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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:

[email protected]

Presented at the annual meeting of The Canadian Orthoptic

Society, Halifax, NS, Canada, June 2003.



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.



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

STRABISMUS

146 Volume 55, 2005



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


FIGURE 2: Apatient may tilt the frame deliberately to induce a prismatic effect.



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.

STRABISMUS

148 Volume 55, 2005



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


FIGURE 3: A Fresnel prism combining both vertical and horizontal correction

may be placed obliquely on the lens.



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.



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


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.



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.



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-

ROPER-HALL




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.



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


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.



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

STRABISMUS

156 Volume 55, 2005



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

1. Prism Adaptation Study Research Group: Effi-

cacy of prism adaptation in the surgical man-

agement of acquired esotropia. Arch Ophthal-

mol 1990; 108:1248–1256.

2. Rubin ML: Prentice Rule. In: Optics for Clini-

cians. 2nd ed. Gainesville: Triad Scientific Pub-

lishers; 1977. p. 243.

3. Rubin ML: Slab-off grinding. In: Optics for Clin-

icians. 2nd ed. Gainesville: Triad Scientific Pub-

lishers; 1977. p. 276–278.

4. Moore S, Stockbridge L: Fresnel prisms in the

management of combined horizontal and verti-

cal strabismus. Am Orthopt J 1972; 22:14–20.

5. Pratt-Johnson JA: Acquired central disruption

of fusional amplitudes. Br J Ophthalmol 1973;

57:347–350.

6. Pritchard C, Ellis GS: Management of central

sensory deficits following trauma. Am Orthopt J 2004; 54:45–48.

7. Roper-Hall G: Diagnosis and management of

central motor deficits following trauma. Am Or-

thopt J 2004; 54:49–56.

8. Burgess D, Roper-Hall G, Burde RM: Binocular

diplopia associated with subretinal neovascular

membranes. Arch Ophthalmol 1980; 98:311–317.

9. Guyton DL: The dragged-fovea diplopia syn-

drome. In: Balkan RJ, Ellis GS Jr., Eustis HS,

(eds.) Proceedings of the 52nd Annual Sympo-

sium of the New Orleans Academy of Ophthal-

mology, New Orleans, LA, February 2003. The

Hague: Kugler Publications; 2004. p. 169–172.10. Roper-Hall G: Ocular difficulties in the near gaze

position. Am Orthopt J 1980; 30:109–120.

11. Roper-Hall G: Vertical gaze anomalies. Br Or-

thopt J 1976; 33:58–66.

12. Weiss NJ: An unusual application of prisms for

field enhancement. J Am Optom Assoc 1990;

61:291–293.

13. Rossi PW, Kheyfets S, Reding MJ: Fresnel

prisms improve visual perception in stroke pa-

tients with homonymous hemianopsia or uni-

lateral neglect. Neurology 1990; 40:1597–1599.

Key words: prisms, anisometropia,

Prentice Rule, slab-off lens, optimal prism,

Fresnel prism, oblique prism, version

prisms

ROPER-HALL


69 optical management in strabismus_ simple, advanced, and unconventional techniques

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