Ординатура / Офтальмология / Английские материалы / Diagnosing and Treating Computer-Related Visual Problems_Sheedy, Shaw-McMinn_2003

94 Diagnosing and Treating Computer-Related Vision Problems

However, in many other cases, an unusual computer location indicates a problem that should or can be fixed before prescribing. Some patients who are “fighting” the onset of presbyopia have learned to push their computer away from them. Patients with myopia in the range of 2.00–3.50 diopters (D) may be taking their glasses off for computer work and, therefore, working at a distance that is too short. Computer screen locations that are too high are common, especially for shorter people. If the screen is too high and the patient has an upright posture at the computer, then he or she should be counseled about lowering the computer screen. This can often be accomplished by removing the computer from atop the central processing unit. Computer locations that are too low can easily be made higher with spacers—a phone book often works well.

If the current screen location is appropriate or if it cannot be altered, then this information can be used for prescribing. If not, then the prescription should be designed for the altered location of the computer screen.

If specific information about the computer location is not available, it is probably best to design the prescription and lenses for a 60-cm (24-in.) viewing distance and a screen center located 15 cm (6 in.) below eye level. This assumption has a greater likelihood of being valid if the person is working at a fairly standard computer workstation for word processing. However, many workstations have visual requirements that are significantly different from this arrangement, especially for special applications.

Determining the Addition for the Computer Display

In some cases, beginning presbyopes are able to use their normal glasses at the computer, because they do not yet require an add for the intermediate distance and are using the distance portion of their glasses. This can be the case if they have bifocals or PALs. However, once a patient begins to require an add for the computer viewing distance, his or her usual correction is seldom acceptable.

It is very common for a presbyopic individual to require his or her first near correction for computer work, although the computer viewing distance is farther from the eyes than most other near tasks, and it

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might be expected that this task would not require a correction until later in life. This may be because of the extended near viewing that is often required by work at a computer and also because of the fact that most computer users also need to see at normal reading distances for part of their workday. It is very common for patients who would normally be considered prepresbyopic to require a near prescription at their computer. These patients can be in their thirties and often exhibit a reduced amplitude of accommodation for their age or poor accommodative facility. A near prescription can be very successful at improving their function and comfort at the computer.

Final prescription determination (add power) is often best determined by demonstration. For advanced presbyopia, the required add for the computer distance is usually 0.50–0.75 of that required for 40 cm. A straight calculation based on the 40-cm add often does not work for various reasons. It is important to determine the needed add at the viewing distance of the computer and to measure the range of clear distances through the add. Demonstration to the patient is important.

The measurements and demonstration can be made with a card on the near point rod of the phoropter, although some near point rods are not long enough to enable testing at longer computer-viewing distances. It is often very effective to make the final prescription measurement in free space by seating patients at a computer workstation and using a meter stick to locate their eyes at their usual distance. Simulated computer screens, including the PRIO unit, are available to serve this purpose. This also reminds your patients about the special care they are receiving for their computer work. Lens flippers can be very useful to quickly demonstrate different add levels. In combination or separately, lens flippers can be used over the patient’s old prescription or distance prescription in a trial frame to demonstrate different adds and to measure working distance ranges.

The patient’s intermediate needs, near viewing needs, and distance viewing needs should be determined by history, and the prescription and lens design determined accordingly. Although the primary visual needs are at the intermediate distance, nearly all computer users also have some viewing requirements at 40 cm. Distance visual needs are often considerably less important and can often be compromised in the interests of best meeting the intermediate and near needs.

96 Diagnosing and Treating Computer-Related Vision Problems

Now you have the distance refractive correction, the normal 40 cm add, and the add required for the computer. The prescription that is written depends on the lens design selected for the patient.

Single Vision Lenses

Younger presbyopic computer users are often successfully fitted with single vision lenses designed for the computer distance. This is because they have enough remaining accommodation to see clearly at 40 cm with the add for the computer distance (nearly all computer users still have extensive viewing needs at 40 cm). In demonstrating the add, it is important to measure the range of clear vision to ensure that it also includes the patient’s near visual needs if you are considering prescribing single vision lenses. In most of these cases, an add of +0.75 to +1.25 is appropriate. If it is possible to accomplish without compromising the patient’s near vision, trim the add on the low side to decrease the distance blur.

The distance blur through this single vision prescription should be demonstrated to the patient. For most patients, the distance blur is acceptable because their distance visual needs in the office are limited, and the amount of blur created by the relatively low add is small. Usually, vision for conversation with others and for walking around the immediate confines of the office is good. However, if the distance blur is disturbing to the patient, another design is indicated.

A single vision correction offers the advantages of a large, clear field of vision, and it is also the least costly alternative.

Occupational Progressive Addition Lenses

Occupational progressive lenses are specially designed lenses that often can meet the presbyopic patient’s computer vision needs very successfully, especially for those patients who wear PALs for general wear. In fact, most PAL wearers would find it unacceptable to wear a pair of bifocals. Clinical trials support the acceptance of occupational progressives by computer users (Bachman, 1992; Brischer et al., 1994; Hanks et al., 1996; Butzon and Eagles, 1997). Most major lens manufacturers have a

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lens design in this category. These lenses are seamless (i.e., they are based on PAL technology) and are designed to provide the appropriate add power for the required viewing angles in the computer environment. All of the lenses provide the full near add (40 cm) in the bottom of the lens. The central portion of the lens provides an intermediate power for the patient, and the add power generally declines further toward the top of the lens. This allows patients to look up and see at farther distances in their office environment. However, the top of the lens typically does not include the distance refractive correction (except the Technica [American Optical, Southbridge, MA] and Tact [Hoya, Bethel, CT], which provide a small area containing the distance correction).

For the convenience of the practitioner, these lenses are prescribed by writing the usual distance prescription with the usual 40-cm add, and then specifying the lens design from those available (see Table 6-1 for the occupational lens designs). The lenses are characterized by having a lens power degression (i.e., the amount by which the add power decreases from the bottom to the top of the lens). The prescribed near power is provided in the bottom of the lens (this is the power to which the optical laboratory finishes the lens), and the power in the top of the lens is determined by the lens degression. In nearly all cases, there is some remaining add in the top of the lens. The available degressions for each lens are listed in Table 6-1.

Some clinicians can be successful by using just one or two of the lens designs. It can be very effective to simply write the usual distance-near prescription and specify the occupational lens of choice. For lens

TABLE 6-1. Occupational progressive lenses

100 Diagnosing and Treating Computer-Related Vision Problems

FIGURE 6-2. Access 1.25 (SOLA, Petaluma, CA). Contour plots (0.25 diopter steps) of spherical equivalent (–0.25 to +1.25) and astigmatism (0.00–1.75). Measurements by COLTS Laboratory, Clearwater, FL.

tion cannot meet both the intermediate and near visual needs, especially for patients who wear flat top bifocals to meet their general visual needs. In this case, the top of the lens contains the intermediate prescription, and the bottom contains the near prescription. The distance blur through the top of the lens must still be demonstrated to the patient; the

FIGURE 6-3. Cosmolit Office 1.00 (Rodenstock, Columbus, OH). Contour plots (0.25 diopter steps) of spherical equivalent (+0.25 to +1.50) and astigmatism (0.00–0.75). Measurements by COLTS Laboratory, Clearwater, FL.

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FIGURE 6-4. Cosmolit Office 1.75 (Rodenstock, Columbus, OH). Contour plots (0.25 diopter steps) of spherical equivalent (0.00 to +1.75) and astigmatism (0.00–1.50). Measurements by COLTS Laboratory, Clearwater, FL.

intermediate power can be trimmed a bit to lessen the distance blur. The prescription should be written with a note that it is for computer use, with the intermediate prescription as the main prescription and the add being the supplemental amount required to see at near. The bifocal can be fitted at the lower limbal margin as in most bifocal fittings.

FIGURE 6-5. Gradal RD (Zeiss, Chester, VA). Contour plots (0.25 diopter steps) of spherical equivalent (+0.50 to +1.75) and astigmatism (0.00–1.00). Measurements by COLTS Laboratory, Clearwater, FL.