Low Vision Refraction                               

Benjamin Freed, O.D., 2000

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A careful refraction may be the single most important test in the sequence of the low vision evaluation. Performed properly, the examiner obtains not only a string of numbers called the refractive error, but information essential for the  effective prescription of low vision devices, such as the level and quality of visual acuity, sensitivity to blur, effects of glare and the quality of fixation. The optical theory involved is of course the same as when refracting the normal eye, but special considerations must be made for low vision patients.

     Refraction is of particular importance in these patients because, for a variety of reasons, they tend to have a high prevalence of uncorrected error. First, certain ocular pathologies have characteristic refractive findings that may be undetected or change more rapidly compared to healthy eyes. Second, some individuals may overlook their own visual needs, perhaps accepting their blurred vision as a result of their ocular pathology or as a natural consequence of aging. Third, many patients referred for this service have come from settings where refractive care is simply not emphasized, or is performed by unqualified personnel. Very often the low vision specialist will confide that all the patient really needed was a good refraction.

     Refracting the low vision patient is not simply a matter of asking the patient to discriminate clarity between two lenses, but the integration of the following measurements and information which direct the examiner towards the refractive error and finally the lens prescription.

The History. The history of ocular pathology yields clues useful in solving the refractive puzzle. The following is a short list of entities commonly seen in low vision patients and associated with typical refractive errors:

  -pseudophakia: up to three or four diopters of against-the-rule corneal astigmatism is not uncommon

  -cataracts: as much as a ten diopter shift towards myopia, can be unilateral

  -diabetic retinopathy: fluctuations in the “best sphere”, dependent on blood glucose levels

  -degenerative myopia: shift toward higher and higher degrees of myopia with the progression of time, up to as much as -35.00D

  -penetrating keratoplasty: high corneal astigmatism, as much as twelve diopters, seen at any axis.

  -albinism: three or four diopters of corneal astigmatism, often with-the-rule, and usually symmetrical.

     In addition to the pathological history, the functional visual history goes a long way toward arriving at the correct refractive result. A determination must be made about task-specific visual disabilities. In particular, it is essential to specifically review the patient’s current near visual abilities and disabilities.  Many patients may not realize that they already have enough magnification for reading; their reading disability may simply be related to inadequate illumination. Moreover, this information may give an indication of the accuracy of their current distance correction. For example, a patient who reads small print at 33 centimeters with their +1.50 bifocal segment may be “over-plussed” by 1.50D in their distance correction.

The purpose and adequacy of each prescription must be carefully determined by the clinician as patients will not always offer complete or orderly information regarding symptoms or usefulness of various pairs of glasses. For example, a patient who presents with a pair of +3.00 spheres for a correction of distance hyperopia may reveal that they prefer not to wear them; knowing this may discourage you from prescribing the change to +3.50 spheres that may have been tested subjectively.

Visual Acuity. Distance visual acuity testing for low vision patients is routinely done at a practical distance between one and six meters, depending on the size of the room, the size of the letters on the chart, and the patient's acuity and fixation ability. The refractive error may also dictate the test distance. For example, if a known -1.00D myope left his glasses at home, a valid distance acuity could be measured at one meter without correction.

     Be accurate in measuring test distances; errors in patient-to-chart distances can be significant when testing at less than four meters compared to the same errors at distances greater than four meters. For example, if you have misplaced the chart at 180 centimeters, intending it to be at two meters, you have overestimated the acuity by 10%. The same 20 centimeter error at 6 meters incurs a 3% overestimation.

     Record the entering distance and near acuity as a Snellen fraction, with the numerator reflecting the actual distance at which you tested. If you test at two meters and the patient reads the 16M line, then record the acuity as 2M/16 so you will know for future reference what the functional test distance was. Don't mix unrelated units when recording acuity measurements; if your patient reads 1M print at 40 centimeters, record the acuity precisely as ".4M/1" rather than "20/50 at forty centimeters."

     Do not omit measuring and recording the entering near visual acuity, as it will give an immediate indication of the degree of the functional reading impairment. In particular, carefully assess the uncorrected near vision of myopes complaining of an inability to read. Their punctum remotum, or far point, is a good indicator of their refractive error in a myope. For example, if a progressive myope presents with an old distance correction of -12.00 and without glasses reads small print at 5 centimeters (and no further), he is now a -20.00D myope. For the same reason, use this method to rule out the existence of nearsightedness in any eye that was previously considered to be "finger counting" and nonfunctional, especially if the retinoscopy reflex is unreadable. An example would be an eye with a nuclear cataract causing a significant shift towards myopia. If such a patient reads 1M at 10 centimeters without glasses(.1M/1), test carefully for the subjective distance correction, which may turn out to be -10.00D.

     Finally, the use of a pinhole to indicate the possible existence of uncorrected refractive error is helpful, but keep in mind that in many pathologies, acuity can improve through a pinhole even while wearing the best possible spectacle correction. Reasons for this include the reduction of light scattering and of the spherical aberrations of irregular corneas.

Lensometry. Low vision patients sometimes present with eyeglass prescriptions greater than +/-20.00, beyond the range of many lensometers. An accurate clinical method to measure the back focal length of these lenses is to "shift" the scale on the drum of the lensometer, which in effect extends its range. This is accomplished by placing a moderate strength "compensating lens" of opposite sign to the unknown lens in such a position in the lensometer as to create a constant and linear shift in the measuring scale. The best position to place this compensating lens is at the objective of the lensometer telescope(fig. 1) and the shift created is the effective power of this lens at the stop. As an example, if you are measuring a -25.00D spectacle, place a +5.00 compensating lens at the telescope objective. Its effective power at the stop will be about +7.00(depending on the lensometer), the amount you have linearly "shifted" the lensometer scale. Now +7.00 can be considered the new "zero" position. With the compensating lens in position, place the patient's lens at the stop and focus the mires. If you now read -18.00D on the lensometer drum, you must now add -7.00 to this amount, yielding a true back vertex power of -25.00. Similarly, plus lenses stronger than 20 diopters can be measured by using a minus compensating lens.

Keratometry. The K-reading is useful as an estimate of refractive cylinder power. It is particularly useful to confirm a finding of very high cylinder found in retinoscopy or the subjective. Although not always comparable to the cylinder found subjectively in cases such as keratoconus or cataracts, the measurement is comparable in, for example, aphakia, albinism, and penetrating keratoplasty. The range of powers can be extended if needed for very flat or steep corneas.1

Retinoscopy. A skilled refractionist relies on the retinoscope as an invaluable tool to determine not only the refractive error, but media clarity and pupillary size and location. It is generally more difficult to perform in the case of the low vision eye compared to that of the healthy eye because of media opacities and optical irregularities, and a special technique is recommended.

     The use of hand-held trial lenses precludes the need to use a trial frame or phoropter for this test and is more comfortable for the patient and faster for the examiner(fig 2). This method allows the examiner to move off axis to obtain a better reflex while phoropters with their multiple lenses tend to make the reflex very difficult to see. Hold the free lenses in the spectacle plane with the patient's glasses off, because if you scope some remaining cylinder over their old sphero-cylindrical correction, you may have to deal with the problem of adding oblique cylinders to ascertain the objective finding. Turn all the lights off, and determine "neutrality" by moving closer(seeing with motion) and further(seheing against motion) from the patient. Subtract the dioptric equivalent of the neutrality distance(the working distance) from the gross lens power in your hand to obtain the net objective refraction. A short working distance, between 20 and 50 centimeters, will brighten a reflex that may be too dim to interpret when viewed from as far as 66 centimeters. For example: 

          -a +5.00D sphere is held in the spectacle plane

          -neutrality is observed in the 90^th meridian at 33cm.

          -neutrality is observed in the 180^th meridian at 25 cm.

          -the net objective finding is +2.00 -1.00 X 90

     Finally, the examiner should not be influenced by the patient‘s current eyeglass correction while performing the objective and subjective refractions, but should approach each eye’s refractive state objectively.

Subjective. Although it is sometimes easier to use a phoropter to perform the subjective test on some low vision patients with mild impairments of vision, a trial frame is necessary to test many low vision patients, especially those with significant visual impairment who may find it difficult to fixate through a phoropter. Make the following six adjustments of the trial frame:

1.temple length

2.bridge height

3.pupillary distance

4.pantoscopic tilt

5.horizontal tilt(level)

6.vertex distance

Take note of the vertex distance in prescriptions over +,-10.00D. Small errors in the estimation of vertex distance create clinically significant differences in refractive corrections. For example, let’s say you refracted a myope at a 12 millimeter vertex distance with  -15.00D and proceeded to prescribe -15.00D. If this myope is fitted with glasses that place the back surface of the -15.00D lens at a distance of 8 millimeters from the cornea, you have “over-minused” the patient by almost a diopter.

     The key to refracting low vision patients is to present the eye with enough of a lens change for the patient to discriminate changes in blur. The amount of spherical lens power needed to elicit an appreciable change in clarity or blur is called the "just noticeable difference" lens, or the JND. The lower the acuity, the larger the JND, which may also depend on the pathology and individual sensitivity. The denominator of the 20 foot Snellen acuity is a good rule-of-thumb estimator of the JND for a given eye. For example, a 20/150 eye will be sensitive to a lens change of approximately 1.50 diopters using this rule.

     The first step in subjective testing is to find the best sphere. The starting lens in the trial frame(spheres in the back barrel, cylinders in the front) should be the retinoscopy finding, or if unobtainable, the patient's past spectacle prescription. If this is also unavailable, use your best judgment based on their eye history and entering acuities.

     Direct the patient's attention to a line large enough to maintain fixation but small enough to detect differences in blur. Present the patient with the JND interval using a plus and minus lens of equal absolute value and ask to compare the clarity(fig 3). For example, a 20/200 eye with a JND of 2.00D should be shown a +1.00 sphere and a -1.00 sphere sequentially(total lens change = 2.00D). If they have a preference for either lens, change the sphere in the back barrel of the trial frame in the appropriate direction. A reasonable amount of change would be the JND.

          Example:   Patient A

                     Entering acuity, OD=20/200, OS=NLP

                     retinoscopy unobtainable

                     old glasses unavailable

                     JND= 2.00D

                     Trial frame has no lenses in it

Find the best sphere:

1. Ask the patient to compare +1.00sph. to -1.00sph. Patient states that +1.00 sph. is clearer. Place +2.00sph. in the trial frame.

2. Again ask the patient to compare +1.00sph. to -1.00sph, this time through the +2.00 sph. in the trial frame. If the patient still prefers the plus lens to the minus, replace the +2.00 in the trial frame with a +4.00sph.

3. Again ask the patient to compare +1.00sph. to -1.00sph this time through +4.00 sph. in the trial frame. If they now prefer the -1.00 to the +1.00, this is called a reversal and you now know the "best sphere" is more than +2.00 and less than +4.00. You can enter a +3.00 in the trial frame and continue refining the best sphere by letting the patient compare the JND lenses in front of the +3.00 sphere in the trial frame.  Notice that you are determining the best sphere by bracketing around it with stronger and weaker lenses. In this way, the refractive error can be arrived quite accurately and reliably in patients with significant loss of sensitivity to blur.

     Notice that we are changing the corrective lens in the trial frame, but not changing the +,-1.00 JND lenses that the patient is being asked to compare. However, if the acuity improves, the JND should naturally be decreased. For example, in this case, if the acuity improved to 20/100, ask the patient to compare +0.50and -0.50 JND lenses.

     After finding the best sphere, test for astigmatism. If the K reading or retinoscopy indicate astigmatism, refine axis first, then power using a hand held Jackson cross cylinder, its strength chosen using the same JND rule of thumb.  Use  a  +,-1.00 JCC to test the patient in the above example. It is recommended to keep the following set of JCC's on hand: +,-0.25, +,-0.50, +,-0.75,and +,-1.00.

     For testing axis, carefully line up the handle of the JCC with the axis of the cylinder being tested(fig.4)and proceed to test axis in the same way as is done in a routine subjective. Smaller changes in axis should be made for larger amounts of astigmatism. For testing axis, this author does not advocate the setting aside of the cross cylinder and manually rotating the cylinder while asking the patient to say when the image is clear. Instead, the conventional forced-choice method is more reliable.

     For testing power, change the amount of cylinder by the JND and get a reversal in the same manner that you would for a "normal" eye. Remember to keep the spherical equivalent constant when changing cylinder powers. If large changes in cylinder power are accepted by the patient, do not hesitate to recheck and refine the spherical component to obtain valid findings. As a time saving trick, correcting cylinder power may be increased or decreased quickly with the use of a second Jackson crossed cylinder. A +,-0.50 JCC held in the free hand at the proper orientation instantly changes the cylinder power by a full diopter while maintaining the spherical equivalent of the correcting cylinder(fig. 5).

     After the test for astigmatism, the final step in the subjective is to retest for best sphere with the JND lenses.

     If keratometry or retinoscopy indicate a spherical(non-astigmatic) error, test subjectively for the presence of astigmatism after the best sphere has been determined. Check for the  presence of cylinder power, with or against-the-rule, by flipping the cross cylinder in front of the best sphere with its handle oriented at 45 degrees(the power meridians will then be at 90 and 180). If the responses indicate astigmatism, place a JND amount of cylinder in the trial frame at the appropriate orientation, adjust the sphere to keep the spherical equivalent constant, and then refine axis and power. If the responses indicate equal blur on both sides of the flip, the patient either has no astigmatism, or has cylinder axis 45 or 135, and you should test for the  possible existence of this oblique cylinder by changing the orientation of the JCC handle by 45 degrees and flipping again. If the responses indicate equal blur on both sides of the flip, you can be confident that there is no cylindrical component. But if the responses indicate a preference for oblique cylinder, place a JND amount in the trial frame at the appropriate axis, keeping the spherical equivalent constant, and refine axis and power. After refining cylinder, go back and retest for best sphere using the bracketing method discussed above.

    The Snellen numerator of the recorded final best corrected acuity should reflect the subjective test distance to indicate any over-correction incurred. For example, if the subjective refraction at one meter is +2.00 -1.50 X 90, remember that the true "distance" refraction is +1.00 -2.00 X 90. If the smallest line read was the 10M line, record the subjective as +2.00 -1.50 X 90, 1M/10.

     Be sensitive to the patient's heightened anxiety for the subjective test; test slowly and carefully, giving the patient enough time to discriminate blur. Repeated presentations to certain patients are necessary to yield valid results.

     The effective prescription and ultimate use of low vision devices is usually dependent on the accuracy of the refraction. Careful and logical technique will improve the validity of this challenging and important test, with improved visual function for your patient as the great reward.