Prescribing Spectacles for the Normally Sighted PDF

PRESCRIBING SPECTACLES FOR THE NORMALLY SIGHTED Most older patients require optical corrections for both distance and near vision tasks. Driving, watching television and movies, watching public events, and referring to distant informational signs cause good distance vision to be important for most i...

PRESCRIBING SPECTACLES FOR THE NORMALLY SIGHTED Most older patients require optical corrections for both distance and near vision tasks. Driving, watching television and movies, watching public events, and referring to distant informational signs cause good distance vision to be important for most individuals. At near, tasks ranging from writing and reading personal and business correspondence; reading labels on foods and medicines; reading price tags; reading directories; and recreational or educational reading of books, newspapers, and magazines are all commonly encountered as part of regular daily life for most people. Obviously most people want easy access to clear distance and near vision. Progressive addition lenses, bifocals, or trifocals are worn by older patients. Small in number are the emmetropes who do not need distance glasses, myopes who never need near vision glasses, and people who choose to use only single-vision glasses and switch spectacles when they change from distance to near viewing. Progressive addition lenses have two benefits. One is that they are preferred by people who do not like the appearance of, or are distracted by, the sharp dividing line of standard bifocal or trifocal segments. Progressive addition lenses also provide a channel of progressively increasing power between the distance viewing point and the near viewing point. In effect, this provides a continuous sequence of focus for all possible intermediate distances. Objects at intermediate distances will be seen in good focus when the head and eyes are positioned so that the most appropriate portion of the lens is being used. The patient can experience some diminished clarity and some spatial distortion when viewing through the areas in the lower half of the lens that are outside the channel of progressive power change. Many patients are not bothered by this. Others find it annoying and distracting, but most adapt in a few days or weeks. In optical corrections for presbyopia, the practitioner’s starting point is usually a consideration of the dioptric range of clear vision. What is the near point of the patient’s range of clear vision with the best “distance” correction? What will be the far and near limits of the range of clear vision through the best “near” portion of the lens? What working distances are most important for the patient? What is the closest working distance required for the patient? Whether that presbyopic correction is to be in the form of a progressive addition or a bifocal or trifocal lens, a fundamental decision must be made about the power of the addition. This, combined with the depth of focus and any remaining accommodation, determines the closer viewing distances at which best visual acuity may be obtained. The “distance” portion of the spectacles may be designed for a distance that is significantly closer than optical infinity, especially for glasses to be used in office environments. Providing good vision within the intermediate space between the clear vision ranges for the “distance” and” near” portions of the lens can be most easily handled by choosing progressive addition lenses. For patients with no accommodation, bifocal lenses might leave a relatively deep range of intermediate distances over which vision will be blurred. Trifocals typically have an intermediate segment that is half the power of the stronger reading segment, and best (or close to best) visual acuity will usually be obtainable at all intermediate distances. For progressive addition lenses, the intermediate portion of the lens is necessarily a relatively narrow channel; thus the lateral extent of the field of good vision will be somewhat limited, and more exacting head movements may be required for many tasks. Trifocals and bifocals provide wider fields of clear near vision than do progressive additions. The size and position of bifocal or trifocal segments should be chosen to suit the patient’s functional needs and particular wishes. The width of the field of clear near vision, prismatic jump, and chromatic effects can influence the recommendations made (see Chapter 11 for a further discussion of vision corrections for the older adult). Monovision solutions for enabling good vision for both distance and near tasks have become more common in recent years. Cataract surgery, contact lens corrections, and sometimes refractive surgery deliberately correct one eye for distance vision and the other for near. Older patients are more likely to have ametropia of high magnitude. In progressive myopia, the magnitude of refractive error can continue to increase throughout life. Also, fairly large myopic shifts can occur from changes in the crystalline lens. At the other end of spectrum, some older patients become highly hyperopic in cases where intraocular lens implants are not used after cataract extraction. Correction of high refractive errors requires special consideration. When performing the refraction, care should be taken to ensure that the vertex distance and pantoscopic angle of the lenses being worn are similar to those expected to be present in any new glasses that might be worn in the future. The position of the optical centers relative to the patient’s pupil can become much more significant when lens powers exceed approximately 8.00 D. A basic principle guiding lens positioning is that the optic axis of the lens should point toward the center of rotation of the eye. Lens designers assume this when lens design parameters are chosen to avoid unwanted aberrational effects. The center of rotation is typically approximately 27 mm behind the spectacle plane. Pantoscopic tilt of the spectacle plane is a relevant factor in determining the vertical placement of the optical centers. The greater the pantoscopic angle, the lower the centers should be. Asimple method for measuring pantoscopic tilt is to take a protractor with a plumb line (or a straightened paper clip) attached to its center. The base of this protractor can be held so that it is against or parallel to the spectacle place. With the patient’s head in its natural or habitual posture, the plumb line will indicate the most usual pantoscopic tilt. To compensate for the effects of pantoscopic tilt, the optical centers should be made lower than the center of the pupils by 1 mm for each 2 degrees of pantoscopic tilt. Ten degrees of pantoscopic tilt is common, and for this the height of the optical centers should be 5 mm below the pupil center. This will be about level with the corneal limbus. The correctness of the optical alignment can be verified by having the patient tilt his or her head back while fixating a bright light at an appropriate distance. The optometrist, with the eye close to the light, should observe the reflex from the cornea being in line with those from the lens surfaces. Aphakic spectacle corrections, though currently uncommon, require special lens design considerations. Bifocals, and rarely trifocals, become virtually essential. The prescribing practitioner is obliged to consider a number of lens design options. For example, should the lens material be plastic to minimize weight? Glass because of its scratch resistance? Or highindex material to minimize thickness? Aspheric lens surfaces are likely to be used, and the purpose may be to enhance appearance by reducing thickness and sagittal depth or provide imagery of better quality when the patient views through more peripheral regions of the lens. Some aspheric lenses available today have been primarily designed to achieve a better cosmetic appearance. Others are designed to minimize aberrational effects. In general, this kind is a little less effective in minimizing thickness. The responsible optometrist stays abreast of developments in lens designs and is prepared to consider the weight, appearance, durability, and aberrations of the lenses. Of course, many of the problems associated with spectacle lenses of higher power can be avoided or minimized by the use of contact lenses. PRESCRIBING FOR PATIENTS WITH LOW VISION Most older patients with low vision can benefit from optical aids to enhance their visual performance. The low-vision aids for individual patients depend on the range and relative importance of the visual tasks they want to perform, their vision characteristics, and their psychological attitudes toward their disability and the use of optical aids. Patients who need low vision aids usually need more than one special optical aid. In prescribing optical aids, the magnification effect is usually the first optical parameter considered. The field of view, distribution of image quality, image brightness, adjustability of focus, appearance, portability, convenience, cost, working distance, and maintenance requirements are other factors that enter the decision-making process. Optical low vision aids can be placed in three categories: (1) magnifiers for distance vision (telescopes), (2) magnifiers for near vision, and (3) nonmagnifying aids to vision (see Chapter 14 for a discussion of vision care of the older adult who is visually impaired). Magnification for Distance Vision: Telescopes Telescopes are commonly used by low vision patients to enhance the resolution of detail in signs, distant faces, television, movies, or other visual displays and scenery. Many older patients with low vision need to travel independently; if their vision is not adequate for driving, however, they may have to rely on public transportation. Dependence on public transportation, in turn, necessitates reading bus numbers, street signs, and traffic signals. Prescribing a telescope (or telescopes) for a low vision patient involves first determining the required magnification and then selecting an appropriate telescope. Achieving the Required Acuity The practitioner first determines visual acuity with best spectacle correction and estimates the resolution performance required to meet the patient’s specific needs. Most commonly, an acuity performance of 20/30 to 20/40 is set as a practical and useful goal for visual acuity with the telescope, but higher resolution is sometimes sought. When acuity is poor, the goal may be reduced to approximately 20/60. If acceptable telescopes do not improve visual acuity to 20/80 or better, the value of a telescope prescription becomes questionable. Calculating the required magnification is simple; it is a matter of ratios. For example, a patient with a visual acuity of 20/200 who wishes to read bus numbers might require a 6× magnification to reach an approximate 20/32 level of performance. Most low vision patients do obtain the expected improvement in resolution. Exceptions may occur when using acuity charts that make the task more difficult (more letters, closer spacing, or both for the smaller letters). In these cases the improvement may be less than simple theory predicts. The optometrist should verify that the patient does, in fact, obtain the expected visual acuity with the magnification originally predicted. Occasionally some modification of the magnification value will be necessary. Certain techniques are important when testing patient performance with telescopes, especially with older patients. For best vision, the telescope must be focused properly. The greater the magnification, the more critical this becomes. If the patient has only a small refractive error, the clinician may focus the telescope for his or her own eye, observing the chart from the correct viewing distance. Then only smallfocus adjustments should be required of the patient. When patients have higher refractive error and remove their spectacles to use the telescope, then the clinician may use a trial lens to simulate the patient’s refractive error and again adjust the telescope. Thus, for a 6.00 D myope, the clinician should look through a telescope while holding a +6.00 D lens between the telescope eyepiece and his or her own eye (or glasses, if worn) and, being careful to be at the correct observation distance, focus to obtain clearest vision. The telescope should then be close to correct adjustment for the patient. Remember that increasing the telescope length adds plus power to either correct hyperopic refractive errors or to focus for closer viewing distances. Shortening the telescope length adds minus power. Some telescopes of 6× or greater magnification do not provide enough focusing range to enable a focus for the commonly used 20- or 10-foot distances. This is especially true for binocular telescope systems. If the range of focus on the chart is insufficient, the clinician can effectively simulate optical infinity by moving the chart to 4 m (12.5 ft) and then hold a +0.25 D trial lens against the objective of the telescope. A chart observation distance of 2 m with +0.50 D lens in front of the telescope achieves the same effect. Ensuring in-focus vision is necessary when determining whether the patient will achieve the resolution goal sought. The clinician should always verify that the prescribed telescope does indeed focus at the distance required. The patient’s actual working distance should be established and simulated in the office or local environment. The patient’s required observation distance may be 30 ft (e.g., a lecture theater) or 8 ft (an overhead menu at a fast-food restaurant); whatever this observation distance is, however, it should be simulated, and the clinician should be sure that the telescope’s focusing range is adequate. When more than one viewing distance is required, prescribing a removable lens cap of appropriate power may be necessary to achieve the shift in focus. Today, most series of monocular telescopes that have been designed for low vision patients provide wide focusing ranges extending from infinity to distances well within arm’s length. Another important optical parameter is the exit pupil of the telescope, which defines the size of the beam of rays that can emerge from the telescope. The exit pupil can be calculated by dividing the diameter of the objective lens by the magnification of the telescope. Thus an 8 × 20 telescope has an 8× magnification and a 20-mm objective lens diameter; the exit pupil size is 20 ÷ 8 = 2.5 mm. An 8 × 40 telescope has a 40-mm objective, and thus has a 5-mm exit pupil. Telescopes of 4 × 20, 6 × 30, 8 × 40, and 10 × 50 all have 5-mm exit pupils. The exit pupil can determine image brightness. If the exit beam is smaller than the eye pupil diameter, the image brightness is reduced as though the eye pupil had constricted to become the same size as the exit pupil of the telescope. An 8 × 20 telescope with a 2.5-mm exit pupil used by an eye with a 5-mm pupil will cause a fourfold decrease in retinal image illumination. In this example, the effective diameter of the pupil is reduced by a factor of 2, so the effective area is reduced by a factor of 4. If the exit beam of the telescope is larger in diameter than the eye pupil, the image brightness should not be reduced except for the small loss by reflectance and absorption. Lens coatings can reduce this kind of light loss. The discussion of image brightness and telescopes applies to the observation of most objects. Different arguments and conclusions apply to the observation of stars, because for point sources the size of the retinal images is not affected by the magnification of the telescope, but image brightness is. Telescopes with smaller exit beams are generally more difficult to use, especially if the patient is inexperienced or has unsteady hands. Maintaining a small exit beam in alignment with the eye pupil can be difficult. Older patients who have difficulty using telescopes to view the visual acuity chart are helped if the magnification is lower, the exit pupil is larger, or the field of view is larger. Sometimes developing the patient’s skills gradually by starting with telescopes of lower magnification is necessary. Patients with alignment difficulties can be assisted by reducing the room lighting and increasing the illumination on the chart. If the telescope is not aligned properly, the exit beam will be visible to the clinician as it illuminates the iris, sclera, or eyelids. The patient can be guided so that the exit beam enters his or her pupil. Even when the telescope is aligned, it may be necessary for the clinician to continue to help maintain alignment and assist with the focus adjustment. In general, older patients need significantly more training and guidance to become proficient at using telescopes. Selecting a Telescope Once it has been verified that a patient can achieve the desired visual acuity with the use of a telescope of a particular magnification, the clinician must select the type of telescope that produces the required magnification and most conveniently satisfies the patient’s needs. The prescribed telescope may be monocular or binocular. Binocular telescopes may be preferred when the visual acuity is similar for the two eyes. Some patients find binocular telescopes easier to hold because the two eyecups can touch the eyebrows and provide some support or tactile feedback to help the patient maintain proper alignment. However, binocular telescopes are bulky and heavy, which detracts from their portability and comfort, and their focusing range can be limited. Patients who use telescopes to assist in their independent travel abilities typically prefer monocular telescopes that are lightweight, concealable, easily carried in a pocket or purse, and easy to use. Wrist straps, neck cords, or small finger-ring mountings may help keep the telescope easily accessible for use as needed. Difficulties in holding a higher powered telescope with adequate steadiness can sometimes by reduced or eliminated by the use of a tripod, monopod, or other supporting structure. Telescopes mounted in a spectacle frame or similar mounting have their advantages. They can prove most useful when observation with the telescope is used for protracted periods (for example, when watching sporting events, stage presentations, or movies). Ready-made sports glasses, which are a pair of adjustable telescopes mounted in a spectacle-style frame, can be relatively inexpensive. Hook-on monocular telescopes can be attached to existing distance spectacles, thereby simultaneously correcting the refractive error and providing magnification. Patients often request a head-mounted telescope system for viewing television. In general, the patient is better off moving closer to the television. Because telescopes restrict the field of view, in many television-viewing situations telescopes will not allow the patient to see the whole screen at one time. Keplerian telescopes provide wider fields of view, with the field widths usually being about equal to the equivalent viewing distance (EVD). For telescopes, the EVD is equal to the viewing distance divided by the magnification. For example, a 4× Keplerian telescope and a viewing distance of 2 m will create an EVD of 50 cm; the expected field of view will be approximately 50 cm. The field of view for Galilean telescopes is typically substantially smaller than the EVD. Many older patients resist sitting close to the television because they believe it is harmful to the eyes. Such misconceptions should be corrected. Bioptic telescopes are spectacle-mounted telescope systems that are arranged to enable a quick transition from viewing through the telescope portion to viewing through the lens in which the telescope is mounted. These systems are more commonly prescribed for younger adults, but some older patients benefit from them. Bioptic telescopes can be used in driving, permitting the wearer quick access to telescope viewing for short-term observation of signs and traffic signals. Many states permit driving with bioptic telescopes provided that the usual visual acuity standard is met when the wearer views through the telescope. Some states have requirements regarding the visual acuity through the nontelescope portion of the bioptic telescope system. Drivers wearing bioptic telescopes may have some general or individual restrictions on their driver’s licenses. Older patients who use a bioptic telescope system for driving might want a bifocal addition for viewing the speedometer and other gauges and displays. Some telescopes have auto-focus that quickly adjusts the focus when the telescope is pointed toward objects at different distances. Spectacle-mounted bioptic telescope systems are available in magnifications up to 8×, but 3× and 4× seem to be most useful. For highmagnification bioptic telescopes, the small field and difficulties with steadiness and aiming the telescope make the system harder to use. Older patients, who are often self-conscious about their visual handicap, may be reluctant to even consider the use of any kind of telescope. Most patients with a visual acuity of 20/60 or poorer should be given information about telescopes so they can understand their potential advantages. Encouraging patients to borrow a telescope for use at home for a week or so can produce some appreciation of the benefits and may change attitudes about telescopes. Magnification for Near Vision Magnification devices to provide low vision patients with assistance for near vision tasks can be grouped into six categories: (1) high-addition reading glasses, (2) handheld magnifying glasses, (3) stand magnifiers, (4) head-mounted loupes, (5) near-vision telescopes, and (6) video magnifiers. Three basic steps are involved in prescribing a magnifier to provide a level of resolution that will meet the patient’s visual needs: 1. Determining the magnification or the EVD required 2. Deciding what kind of magnifier (e.g., handheld, stand) would be most appropriate 3. Given the required EVD and the kind of magnifier, determining which of the available models has the best combination of features to satisfy the patient’s requirements Determining Magnification Requirements Magnification for near vision can be a difficult topic to discuss many conflicting definitions of magnification are used. To illustrate the difficulty, a 5.00 D lens held at a full arm’s length might produce an apparent magnification of 5×, and the resolution could be 2× better than that obtained with a previous 2.50 D addition that gave clear vision at 40 cm. However, the manufacturer likely has this magnifier labeled 2.25×; in addition, the clinician may recall a simple formula (M = F/4) that suggests the magnification should be 1.25×. The word magnification implies a comparison and demands the question, magnified compared with what? Because this question has several alternative answers, several alternative definitions of magnification are used, and which definition is most appropriate is not always clear. The example above shows four magnification values arising from four different definitions. Much of the potential confusion surrounding the use of the term magnification can be avoided by using the concept of EVD to quantify the magnifying effect that optical systems provide. The EVD is the distance at which the object would subtend the same angle as that being subtended by the image seen with the magnifying device. For example, a 4× photographic enlargement of a sample of print that is viewed from a distance of 40 cm presents the observer with an image whose angular size is equivalent to that obtained if the original print sample was viewed from a distance of 10 cm; that is, the EVD is 10 cm. Similarly, a video magnifier giving an image that is 50 cm from the eye and enlarged 10 times provides an EVD of 5 cm. Many optical magnifiers can be used to create an image at optical infinity, in which case the EVD is equal to the equivalent focal length of the magnifier. Thus a +20 D lens being used to give an image at infinity will provide an EVD of 5 cm. Frequently optical magnifiers create an enlarged image that is located not at infinity, but at a finite distance. The EVD is then the eyeto-image distance divided by the enlargement ratio (or transverse magnification). For example, consider a fixed-focus stand magnifier that gives an image that is enlarged by a factor of 3 and is located 20 cm behind the lens of the magnifier. If the patient uses this magnifier so the separation between the eye and magnifier is 10 cm, this will achieve an EVD of (10 + 20)/3 = 30/3 = 10 cm. For a given patient, systems that provide the same EVD will enable the same visual resolution if, of course, the image is in satisfactory focus. A patient with presbyopia would be expected to obtain the same resolution with each of the optical systems listed in Box 7-2. BOX 7-2 Providing an EVD of 8 cm by Several Different Means A +12.5 D spectacle addition; print at 8 cm. A +12.5 D handheld magnifier regardless of how far it is held from the eye while the patient uses distance vision glasses (the lens-page separation will be 8 cm). A 2.5× telescope with a +5 D cap to provide a 20-cm working distance. A stand magnifier with a +20 D lens whose image is 20 cm (5 D) below the lens with the patient using a +2.50 D addition. The separation between the object and the magnifier must be 4 cm (25 D), and the enlargement ratio will be 5×. If the eye-lens separation is 20 cm, the eye-to-image distance is 40 cm. A video magnifier giving an enlargement of 10× when the viewing distance is 80 cm (+1.25 D addition). All five systems provide an EVD of 8 cm (or equivalent viewing power of +12.5 D). All afford a resolution equal to that expected if the patient were to hold the object of interest 8 cm from the eye while maintaining a clear image by accommodation or using an addition. In any evaluation of near-vision performance, the patient should wear an appropriate addition and hold the test material so that it is in good focus. Determining the Equivalent Viewing Distance Required Although using distance visual acuity to estimate how much dioptric power is required to enable a patient to read print of a certain size is possible, this is not the surest approach. Letter charts used for distance vision and reading cards used for near vision present tasks of different complexity; no strong concordance exists between letter chart acuity and reading chart acuity, especially when macular disturbances are present. Patients with presbyopia almost invariably have a reading correction, and the most convenient and appropriate way to begin the power determination is to measure the patient’s reading acuity while he or she uses an existing reading correction. For example, a patient might have eyeglasses that incorporate a +3.00 D addition. Holding the test card at 33 cm, this patient may be able to read 2.5 M print (which could, on some charts, be labeled 20 points or 20/125). The EVD required to reach a resolution goal then can be determined by simple ratios or proportions. For example, the clinician might decide that the resolution goal should have the patient able to read text of newsprint size (1 M, 8 points, or 20/50 in size) with the same level of confidence or difficulty exhibited when the patient read the 2.5 M print at 33 cm. Whatever units are used, a 2.5× improvement in resolution is required. The required improvement in resolution can be achieved by changing the viewing distance from 33 cm by a factor of 2.5×, so that an EVD of 13 cm is indicated. For this presbyopic patient, the power of the addition needs to be increased from 3.00 D to 7.50 D. The clinician should then verify that the patient does, in fact, achieve satisfactory reading of the 1 M print (8 points or 20/50) that had been set as the goal. This testing is usually done with spectacle lenses in a trial frame or trial lens clip. When working over existing bifocals, the lens clip might not allow full access to the bifocal portion of the lens. In such a case, the lens clip can be raised so that only the distance portion of the lens is being used, and the full required addition can be introduced into the lens clip. Special care must be taken to ensure that an appropriate working distance is being used because some older patients strongly resist working at close distances. Selecting the Magnifying Aid Once the EVD needed to provide the desired reading resolution performance has been determined, the optometrist must decide which of the various aids should be used to provide the required EVD. Spectacle lens corrections afford the widest fields of view, leave both hands free to support or manipulate task materials, are convenient to carry, and are relatively inconspicuous. Their disadvantage is the close working distance they may impose. If a spectacle prescription is to be issued, the lens form must be considered. Will it be single-vision lenses, bifocals in a standard configuration, bifocals with high placement of the segments, special series lenses, or aspheric lenses? Binocularity issues should be addressed. If binocular vision is to be achieved (it is usually achievable if addition powers are +8.00 D or less), the prism or decentration must be considered. Fonda15 recommends as a rule of thumb that, relative to the distance prism, 2 mm of total decentration be given for each diopter of addition power. Thus 8 mm of total decentration should be present for a 4.00 D addition, 16 mm for an 8.00 D addition, and so on. This method provides a small net amount of base-in prism. Many patients with low vision must perform near-vision tasks monocularly, either because there is substantial inequality between the two eyes or because the required lens powers are too high. Even though the other eye might not be used during the main reading tasks, some attention should be given to the lens that will be worn in front of the second eye. A simple balance lens may be indicated. Often monovision possibilities should be considered. Having a single-vision lens before the poorer eye to give in-focus distance vision or provide clearest focus for intermediate or near vision may be useful. A bifocal or trifocal lens before the poorer eye might best satisfy the patient’s overall visual needs and convenience. Handheld magnifiers have as their main advantage the adjustability of working distance. In one extreme, the patient can hold the magnifying glass in the spectacle plane, in which case the reading material will need to be in the appropriate focal plane, usually relatively close to the lens. At the other end of the scale, the magnifier may be held at a full arm’s length; again, the object of regard will need to be in the proper focal plane. The further the lens is held from the eye, the smaller is the field of view. The field of view is equal to the EVD multiplied by the ratio of the lens diameter to its distance from the eye. For example, a +12.5 D lens held 8 cm from the page and 50 cm from the eye will give an EVD of 8.0 cm. If, in this example, the lens diameter is 5 cm, the diameter-to-distance aspect ratio is 5/50 cm, then the field of view— 8.0(5/50)—is 0.8 cm. Provided patients are using their distance glasses, the resolution expected from a handheld magnifier can be determined directly from the equivalent power of the lens. The EVD will simply be the focal length of the magnifier. Bifocal wearers should not view through their bifocal segments unless the magnifier is being held close (i.e., closer than one of its focal lengths) to the spectacle lens. Holding the magnifier against the reading addition in spectacles effectively provides the sum of the two powers. The combination then will act as a strong spectacle lens. Handheld magnifiers provide portability and flexibility. They are ideally suited for looking at price tags, reading maps, and checking labels on containers in a store. Stand magnifiers are most commonly used for short-term reading tasks. They can be particularly helpful to older patients because steady hands are not necessarily needed. If strong dioptric powers are required, and even if no hand steadiness problems are present, stand magnifiers provide a level of easy and reliable control not attainable with spectacles or handheld magnifiers. For reading tasks of limited duration (e.g., reading telephone books and television schedules, checking bills, and reading greeting cards), stand magnifiers can be of value. Most incorporate a light to illuminate the task. Stand magnifiers may be somewhat bulky and therefore less convenient to carry, and the working situation becomes much more rigidly defined. The practitioner must understand a few basic optical principles to prescribe stand magnifiers intelligently. First, fixed-focus stand magnifiers produce images that are relatively close to the lens of the magnifier. Rarely is the image farther than 50 cm behind the magnifying lens, and most image locations are in the range of 3 cm to 40 cm behind the lens surface. Consequently, patients with presbyopia must wear a reading correction to obtain a clear view of the image. The image location and the power of the spectacle addition determine the separation required between the magnifier and the spectacles. If the image of the magnifier is 10 cm below its surface, and the patient is focused for 40 cm because of a +2.50 D reading addition, then the required separation between the eye and the magnifier is 30 cm. Had the spectacle addition been 5.00 D, the required eye-to-image distance is 20 cm so the required eye-to-magnifier distance becomes 10 cm for a stand magnifier. The practitioner needs to know the image location for the stand magnifier being used. The second basic optical principle to be understood is that the image produced by the stand magnifier is larger than the original object by a fixed ratio. The net effect of the magnifier is to produce an image that is both larger and more remote than the original object. The power of the magnifier lens and the object-lens separation determine the size of the image. The enlargement ratio (which may also be called transverse magnification) is constant for a given magnifier, and the enlargement ratio should be known to the clinician because it influences the final resolution. Third, when the patient views the enlarged image formed by the magnifier, the resulting EVD can be determined by dividing the eye-toimage distance by the enlargement ratio. Some clinicians prefer to consider the EVD in dioptric units; then the equivalent viewing power may be determined by multiplying the accommodation demand by the enlargement ratio. If a stand magnifier forms an image that is 20 cm below the lens and the enlargement ratio is 3×, then a patient whose eye is 10 cm above the lens will have eye-to-image distance of 30 cm, so the EVD is 10 cm. The accommodation demand is 3.33 D, so that the equivalent viewing power TABLE 7-1 Comparison of Two Stand Magnifiers Li g ht h o u s e Mag C P nifie O o r I w L er 5 M 2 a 8 g 9 9 5 2 8 Labe + + led 2 2 pow 8 8. er/. 0 0 0 0 D D /8 / X 8. 0 X  m ag nif ic ati + on 2 + Equi 3 2 vale. 7. nt 4 4 pow 0 0 er D D Imag 2 5 e 5 3 posit c c ion m m Enla 6 1 rge. 4. ment 9 6 ratio X X is 10 D. The EVD can be used in predicting the resolution that the patient will be able to achieve with the system. The following demonstrates some of the important considerations that should be made when using stand magnifiers. Two widely used stand magnifiers, one from COIL (Slough, England) and one from the Lighthouse PowerMag series (Optelec, New York, NY), have the same magnification ratings assigned by the manufacturers. However, comparison of their optical properties shows how they have different optical effects (Table 7-1). If the patient’s eye were 15 cm from the lens, then the eye-to-image distances would be 40 cm and 68 cm for these two magnifiers, and the ideal additions would be +2.50 D and +1.50 D, respectively. The EVDs that would be achieved would be 40/6.9 = 5.8 cm and 68/14.6 = 4.7 cm. The visual acuity difference would be equal to one row on the chart. If, however, the eye were placed 5 cm from the lens, the eye-to-image distances would be reduced by 10 cm and the EVDs would become 4.3 cm and 4.0 cm, respectively. Unfortunately most, but not all, manufacturers do not specify image location or the enlargement ratio; even their nominal lens powers or magnification ratings are often inappropriate or wrong. However, simple in-office methods are available for measuring these key optical parameters.1-3,9 Head-mounted loupes are positive-powered lenses mounted so they sit in front of the spectacle plane. They are mainly used for viewing manipulative tasks. A variety of such devices are available. Some are single lenses that attach to the spectacle frame or spectacle lens, and the lens generally is mounted on a pivoting bracket that allows it to be conveniently removed from or inserted into the line of vision. These can provide monocular viewing, and lens powers usually range from 10.00 D to approximately 30.00 D. Binocular loupes, which are available in powers up to approximately 10.00 D or 12.00 D, usually incorporate some prism or decentration to reduce the convergence demand. Some binocular loupe systems attach to spectacles, but most are mounted on a headband that positions the lens bracket 2 cm or so in front of the spectacle place. Many can be flipped up and down as needed. Another potential advantage of head-mounted loupes is that mounting the lenses a few centimeters away from the spectacle means the object of regard may be moved a few centimeters further from the face. For some manipulative tasks, this modest change in viewing distance is useful. Near-vision telescopes (sometimes called telemicroscopes) are commonly prescribed for older patients, but they have distinct advantages that sometimes make them essential. Near- vision telescopes are called for when a certain level of dioptric power is necessary to provide the EVD required to achieve a given resolution goal, but the patient is compelled to have a long working distance and unrestricted or bimanual access to the task. Performing surgery and viewing computer terminals are two examples of tasks for which near-vision telescopes might be considered. The advantage of near-vision telescopes is the increased working distance, but this must be balanced against the principal disadvantage: the reduced field of view. The depth of field is also quite small, so the working distance must be accurately maintained. The small fields of view almost invariably make near-vision telescopes unsuitable for use at computer screens. Near-vision telescopes can be created most simply by adding a lens cap to the objective lens of a distance telescope. The EVD achieved by the system can be computed easily; it is simply the focal length of the lens cap divided by the telescope magnification. For example, a system made from placing a 4.00 D lens cap on the front of a 2.5× telescope creates a focus for 25 cm. The EVD = 25/2.5 = 10 cm (equivalent viewing power = 4.00 × 2.5). This will provide the same resolution as any other system that achieves an EVD of 10 cm. In this example, the working distance is 25 cm because it is set by the focal lens of the lens cap. Other near-vision telescope systems to achieve an EVD of 10 cm could be created by combining a 2.00 D lens cap and a 5× telescope (working distance, 50 cm), a 3.00 D cap and a 3.3× telescope (working distance, 33 cm), a 5.00-D cap and a 2.0× telescope (working distance, 20 cm), or many other combinations. Many near-vision telescope systems are created by taking a distance-vision telescope and increasing its optical path length to achieve a near-vision focus. The EVD can be determined from the formula EVD = u/M − foc, where u is the telescope-to-object distance, M is the magnification of the telescope, and foc is the focal length of the ocular. For most Keplerian telescopes the value of foc is approximately 1 cm. The critical optical parameters for prescribing near-vision telescopes are the working distance and the EVD that the system provides. Electronic Display Systems Computer displays and video magnifiers are becoming more widely used by older people to compensate for visual impairments. The use of electronic systems will continue to increase rapidly as information technology becomes more user friendly and as technophobia becomes less prevalent. Video Magnifiers In video magnifier systems (often called CCTVs), the image from a video camera is imaged on a cathode-ray tube or flat-panel display screen. The size and other characteristics of the image can be manipulated by adjusting the optical or electronic components of the system. Most commonly the camera is in a stand pointing down toward an x-y table on which printed material is placed. The table surface may be easily moved left and right and back and forth to position the required area of the reading material under the camera. The height of the image of the print on the screen depends on the size of the screen and the characteristics and state of adjustment of the camera system. Almost all systems use color cameras, and easy controls enable the image to be changed to highcontrast black-on-white or reversed-contrast with white-on-black. Typically a variety of combinations of background and foreground colors are available. The range of magnification changes by a factor of approximately 10 or 12 times in most cases. For most stand-mounted cameras, at minimal magnification the field of view has a width of 10 to 15 cm. At maximal magnification, the displayed print becomes so large that only a few letters are seen across the width of the screen. The enlargement ratio is equal to the screen width divided by the field of view. The equivalent viewing distance is equal to the eye-to-screen distance divided by the enlargement ratio. That is, EVD = d/(W/FoV), where d is viewing distance, W is screen width, and FoV is field of view. Patients typically sit closer when the screen is smaller. The patient typically adopts a viewing distance that is approximately equal to the width of the display screen. If the viewing distance is equal to the screen width, then the EVD becomes equal to the width field of view. When the viewing distance is equal to half the screen width, the EVD will be half of the field of view. Given that the magnification levels are easily adjustable with a knob or a lever, and some scope can change viewing distance, patients can easily balance enlargement ratios, fields of view, and EVDs according to the size of the print they are reading. The special advantage that video magnifiers have over optical magnifiers is that they can give a larger field of view when strong magnification is required. The field of view of optical magnifiers is usually equal to or less than the EVD, but for a video magnifier giving same EVD, the field of view may exceed the EVD, provided the viewing distance is less than the screen width. The most widely used video magnifiers have a camera with an easily adjustable zoom lens mounted above a moveable x-y table; the display screen sits above the camera stand. Other camera systems are available. Handheld cameras, some as small as a computer mouse, can be moved over the page. Some cameras mounted on stands or movable carriages allow the axis of the camera to be rotated and redirected from pointing down at the tabletop to distant objects or displays. The output of some of the small handheld camera systems can be fed into a standard television or computer display. Some specialized systems have the camera head mounted so that hat shifts in the point of regard are achieved by moving the head. Display screens have traditionally been cathode-ray tube video monitors separated from the tabletop by the camera system, a work space, and the x-y table. Such models are relatively heavy and bulky, and they require a dedicated workstation. Flat-panel displays have enabled more portability and more flexible screen placement. A few systems have used head-mounted displays that have a small digital display screen and a high-powered viewing lens mounted within a goggle or visor device. A recent addition to the armamentarium of electronic magnifiers are small units that have a 10- to 15-cm wide display screen and a camera system built into a small, thin box that rests on the page. Such systems currently have limited adjustability of magnification, but this will change and they are likely to become a popular replacement for optical hand and stand magnifiers (see Chapter 15 for further discussion of video magnifiers). Computers Computer display systems create their displays by taking stored digital information that can be manipulated to provide outputs with the possibility of a diverse range of characteristics. The display outputs can use senses other than vision. Auditory and tactile displays are readily available in the form of synthesized speech and Braille outputs. With the visual displays, the size and appearance of printed material can easily be selected to suit the patient’s needs. Some modifications can be made to enhance the accessibility of information from graphic or pictorial displays. The input of the information into the computer may come from the keyboard and mouse; from scanning hard copy documents; or to the computer already in digital form accessible over the Internet by e-mail, web browsers, or a variety of modes of storage media ranging from floppy disks to compact disks. Voice-recognition software convert’s a user’s speech to digital text documents that can be stored or displayed as needed. With electronic visual displays the size, font, style, spacing, and foreground and background colors can all be manipulated by the user to optimize the appearance of displayed text. When the print needs to be large, the amount of information than can be displayed on the screen is reduced. For a patient with low vision, some of the simplest ways of enhancing the visibility of the screen displays may come from choosing to use a large screen or from using standard control panel options to reduce the number of pixels displayed on the screen. Modern computer operating systems offer ways of easily changing the visibility of the cursor and making simple adjustments to the size and color and contrast features of the screen images. Speech output options are also available. Most current word processing or spreadsheet programs have options for making the printed material within the document smaller or larger by a ratio that the user may select. For some patients, more sophisticated enhancements of displayed material are needed, and special software may be required. Dedicated programs to enhance access to computer information may facilitate quick changes in magnification or other visual display characteristics over the whole screen or parts of the screen. Speech or other nonvisual outputs may be readily engaged or disengaged. The display layout characteristics may be changed to a horizontal streaming mode, by which the text is presented in a single row that moves from right to left across the screen; a vertical streaming mode, in which the column width is made equal to the screen width at all print sizes; or the “rapid serial visual presentation” mode, in which the words appear one at a time in one location on the screen. For each of these presentation modes, the user controls the speed of presentation, and a facility exists for switching back to the original display layout to ascertain place or shift to a new region of the document. Display technology will continue to develop rapidly, and the older population, especially those with visual impairment, are sure to benefit from enhanced and simplified access to information technology, from more flexible flatpanel and head mounted visual displays, and from nonvisual input and output systems mediated through touch and sound. Nonmagnifying Aids to Vision For all near-vision magnification systems, care should be taken to adjust the illumination to suit the patients’ needs. Most older, low vision patients require more illumination than usual, but they are more susceptible to problems with glare. Consequently, more than the usual attention should be paid to positioning the light and the task material so that potentially troublesome glare is avoided. A device that has long been useful for glare control is the typoscope, or reading mask. This is a black card with a rectangular aperture that is usually made large enough to accommodate three lines of one-column-width print. The typoscope can enhance reading acuity and comfort by reducing glare from white paper close to the immediate fixation point. This device also serves as a line guide and helps patients with field defects maintain their place when reading. Yellow filters also are beneficial to some patients, who report that they make vision clearer and more comfortable. Illumination control by varying the quantity and quality of the task and ambient lighting should be routinely included in the assessment of the patient’s nearvision magnification needs. Filters also can be beneficial to many older patients. For reasons that are not fully understood, many patients with retinal disorders and some with cataracts report seeing better when yellow filters or some other “minus blue” filter is worn (see Chapter 2). Older patients, especially those with visual disorders, tend to be more sensitive to bright light; thus sunglass filters (sometimes very dense ones) are often required. The approach to prescribing tints is largely empirical. Decisions are based on reported symptoms and the patient’s perception of the effect of the filters. Contact lenses can offer special advantages to some low vision patients. They can substantially reduce the effect of corneal irregularities caused by corneal or anterior eye scarring or dystrophies. When the corneal distortion is more pronounced, soft contact lenses are not as effective because some of the corneal distortion may be translated to the front surface of the lens. Rigid lenses can be more effective in nullifying the optical effect of distortion, but they are more likely to cause potentially troublesome pressure spots on the distorted cornea. The other major optical benefit of contact lenses is that they substantially reduce aberration and prismatic effects that can produce vision difficulties with the use of stronger spectacle lenses. Because the contact lenses move with the eye, the visual axis of the eye is always close to the optic axis of the correcting lens; this means that peripheral aberration problems are eliminated or at least greatly reduced. Differential prismatic effects that occur with spectacle corrections for anisometropia are much better controlled with contact lenses. Patients with aphakia often have perceptual and depth judgment difficulties when they are introduced to spectacle corrections, and they may be bothered by the field restriction and the “jack-in-the-box” effect created by the high plus spectacle lens. These effects are avoided or minimized by using contact lenses. Many older patients have impaired manual dexterity or reduced vision that may create difficulties for handling and caring for contact lens, thus making them contraindicated (see Chapter 12 for further discussion of contact lenses and the older adult). Visual field defects accompanying pathological changes can produce functional difficulties for patients, especially in mobility tasks. Three kinds of optical devices can help patients with particular kinds of visual field defects: reversed telescopes, hemianopic mirrors, and partial prisms. Reversed telescopes can be useful for some patients who have a concentric loss of visual field such as what occurs in advanced retinitis pigmentosa or glaucoma. Reversed telescopes minify and obviously reduce visual acuity, but they can provide an enlarged visual field. Most patients who use reversed telescopes only do so for navigational purposes, and then only when the local environment contains repetitive or potentially confusing details. An example of such a situation is an intersection at which many roads or paths meet. Most reversed telescopes are handheld and are of a moderate range of magnification (2× to 6×). A handheld minus lens positioned 30 to 50 cm from the eye can achieve a similar minification, fieldexpanding effect. Partial prisms are prisms that cover only part of the spectacle lens area and can be prescribed to help patients with field problems. Their main use is in hemianopsias, but they are sometimes used for a concentric loss of visual field. Partial prisms are usually in the form of Fresnel membrane prisms of high deviation (15 to 40 prism diopters) that are most commonly placed on the lens so that they are totally within the blind field when the patient looks straight ahead. The base direction of the prism is always away from the primary line of sight. When the patient makes an eye movement toward a blind part of the field, the prism will be encountered after a certain degree of eye rotation. The prism optically shifts things in from the periphery. This means that smaller eye movements will be required to view more peripheral objects on the blind side. Stated another way, an eye movement of a given magnitude allows the patient to see further to the periphery when viewing through the prism. The prism, however, creates a blind spot, and the patient may be distracted by the apparent jumping or disappearance of objects when eye movements traverse the edge of the prism. The prisms are placed so they will remain unnoticed within the patient’s blind field most of the time when relatively normal, small-magnitude lateral eye movements are being made. When the patient wants to inspect more peripheral regions briefly, larger eye movements are called for and are supplemented by the effect of the prism. A fairly typical configuration of the partial prisms for a patient with right hemianopsia would be a 30 prism diopter base-out prism mounted on the right lens so that it covers the full vertical height of the lens beginning at a point approximately 6 mm to the right of the primary viewing point. Partial prisms are sometimes used for both eyes; for the case presented here, the left lens would have 30 prism diopter base-in, and it would be smaller in area. Again, the vertical edge of the prism would be approximately 6 mm from the primary viewing point. Because Fresnel membrane prisms can be removed and replaced easily, experimenting with this kind of correction is not difficult or expensive. An alternative arrangement of partial prisms is to have a horizontal strip of prism across the width of the lens above or below (or even both) the primary line of sight. Vision through these strong prisms is necessarily degraded by chromatic aberration and other contrast reductions. The purpose of these various partial prism systems is to make the patient more aware of objects of potential interest that are off to the blind side. Once located, the patient makes appropriate head and eye movements to look at the object of interest directly so he or she is not looking at it through the prism. Hemianopic mirrors can occasionally help patients with homonymous hemianopia. A mirror is mounted on the nasal eyewire of the spectacles in front of the eye that is on the same side as the field loss. The mirror is angled so that it is approximately 5 or 10 degrees with respect to the primary line of sight and approximately 20 mm to 40 mm in width. For a patient with a right homonymous hemianopia, the mirror would therefore be mounted on the nasal portion of the right eyewire and angled slightly toward the right eye. By reflection, this mirror will present part of the right-hand field of view to the right eye. This segment from the blind field seen through the mirror will seen by the right eye to be reversed and unstable, and it will be projected so that it seems to be superimposed on the left-hand field. The purpose of the mirror is to provide some awareness of events and hazards on the blind side. The patient must learn not to give close attention to detail seen in the mirror. When the patient becomes aware of an object or event deserving attention, the head should be turned so that the full inspection and any decision making can be made with benefit of direct vision. Few patients with hemianopia actually have mirrors prescribed, but optometrists nevertheless should be aware of them as a possibility. Some have advocated prisms for patients with central scotomas to “relocate” the image that would normally fall on the fovea and position that image on a more central or paracentral region of the retina. Such relocation is logically impossible with simple prisms. However, these prism systems may have some advantage through encouraging changes in habitual head and eye postures that might somehow facilitate eccentric viewing. Training in the Use of Optical Aids Adaptation, practice, and training are often necessary if patients are to receive maximal benefit from their low vision aids. Many aids demand that new skills be learned. Older patients are generally less able to adapt, and they require more training. For example, structured, guided practice in techniques for sighting and focusing with telescopes can be vital to success. Furthermore, initial training with telescopes that have lower magnification and larger exit pupils is beneficial because they are easier to use. A variety of skills may need to be developed for reading, including positioning the head, the eyes, the aid, and the material to achieve clear focus and then making the required relative movements to enable the most fluent reading. Sometimes only brief instruction is needed, whereas other situations may require extensive training and supervision. Many low vision patients have central scotomas, and their visual performance and efficiency may improve if they can learn eccentric viewing strategies. Similarly, many patients may need training to develop more efficient scanning and search techniques. Ensuring that a patient has reasonable proficiency with any optical aid before it is issued is a good policy. Training the patient to be efficient in the use of an aid or the eyes can be the key to success. ADVICE AND RECOMMENDATIONS When all the clinical data have been collected, the practitioner should pause and take stock. Has all the relevant information been uncovered? Again, the following questions should be asked: “What does the patient really want?” “What do I want the patient to have?” “Really, why did the patient come to see me?” The clinician should decide on the treatment options and then consciously consider the strategy for presenting recommendations and advice. For example, should others be present when the advice is given, and to what extent should they be involved? What issues need to be given strongest emphasis, and what should be sidestepped or downplayed? How strongly should the various treatment recommendations be advocated? What does the patient really need to know? What will the patient like to hear, and what will not be accepted easily? The advice and recommendations the optometrist submits to older patients need not be confined to optical, visual, and ocular health matters. As a health care practitioner, the optometrist has a responsibility to the patient’s general health, and any need for rehabilitative attention that may contribute to the patient’s overall well-being should be discussed. The optometrist can be a critical link in the health care chain by taking the initiative in directing patients toward appropriate and broadly based care for their health and well-being. Eye care professionals need to stay abreast of new developments in the treatment and understanding of eye disease. New scientific advances in treatment of eye disease often get considerable publicity, but the reports in the media are commonly exaggerated or wrong. The public access to technical and medical information through the Internet adds to health care practitioners’ responsibility to stay well informed so that they can offer appropriate advice and opinion to their well-informed, as well as misinformed, patients. Optometrists should remain well informed about health care and other supporting services available in the local community. In particular, the practitioner should maintain contacts with the medical community and rehabilitation counselors or social workers as well as be familiar with the activities and services of organizations serving persons who are visually impaired.