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144
Optical Management in
Strabismus: Simple, Advanced,and Unconventional Techniques
Gill Roper-Hall, D.B.O.T., C.O., C.O.M.T.
©2005 Board of Regents of the University of Wisconsin System, American Orthoptic Journal, Volume 55, 2005, ISSN 0065-955X, E-ISSN 1553-4448
ABSTRACT
The relationship between prisms and lenses can be an as-
set in the management of strabismus. Relief from diplopia in
incomitant strabismus is challenging and may be compli-
cated by large refractive errors or ill-fitting spectacles. Prac-
tical optical techniques include crossing and splitting in-
corporated prisms, using slab-off lenses or oblique prisms,
and combining contact lenses with a spectacle correction or
prisms. Spectacle frame selection and fit may induce an oc-
ular deviation or influence an existing one. Selecting an ap-
propriate frame, particularly if prisms will be incorporated,
contributes to binocular comfort.
INTRODUCTION
Performing a formal motility evaluation
in incomitant strabismus provides baseline
measurements and identifies the positionof greatest deviation. This is helpful in
the diagnosis of the primary weakness of a
muscle. It also identifies two other impor-
tant parameters: the position of least devi-
ation and the extent of any field of binocu-
lar single vision. These are useful whenconsidering the therapeutic aspects of the
deviation.
Prisms are used routinely in the man-
agement of strabismus for the correction of
diplopia and in adapting patients to the ap-
propriate angle of deviation before ex-
traocular muscle surgery.1
The practical combination of prisms and
lenses can become complex in large devia-
tions, those with lateral or vertical incomi-
tance, when mechanical restrictions are
present, and in the presence of abnormal
From the Saint Louis University Eye Institute, St. Louis,
Missouri. Supported in part by an unrestricted departmen-
tal grant from Research to Prevent Blindness, Inc., New
York, New York.
Requests for reprints should be addressed to: Gill Roper-
Hall, D.B.O.T., C.O., C.O.M.T., Saint Louis University Eye
Institute, 1755 S. Grand Blvd., St. Louis, MO 63104. e-mail:
Presented at the annual meeting of The Canadian Orthoptic
Society, Halifax, NS, Canada, June 2003.
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head postures. Patients with combined hor-
izontal and vertical deviations, torsion, gaze
pareses, facial asymmetry or deformity add
to the challenge of determining the correct
amount and application of prism.Large refractive errors may produce
lenses with steep base curves preventing
easy application of a Fresnel prism, or pre-
cluding incorporation of a prism into an al-
ready thick lens. Patients with significant
anisometropia, those who choose to fixate
with the unaffected eye or have concurrent
visual field defects cause additional prob-
lems in selecting and prescribing prisms.
RELATIONSHIP BETWEEN LENSES
AND PRISMS
All lenses create a prismatic effect when
objects are viewed off the optical center.
Equal or similar refractive errors in each
eye create similar displacement and this
may not cause any visual disturbance. How-
ever, anisometropia induces an asymmetric
prismatic effect that can induce diplopia.
A concave lens may be thought of as two
prisms placed apex to apex whereas a con-
vex lens may be thought of two prisms
placed base to base (Figure 1). By observ-
ing the direction of displacement through a
lens, the direction of an induced prism can
be determined. In anisometropia, a signif-
icant difference in the strength of each lens
will create a prismatic effect and the direc-tion of induced prism and the resultant oc-
ular deviation can be determined.
An anisometropia-induced prismatic ef-
fect is commonly seen after cataract sur-
gery causing diplopia in the reading posi-
tion. An optical technique known as a
slab-off lens rectifies this discrepancy.
CALCULATION OF THE AMOUNT OF
INDUCED PRISM BY THE PRENTICERULE
Calculation of the induced prism amount
is based on the Prentice Rule, as follows:
∆ = D × d
(where ∆ = amount of induced prism,
D = the power of the lens in diopter spheres
and d = amount of decentration in cm)
The amount of decentration is calculated
for each eye and the difference subtractedto obtain the induced prism amount.2
Example:
OD –5.00 sph, OS –11.25 sph, with patient
reading 8 mm below optical center.
OD ∆ = 5 × 0.8 = 4 ∆ base down
OS ∆ = 11.25 × 0.8 = 9 ∆ base down
Net = 5 ∆ BD OS
ANISOMETROPIA—HORIZONTAL
GAZE EFFECT
It should be note that induced prism can
be vertical or horizontal (Figure 1).
Example:
–OD 1.00 D.S. and OS –9.00 D.S.
In this patient, the correction induced a rel-
ative base-in prismatic effect on right gaze
and a base-out effect on left gaze. This can
influence horizontal gaze measurements
through the spectacle correction and be
misleading. It can induce diplopia that is
optical and not neurological in etiology. This
ROPER-HALL
American Orthoptic Journal 145
FIGURE 1: A concave lens resembles two prisms
placed apex to apex; a convex lens resembles two prisms placed base to base. The prismatic effect can be vertical
or horizontal, depending upon the direction of gaze.
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can be important when evaluating a pa-
tient with subtle third or sixth nerve palsy.
PRISMATIC EFFECT OF A CYLINDER
Significant amounts of plus or minus
cylinder may add to the spherical amount
to induce prism. This depends largely on
the axis of the cylinder. The effect in the
down-gaze or reading position is calculated
as this direction is most often affected by
the phenomenon. Since cylindrical power
is created at 90° to its axis and there is no
effect at an axis of 90°, there is maximal ef-
fect with an axis of 180°. A cylinder at 90°must be transposed to 180° to calculate the
prismatic effect. Similarly, the effect of a
cylinder at an oblique axis must be trans-
posed to its effect at 180°.
The calculation of cylinder power at an
oblique axis is as follows:
C1 = C(sin*)2
(where C = cylinder power in DS,
* = angle of axis, C1 = resultant cylinder
power in prism diopters)
Example:
Plano +3.00 × 45°
C1 = 3(sin 45)2 = 1.5∆
This amount is added to the spherical
amount and incorporated into the Prentice
equation. From a practical standpoint,
merely observing the fact that there is sig-
nificant cylinder and estimating its effect
often suffices.
USE OF A SLAB-OFF LENS
When significant anisometropia is pres-
ent, a slab-off lens is ordered. This bi-centric
grinding technique equalizes the effect of
the anisometropia by removing the induced
prism. In most cases, the excess lens ma-
terial is “slabbed-off,” which removes base-
down prism from the reading area.3
In the days of glass lenses, the slab-off
was placed on the least myopic side; in
modern day plastic lenses, it goes on the
more myopic one. It is not usually needed
for anisometropic differences less than 2.00
D.S., and may correct up to a difference of
10.00 D.S. or higher. The prismatic reduc-tion achieved is typically effective up to 6∆.2
USING ANISOMETROPIATO
CORRECT DIPLOPIA
In patients without strabismus, ani-
sometropia over a few diopters becomes a
problem. However, in some instances, a pa-
tient with an ocular deviation may benefit
from the induced prism caused by this dif-ference in refractive correction in each eye.
This works particularly well in vertical
strabismus and when the deviation is in-
comitant, measuring more for distance
than near and vice versa.
The first step is to estimate whether there
is (or will be, if not yet prescribed) any in-
duced prism in a patient’s refractive cor-
rection (Figure 1). Comparing this with the
direction of any deviation reveals whether
the induced prism will help or hinder the
existing deviation. The exact amount may
be calculated if desired.
Example:
Following a retinal buckle procedure, a pa-
tient has a small left hypertropia on down-
gaze, but not in primary gaze. His lenses were
–1.75 OD and –3.75 OS. This induced a base-
down effect OS which was helpful. Therefore
no slab-off or incorporated prism was needed,
and the induced prism remained as a useful
tool to correct the incomitant deviation.
This technique can be utilized when prisms
are to be incorporated. It may add to, or
lessen, the amount of prism in the reading
position.
Example:
A patient with thyroid restrictive eye disease
had a deviation measuring LHT 12∆ for dis-
tance and LHT 6∆ at near. She wore a refrac-
tive correction with significant myopia OD
and hyperopia OS. She had been wearing a
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12∆
BU Fresnel prism OS to correct the left,hypertropia for distance. If this amount were
incorporated, the near deviation would be
overcorrected and require either a different
Fresnel prism or the reading segment oc-
cluded. Although a stacked bifocal is possible
(see Jarvis Method below), incorporating dif-
ferent amounts of prism into the distance and
near portions of a lens is expensive and cum-
bersome. Since her refractive correction is
myopic OD and hyperopic OS, a base-down
OS effect will be induced in the reading posi-
tion (the opposite of the ocular deviation). The
lenses may be incorporated with 6∆ of incor-
porated prism in each eye (base-up OD and
base-down OS). The anisometropia will neu-
tralize some of this prismatic effect at near.
PARTIAL SLAB-OFF TECHNIQUE
In some cases, a portion of an induced
prism is needed. It is possible to order a
partial slab-off to retain some of the pris-
matic effect.
OTHER EFFECTS OF GLASSES ON
THE DEVIATION
Other influences from the glasses can af-
fect the measurement of a deviation. In pa-
tients with a significant refractive error,
the correction is usually worn while the de-
viation is measured. Negative influences
on the deviation include ill-fitting frames
with optical centers incorrectly aligned,
bent, or twisted frames that can induce aprismatic effect, unequal reading add
heights and overlooking a small incorpo-
rated prism. Sometimes a patient will tilt
the frame deliberately to create a prismatic
effect (Figure 2). The amount can be calcu-
lated using the formula given, or direction
of prism estimated using the technique
shown (Figure 1).
Taking an additional primary gaze mea-
surement without correction for comparison
is sometimes helpful to determine whether
the glasses are influencing the deviation.
SELECTING AN OPTIMAL PRISM
Once measurements in all gaze positions
have been obtained, the least amount of
prism needed to fuse in primary gaze for
distance is then assessed. This is approxi-
mately one-half to one-third of an exodevi-
ation and close to the full amount of an eso-deviation or vertical deviation unless the
strabismus is longstanding; in the latter,
less prism is required as fusional ampli-
tudes are more generous.
In a concomitant deviation, the prism se-
lected for primary gaze will be effective on
lateral gaze to either side. The near mea-
surement may be more or less than dis-
tance and a compromise between these two
settings is sought. If an esodeviation is
present for distance that measures less at
ROPER-HALL
American Orthoptic Journal 147
FIGURE 2: Apatient may tilt the frame deliberately to induce a prismatic effect.
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near, overcorrecting the near deviation to
a small exophoria is usually tolerated.
When lateral incomitance is present, the
selected prism is modified, if possible, to al-
low an equal range of single vision to eitherside of midline. In some cases, the devia-
tion is too large or incomitant for prisms.
Factors that make this task more difficult
include marked lateral incomitance, re-
stricted eye movements, or paralysis. Over-
correcting an esodeviation slightly to one
side is usually acceptable as it produces a
small, easily controlled exophoria.
Example:
A patient with a sixth nerve palsy has a resolv-
ing deviation that measures ET 10∆ in primary
gaze, decreases to E 6∆ on left gaze and in-
creases to ET 18∆ on right gaze. Giving 8–10∆
base-out in primary gaze will give single vision
in that position and also in right gaze where
a small exophoria is produced. However, this
may be inadequate for left gaze leaving the
esodeviation undercorrected. Increasing the
base-out prism will offer more correction in
left gaze, but care must be taken to avoid too
great an overcorrection in the opposite direc-
tion. Ideally, the prism centers the range of single vision on either side of the midline.
With the selected prism in place, the pa-
tient’s head is moved slowly to each side
while the patient fixates a distance target.
A small adjustment to the prism strength
may be required to achieve single vision. If
the images remain aligned, the head is
then moved slightly up and down in pri-
mary gaze to see if single vision is main-
tained. A final check of the deviation atnear determines whether the optimal prism
has been found.
CONSIDERATIONS IN FITTING
FRESNEL PRISMS
In general, the smaller the amount of
prism the clearer the acuity will be through
the prism. However, if larger amounts are
needed, it is still preferable to apply a Fres-
nel prism to one lens in an adult. This in-
curs half the expense and ensures better
binocular acuity than having prisms on
both lenses. In incomitant strabismus, the
Fresnel prism is usually placed over the
paretic eye except when the acuity of that
eye is subjectively much clearer or domi-nant. In a patient with a secondary (larger)
deviation caused by fixating with an eye
with a restricted or paretic muscle, a larger
amount of prism may be needed to fuse.
In cases where there is no compelling
reason to select one eye for motility reasons
(such as in a patient with divergence pare-
sis), the prism is usually placed over the
eye with least good acuity.
Fresnel prisms will not adhere easily tolenses with antireflective coating, a mir-
rored finish, or a very curved surface. They
will adhere well to most plastic lenses, al-
though real glass, rarely seen these days,
provides a superior fit. Other helpful hints
include checking the clarity and strength
of a Fresnel prism before cutting. The
prism should be rinsed and applied using
warm water. Water that is too hot or too
cold may produce a mist or bubbles be-
tween the lens and smooth surface of the
prism. This occurs most often if the outside
temperature is much hotter or colder than
inside and the prism is still wet.
APPLICATION OF AN OBLIQUE
FRESNEL PRISM
If a deviation with a horizontal and verti-
cal component requires prismatic correction,
some patients are given a vertical prism onone lens and a horizontal on the other. This
is not necessary. Instead, an oblique Fresnel
prism may be used and applied to one lens
(Figure 3). Auseful set of tables can be used
to calculate the resultant prisms.3 Without
reference to the tables, the same resultant
prism and its axis can be calculated easily
using simple trigonometric formulae. The
amount of prism may be determined also
from a cross-cover technique, then placed
in a trial frame at the approximate axis
and refined by dialing the lens carrier.
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WRITING INSTRUCTIONS FOR
OBLIQUE PRISMS
Although some orthoptists cut their own
prisms, others refer this task to an opti-
cian. For an oblique prism, the resultant
prism and its axis are calculated. To avoid
confusion, it is also helpful to indicate the
quadrant for the direction of the base, in
part, because the optician will cut the Fres-
nel to fit the back of the lens.
Example:
A prismatic correction requiring 6∆ base-out
and 4∆ base-down OD will result in a 7∆ oblique
prism at 34°. The prescription can be written
with instructions to place the prism base-out
OD, rotated down 34° from the 180° position.
It is also advisable to supply the optician
with the original horizontal and vertical
amount and the direction of their bases if the prisms are to be incorporated, as these
numbers are entered into a computerized
system to grind the lens. It is necessary to
state the axis for the oblique prism. The in-
corporated amount is generally divided be-
tween the two lenses.
APPLYING A FRESNEL PRISM IF A
PATIENT DOES NOT WEAR GLASSES
If a patient does not wear glasses, light-
tinted sunglasses (not mirrored or exces-
sively curved) may support a Fresnel prism.
Gradient tint sunglasses with a lighter tint
inferiorly may allow indoor use. Plano car-
rier glasses can sometimes be made up by
a local optical shop at minimal expense to
the patient. Sometimes an attractive opti-
cal quality frame can be purchased initially
for this use and later used to incorporate
permanent prisms. Or, if the condition
resolves, they may be converted to sun-
glasses.
INCORPORATING PRISMS
Considerations in incorporating prisms
include first and foremost the total strength
of prism required and the refractive error
in the patient’s spectacles. In minimal re-
fractive corrections it is possible to incor-
porate up to 10 diopters of prism into eachlens, however, 7∆ is generally considered
maximal. Using high index lens materi-
als may permit more prism to be incorpo-
rated but acuity may be compromised and
the expenses are higher. Note that large
amounts of base-out prism may be visible
in most spectacle frames, the base-up
prism in a pair of vertical prisms will be
less noticeable than its base-down coun-
terpart, and large amounts of base-in
prism may be limited by the position of the
nose pads.
ROPER-HALL
American Orthoptic Journal 149
FIGURE 3: A Fresnel prism combining both vertical and horizontal correction
may be placed obliquely on the lens.
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SYMMETRIC OR ASYMMETRIC
SPLITTING AND CROSSING
Although a unilateral prism is recom-
mended when using Fresnel prisms, theamount should be divided between the two
lenses when incorporating. This is a tech-
nique known as “splitting and crossing”
and when the refractive correction is simi-
lar in each eye the amount is equally di-
vided. When anisometropia is present,
prisms may be incorporated asymmetri-
cally adding more to the thinner lens to
produce an even result. This reduces weight
and edge thickness and is more cosmeti-cally appealing.
Example:
A patient seen in our practice was given a 9∆
Fresnel prism placed base-out OS. When his
local eye care provider decided to incorpo-
rate this, the total amount was incorporated
OS. It would have been better ordered as
4.5∆ base-out OU (Figure 4). Another of our
patients wore a 5∆ base-down OS Fresnel
prism on plano glasses. These were later in-
corporated as plano 3∆
base-down OD andplano 2∆ base-up OS. The prisms were barely
noticeable.
DISTANCE AND NEAR
INCOMPATIBILITY
When fitting prisms in patients with dip-
lopia for distance and near, the position of least deviation is generally addressed first. If
the measurements are significantly differ-
ent, it may be necessary to address distance
and near separately. Different strengths of
Fresnel prisms may be applied for each dis-
tance. If one position is more easily cor-
rected, a Fresnel prism may be applied (ei-
ther to the whole lens or just one portion)
and the remaining segment occluded with
3M Blenderm ®
occlusive tape (Figure 5).Separate single focus prism glasses are
sometimes indicated when prismatic
amounts are quite different for distance
and near. Patients occasionally request
separate distance glasses, but most prefer
some form of bifocal for everyday tasks.
When incorporating, the prismatic correc-
tion in the distance position becomes para-
mount since the patient will wear their
bifocals or progressive lenses as multipur-
pose glasses. One reading segment may be
occluded, or the patient may close one eye
STRABISMUS
150 Volume 55, 2005
FIGURE 4: When incorporating prisms, the amount should be split equally or
evenly between the lenses, not as shown here with the total amount of prism in-
corporated into the left lens.
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for casual near tasks. A separate pair of
prism reading glasses can be prescribed for
extensive reading. Although cumbersome, it is possible to
incorporate different amounts of prism in
the distance and near portion of the spec-
tacles. These are sometimes called stacked
bifocals (Figure 6). We refer to this in our
institution as the “Jarvis” method, named
with permission after a delightful patient.
He had owned the same frame for thirty
years, and required more base-out prism in
the distance portion to correct a decom-
pensating esodeviation of the divergence
weakness type. His glasses were quite top-
heavy and unattractive (Figure 6 top).
In order to create a stacked bifocal two
separate pairs of lenses, each with appro-
priate prisms, must be ground and then cut
in half at bifocal height. Each matching
half is then placed in the frame and glued
or bonded together. In his case, the lenses
were not glued but tightened in the frame
so they could be modified when necessary. Another patient required base-out prisms
to read and was given stacked bifocals cost-
ing over six hundred dollars (Figure 6 bot-
tom). She disliked the wide line this cre-
ated between segments, but she tolerated
this as she did not want to wear separate
distance and near glasses. In our experi-
ence, most patients do not like the appear-
ance or expense of stacked bifocals, prefer-
ring a separate pair of inexpensive prism
readers.
WHEN AN OPTIMAL PRISM IS
UNATTAINABLE
In some patients, due to the size of the de-
viation, the presence of torsion, or incomi-
tance, no single prism will provide adequate
relief from diplopia. For a prism to work, it is
necessary not only to reduce the deviation in
primary gaze to control diplopia but provide
a practical range of binocular single vision. A
combination of methods may be tried, such
as using a compensatory head posture with
a prism, placing different Fresnel prisms
top and bottom, or combining prisms and
occlusion (Figure 5). In some cases, surgery
may be indicated and these non-surgical
methods pursued again postoperatively.
WHEN DIPLOPIA BECOMES
INTRACTIBLE
There are fortunately only a few forms of
diplopia that cannot be helped by one of the
methods already described. These includegrossly incomitant deviations, especially
restrictive deviations such as those seen af-
ter severe orbital and facial fractures or
scleral buckling procedures. Certain tor-
sional deviations or those associated with
supranuclear disturbances may not be
amenable to surgical correction. Patients
with extreme loss of peripheral visual fields
such as in glaucoma may not be able to
fuse. Patients with central loss of fusion fol-
lowing severe head trauma are some of the
ROPER-HALL
American Orthoptic Journal 151
FIGURE 5: A Fresnel prism may be combined with occluding tape. If the prism
covers the entire inside surface of the lens, the tape is applied to the front surface.
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most challenging patients to treat.5,6,7 Oth-
ers include those with foveal displacement
syndrome from retinal conditions such as
macular pucker.8,9 The use of a Bangerter
foil or a permanent Min occluding lens may
be appropriate (Figure 7).
USING CONTACT LENSES WITH
SPECTACLES TO MINIMIZE EFFECTS
OF ANISOMETROPIAAND OTHER
REFRACTIVE ERRORS
In a patient with high myopia or ani-
sometropia, better acuity and more com-
fortable binocular vision may be obtained
by wearing contact lenses rather than
spectacles. When a decision has been made
to try contact lenses, most clinicians try
to order the full correction, including cyl-
inder. Fitting toric contact lenses may be
time-consuming, frustrating, and uncom-
fortable for the patient and examiner.
A useful management approach is to re-
duce the large spherical correction in each
eye and replace it with a soft lens. This
leaves a residual spherical amount to be
corrected with any astigmatism or need for
prisms and incorporated into a lightweight
spectacle correction that looks “normal” to
others.
Example:
A 57-year-old patient with high myopia had
worn thick lenses since childhood. Contactlenses were tried when she was younger, but
astigmatism prevented good acuity. She de-
veloped a decompensating esodeviation. Her
refractive error was –10.75 + 1.00 × 113 OD
and –11.25 +1.25 × 71 OS with a +1.75 add.
We ordered –6.50 OD and –7.50 OS in soft
contact lenses. A residual correction of –2.00
+ 0.50 × 120 OD and –2.00 + 0.25 × 65 OS was
required to obtain 20/20 acuities. This was
ordered with base-out prisms measuring 3.5∆
OU and a +2.25 progressive add producing
excellent binocular comfort in lightweight
spectacles.
STRABISMUS
152 Volume 55, 2005
FIGURE 6: Stacked bifocal corrections allow prisms
to be incorporated in different amounts for (top) dis-
tance and (bottom) near.
FIGURE 7: A permanent occluding lens such as a
Min lens may be used to alleviate diplopia.
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In cases of anisometropia, the sphere in
the eye with the higher refractive error
may be corrected with a contact lens. The
remaining correction is given in the spec-
tacles. This not only reduces any aniseiko-nia but produces a pair of lenses that are
even in weight, thickness and appearance.
Example:
A graduate student with anisometropia and
asthenopia wore spectacles measuring +2.00
+ 1.50 × 65 OD and +6.50 + 1.25 × 117 OS. By
giving her a +3.00 soft contact lens OS, her
spectacle correction became more symmetric
and both her acuity and asthenopia improved.
INCORPORATING PRISM INTO A CONTACT LENS
A successful contact lens wearer who de-
velops diplopia may have to revert to a
spectacle correction in order to benefit from
prisms. Even after extraocular muscle sur-
gery, some prism may be needed. It is pos-
sible to grind small amounts of prism into a
contact lens. Up to 2∆ placed in a base-down
direction is easily accomplished and tolerated
by the patient; larger amounts are uncom-
fortable and do not fit well. If base-up prism
is desired, the contact lens should be fitted
to the opposite eye as a base-down prism.
Horizontal prismatic correction is not usu-
ally possible. Even if constructed like a toric
lens, due to gravity, the weight will make the
contact lens dial into a base-down position.
SELECTION OF SPECTACLE FRAME
When incorporating prisms, the shape
and size of the spectacle frame must be con-
sidered. In general, the frame should be
small but suitable for the patient’s facial
features. Current styles are smaller and
more lightweight than in previous decades,
although industrial safety glasses still re-
quire a frame providing broader protective
coverage. The shape, depth, and width of a
frame including the bridge, must be con-
sidered in relationship to the direction and
amount of prisms to be incorporated and
whether any magnification will be included.
Base-out prisms will be lighter with less
edge thickness if a narrow frame is se-
lected. Vertical prisms are better placed ina shallow frame or one that is squared off
inferiorly rather than rounded. Frames for
base-in prisms must be carefully selected
so that the nose pieces are uninhibited by
the extra thickness nasally.
FITTING A SPECTACLE FRAME
CORRECTLY
An experienced optician will do this auto-matically. However, ill-fitting spectacles can
influence the ocular deviation so it is impor-
tant to observe whether the frame is the cor-
rect size for the patient’s face and whether
the optical centers, interpupillary and ver-
tex distances are correct. The reading add
should allow adequate room to read, and not
sit too low. The correct position for a lined bi-
focal is level with the lower eyelid margin
when the patient is fixating in primary gaze;
a trifocal may sit a few millimeters higher,
bisecting the lower border of the pupil. Apro-
gressive lens should be positioned with the
top of the magnification centered through
the pupil. Another nuance is that myopes
prefer the top of the add in a progressive lens
to sit a little lower in the frame than their
hyperopic counterparts who can absorb plus;
short people also may prefer the magnifica-
tion to start 1–2 mm lower than tall people.
Lenses should carry a slight panoramic(curved horizontally) and pantoscopic (tilted
inward inferiorly) tilt. The plane of a pan-
toscopically tilted lens will be parallel to
the angle of the printed material held in
the reading position (Figure 8).
SELECTION OF MAGNIFICATION
TYPE
This again is the purview of the optician,
but it is important to understand the dif-
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ferences and not switch patients casually
from a design that has been comfortable for
them in the past. Kryptok ® lenses (small
curved adds) and executive lenses (a line
extending across the entire lens) are be-
coming less prevalent in the era of pro-
gressive lenses, and in some areas are no
longer available. Progressive lenses are ac-
tually blended multifocal lenses despite the
popular term “no-line bifocals.” Flat-top
adds are common and are available in var-
ious different widths in a lined bifocal ortrifocal. A patient wearing a Kryptok ® type
of lens can usually be coaxed into a narrow
D25 lens whereas an executive enthusiast
may be convinced to wear a D35 or D45
lens (Figure 9). Prisms can be incorporated
into all these lenses. Tints and transitions
can be combined with prisms too.
CHOICE OF LENS MATERIAL
Most modern lenses are made of plastic,
although glass in selected strengths is still
available for those who request it. Plastics
have become tougher, thinner, and more
scratch resistant. Thinner lenses using ma-
terial with a higher refractive index are
available in most prescriptions at an in-
creased cost. Lenses up to plus or minus
10.00 diopter spheres can be made in thin-
ner lens material. Higher strengths are
possible in custom lenses depending upon
the prescription. Incorporation of prisms
up to 7∆ on each side is common with 10∆ or
STRABISMUS
154 Volume 55, 2005
FIGURE 8: In a pantoscopic tilt the lenses are angled
inward inferiorly, parallel to the reading material.
FIGURE 9: Different types of magnification in a lined bifocal correction. Pro-
gressive lenses are not shown.
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greater possible, depending upon the re-
fractive error and whether adds are neces-sary. High index lens materials permit large
refractive corrections that include prisms
to be manufactured in thinner lenses. This
reduces the weight and edge thickness of a
lens making it more comfortable and cos-
metically acceptable. The days of the “coke
bottle” glasses are gone.
Safety glasses require a minimum cen-
ter thickness of 3 mm and polycarbonate is
a popular choice. Lens materials can be
provided with a refractive index as high as
1.71. Some specialty glass lenses with a re-
fractive index as high as 1.80 have been ad-
vertised. High-index lens materials cost
more than ordinary plastic and in some in-
stances result in some subjective blurring
of acuity as a pay-off for lighter weight and
a thinner appearance.
RELATIONSHIP OF FRAME AND
SEGMENT HEIGHT TO STRABISMUS
The normal height for reading adds has
been mentioned above. In some conditions,
looking into the downward position exacer-
bates a patient’s symptoms. These condi-
tions include the induced vertical prismatic
effect from anisometropia, and strabismus
types such as A-pattern exotropia, V-pattern
esotropia, and unilateral or bilateral supe-
rior oblique palsies. Other conditions seen
with downgaze difficulties include patients
with progressive supranuclear palsy, Par-
kinson disease, downbeat nystagmus, or a
down-gaze palsy.10
Patients will sometimes avoid down-gaze
and hold their glasses up to use their mag-
nification toward midline (Figure 10).
These problems can sometimes be allevi-
ated by the simple technique of selecting a
shallow frame to prevent the patient from
looking down too far when reading, or set-
ting the reading segment higher to allow
reading close toward the midline (Figure
11). Another technique is to prescribe a sep-
arate pair of single focus readers. This al-
lows the patient to hold the reading mate-
ROPER-HALL
American Orthoptic Journal 155
FIGURE 10: A patient with down-gaze paralysis raises her glasses to read
through the bifocal in primary gaze.
FIGURE 11: The bifocal segment in the large frame
(top) is too low and forces the patient to read where her
diplopia occurs on down-gaze. The new shallow frame(bottom) is much smaller, and diplopia is eliminated
in the reading position without requiring prisms.
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rial closer to midline to see single, and still
see clearly. This may eliminate diplopia
without requiring any prisms.
HANDLING OF SPECIAL CASES
Lateral Incomitance
Apatient with a horizontal gaze palsy or
lateral incomitance in conditions such as
sixth nerve palsy or Duane syndrome may
utilize a face turn to obtain binocular single
vision. These patients will not tolerate a pro-
gressive lens well and will do better with an
executive style or wide flat-top bifocal type.
Version Prisms
These can be defined as prisms placed
with their bases in the same direction to
displace both images into the same position
of gaze.11 They may be of some assistance
in patients who cannot turn their eyes into
a certain gaze direction. This has limited
benefit in patients with severe horizontal
or vertical gaze palsies, but has some use
in those with a nystagmus null point, an
immobile neck, or who are confined to bed
in a supine position. There have been some
reports that version prisms can assist
awareness of peripheral vision in patients
with homonymous hemianopia.12,13
Distance/Near Disparity
Some patients with deviations notablydifferent for distance and near request
glasses for computer and reading only. A
pair of glasses can be ordered with the top
portion made for an intermediate range
and the bottom with a high segment for
reading and a uniform amount of prism
throughout the lens.
Poor Posture
In a patient with a postural problem
such as a forward stoop in osteoporosis or
Parkinson disease, the frame may fit well,
but the patient is looking over the optical
centers and through the edges of the
lenses. This can create blur or induce prism
in anisometropia. It is possible to mark theposition used by the patient through the
frame and ask the optician to remake the
lenses raising both optical centers.
Convergence Paralysis
Patients with complete paralysis of con-
vergence, especially when accommodation
is also involved, require special handling.
Assuming the patient’s deviation was ini-tially orthophoric or slightly exophoric, the
near deviation will present as an exodevi-
ation measuring 18–20∆. This is because
the amount of convergence necessary to
converge the eyes to a target at 33 cm in an
individual with an average interpupillary
distance of 60 cm is 18∆. So to align the vi-
sual axes at 33 cm, 18∆ will be needed to su-
perimpose images. Since there is also no
accommodation, plus lenses will be needed
to focus a clear image on each retina. Since
the distance deviation is unaffected, this
correction can be prescribed only at near.
One option is to give single focus prism
readers, such as +3.00 D.S., 9∆ base-in OU.
In practice this will only work at a fixed dis-
tance and may be impractical. Abetter plan
is to give a lined bifocal and apply a Fresnel
prism to the reading portion, or provide
progressive lenses without prism and oc-
clude one eye. Convergence exercises areof limited value in patients with true paral-
ysis, but may be effective if recovery begins.
SUMMARY OF UNCONVENTIONAL
TECHNIQUES
These techniques are based upon practi-
cal management ideas that evolved from
unconventional optical situations. These
include limiting a patient’s down-gaze ex-
cursion by selecting a shallow frame, rais-
ing a bifocal segment to avoid down-gaze
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or ordering separate single focus readers,
with or without prisms, to be used at mid-
line. Other ideas include using an asym-
metric splitting and crossing technique to
minimize weight and edge thickness, using anisometropia to create prism or a partial
slab-off to retain a prismatic effect, order-
ing an intermediate/near prismatic correc-
tion in cases of distance/near disparity, and
displacing optical centers upward in cer-
tain postural defects. The combined use of
contact lenses with a minimal residual
spectacle correction is also valuable in pa-
tients with large amounts of anisometropia
or high refractive errors.
REFERENCES
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3. Rubin ML: Slab-off grinding. In: Optics for Clin-
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Key words: prisms, anisometropia,
Prentice Rule, slab-off lens, optimal prism,
Fresnel prism, oblique prism, version
prisms
ROPER-HALL
American Orthoptic Journal 157
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