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
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 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
-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 patients 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.
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
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.
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.
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
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:
sphere is held in the spectacle plane
is observed in the 90th meridian at 33cm.
is observed in the 180th meridian at 25 cm.
objective finding is +2.00 -1.00 X 90
Finally, the examiner should not be influenced by the patients current
eyeglass correction while performing the objective and subjective refractions,
but should approach each eyes refractive state objectively.
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:
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, lets 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
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.
Entering acuity, OD=20/200, OS=NLP
old glasses unavailable
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
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.