Chapter 11 – Subjective Testing of Refraction
CLIFFORD A. SCOTT
• The neutralization of an individual’s refractive error using a variety of tests in which the patient’s responses determine the lens power that best produces a sharply focused image on the retina.
• The selection of a prescription for corrective lenses that balances optical clarity with other important physical and psychological factors, such as equality of magnification, single vision, and comfort.
• The determination of the most appropriate form of optical correction based on the patient’s visual needs and on environmental factors.
Many people equate an eye examination with a refraction for glasses. The confusion is understandable because for the vast majority, especially those in the preretirement age group, eyeglasses or contact lenses resolve the main complaints they have about their eyes. Also, a refraction is almost always part of a comprehensive eye examination, not only to provide a prescription for corrective lenses but also to determine the best acuity that an eye is capable of achieving.
As the rest of this textbook attests, refraction is only one of the many methods used to determine the function and health of the visual system. Because of the value of the results, it is important to develop an efficient and accurate basic refractive technique that can be modified when unusual variations present themselves.
Although often relegated as a purely technical task in the spectrum of high-technology examination and treatment procedures that characterize contemporary ophthalmic practice, refraction provides relief for one of the world’s most common physical defects. An understanding of the concepts used to identify and measure refractive errors is the basis for prescribing individual corrections that offer patients improved quality of life. Equally important, refraction is one of the few procedures that consistently provides doctors with immediate gratification for their knowledge and efforts.
Spectacles were first described during the Middle Ages. In 1266, Roger Bacon magnified print in a book using a segment of a glass sphere. A painting completed in 1352 shows a prelate wearing lenses in a mounting. In the late fifteenth century, merchants sold spectacles to buyers who chose them on the basis of their own judgment of how vision improved. As the trade of lens making proliferated throughout Europe, it became organized into a guild. Although cylindrical lenses had been manufactured since 1827, it was not until Donders published his methods of refraction that correcting astigmatism became an exact science. In 1893, when American Optical developed the trial case of lenses, opticians, rather than spectacle peddlers, became the primary providers of eye examinations. Although instrument makers have dramatically improved the ability of examiners to provide accurate and repeatable lens prescriptions, most subjective techniques still rely upon a comparison of views through different lenses.
PURPOSE OF THE TEST
One of the most common reasons that patients seek eye care is to obtain correction of their refractive error. However, refraction is also a diagnostic tool used to differentiate decreased acuity caused by uncorrected or incompletely corrected refractive error from blurred vision related to eye disease.
In most cases, the final determination of the refractive correction is based upon the patient’s appreciation of the lens power that provides the clearest vision at the desired viewing distance. This procedure, subjective refraction, is a time-honored combination of the technical skill required to select a lens that produces a sharply focused image on the retina tempered with the fine art of determining the best overall correction, incorporating other factors such as the balance between the two eyes, the patient’s visual needs, the patient’s age, and the rate of change of the refractive error. The concepts and procedures described here refer to neutralization of the refractive error with spectacles, but most of these principles and techniques also apply to correction using contact lenses.
UTILITY OF THE TEST
Subjective refraction is usually performed after an in-depth history has been obtained, which includes ascertaining that clear vision has been achieved previously, describing visual symptoms and any relief provided with the current correction, and specific visual requirements related to work and avocations. It should be performed before any other test that might alter the patient’s responses because of physical changes to the eye, including Goldmann tonometry and gonioscopy. Any examination procedure that uses bright lights, such as ophthalmoscopy or slit-lamp evaluation, can produce a photostress response. Refraction should be done either before performing these tests or after an appropriate recovery period.
Cycloplegic eyedrops can be used to eliminate accommodation during the examination. In most cases, the results of a cycloplegic refraction are not prescribed as a correction. Rather, this type of examination is used in select circumstances to determine the baseline refractive status of the eye. There are two common situations in which this is valuable.
• In young individuals who are suspected of accommodative spasm, especially when it is accompanied by esophoria or esotropia, it is important to prescribe the strongest plus power correction in order to relax accommodation. A follow-up examination, not under cycloplegia, is usually required to determine the maximum amount of lens power that can be tolerated in the natural state.
• Recent protocols for refractive surgery dictate that the cycloplegic refractive power of the eyes be determined prior to undergoing the procedure.
Although a totally subjective test is possible, most examiners use a baseline starting point, such as an evaluation of the patient’s previous eyeglasses, retinoscopy, or the results of an automated refraction, which they then refine to meet the patient’s requirements. In general, the goal is to determine the maximum plus power correction that provides clear vision at far and a near correction that provides clear vision at the desired distances. Many methods have been developed to determine the “best” correction, any of which an adept refractionist can call upon to resolve a specific refractive quandary. For the purposes of this chapter, only the most widely accepted methods are described.
As with most health care procedures, many levels of sophistication in the instruments are available to perform this technique. They range from highly automated scanners and analyzers that provide an objective measure of the eye’s refractive error in seconds to the centuries-old method of placing loose lenses by hand into a trial frame worn by the patient. Each method has its proponents and, in particular situations, each method has its advantages.
Automated refractors analyze the focal power of emitted light from the eye and convert it into a dioptric correction. They are very fast, require minimum skill levels to operate, and are fairly accurate. They are also very expensive. Certain high-end models have subjective refraction capability so that the correction can be refined in the instrument. Portable automated refractors are now available.
Most practitioners rely upon the manually operated refractor or Phoroptor that contains a battery of lenses arranged in wheels that can be positioned in front of the patient’s eyes; the lenses can be changed quickly to provide a wide array of plus and minus spherical lenses as well as a range of cylindrical lenses, available in both minus cylinder and plus cylinder configurations, that can be rotated to the appropriate axis. Earlier in this century, when most ophthalmic lenses were manufactured in plus cylinder form, it made sense to use plus cylinder examination lenses that mimicked the final lens design as closely as possible. Contemporary ophthalmic lenses are made in minus cylinder form, so it now may make sense to refract using minus cylinders.
A trial frame can be used to mount loose trial lenses in front of the patient’s eyes. Trial frame refraction is a time-consuming procedure, and because of the thickness of individual lenses, especially at stronger powers, a power shift is induced when several lenses are stacked together. This error can be minimized by placing the strongest spherical lens in the rear well closest to the patient’s eye. A variation of this technique is to use a clip-on trial lens holder which can be mounted on the patient’s current glasses or on a “loaner” pair of glasses made up in a spherical power close to the patient’s required correction. This works exceptionally well when the existing eyeglasses contain a strong spherical or cylindrical component. The most practical use of a trial frame or clip-ons is to allow the patient to experience the change in correction before investing in a new pair of glasses.
Monocular Subjective Refraction
There are three basic components in the operation of a spectacle lens: the spherical power, the astigmatic cylinder power, and the axis. An accurate determination of the spherical component is predicated on having fully corrected the astigmatic error to ensure that a point focus is obtained with the final correcting lens. Therefore, subjective examinations proceed in that order. In eyes with astigmatism, each of the principal meridians produces a linear image at its focal distance. In the space between foci, the interval of Sturm, the image has a progressive change in its elliptic profile and, at the focal distance of the dioptric average of the two principal powers, the image is round, the circle of least confusion. In an eye uncorrected for astigmatism, the best acuity occurs when the circle of least confusion falls upon the retina ( Fig. 11-1 ). At all other points within the astigmatic pencil, the image is distorted along the principal meridians whereby each point source produces an oval image. The oval images of two or more adjacent point objects overlap along one of the principal meridians and appear darker along their long axes. Some refractive techniques use this effect to neutralize subjectively the astigmatic focus.
Figure 11-1 An uncorrected astigmatic eye with the circle of least confusion on the retina. The horizontal and vertical focal lines are dioptrically equal in front of and behind the retina.
The “clock dial,” a standard target in most ophthalmic projector systems, is a circular chart with radii drawn at 30° intervals. When the correct power toric lens is interposed along the appropriate axis, each image is circular and all of the radii appear equally dark ( Fig. 11-2 ). The starting point of the test is to have sufficient plus lens power in the tentative correction so that the focal points of both principal meridians are anterior to the retina, yet are recognizable. This “fogging” technique serves to inhibit the natural accommodative response to blur; any focusing effort only further blurs the image. In practice, the initial starting sphere (obtained by omitting the minus cylinder from the net retinoscopy result, the previous spectacle correction, or the autorefraction result) is placed before the eye under test. For an eye correctable to 20/20 (6/6), sufficient plus lens power is added to blur the 20/40 (6/12) line of letters, usually at least 1.00D.
In eyes with more astigmatism, enough plus power must be added to fog the least myopic or the most hyperopic meridian. The clock chart is projected and the patient is asked, “Which, if any, spokes on the wheel are darker?” Because the details on the chart are standardized at the 20/30 level, incremental reductions in plus power are required until some of the lines are clear. If no astigmatism is present, all of the spokes remain equally blurred as plus power is reduced. In astigmatic eyes, the focal line produced by the flatter principal meridian is closer to the retina and appears darker or bolder. With high values of astigmatism, one or two lines are prominent, whereas at lower values several lines may initially appear equally dark. The center of the group is estimated by the patient. A direct method of communicating the correct axis is to have the patient point out the darkest meridian with a laser pointer. The axis of the correcting minus cylinder is placed at 90° to this line.
Another simple method is to use the lowest clock time of the darkest line and multiply by 30. For example, if the vertical line was darkest, the patient would respond, “The 6 o’clock/12 o’clock line.” The correcting cylinder should be placed at axis 180° (6 × 30). To maintain the refractive fog, a +0.25D sphere is added for each -0.50D of cylinder that is added. Minus cylinder lenses are then added in 0.25D increments until all the spokes are equally dark. When equality of the spokes is reported, the process should be continued until reversal occurs to ensure that the full cylinder power has actually been achieved.
Because the meridians on the clock chart are 30° apart, the true axis may lie between them. There are several other commonly used charts that can refine the axis more precisely. The sunburst chart has radii that are only 15° apart; however, it is often
Figure 11-2 The principal meridians of the clock chart. A, As seen by an eye with uncorrected astigmatism in which each image appears as a vertical oval. The overlapping ovals make the vertical line darker. B, The same eye when the correct amount of cylinder is in place and the fogging lens has not been removed. Each image appears as a blurred circle so that all the lines appear equally dark. C, The same eye with the full spherocylindrical correction in place. Each image appears as a sharp point, giving an even, well-focused appearance to the chart.
difficult to communicate the precise axis to the examiner because of fluctuations in response related to minor head movements. The Paraboline rotary slide ( Fig. 11-3 ) has two symmetrical parabolic arcs whose asymptotic ends approximate the image of an arrowhead. With the eye in a “fogged” state, the slide is rotated until both halves of the arrowhead appear equally dark. The axis can be read from a protractor projected onto the screen. Along the principal axes of the pattern is a cross of dotted lines, which is then used as described before to determine the correct cylinder power.
The Jackson Cross-Cylinder (JCC) test is perhaps the most commonly used method for subjectively determining the presence of astigmatism and for refining the power and axis of a refractive cylinder. It relies on the principle of placing the circle of least confusion on the retina. A crossed cylinder is a lens whose principal powers are equal and opposite in sign. The standard power JCC has powers along its principal meridians of +0.37D and -0.37D to produce an astigmatic range of 0.75D. Each end of the minus axis is marked with a red dot, whereas the plus axis has white dots. The rotating handle in the manual
Figure 11-3 The Paraboline rotary projection chart. The axis of the correcting lens is determined by rotating the slide until both arms of the pattern appear equally dark.
Figure 11-4 The handheld Jackson Cross-Cylinder. Note the red circles at the ends of the minus axis.
lens and the pivot axis in the refractor-mounted JCC is 45° away from the principal meridians to allow the lens to be flipped quickly into two primary positions. When the circle of least confusion lies on the retina, meridians are equally out of focus (see Figs. 11-4 and 11-5 ).
A cylindrical lens whose minus axis is lined up exactly along the astigmatic plus axis of the eye produces a resultant spherocylinder whose axis is also coincident. However, if the correcting lens axis is not aligned with the astigmatic axis of the eye, the resultant cylinder’s axis lies oblique to the other two axes. The power of the resultant cylinder varies in proportion to the amount of axial rotation between the axis of the eye and the axis of the lens. The JCC is used to determine the correct axis and power of the correcting lens by producing larger or smaller circles of least confusion on the retina in each of its flipped positions. The correcting cylinder can be changed until the sizes of the blur circles are equal. As in all astigmatic correction techniques, the cylinder axis needs to be established before the power can be determined.
When the JCC is used to locate the correcting axis, the images are equally blurred when the principal meridians of the JCC are equally misaligned with the true correcting axis of the eye. To refine the astigmatic correction, the starting point correction is placed before the eye. The handle of the JCC is aligned along the minus axis of the spectacle lens, placing the JCC’s principal meridians each 45° away. Using a target one line larger than the best acuity obtained through the tentative correction, the examiner flips the JCC and asks the patient the famous question, “Which is better—one or two?” The end point is the answer, “They are equally blurred.” If one position of the JCC produces a better image, the axes of both the tentative correcting lens and the JCC are moved 5° in the direction of the red dots on the JCC. In the refractor models, the cylinder axis and the JCC rotate together. The lens is flipped again and the patient is given the opportunity to compare the image through each lens. At the axis at which equality of blur is located, the lenses should be rotated another 5° in the same direction. If the previous response was accurate, the new position should produce a reversal in direction.
To determine the correct minus cylinder power, the JCC is rotated until one set of principal meridians overlies the minus axis of the correcting lens. The handle of the manual JCC is now 45° away. On the refractor units, the position is marked by a click-stop detent. Using the same line of letters, the JCC is flipped and, again, the patient is asked to choose the better of the two images. If the JCC’s minus axis, marked by the red dots, is
Figure 11-5 The refractor-mounted Jackson Cross-Cylinder in place to check for axis orientation of a cylinder axis 45°.
aligned with the cylinder axis, a cylinder of 0.25D more minus power is added to the correction. If the better image is produced when the plus axis, marked by the white dots, is aligned, the cylinder power is reduced by 0.25D. To ensure that the circle of confusion remains on the retina, for each 0.50D of cylinder that is added, a 0.25D sphere of the opposite power should be added. The procedure is repeated until the two images are equally blurred. Going one lens past to reversal ensures that the correct lens is selected.
Once the proper cylindrical correction has been established, the final sphere needs to be determined. One simple technique is to “refog” the eye using plus lenses to minimize the effects of uncontrolled accommodation. Obviously, this is more of an issue in younger individuals who have a large accommodative reserve. Using the line of expected best acuity, the power is reduced by 0.25D at a time with a sufficient intervening pause to allow the patient to attempt to interpret the letters. Once the letters are identified, the next smaller line is presented. If these letters are not clearly recognized, power is reduced by another 0.25D. If the letters are still not clear, the previous lens probably produces the sharpest unaccommodated focus. If the letters are seen, the process is repeated with the next smaller line. Many individuals have the capability to see details smaller than those on the 20/20 line. It is important to record the best acuity to establish a baseline for future comparisons.
Another technique commonly used to refine the final sphere is the duochrome test, which makes use of the chromatic aberration of the eye. White light entering the eye is refracted according to its component wavelengths. In an emmetropic eye, blue light focuses about 1D myopic, whereas red light focuses about 0.5D hyperopic but equidistant from the retina. The duochrome test uses a pair of colored filters built into the projector chart, the peak transmission of one at 530?m (green) and of the other at 670?m (red) ( Fig. 11-6 ). In corrected emmetropia, a matched presentation of letters is equally blurred on each side of the chart. With the best spherocylindrical correction, the patient is asked to look at the letters on the green side. They remain in focus only when accommodation is relaxed. Because the letters on the red side can be made clearer by accommodating for them, the patient is asked to look quickly at the letters on the red side and then back at the green and compare their clarity. If the letters on the green side are clearer, the correcting sphere is changed by 0.25D in the plus direction. If the letters on the red side are clearer, 0.25D is added in the minus direction. This test is sensitive enough for 0.25D to cause a reversal in clarity.
Figure 11-6 When letters on the green side of the chart are clearer, more spherical plus power needs to be added.
The entire procedure is repeated for each eye to produce two monocular subjective prescriptions. Assuming that the patient has clear, single binocular vision, the effects of compensating for an existing heterophoria or the effects of summation of vision from both eyes may alter the lens powers chosen for the binocular subjective prescription. The process is usually accomplished in two steps.
The first is to ensure that equal accommodative effort is present between the two eyes. If the best-corrected vision is approximately the same in each eye, vision is fogged with +0.75D lenses. Sufficient vertical prism is placed in front of each eye to produce two separate images of the isolated 20/40 (6/12) line. The patient is asked to compare the clarities of the upper line and the lower line. If they appear equally blurred, +0.25D is added to one eye and they are compared again. The other eye should now see slightly more clearly. The lens is removed and the process repeated for the other eye. Adjustments are made until the images are as equally blurred as possible. If there is no pair of lenses that produces an equality of blur between the two eyes, the pair that gives the slightly better image in front of the dominant eye is often preferred.
The best acuity line is then isolated on the chart. The fogging lenses are reduced from both eyes by 0.25D at a time, allowing sufficient time between stages for the patient to adjust to the lens change. In the same way as with the monocular subjective test, the lens power that gives best acuity without inducing accommodation is usually the final choice. The duochrome test offers an alternative method of determining the lens powers that produce a sharp unaccommodated retinal image.
The same technique can be employed with eyes that have a moderate discrepancy in best-corrected vision, either from amblyopia or from some other abnormality. The lens powers can be balanced using a larger line of letters, for example, the 20/80 (6/24) line, and then reduced to the best acuity, which is that of the better eye. This solves the dilemma of trying to determine the best monocular subjective correction in an eye with poor visual discrimination.
Binocular refraction is an infrequently used technique in which both eyes are fixating while the monocular refraction is measured. Most contemporary devices use some form of vectographic separation in which a polarized target is presented to each eye through interposing polarized analyzers with a different axis in front of each eye. This has the advantage of mimicking the normal form of seeing, incorporating all of the patient’s binocular efforts including horizontal and vertical phorias.  In addition, this method offers the only way to identify a cyclophoria in which the astigmatic axes of the eyes are different under binocular conditions than when observed monocularly.
Trial frame confirmation of the final prescription is often overlooked but is an extremely valuable verification of the comfort and acuity of the new lens power. Although an examination room of length 20?ft (6?m) is considered to be the equivalent of optical infinity, .17 D of accommodation is still required at that distance. It is psychologically reassuring for the patient to step out of the examination room and view the end of the hallway or, better still, the other side of the street through the new lenses. This small investment of time may save lengthy follow-up visits that could result from miscommunication in the examination room.
If the cylinder correction is similar to that of the patient’s old glasses, it is relatively straightforward to have the patient hand-hold spherical trial lenses in front of the glasses and compare vision with and without the change in prescription. This is a simple way to determine which is the more satisfactory lens correction when there is a discrepancy between the monocular subjective and the binocular subjective tests. As the monocular subjective test’s end point is best acuity and the binocular subjective test’s end point is equality of accommodation, some patients may have a slight difference in right and left eye acuities through the binocular prescription. This refinement offers them the opportunity to observe the difference between the two corrections and to make a practical choice between them.
If there is some doubt about the visual comfort of the change, the lenses can be held in position with a clip-on lens holder while the patient takes the opportunity to walk around and adjust to the difference. In some cases, it may be beneficial to allow patients to borrow the lenses and holders overnight in order to evaluate the lens changes in their own environment. It is important to mark the right and left lenses and, if cylinders are required, to provide a sketch to help align the axis marks.
A similar procedure can be used when the change in correction is a spherocylinder. It is unwieldy to place and remove more than one lens in front of the patient’s glasses. If the new cylinder axis is different from that of the old eyeglasses, a calculation of resultant cylinder axis and power is required to determine the appropriate lens to hold in front of the glasses. In such a situation, it is more practical to place the new correction in a trial frame and to let the patient alternately view at a distance through the trial frame and the old glasses. The trial frame interpupillary distance, the vertical lens position, and the pantoscopic angle should be adjusted correctly, especially with strong lens powers.
The near correction is the distance correction with sufficient plus additional power (the “add”) to satisfy individual needs for clear, comfortable single vision at a desired near point. Although there are normative tables for determining an add according to the patient’s age, these simply function as benchmarks to help the examiner recognize a potential overcorrected or undercorrected condition. This is an important time to listen to your patient. Although patients are notoriously inaccurate when estimating their working distances, the description of how they use their eyes at near helps to determine not only the strength of the lens power required for tasks at near but also the form in which the correction will be most effective. For example, a presbyope who requires a +2.00D add for reading may be very satisfied with a bifocal correction for most activities but may require a +1.25D add in single-vision lenses to work at a computer terminal.
One rule of thumb that has gained wide acceptance is that the near add at a given distance should allow half of the patient’s accommodative amplitude to remain in reserve. The amplitude is
determined by measuring the closest point at which an individual can maintain focus through the distance correction. For a prepresbyope, this simply means measuring the distance at which a fine line of print can no longer be focused. This distance, measured in centimeters, is divided into 100 to convert it into amplitude of accommodation. A presbyope needs to place a plus lens over the distance correction to be able to see the fine print. The closest distance to which the print can be moved before blurring is again converted into diopters and the power of the interposed lens subtracted to give the amplitude ( Box 11-1 and Fig. 11-7 ).
A clinical method commonly used to measure the near add is the Fused Cross-Cylinder test. A cross made up of multiple horizontal and vertical lines is presented to the patient at a distance of 40?cm. A JCC with its minus axis vertical is placed in front of the distance correction. The patient is asked to compare the boldness of the horizontal and vertical lines of the cross. If no add is required, the lines are equally dark. If the horizontal lines are darker, plus power is added binocularly in 0.25D increments until the lines are equally black or until the vertical lines become more prominent. This lens power becomes the tentative add.
The final add is determined by verifying that the add is appropriate for the patient’s visual needs. The range of clear near vision is the linear distance between the far point of the near lens (usually the reciprocal of the add power) and the near point of accommodation through the add. Because the range of vision is inversely proportional to the power of the lens, many experienced refractionists prescribe the weakest add that meets the patient’s demands. For most individuals, having a larger range in which objects are clear overrides the desire to see extremely fine print at a close distance. It is often helpful to patients who are receiving their first presbyopic correction to have lenses held in
Calculated Near “Add” at Any Distance Should Keep Half of the Patient’s Accommodative Amplitude in Reserve
With an extra +1.50D lens, the near point of accommodation is 40?cm (2.50D).
The patient’s amplitude is 1.00D (2.50D – 1.50D).
For a working distance of 50?cm, 2.00D of accommodation is required.
Therefore, the patient’s “add” for that distance should be +1.50D [2.00D -1/2(1.00D)].
Figure 11-7 A near card is placed in front of the Phoroptor and slid back and forth to determine the closest distance at which the print can be seen before blurring takes place.
place to demonstrate that their near correction will, of necessity, blur their distance vision.
In situations where an anisometropic distance correction is required, it is wise to measure the ranges monocularly to account for any optical effects related to the unequal strength of the lenses. Unequal adds may also be prescribed in certain other situations to keep the near and far points of the ranges at similar distances. As with any significant change, a trial frame evaluation of the new correction may help to identify any potential difficulties before glasses are fabricated. In some cases of anisometropia, bifocals may produce reading discomfort because of an induced vertical prismatic effect in the reading position of gaze. Specially designed slab-off lenses or single-vision reading glasses may be required.
Patients who require higher bifocal adds may not have sufficient accommodation to overlap their distance and near ranges of vision. This “dead zone” is problematic in certain jobs and avocations. An accountant may not be able to see a calculator clearly in its normal desktop position, and a violinist may have difficulty reading stand music. Although trifocals or progressive lenses may be satisfactory, special use lenses, such as low-add bifocals with a high segment line, may be required.
Computer users who must also read place a unique set of demands upon their glasses. The video screen is usually just below eye level at arm’s length or slightly closer, whereas reading material and the keyboard are positioned lower and somewhat closer. It is often worthwhile to have patients adjust one of the computer terminals in the examiner’s office to simulate their workstation conditions. Eye-to-screen and eye-to-keyboard measurements can be used to determine the necessary add powers. Many presbyopic computer operators have occupational bifocals in which the top section of the lenses has the intermediate
Figure 11-8 Plus and minus racks of spherical lenses used for retinoscopy screening. They are also useful to determine an approximate subjective spherical equivalent lens.