a 1.25 add to achieve 20/20 acuity at near, he or she may need only 0.75 or 1.00 over the distance correction to meet the general near visual demand. Most patients can eventually tolerate a reduction in distance acuity to 20/60 or 20/ 70 in the near eye. This corresponds to an add of approximately 1.75 D. More than 2.00 D of undercorrection in the near eye may lead to asthenopia due to a loss of stereopsis.

Prescribe spectacles with full distance correction in both eyes and the appropriate add for prolonged, detailed visual tasks. Also consider prescribing ‘‘driving glasses’’ (spectacle correction to fully correct distance refractive error in the ‘‘near’’ eye) or an extra contact lens for the near eye that fully corrects distance refractive error, particularly if the patient’s distance acuity is less than 20/40 in the ‘‘near’’ eye. Most contact lenses are very stable on the surface of the eye, but a monovision patient could have difficulty if the lens that corrects distance refractive error becomes dislodged for any reason while driving. Warn beginning monovision wearers that their depth perception may initially be reduced and instruct them to take extra care when driving.3

In general, a patient is most comfortable when his or her dominant eye is corrected for distant visual tasks.2 However, there are some exceptions to this rule. If patients spend a majority of their time on detailed near tasks, they may prefer to use the dominant eye for near work. Individuals who have better visual acuity in one eye generally prefer this eye for the distance correction. For example, a spherical hydrogel contact wearer with unilateral astigmatism will probably prefer to use the eye without astigmatic refractive error for distance tasks.

4.What are the advantages and disadvantages of monovision correction?

Advantages

1.Easily demonstrated.

●To show a patient how monovision works, simply hold a plus lens over one eye while the patient is wearing full distance correction and allow him to look at both distant and near objects binocularly.

2.Considerable flexibility.

●Any single vision lenses can be used in monovision correction. If the patient finds the distance acuity compromise with single vision lenses unacceptable, a multifocal lens can be used for the ‘‘near’’ eye.

3.Relatively low cost.

●Because single vision lenses are most frequently used, this method of correction costs no more than wearing full distance correction.

Disadvantages

1.Loss of stereopsis. Monovision decreases fine stereoacuity.45 Patients are most aware of this loss of depth perception immediately after

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beginning monovision contact lens wear. This discomfort generally subsides as the patient’s ability to preferentially suppress the image from one eye improves. Nevertheless, it is best to avoid monovision correction in individuals whose daily tasks require excellent stereoscopic vision.

2.Reduction in distance acuity. Most individuals see better when using both eyes than when using just one eye.6 Thus, when we intentionally decrease the distance acuity in one eye, patients frequently report that their vision does not seem as clear as it was with full distance correction in both eyes. Again, most patients adjust to this within a few weeks of monovision wear. Because of this, however, monovision should be avoided in individuals who require excellent distance acuity in both eyes, such as professional drivers and aviators.

3.Difficulty with night driving. In low ambient illumination, the pupils dilate. This increases spherical aberration. Lack of visual contrast can increase image confusion, especially when one eye is not corrected accurately for distance. As discussed above, patients may benefit from a pair of glasses that brings the undercorrected eye to 20/ 20 distance acuity, or by having a third contact lens (or lenses) with full distance correction for the habitual near eye.

4.Reduction of contrast sensitivity. The decrease in contrast sensitivity is similar to that encountered in bifocal contact lens users with bilateral simultaneous vision.7

5.What are the favorable and unfavorable

factors for success in monovision contact lens correction?

Favorable Factors

●Patients who have historically accepted less-than-optimal distance correction typically have little difficulty adjusting to monovision’s compromise in acuity. For example, if patients have been satisfied with 20/25 spectacle acuity in each eye, they may not even notice the slight decrease in acuity caused by monovision correction.

●Emerging presbyopes are good candidates for monovision. Because the difference between their distance and near correction is small, the disparity between the two eyes is minimized. The near visual complaints can often be addressed with as little as 0.75 over the ‘‘near’’ eye. This would minimally impact distance acuity and stereopsis.

●Individuals whose near visual demands are not primarily in downgaze, such as computer users, will appreciate the clear vision in any field of gaze with monovision contact lenses.

●Patients with well-developed suppression patterns are good candidates for monovision. For example, an asymptomatic intermittent alternating exotrope’s visual system has already developed the ability

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Figure 12.5. A: Bifocal contact lens with diffraction design (frontal). B: Bifocal contact lens with diffraction design (profile).

a distant object, light from that object passing through the distance portion of the contact lens focuses a clear image on the macula. Light from the distant object also passes through the near correction zone and forms an image in front of the retina. This image appears blurred and is superimposed on the clear image. When the patient focuses on near objects, light that passes through the near correction zone of the lens is focused on the retina, and light rays passing through the distance correction focus behind the retina. The contact lens wearer may be bothered by the ‘‘shadows’’ or ‘‘ghost images’’ (superimposed blurred images) around clear central images when looking at both distant and near visual targets. As in monovision, visual adaptation occurs with time. Patients who continue to wear the lenses in spite of the initial visual disturbance report a decrease in visual symptoms within the first several weeks.

8.What are the major categories of simultaneous image contact lens designs?

Simultaneous image lenses can be categorized into three basic types: concentric design, aspheric design, and diffraction-based design. Each design uses a different method of providing both distance and near correction, but all three simultaneously focus light entering the eye at various points in relation to the retina. Simultaneous image designs are available in both rigid gas permeable and hydrogel materials.

9.What are the defining characteristics of concentric simultaneous vision lenses?

Concentric simultaneous vision lenses have well-defined zones of correction for various distances arranged in a bull’s-eye pattern. The lenses can have either the distance or near correction in the center of the lens. Some lenses also incorporate zones of correction for intermediate distances. The presence of distinct zones of correction for various distances is the defining characteristic of lenses with this design.

The AcuVue Bifocal (Vistakon), a lens designed for 1- to 2-week

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replacement, incorporates this design. AcuVue Bifocal lenses feature distance correction in the central zone with four additional alternating concentric rings of near and distance correction. The design, with its multiple correction zones for both distant and near tasks, attempts to minimize visual compromise in dim illumination. The Horizon 55 BiCon (Westcon) is a conventional (annual replacement) hydrogel lens with a central near zone with a peripheral distance correction zone. The Lifestyle MV2 (Lifestyle Company) is a monthly replacement hydrogel lens. This lens incorporates a concentric design with a central zone for intermediate vision combined with either peripheral distance correction or peripheral near correction. Generally, a lens with peripheral distance correction is placed on the patient’s dominant eye, while a lens with peripheral near correction is placed on the nondominant eye. Computer users and others who have specific intermediate visual demands may appreciate the central zone of correction for their primary visual tasks.

10.What factors must be considered when fitting concentric bifocal lenses?

Adequate pupil size is essential for success in fitting any lens with a concentric design. If the pupil is too small, light passing through more peripheral correction zones may not be able to enter the eye. If the pupil is too large, the patient may notice more ghosting as light passing through the peripheral zones of the lens focuses in front of or behind the retina. Consider a patient with 3-mm pupils. The patient should be able to read well with a concentric design with central near correction. However, the patient would probably report unsatisfactory distance vision with this lens, because the small pupil size would keep light from passing through the peripheral distance correction zone from entering the eye. If we fit a lens with a central distance correction zone, the patient’s distance vision would improve, but he or she would not be able read comfortably. A patient with 7-mm pupils will experience more image confusion with either distance center or near center designs because the large pupil will continuously allow light from both zones to enter the eye.

The lenses also need to center well and move minimally to avoid fluctuations in vision. Some concentric designs allow the contact lens fitter to specify the size of the central zone to maximize visual acuity and minimize ghosting.

11.What are the defining characteristics of aspheric simultaneous vision lenses?

Contact lenses with an aspheric design are also called multifocals. The gradual flattening of curvature from the center to the periphery of the lens accompanies progressive power change. This progressive power change resembles the power change present in progressive addition spectacle lenses. Aspheric lenses with both distance and near central zones of correction are available.

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Focus Progressives (CIBA) are aspheric lenses and are available as either 2- to 4-week replacement or single use disposable lenses. These lenses have a concentrated central zone of near correction surrounded by progressively increasing distance correction. SofLens Multifocals (Bausch and Lomb) are also aspheric lenses with near central correction. These lenses are available as 2- to 4-week replacement lenses, and they allow the clinician to specify either low or high add power. Frequency 55 Multifocals (CooperVision) are available as monthly replacement lenses. Like the MV2 , this lens system features two designs: one with central distance correction and the other with central near correction. Rigid gas permeable aspheric multifocals are also available. Examples include the Unilens (Unilens Corp.) and the Essential RGP Multifocal (Blanchard Contact Lens).

12.What factors must be considered when fitting aspheric simultaneous image contact lenses?

Concentric and aspheric multifocal contact lenses all use the same basic optical principles; light is focused differently through central and peripheral zones of the lens. Thus, pupil size and lens centration are very important factors to consider when fitting aspheric simultaneous image lenses.

Because aspheric lenses flatten toward the periphery, the proper fit for a back-surface aspheric rigid gas permeable lens requires a base curve that is steeper than one would expect based on keratometry readings. It is possible to order these lenses based on the patient’s refractive error and keratometry readings, but the evaluation of the performance of trial lenses enhances success. Most trial sets include fitting guidelines for the specific designs, and these guidelines provide an excellent starting point for the contact lens fit.

Aspheric lenses correct vision at a range of working distances, just like progressive addition spectacle lenses. Patients with a fair amount of intermediate work may therefore prefer aspheric lenses to concentric designs.

13.What are the defining characteristics of diffractive simultaneous vision lenses?

Diffraction of light occurs when light waves interact with obstacles. This interaction changes the trajectory of the light rays. Diffraction bifocal contact lenses utilize this optical principle by incorporating a diffraction plate (a series of concentric ridges) into the center of the lens. Differences between the curvature of the front and back of the contact lens correct distance refractive error, and the plate diffracts light to provide the additional correction necessary for near tasks. Thus, these lenses use both refraction and diffraction to provide good vision at all distances.

The Hydron Echelon (Ocular Sciences, Inc.) is the only hydrogel lens that uses this design. Unfortunately, the lens is not available as a

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planned replacement lens. Patients also report ‘‘ghosting’’ of images with this lens. Furthermore, clear vision heavily depends on excellent illumination, because much light is diffused as it passes through the diffraction plate. Good vision is not as dependent on pupil size as in other simultaneous vision lenses because all light passing through the diffraction plate contributes to images of both distant and near objects. The lenses have not been widely used because of the limited range of parameters available, conventional (annual replacement) use, and patient reports of poor contrast sensitivity in anything but perfect illumination.

14. What is modified monovision?

There are two types of modified monovision:

1.Modified monovision I utilizes bifocal or multifocal contact lenses for both eyes. The dominant eye is more fully corrected for distance, and the nondominant eye is more fully corrected for near tasks.

2.Modified monovision II utilizes a single vision lens that fully corrects distance refractive error in the dominant eye, and a simultaneous vision bifocal contact lens in the nondominant eye.

15.What are the advantages and disadvantages of simultaneous image lenses?

Advantages

●Ease in fitting. Each brand of hydrogel simultaneous image lenses has a specified range of parameters. With available diagnostic trial lens sets, it is easy to see whether the lenses will work for a given patient. For example, if a patient is interested in AcuVue Bifocals , trial lenses can be inserted. Because AcuVue Bifocals are available in a single base curve, whether the lenses will provide an adequate fit can be determined quickly. If the lenses move, cover, and center well, the power can be adjusted. If the fit is not acceptable, another option can be considered.

●Good comfort. Patients who have worn single vision contact lenses (either hydrogel or rigid gas permeable) will notice little difference between the comfort of their single vision lenses and simultaneous vision lenses. With the introduction of the frequent replacement hydrogel bifocal lenses, hydrogel wearers can continue to enjoy the comfort and health benefits of disposable lenses.

●Acceptable intermediate vision. Aspheric lenses provide some correction for intermediate visual tasks. Some concentric designs, such as the MV2 , also provide intermediate correction. Simultaneous vision lenses can be used in modified monovision to provide even better intermediate clarity.

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Disadvantages

●‘‘Ghost’’ images. All simultaneous vision lenses work by focusing light in various planes parallel to the retina. Therefore, it is impossible to completely eliminate the blurred image that is perceived along with the clear image of the object of regard. Reassure patients that their awareness of the ‘‘ghost’’ images will fade with time.

●Dependence on pupil size. As discussed above, the quality of the patient’s vision will be reduced if pupils are either too large or too small. Simultaneous vision lenses may not be the best choice for the blue-eyed patient with 7-mm pupils in normal room illumination or for the 68-year-old with 3-mm pupils in a dark room.

●Fitting requirements. Because good vision depends on excellent centration and little movement, these lenses are frequently fitted rather tightly. Undesirable physiologic effects, such as limbal vascular congestion or edema, may ensue.

●Limited add power available. As with monovision, visual disturbances (halos, glare, ghost images) increase as add power increases. Mature presbyopes may have difficulty adjusting to these symptoms.

16.What are the favorable and unfavorable factors for success in simultaneous image contact lens wear?

Favorable factors

●Patients with realistic visual expectations are more likely to wear simultaneous vision lenses successfully. Before beginning the fitting process, make sure that the patient understands that contact lenses cannot restore pre-presbyopic vision. A successful bifocal contact lens fit is one in which the visual needs of the patient are met. This may mean that distance vision is not as clear as with single vision distance correction. A successful bifocal contact lens fit may not provide equal acuity in both eyes; in some cases, a modified monovision technique must be employed.

●Patients who are highly motivated to wear contact lenses are more likely to succeed with simultaneous vision lenses. Adjusting the design and power of the lenses to maximize visual performance takes more time than fitting single vision lenses. Motivated patients are more likely to complete the process.

●Spherical hydrogel multifocals work best for patients with minimal astigmatic refractive error. Patients with more than 0.75 D of astigmatism should consider rigid gas permeable lenses, multifocal toric hydrogels, or monovision.

●The use of diagnostic trial lenses during the initial fitting visit allows the contact lens fitter to observe the performance of the contact lens on the eye and the visual response of the patient. Diagnostic lenses also allow the patient to experience the vision provided by these lenses and to develop realistic expectations for final visual acuity.