Ординатура / Офтальмология / Английские материалы / Hyperopia and Presbyopia_Tsubota, Boxer Wachler, Azar_2003

1. Advantages of Soft Contact Lenses

Immediate comfort is clearly the advantage of these lenses. Soft contact lenses are generally large in diameter and extend beyond the cornea and limbus and fit under the upper lid margin. Though there is slight movement with each blink because of the size of the lens and the flexibility of the material, little sensation is associated with soft contact lens use. Adaptation to soft contact lenses is rapid and patients can begin enjoying quality vision almost from the moment the lenses are placed.

Key advantages of soft contact lens use are

Adaptation

Comfort

High-quality visual acuity

Ability to use on an intermittent basis

Ease of fitting

Ease of contact lens care

Correction of a wide range of refractive errors

Soft contact lenses can usually be fitted in one visit, with a brief follow-up to ensure that they continue to provide excellent comfort and vision. Few unscheduled visits are required, and patients typically return every 6 to 12 months for a follow-up that includes evaluation for other ocular diseases, such as glaucoma and to ensure the general ocular health as well continued proper use of the contact lenses.

2. Disadvantages of Soft Contact Lenses

Unlike myopes, hyperopes may have difficulties with visualization of contact lenses during handling, since these patients do not have any near point in focus. Deposits can develop on the lenses, which can interfere with comfort and vision. Patients who may be exposed to environmental hazards such as volatile chemicals and those who have poor or inadequate tear film are presently not good candidates for the use of soft contact lenses.

3. Lens Selection

There are a variety of things to be considered in picking a soft contact lens for a particular patient. Most soft lenses come in predetermined parameters from the manufacturer, though some can be custom-ordered for a particular prescription, such as lenses for patients with high astigmatism. Soft contact lenses vary in water content: low range (30 to 45% water), medium content (40 to 58% water), and high content (60 to 80% water). The amount of water is a factor in the oxygen permeability of the lens and also influences comfort and ease of handling. Contact lens parameters include the base curve of the central optic zone and the diameter of the lens. Typically, a trial lens will be placed to evaluate the fit and determine the refractive correction needed to account for the reduced vertex distance and residual astigmatism. Many manufacturers supply practitioners with trial lenses that can then be dispensed on the initial visit. Replacement lenses can then be sent directly to the patient’s home or work for added convenience.

Another key consideration in fitting a soft lens is the wear schedule. For patients who might need intermittent correction, as for social events or sports, one-day disposable lenses may very well be suitable. Other patients do well with daily-wear disposable lenses that are replaced weekly or biweekly. Soft lenses can also be used on a flexible replacement

schedule: weekly, monthly, or quarterly. They can also be dispensed as extended-wear lenses, typically being left in place for 6 to 7 days and then removed and disposed of and replaced by new contact lenses. Past studies suggest that the use of extended-wear contact lenses significantly increases the risk of corneal infections. However, new lens materials have become available that have significantly increased oxygen permeability and disposability of lenses and possibly may have reduced the risks of extended-wear usage. Conventional lenses that are used day in and day out are still available but are less commonly prescribed, since they offer none of the advantages of disposable lenses, including the regular use of a Fuch sterile lens offering assured quality vision and comfort with each new lens.

D. RIGID GAS PERMEABLE CONTACT LENS

The original cosmetic lenses made of a hard material (PMMA) and are still available today but are rarely used in new fittings in the United States. Newer materials afford increased oxygen to the cornea and greater comfort while providing better rigidity than soft contact lenses. Several rigid gas-permeable (RGP) materials are available, including silicone acrylate and fluorine copolymers as well as others.

1. Key Advantages of RGP

The key advantages of RGP contact lenses are that they are manufactured “to order,” allowing for adjustments for an individual’s visual needs to achieve the best fit. RGPs allow for sharp, excellent visual quality and lens durability.

The advantages of rigid gas permeable lenses are

Quality of vision

Durability of lens material

Ability to correct astigmatism

In-office modification possible

Resistance to formation of lens deposit

Increased suitability in patients with poor tear film

Ease of lens handling

2. Disadvantages of Gas-Permeable Contact Lenses

A period of adaptation is needed for the patient to become comfortable with the lenses. This varies from patient to patient but usually is about 2 weeks long. Comfort is not “instantaneous,” and these lenses are less likely to provide the “wow factor,” which may be routine with soft contact lenses. Fitting RGP lenses can be more challenging, but modifications can allow for the best fit for an individual patient. Possible corneal thinning with long-term contact lens wear is another disadvantage.

3. Lens Selection (RGP)

Lens selection criteria for RGP materials will include choosing the material based on its oxygen permeability and its ability to resist deposit formation. Other parameters that need to be considered are the base curve and the peripheral (secondary and tertiary) curves to provide a good fit. Aspheric lenses are also available; these can provide less optical distortion and better-quality visual acuity. As with other lens modalities, wear schedules will

depend the material chosen and patient’s visual needs, whether these involve daily wear or extended-wear use. More recently, frequent lens replacement has been introduced even for rigid gas permeable lenses. Intermittent use is inadvisable, since an adaptation period is needed to achieve maximum comfort with RGP lenses.

E. ASTIGMATISM

Soft contact lenses can be used in patients needing astigmatic corrections. If the patient’s refractive error demonstrates only a small amount of astigmatism (under 1.00 D) and the astigmatism is a small amount of the spherical correction (less than one-third), a soft spherical contact lens will adequately correct vision. However, greater degrees of astigmatism can easily be corrected with toric soft contact lenses. RGP lenses can be used to correct astigmatism whether it is corneal or lenticular in origin. Various methods have been developed to stabilize the soft lens to match the orientation of the astigmatic correction. These include prism ballast (weighting the lens more heavily on the bottom), truncation (removing a section of the upper and/or lower part of the lens), or a combination of the two methods, and “slab off” (a change in the lens periphery using pressure from the eyelids to maintain the position).

The fitting of soft toric lenses usually involves using a trial set and then ordering the appropriate lenses for an individual patient. These lenses have a surface orientation mark to demonstrate whether there is lens rotation, indicating that the astigmatism might not be corrected. One must observe the orientation mark on the soft contact lens when it is fitted on the eye. Typically, the mark is located at the 6 o’clock position, and one must observe whether the rotation is clockwise or counterclockwise. A mnemonic that can help the fitter to remember how to order the appropriate lens is LARS: left add, right subtract. This means that if the lens rotates to the left, one adds to the amount of trial lens rotation to the axis from the spectacle refraction. However, if the rotation, is to the right, then one subtracts the amount of lens rotation from the axis obtained in the spectacle refraction. Soft lenses come in a variety of astigmatic corrections, which are available in disposable or frequent lens replacement styles, for correcting 2 to 3 D. These lenses are not indicated for greater amounts of astigmatism and irregular astigmats. RGP contact lenses, on the other hand, easily correct corneal astigmatism. Typically a spherical RGP contact lens may correct up to 3 D of astigmatism. For greater amounts of astigmatism, a toric RGP lens can be made. Most manufacturers provide customer service information over the telephone or by e-mail through the Internet. This procedure can help in fitting patients who have significant astigmatism that may not be adequately corrected with standard lens materials.

F. PRESBYOPIA

1. Monovison

Currently refractive surgery also offers presbyopic correction primarily with the use of monovison, where one eye is corrected for distance and the other eye for near use. This procedure has been used successfully for many years with contact lens patients. In fact, before refractive surgeries, often a trial monovison using contact lenses is indicated to help a patient decide if this is appropriate for the planned permanent refractive procedure to be done. Monovision offers simplicity both for the patient and the contact lens fitter.

However, some patients do not get used to the 2 D of anisometropia induced with monovision fitting and note reduced stereopsis; they sometimes need an overrefraction in spectacles for driving and other demanding visual tasks. Another option in such patients is to reduce the anisometropia to approximately 1.25 D, which usually resolves symptoms, but by doing so the patient should understand there will be an increased need for glasses for closer vision compared to midrange vision.

2. Bifocal Contact Lenses

There are basically two designs in bifocal contact lenses. Alternating-vision bifocal contact lenses provide for a slight range movement of the patient’s gaze from distance to near. A slight twist of the lens provides an altered path for the light rays into the eye and how it is refracted. The lenses can be made in a segmented bifocal-style, similar to bifocal spectacles, or they can be made in a concentric bifocal style. The distance vision is in the central part of the lens and the peripheral portion of the lens is for near vision. In both styles, the lens must move slightly for patients to have good vision as they change their focus from distance to near.

An alternative bifocal contact lens design provides simultaneous vision, whereby light rays from both distant and near object pass through the lens and pupil. The patient’s brain then selects the object to regard and bring into focus. This type of lens is available in several different types, including the concentric bifocal which has an annular design, with distance vision in the center and the near vision in the peripheral part of the contact lens; an aspheric “multifocal” design, where there is an increase in plus power as one moves from the center to the periphery of the lens as a result of the changing curvature of the lens as oppose to a single based curve; and the diffractive bifocal type, which has small concentric circular facets of varying refractive ability that are alternated to provide the appropriate additional power needed for near vision near the center of the lens. Lenses differ in their ability to be fitted, requiring careful attention to centering of the lens and the relationship of the lens size to the patient’s pupil size. Typically, a practitioner becomes expert in one or two bifocal lens types and acquires the experience and knowledge to pick patients appropriately and fit them quickly. The availability of soft bifocal contact lenses, which may also disposable, allows for an easier fit with the use of trial lenses. As with refractive surgery, however, monovision probably continues to be the mainstay, considering the presbyopic patient today.

G. CONCLUSION

In conclusion, we will probably always have patients who prefer to use contact lenses and spectacles for their refractive correction. These modalities offer quality visual acuity as well as stability and affordability. In addition, the development of new materials for contact lenses, particularly those offering extended wear, may very likely present a competitive alternative to refractive surgery. This is especially true if the material for extended wear demonstrates increased safety and comfort compared to older lens materials. It remains to be seen whether this goal of extended wear and comfort can in fact be associated normal corneal physiology and the maintenance of a risk-free use of contact lenses. With further research and an increasing number of people seeking “hassle free” vision correction, we may yet see the emergence of “permanent” contact lenses.

Figure 3 Importance of corneal diameter in hyperopic refractive surgery.

Also, hyperopic ablation generates a negative longitudinal spherical aberration with worse vision quality, due to the fact that para-axial light rays will pass through the first curvature flexus, which imparts a hyperopic shift (Figs 4 and 5).

However, it is important to remember that these problems are sometimes reduced by the anatomical characteristics of hyperopic eyes. Generally speaking, the size of the optical zone is less important for a hyperopic patient than for a myopic one. The hyperopic eye features smaller axial length, anterior chamber, and corneal diameter. Given the same ablation diameter, an eye with a shallow anterior chamber will enjoy a larger optical zone than one with a deep chamber (Fig. 6), and will have a larger percentage of corneal surface involved by the treatment. Moreover, the two transition zones mentioned above will lie peripherally, positioned in a corneal area with lesser curvature and lesser influence on refraction. In a hyperopic eye with a shallow anterior chamber, the treatment results will be less influenced by pupil diameter: even an optical zone of small size may cover the pupillary area sufficiently, since the treated corneal arc will be closer to the pupillary area and thus able to cover the pupil halos during mydriasis (Fig. 6). What is really important is that the optical zone be truly homogeneous.

Figure 4 Myopic ablation. With a wide ablation area, the optical zone is wide and uniform, without aberrations induced on para-axial rays.

the way the cornea changes from a flatter periphery to a more curved central portion. Eccentricity values (e values) are positive when the cornea is prolate, negative when it is oblate. Normal eccentricity values range between 0.5 and0.6 (normally prolate cornea, curved in the center, flat in the periphery). A hyperopic treatment increases the e value; Figures 7 through 11 show cases with progressively higher e values, approaching a keratoconus-like situation. Central keratoconus, featuring a high eccentricity, with e values of 1.5 or more, amplifies the physiological situation of transition from a curved central cornea to a flat periphery. On the contrary, a myopic treatment induces negative eccentricity, inverting the normal morphology.

2.Longitudinal spherical aberration (LSA). LSA expresses the aberration induced by corneal multifocality. It is a measure of spherical aberration; its increase indicates a decrease in contrast sensitivity.

3.Root mean square (RMS). RMS is a measure of the irregularity of curvature, expressing the amount of deviation from a regular corneal curvature.

4.Surface asymmetry index (SAI). SAI is a measure of corneal symmetry in the pupillary area. Its increase leads to an increase in coma.

In the past, it was generally thought that excessively high corneal curvature values would lead to a keratoconus-like situation. From our studies, it appears that an important factor for this complication is the corneal curvature gradient (eccentricity). Interestingly,

Figure 7 through 11 Eyes with progressively higher e values, approaching a keratoconus-like situation.