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that are commonly used and can be easily adopted
by an individual practitioner will be discussed.
This chapter will also describe only the trial
frame refraction used most commonly in the
author’s country of practice. However, these
techniques and logic can be easily adapted to
phoropter refraction as well.
4.3 Principle
The fundamental goal of a spectacle prescription
is to provide clear visual acuity to a person at all
viewing distances. To achieve this, the far point
of the person should be conjugate with optical
innity. With the spectacles, one should be able
to see clearly at far distance with nil accommodative effort and at all other closer distances with
optimal accommodative effort (before presbyopia sets in). In the attempt to abolish accommodation for distance, the most common
philosophy followed for spectacle prescription is
“minimum minus for maximum visual acuity
and maximum plus for maximum visual acuity.”
This can be modied as “optimal correction
(plus or minus) for maximum visual acuity with
nil visual stress.” Visual stress can be reduced or
eliminated when we consider other factors of
binocular vision status for prescribing
spectacles.
For example, in the presence of an exodeviation, with a hyperopic refractive error and a high
AC/A (accommodative convergence/accommodation) ratio, a little under-correction of the plus
power can be considered if a good control for
exodeviation is achieved without compromising
visual acuity and stressing the accommodative
system. The lag of accommodation over the subjective acceptance in this scenario can be checked
to ensure that it is within the tolerable limit
(+0.25 D to +0.75 D). Such a spectacle prescription would achieve optimal correction, maximal
visual acuity, and minimal visual stress. Of
course, this could be a rare event. The conventional principle would work in most instances,
but in exceptional situations, the spectacle prescription can be modied to alleviate the signs
and symptoms of the patient. Therefore, there is
no one correct method/technique to prescribe
spectacles. Yet, we will be discussing the broad
guidelines and techniques here.
4.4 Technique
The rst step toward spectacle prescription starts
with history, followed by measurement of visual
acuity (distance and near), objective refraction,
and eventually, subjective refraction for distance
and then near viewing. The near visual acuity
check is typically done at a 40cm viewing distance. For those in the presbyopic age-group or
those whose near visual acuity is not optimal, a
near addition is prescribed to improve the near
acuity.
Proper patient history matters in subjective
refraction. If a patient is very comfortable with
their earlier spectacles and simply wants to
change their frame, it still requires performing
good refraction. However, the nal spectacle prescription should be close enough, if not similar,
to their earlier spectacle correction, especially if
the visual acuity is good for both distance and
near with their current spectacles. If very different, it requires careful examination and perhaps
cycloplegic refraction. In one study, about 10%
of patients with non-tolerance to their new spectacles had adaptation issues even when the refraction was correct [2]. Sometimes, some people
may not tolerate even a ±0.50 D difference in
spherical error [10]. Errors of refraction were the
leading cause (47%) for spectacles non-tolerance
[10]. Hence, it will be important to learn the technique carefully to minimize this error.
It is good to remember the following important points before subjective refraction.
1. Age of the patient matters in how we start the
subjective refraction. For pre-presbyopic
adults and young children, a good starting
point is to leave the working lens (typically
+1.50 DS) and start fogging (actually
defogging).
2. A general rule of thumb is for every 0.25 DS
(or 0.50DC) change, one-line visual acuity
improvement should be expected [11, 12].
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This rule of thumb, however, depends on
other factors like pupil size, and blur adaptation of the patient, etc. Nevertheless, this rule
helps check the correlation between uncorrected visual acuity and refractive error.
3. For example, if a young person has a logMAR visual acuity of 20/200, to improve to
20/20, which is about 10 lines in the logMAR chart, a refractive error of −2.50 DS
could be expected. On the other hand, a
refractive error of −6.00 DS appears unrealistic for that level of visual acuity.
4. In the above case, other factors, such as
squinting/squeezing the eyes by the patient
to read or spasm of near reex [13], should
be considered.
5. This said, sometimes there can be outliers
who have reasonably good visual acuity
despite having a high astigmatic error (e.g.,
3.00DC). This could have resulted from better blur interpretation with one clear meridian (especially, vertical clarity can still help
read English optotypes than a vertical blur)
[12].
6. It will also be necessary to closely observe
the time taken and the ease with which
patients read the letters with or without spectacles. Look also beyond the patient’s verbal
response; look for the patient’s body language, facial frowns, head tilts, etc.
7. Particularly in non-verbal patients, looking
for these signs becomes even more important
in deciding whether the nal spectacle prescription should or should not be given.
8. Some patients can be nicky or sensitive to
any axis change they are not used to. So,
check the axis in their previous prescription
before changing it and make sure it warrants
a change.
9. Cycloplegic refraction is advisable for children, rst-time spectacle wearers, patients
with asthenopic symptoms, sometimes
patients with pre-presbyopia, and in those
when visual acuity, objective, and subjective
refraction are not correlating.
10. PMT (post-mydriatic test) or, more appropriately, PCT (post-cycloplegic test) visit will
be indicated if there is a major change from
the previous spectacle prescription or if the
subjective acceptance and cycloplegic objective refraction are very different. Else, the
spectacle prescription can be nalized in one
visit itself.
The spectacle correction for ametropia is
divided into a spherical and cylindrical correction. Considerations for anisometropic correction
(see Sect. 4.4) and presbyopia correction (Sect.
4.5) are also discussed.
4.4.1 Spherical Correction
Spherical correction is fairly straightforward for
patients with myopia or hyperopia, as long as the
patient’s accommodation is without large (>0.50
D) uctuations and is able to relax fully. Fogging
should be done for children and young adults. It
is also done for those with asthenopic symptoms
with good visual acuity (regardless of age).
Patients with high hyperopia and lower myopic
errors can also show accommodative uctuations
and would need fogging. Fogging is a must for
over-corrected (over-minus) myopic patients as
well. Pre-presbyopes with mild myopic or hyperopic refractive error will also benet from
fogging.
Spectacle prescription is challenging in people with hyperopic correction, especially with
active accommodation (children and young
adults). Different age-groups of children and
magnitudes of ametropia have different guidelines for prescribing spectacles [6]. The Vision in
Preschoolers–Hyperopia in Preschoolers (VIP–
HIP) study group [14, 15] and a systematic
review [16] showed increasing evidence for the
need to prescribe hyperopic correction for children, especially when there are concerns in academic performance. For example, the child may
have 20/20 visual acuity, but the refractive error
could be +5.00 DS; in such cases, it will be necessary also to do a dynamic retinoscopy and
determine the lag of accommodation. Remember,
children spend most of their time doing near and
intermediate tasks. Hence, while they may have
good accommodation momentarily to read a
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chart (distance and near), they may be unable to
sustain their accommodation for longer periods.
In an esodeviation, full correction of the
hyperopic refractive error is mandatory. But even
in the absence of esodeviation, if the lag of
accommodation is high (>+0.75 D), the child
may benet from a spectacle prescription.
Normative lag for different accommodative stimuli for children has been studied [17]. This would
be helpful when considering closer working distance for young children since their Harmon distance (the distance between the elbow and the
knuckle) is smaller. Also, in children with special
needs (e.g., Down syndrome), it would be important to check for their lag of accommodation, and
some children may need a near reading addition
as well [18, 19].
For patients who cannot relax their accommodation, techniques such as the Borish delayed
subjective test [20] will be useful. Briey, this
test requires regular subjective refraction, and
then a negative relative accommodation (NRA)
procedure is done to relax the accommodation. In
NRA, plus lenses in the steps of +0.25 DS are
added binocularly till the patient reports a sustained blur of the reading material held at 40cm.
Post this procedure, the patient looks at the distance visual acuity chart, 20/20 line with the
NRA value. The examiner then defogs until the
patient can again read that line. Through this procedure, the patient can accept additional plus
power. For patients who cannot accept their
hyperopic correction initially, it is important to
educate them about their spectacle prescription
and inform them that they may have to change
their spectacles later (after 3–6months). During
this time, the patient would be able to relax their
accommodation with their new spectacles and, in
their subsequent visit(s), may easily accept a
higher or their full hyperopic correction (see
Case 1). In some ophthalmology practices, homatropine hydrobromide (2%) eye drops are prescribed, and higher (or full) hyperopic spectacle
prescription is given to aid the spectacle adaptation process.
As a clinical note, if leaving a patient with
maximum plus, ensure the binocular acuity is at
least one line better than 20/20 (i.e., 20/16 or
−0.1 LogMAR). This assures that the patient can
easily adapt to their plus spectacle correction
without many complaints of distance blur. A similar check can also be done in myopic correction
(minimum minus). Also, check for correlations
with axial length (A-scan measurements) in cases
of high refractive errors.
Case 1
A 31-year-old male had a history of intermittent blurring of vision and dry eyes
while working on the computer for the past
5years. He was prescribed spectacles and
lubricating eye drops but was still symptomatic. He was advised to undergo psychological counselling since there was no
reason to explain his blurred vision.
Visual acuity with his spectacle prescription: RE: −0.25 DS (20/20, N6); LE:
plano (20/20, N6). Dry Retinoscopy: RE:
+0.50 DS; LE: +0.25 DS; Cycloplegic retinoscopy and acceptance: RE: +2.00 DS
(20/25); LE: +1.75 DS (20/25+2). He was
called for the post-cycloplegic visit and
was rst prescribed +0.75 DS (20/20 − 2)
for each eye. After 2months, his spectacles
were changed to +1.50 DS (20/20+3) in
each eye. He was more comfortable with
the new spectacles and was less symptomatic for his computer work from then on.
4.4.2 Cylindrical Correction
Cylindrical correction is given for astigmatism.
In astigmatism, the two principal meridians in the
eye have different refractive errors. As a result of
this, instead of a single point (stigma), an elongated ellipse (a-stigma) is seen. This ellipse is the
conoid of Sturm, and the clearest image within
this zone is the circle of least confusion. A perfect
refractive correction should aim to collapse this
conoid of Sturm to a clear point that will be
focused on the retina. An indication for a cylindrical correction will be evident when the patient
reads letters like “V” as “Y” or “O” as “Q,” etc.
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It is generally easier to adapt to spherical
correction than cylindrical errors. However,
when the cylindrical correction is large
(>1.00 DC), patients quickly appreciate the
clarity this correction provides and easily adapt
to this. For the rst-time spectacle wearer, the
cylindrical spectacle correction’s accuracy for
both the magnitude of the power and the axis of
the correcting cylindrical lens is crucial.
Patients who adapt to a wrong axis may later
have difculty accepting the correct axis and
may prefer the cylinder axis oriented to their
previous spectacle correction.
There are different types of astigmatism; these
include:
1. With-the-rule astigmatism: refractive power
of the vertical meridian is higher than the
horizontal meridian (e.g., K-readings: 44.70
D @106 and 41.80 D @16, (Fig.4.1).
2. Against-the-rule-astigmatism: refractive
power of the horizontal meridian is higher
than the vertical meridian (e.g., −2.00
DS/−3.00DCx90).
3. Oblique astigmatism: The principal meridi-
ans are within 120°–150° and 30°–60° in an
oblique direction (e.g., −2.00
DS/−3.00DCx40).
4. Regular astigmatism: The principal meridians are perpendicular to each other (e.g.,
44.00 D @20 and 47.00 D @110).
5. Irregular astigmatism: The principal meridians are not perpendicular to each other (e.g.,
44.00 D @30 and 47.00 D @90); this can be
seen in keratoconus or corneal opacities post
trauma.
6. Simple myopic astigmatism: only one of the
principal meridians has a myopic refractive
error; the other is focused on the retina (e.g.,
plano/−1.00 DCx90).
7. Compound myopic astigmatism: both the
principal meridians are in front of the retina
(e.g., −2.00 DS/−1.00 DCx60).
8. Simple hyperopic astigmatism: only one of
the principal meridians has a hyperopic defocus; the other is focused on the retina (e.g.,
+0.50/−0.50DCx90=plano/+0.50 DCx180).
9. Compound hyperopic astigmatism: both the
principal meridians are focused behind the
retina (e.g., +1.00 DS/−0.50 DCx100=+0.50
DS/+0.50 DCx10).
10. Mixed astigmatism: one of the principal
meridians has a hyperopic defocus while the
other has a myopic defocus (e.g., +3.00
DS/−5.00 DCx20 = −2.00 DS/+5.00
DCx110).
Fig. 4.1 Corneal
topography of a patient
who has with-the-rule,
regular astigmatism. The
vertical meridian has
higher power than the
horizontal meridian. The
correcting cylinder for
this patient was −3.00
DCx15°
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The above examples are given using keratometric readings or spectacle corrections in both
minus and plus cylinder forms to provide a broad
understanding of the astigmatism presentations.
However, we usually prescribe and dispense the
spectacles for astigmatic correction in a minus
cylinder. As a convention, “@” indicates the corneal power in that meridian in keratometric
readings. While “x” is used to indicate the correcting cylinder axis in the spectacle prescription.
This axis will be perpendicular to the corneal
meridian with higher power for the minus cylinder. For example, 44.00 D@90/45.00 D@180
can have a sphero-cylindrical correction as
plano/−1.00 DCx90. The vertical corneal meridian will form the horizontal focal line and vice
versa with the horizontal corneal meridian (forms
the vertical focal line). It will be important to
keep this understanding as we learn further about
correcting astigmatism (see also Fig.4.2).
The steps for prescribing cylindrical spectacle
correction are as follows:
1. First, rene the spherical refractive error
through fogging when appropriate.
2. If there is no improvement in visual acuity
upon further addition of spherical lenses
(more minus or less plus), then introduce the
cylinder power. The introduction of cylinder
power is similar to the spherical fogging tech-
nique. The initial cylinder axis position is
based on the objective refraction value.
3. A 0.50DC increase in power should improve
the visual acuity by one line. When a reasonable amount of acuity is achieved (i.e., about
20/63 or better), axis and power renement
can be ne-tuned further by using JCC
(Jackson’s Cross Cylinder). The JCC procedure is explained below.
4. A keratometer and topography can also help
determine cylinder power and axis. It is also a
recommended procedure to detect any corneal
pathologies like keratoconus.
JCC helps in the renement of the power and
axis of the cylinder. It is good for ner renement. If large changes are expected, rechecking
the objective refraction or using topography for
conrmation would be ideal.
JCC has two cylinders of equal magnitude and
opposite signs (plus and minus) aligned perpendicular to each other and is available in different
powers such as ±0.25 D, ±0.50 D, ±0.75 D, and
even ±1.00 D. When the visual acuity is very
good, even the smallest blur can be detected;
choose the lower denomination of the JCC for
those patients. For those with poorer visual acuity, a higher denomination can be used. Usually,
red dots/lines indicate minus cylinder and white
(or black or green) indicates plus cylinder.
Fig. 4.2 Illustration of
compound myopic
astigmatism. Note that
the linear distance is not
equally spaced in the
conoid of Sturm, hence
the circle of least
confusion appears closer
to one image than the
other. The horizontal
corneal meridian (dotted
green lines) forms the
vertical image (solid
green line). Similarly,
the vertical corneal
meridian forms the
horizontal image
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However, it is always better to verify this before
starting the JCC procedure. The spherical equivalent of JCC is always zero. The technique is
explained below.
4.4.3 Jackson’s Cross Cylinder (JCC)
Technique
1. JCC is used after arriving upon the best
sphero-cylinder correction.
2. The procedure is done without any fogging
lens. The patient is asked to look at a line that
is two lines above their best visual acuity
(e.g., if 20/20 is the best acuity, the patient
can look at a 20/30 line).
3. First step is to rene the axis. For this, the
JCC handle is held parallel to the correcting
cylinder axis (Fig.4.3a).
4. The patient is asked to view two positions by
straddling the JCC back and forth and to
report “if 1 is better than 2 or vice-versa or if
both are equally blurred.”
5. The endpoint is when the patient says, “both
are equally blurred” or “both positions are
bad, and removing it is good.”
6. Sometimes the patient may say the letters
slant in one direction in position 1 and the
opposite direction in the other position. This
is also a good indication that the axis is
correct.
7. If the patient prefers one position over the
other, move the correcting minus cylinder
toward the minus cylinder position in
JCC. The rst step size can be large (10°),
and in the subsequent renement, it can be
smaller (5°) when the direction to move the
minus cylinder changes. The endpoint to
achieve is step 5 above.
8. Next, cylinder power is rened. The cylinder
(indicated by dots/lines) in the JCC is kept
parallel to the cylinder axis in the trial frame
(Fig. 4.3b). The JCC is then straddled
between the two positions. The endpoint to
achieve is step 5.
9. If the patient prefers one position over the
other, change the cylinder power accordingly. For example, if minus power is preferred, increase the magnitude of the minus
cylinder power and, at the same time, add
plus sphere power (half of the cylinder value)
to maintain the spherical equivalent. Vice
versa for plus cylinder power increase (i.e.,
the minus sphere will be added).
10. For example, if a patient has
−1.00DS/−0.50DCx90, if with JCC the
patient prefers −0.50DC, the resultant power
in the trial lens should be
−0.75DS/−1.00DCx90.
JCC renement aims to put the circle of least
confusion on the retina. This is why the endpoint
is “both views are equally blurred,” and upon
removing the JCC, the patient reports the clarity
is much better if the correct power and axis are
achieved (Fig. 4.4). This is because, for an
already clear point on the retina, the JCC expands
the conoid of Sturm with the circle of least confu-
a b
Fig. 4.3 JCC (Jackson’s cross cylinder) technique
depicted for (a) axis renement: the handle of the JCC is
held parallel to the cylinder axis and (b) power renement: the axis markings of the JCC is held parallel to the
cylinder axis. The two JCC axes are drawn in red (minus)
and green (plus) thick lines in this picture for better
illustration
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Fig. 4.4 Illustration of the JCC technique. Two positions
(1&2 [blue boxes]) are shown to the patient by ipping
the JCC.The blur circles are smaller in position 1 than 2
(upper panel). Accordingly, the power or axis is adjusted
(see text for description). The procedure continues until
sion on the retina. In positions 1 and 2, the two
principal meridians simply straddle around the
circle of least confusion, giving the perception of
equally blurred. The conoid collapses upon
removal of the JCC.Thus, the patient appreciates
the clarity on JCC removal.
Generally, the cylindrical axis and power in
both eyes tend to be symmetrical. It is easier to
adapt to a symmetrical axis in both eyes.
However, occasionally an asymmetrical value
can also be observed. Sometimes asymmetrical
cylindrical errors are found in accommodative
stress such as spasm or accommodative excess.
In such patients, a cylinder power of
−0.50DCx50in only one eye and a plano in the
other eye (for example) would disappear in
cycloplegic refraction. Hence apply caution to
prescribe an oblique axis, even if there is a slight
improvement in visual acuity, and the patient prefers it. Perform a cycloplegic refraction (especially for rst-time spectacles wearers). This
said, cases of astigmatism in very different axes
between the two eyes, while very rare, can still be
encountered (for example, the cylinder axis in
one eye is 90°, and the other eye is 170°).
the endpoint is reached, which places the circle of least
confusion on the retina (lower panel, left image). The
patient would report equal blur upon ipping the JCC at
this position. Upon removal of the JCC, a clear image is
formed on the retina (lower panel, right image)
In those patients with presbyopia who are
uncomfortable with their spectacle correction
only for near vision, do near refraction to check
for any axis variation in the cylindrical power. In
very rare instances, there might be an axis change.
If the magnitude of the cylinder is high, even a
small change in axis can also cause a lot of visual
disturbance. Consider giving separate spectacles
for distance and near in such cases. In some
patients with high astigmatism (corneal), the
quality of vision obtained with Rigid Gas
Permeable (RGP) contact lenses (CLs) will be
superior to the spectacle correction. Encourage
these patients to give CLs a try. Sharp visual acuity can be a good motivation for them to switch
and be compliant with CLs.
4.4.4 Astigmatic Fan andStenopic
Slit
Astigmatic fan, clock dial charts, rotating T chart,
etc., can be used to determine the axis of the cylinder. The fundamental principle of all these tests
is similar. Hence only the astigmatic fan test will
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Fig. 4.5 Illustration of an astigmatic fan chart viewed by
a patient with compound myopic, with-the-rule astigmatism. Therefore, the vertical line (green) formed by the
horizontal meridian has the least myopic power and is
closer to the retina. Therefore, the patient perceives this
line as sharper, and the horizontal line is the most blurred.
be explained with an example (Fig.4.5). The test
can be done with a weak fogging lens; that way,
the principal meridians are both kept in front of
the retina. Else, accommodative uctuations can
make it difcult to interpret the result, and the
patient response may be variable. The patient is
asked to look at all the lines and to pick one line
that appears the darkest, blackest, and sharpest.
The chosen line (say, for example, 90°) is formed
by the least myopic meridian (or low refractive
power) of the eye (i.e., 180°). Therefore, the most
myopic meridian is perpendicular to this (i.e.,
90°). The correcting minus cylinder axis should
be perpendicular (i.e. 180°) to the most myopic
meridian. So, an easy way to remember this is to
give the minus cylinder perpendicular to the line
chosen to be the darkest, blackest, and sharpest.
A stenopic slit is a handy tool that can help
nd the axis for irregular astigmatism due to corneal pathologies and where retinoscopy and keratometry are difcult to perform. This procedure
can also be done with a weak fogging lens. A
stenopic slit can be considered as an elongated
A stenopic slit oriented in the horizontal direction would
cut the blurred circles formed by the vertical meridian
(horizontal line), and in that position, the patient would
say the image appears clear. The correcting minus cylinder for this patient will be ×180°
pinhole. The orientation of the slit where better
clarity of the chart (visual acuity chart can be
shown) is obtained will be the minus cylinder
axis position. This is because the orientation of
the slit isolates the meridian position that gives
the best clarity (say 40°). The most blurred position, therefore, could be perpendicular to this
meridian (130°). Hence to correct that meridian,
a minus cylinder oriented in the same location as
the stenopic slit (40°) can improve the clarity.
4.4.5 Duochrome Test
After arriving at the refractive correction for each
eye, the duochrome test (Fig.4.6) is done as the
endpoint of the monocular subjective refraction
procedure. Duochrome (also referred to as
bichrome) test works on the principle of chromatic aberration. Therefore, the test can also be
administered to those with a color deciency. In
this test, the red band that has a longer wavelength
will be focused behind the retina. The green band
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Fig. 4.6 Duochrome test. If the accommodation is optimally relaxed and with appropriate refractive error correction, the images from the red and green background will
that has a shorter wavelength will be focused in
front of the retina. Therefore, the black letters on
these colored backgrounds will appear equally
clear if the refractive correction is optimal plus or
minus. Else, letters or symbols on the green background will appear darker and sharper in case of
overcorrection for myopia or under- correction for
hyperopia. It would be vice versa for overcorrection of hyperopia and under- correction of myopia,
i.e., letters or symbols on red will be darker or
sharper. The ideal is to achieve balance or leave
the patient slightly red better so as not to stress the
accommodative system. An important check to
make here is in cases of spasm of near reex with
the accommodative component; the patient may
always report red better, no matter how much
more minus power is added. Therefore, checking
on reversal for the response is a good habit. That
is, the patient is expected to report “green better”
when extra minus power is added (under-correction of hyperopia or overcorrection of myopia)
and to say “red better” when extra plus is added
(overcorrection of hyperopia or under-correction
of myopia). Such a reversal response indicates
that the accommodation system is not on overdrive, and the patient is consistent with their
response and has understood the test.
4.4.6 Binocular Balancing
Most of the routine subjective refraction would
stop after the monocular duochrome check. Upon
be formed behind and in front of the retina, respectively,
thus giving a perception of both being equally clear
nalizing the spectacle power, the patient is
allowed to view the visual acuity chart binocularly. As a good practice, the patient should be
shown the chart with their old spectacles and the
new correction. This would help the patient to
appreciate the difference in clarity and to decide
on changing the spectacles.
For some patients, especially those with asthenopic symptoms, those who are not happy with
their spectacle correction, etc., it would be better
to perform a few additional tests to give them a
comfortable pair of spectacles. One such additional test is binocular balancing. This test aims
to balance the accommodation (not visual acuity)
between both eyes. In those patients who do not
have equal visual acuity, a couple of lines above
the acuity level of the worst eye can be used.
Different methods can be used to achieve binocular balancing. This includes prism dissociation,
alternate occlusion, and duochrome with prism
dissociation. There is also the Turville Innity
Balance (TIB); however, unlike the other methods, which are dissociating fusion, this method
provides a peripheral fusion lock and allows
monocular viewing only for central viewing. For
this reason, this test is included under “binocular
refraction,” which provides a bit more naturalistic viewing to balance the accommodation. Please
also refer to the text Primary Care Optometry for
details of other methods [21]. Prism dissociation
technique will be explained below.
The prism dissociation test can be a quick and
handy test to do for binocular balancing. While
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Fig. 4.7 Binocular balancing using prism dissociation
test. Prisms base down (image perceived up) and base up
(image perceived down) are placed in front of the eyes.
the alternate occlusion is the easiest, where each
eye is occluded alternately and quickly to
compare the images in each eye, it may not be a
very efcient test. The act of occluding one eye
itself can result in some accommodative uctuations. As an extreme example, a report of a case
of spasm of accommodation has been noted in a
patient only when one eye is occluded [22]. Prism
dissociation can be done using the prism lenses in
the trial box and does not need any other
equipment.
Dissociating prisms (4PD base down and 5PD
base up) can be placed in each eye (Fig.4.7). This
way, there will be an equal luminance level for
both eyes, as opposed to placing the prism only in
one eye. The unequal prisms are chosen only
because the trial box has a single prism for a
given power denomination. Vertical dissociation
is easier, as horizontal vergence eye movements
can easily overcome horizontal dissociation.
After arriving at the monocular subjective endpoint, the procedure is done with little fog (each
eye can be added +0.50 DS or +0.75 DS). The
fogging lens will reduce the acuity by about 2 or
3 lines. The patient is instructed to look at that
line and to report whether the top or the bottom
chart appears clearer. To the eye that reports the
chart to be clear (base down prism will form an
Patient is asked to report which chart appears clearer. A
power of +0.25 DS is added to that eye that views the clear
chart
image above and vice versa for base up), a +0.25
DS can be added. If both look equally clear, the
procedure is stopped. If the clarity ips to the
other eye, the +0.25 DS can be removed and
taken as accommodation being equalized. If the
same eye image is still reported to be clear,
another +0.25 DS is added till an equal or reversal endpoint is achieved. After reaching the endpoint, prisms are removed, and binocular
defogging is done. A prescription that allows the
patient to read 20/20 or better is given. Under
binocular viewing, accommodation can be
relaxed more.
4.5 Anisometropia
The steps involved in giving the spherical and
cylindrical correction for anisometropic patients
are relatively similar. However, in anisometropia,
it will be important to check for the effects of
aniseikonia and tolerance to the new spectacle
correction. Adaptation time should be given with
the new spectacle prescription in the trial frame
to ensure the patient can tolerate the difference. A
difference of more than 2.5 D should be checked
for binocular fusion for both near and distance
with the Worth Four Dot Test. However, it must
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