crosscylinder accuracy

7/25/2019 Crosscylinder Accuracy

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Does the dynamic cross cylinder test measure

the accommodative response accurately?

Jaclyn A. Benzoni, O.D., M.S., Juanita D. Collier, O.D., M.S., Kimberley McHugh, O.D.,Mark Rosenfield, M.C.Optom., Ph.D., and Joan K. Portello, O.D., M.P.H., M.S.

State University of New York State College of Optometry, New York, New York.

KEYWORDSAccommodation;

Near testing;

Optometer;

Refraction

Abstract

BACKGROUND: The dynamic cross cylinder (DCC) test is a standard clinical procedure used to assessthe

accommodative response (AR) subjectively. However, because of potential problems arising from the

ambiguous stimulus conditions, it is unclear whether this test provides an accurate measure of the AR.

Theaim of this study was to compare clinical subjective findings with objective measurements of the AR.

METHODS: Subjective findings to a 2.50-diopter (D) accommodative stimulus obtained with the DCC

test (without fogging lenses) were compared with objective measurements of the AR obtained with a

Grand Seiko WAM 5500 optometer (RyuSyo Industrial Co. Ltd., Kagawa, Japan) in 25 young subjects.

As spherical lenseswere introduced to quantify thesubjective finding, objective measures of the AR were

also recorded through these lenses.

RESULTS: The mean AR recorded subjectively and objectively was 2.35 and 1.68 D, respectively

(P , 0.0001). Of the 10 subjects who demonstrated a lead of accommodation subjectively, only 1 had

a lead objectively. For the 8 subjects who showed a lag of accommodation subjectively, all had a lag ob-jectively. Introducing lenses produced a significant change in the mean AR.

CONCLUSION: The subjective DCC test as performed here does not provide an accurate measurement

of the AR to a near target in a young population. We recommend that alternative techniques such as

using an objective, open-field optometer or Cross-Nott retinoscopy be adopted for determining the

within-task AR.

Optometry 2009;80:630-634

The assessment of the accommodative response (AR) to

a range of stimuli is an important part of the clinical

optometric examination.

1

Patients symptoms frequently re-late to near vision activities, and inappropriate responses,

whether under or over-accommodation relative to the object

of regard are a frequent cause of asthenopia.2 Accordingly,

it is essential for the clinician to determine the actual AR to

the stimulus condition for which the patient is reporting

difficulty. In some cases, a more complete examination of

a range of responses may be appropriate, which can be

achievedby plotting an accommodative stimulus-responsecurve.3,4 An example of such a plot is illustrated inFigure 1.

Only for a single stimulus level (the so-called crossover

point) are the accommodative stimulus and response equal.

Whereas the plot illustrated in Figure 1 represents an

average finding, patients presenting with symptoms relating

to near vision activities may exhibit different results. For ex-

ample, some patients show a tendency to overaccommodate

for near targets2 rather than exhibiting the more typical lag

of accommodation.3-6 A lead of accommodation can be a

primary problem resulting from trauma, systemic pathology,

Corresponding author: Mark Rosenfield, M.C.Optom., Ph.D, State

University of New York State College of Optometry, 33 West 42nd Street,

New York, New York 10036.

E-mail:[email protected]

1529-1839/09/$ -see front matter 2009 American Optometric Association. All rights reserved.doi:10.1016/j.optm.2009.07.012

Optometry (2009) 80, 630-634
mailto:[email protected]:[email protected]


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or the administration of either ocular or systemic pharmaco-

logic agents, or secondarytoa vergence abnormality such as

convergence insufficiency.1,7 Although the object of regard

will remain clear as long as the accommodative error does

not exceed the depth-of-focus of the eye, such a patient

may still experience symptoms, possibly resulting from the

effect of excessive accommodative convergence. An exces-

sive lag or underaccommodation relative to the accommoda-tive stimulus may also produce asthenopia. This symptom is

most typically found in early presbyopia, but may also be as-

sociated with both systemic and ocular pathological condi-

tions, pharmacologic therapy, neurologic abnormalities,

and functional disorders such as basicexophoria, divergence

excess, and accommodative infacility.7-9

One of the most commonly used clinical procedures to

assess the AR is the dynamic cross-cylinder (DCC) test.1

Here, subjects view a pattern of intersecting horizontal

and vertical lines through a cross cylinder (typically

60.50 diopters [D]) to create mixed astigmatism, with

the horizontal and vertical lines theoretically equidistantin front of and behind the retina. If the patient is accommo-

dating exactly in the plane of the target, then after introduc-

tion of the cross cylinder, the circle of least confusion

(COLC) will lie on the retina, and the patient will report

that both sets of lines (horizontal and vertical) appear

equally clear (or more accurately, equally blurred). How-

ever, if the patient has either a lag or lead of accommoda-

tion, then the horizontal or vertical lines, respectively, will

appear clearer. Spherical lenses can subsequently be intro-

duced to move the COLC onto the retina, and the lens

power required to make the 2 sets of lines appear equally

clear provides a measure of the accommodative error.A number of different procedures have been advocated

for performing the DCC test. In principle, the test may be

carried out under monocular, binocular fused, or binocular

unfused conditions.1 Although monocular testing should

assess the blur-driven and proximally induced AR only, un-

der binocular fused conditions, convergent accommodation

will be added to the response. Binocular unfused testing

uses vertical prisms to dissociate the patient, thereby elim-

inating disparity-vergence. As might be expected, this

results in a response very similar to the monocular condi-

tion.10 Additionally, some techniques (e.g., the 14A and

14B procedures advocated by the Optometric ExtensionProgram2,11) recommend that additional plus lens power

should be added to the refractive correction so that the ver-

tical lines appear clearer and then the plus power reduced

until the horizontal and vertical lines are equally clear

and dark. This is discussed later in this report.

A number of significant problems with the DCC test

make it of questionable value in prepresbyopic patients

with active accommodation, irrespective of the specific

technique being used. For example, it is assumed that a

patient who has minimal accommodative error under

naturalistic conditions will, during testing, produce an AR

to the dioptrically conflicting, rectilinear target that liesexactly midway between the 2 foci (i.e., places the COLC

on the retina). Little evidence supports this proposal. For

example, Portello et al.12 used an infrared optometer to

measure the AR in subjects with uncorrected astigmatism.

They found that subjects generally exerted the minimum

accommodation necessary to place the anterior focal line

within the depth-of-focus of the eye. However, under

none of the conditions tested was the COLC positioned

on or close to the retina.

The observation that the AR changes after the introduc-tion of lenses provides an additional difficulty with this test.

If a lag of accommodation is observed, plus lenses are

introduced to obtain the required endpoint. However, in a

young patient with active accommodation, the introduction

of additional plus power is likely to stimulate a reduction in

the blur-driven AR, provided the subject is able to detect

the change in accommodative stimulus in the presence of

the uncorrected astigmatism. If the reduction in accommo-

dation is equal to the magnitude of the plus lens, then the

subjective response to the test will remain unchanged.13-16

Accordingly, the current study compared subjective

measurements of the AR obtained with the DCC procedurewith objective measurements taken using an open-field,

infrared optometer to assess the validity of this clinical

procedure.

Methods

The study was performed on 25 visually normal subjects

having a mean age of 23.4 years (range, 20 to 30 years). All

subjects were optometry students at the State University

of New York State College of Optometry, and had best-

corrected visual acuity of at least 6/6 (20/20) in each eye.None had any manifest ocular disease or strabismus. The

Figure 1 Static accommodative stimulus-response curve for a normalsubject. 1 5 initial nonlinear region, 2 5 linear region, 3 5 transitional

soft saturation region, 4 5 hard saturation presbyopic region. The diago-

nal linerepresents the unit ratio (or 1:1) line. Figure redrawn with permis-sion from Ciuffreda and Kenyon4 (1983).

Benzoni et al Clinical Research 631


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study followed the tenets of the Declaration of Helsinki,

and informed consent was obtained from all subjects after

an explanation of the nature and possible consequences of

the study. The protocol was approved by the Institutional

Review Board at the SUNY State College of Optometry.

All of the testing was completed in a single session.

The DCC test was performed without the use of additional

fogging lenses.1,17

Cross-cylinders of60.50 D (minus axesvertical) were introduced before each eye. Subjects were re-

quired to view a high-contrast, black on white, cross cylinder

target, mounted on a near point rod at a viewing distance of

40 cm. The target was viewed through the habitual refractive

correction, which was mounted in a trial frame. Subjects

were asked to indicate whether the horizontal or vertical lines

appeared either clearer or darker. If a preference was indi-

cated, then spherical lenses were introduced before both

eyes until the 2 sets of rectilinear lines appeared equally

clear. This lens value was taken as the accommodative error,

i.e., the difference between the accommodative stimulus and

response. If no equal response was found, then the endpointwas taken as the midpoint between the lenses that produced a

change in subjective response. For example, if the subject re-

ported that the horizontal lines were clearer initially, but the

vertical lines appeared clearer through an additional10.25 D

lens, this was recorded as a 0.12 D lag of accommodation. In

accordance with the conventional clinical procedure, the lu-

minance of the target was maintained at a low level (approx-

imately 10 cd/m2). Additionally, the AR to the DCC target

was measured objectively using a Grand Seiko WAM 5500

infra-red optometer (RyuSyo Industrial Co. Ltd., Kagawa,

Japan). This instrument has been described and evaluated

previously.18-20

Immediately after the subjective testing, ob-jective measurement of the AR was performed with the

habitual refractive correction in place. If the subject indi-

cated subsequently that one set of lines on the DCC target ap-

peared clearer, then spherical lenses were introduced to

achieve the position of subjective equality, and the AR was

measured through each of these supplementary lenses usingthe infrared optometer. All objective data were recorded

from the right eye only, and for each condition at least 10

readings of the refractive state of the eye were taken, con-

verted into spherical equivalents, and averaged. Because

the instrument takes measurements at approximately 1- to

2-second intervals, the time to assess the AR for each stimu-

lus level was approximately 15 seconds. The objective mea-

surements typically took 1 to 2 minutes to complete.

However, subjects received a short break between measure-

ments while the lenses were being changed.

The same experimental set up was used for both the

objective and subjective measurements. Subjects wore atrial frame and lenses with their distance refractive correc-

tion, and additional lenses, when indicated by the subjec-

tive responses, were added to the trial frame as necessary.

For all testing, subjects viewed the nearpoint card mounted

on the infrared optometer, with their head against the

forehead rest and chin placed on the chinrest. The exper-

imental setup is shown in Figure 2.

Results

The mean AR recorded subjectively and objectively was2.35 D (SD 5 0.60) and 1.68 D (SD 5 0.49), respectively.

A paired t test indicated that this difference was significant

(t 5 7.13; df5 24; P , 0.0001). Additionally, the differ-

ence between the subjective and objective findings was cal-

culated for each individual, and the 95% limits of

agreement, calculated as 1.96 multiplied by the standard

deviation of the differences21 was 60.95 D. These differ-

ences are illustrated inFigure 3.

ARs for each individual are shown inTable 1. Linear re-

gression analysis indicated a significant correlation between

the subjective and objective findings (r25 0.39;P 5 0.001),

with a regression line described by the equation: objectiveAR5 (0.51x subjective AR)1 0.47. Ten subjects exhibited

Figure 2 Photograph of the experimental setup. Subjects wore boththeir habitual refractive correction and the 60.50 D cross cylinders in a

trial frame, and viewed a rectilinear target (not visible here as it was on

the side of the nearpoint card nearest the subject) at a viewing distance

of 40 cm. The subject was positioned at the autorefractor throughout,

which allowed objective measurements to be recorded in addition to the

subjective responses.

-1

-0.5

0

0.5

1

1.5

2

0 2 4Objective AR (D)

SubjAR

ObjAR

(D)

1 3

Figure 3 Difference between the subjective AR determined using theDCC procedure and the objective AR measured with an infrared optometer

as a function of the objective response. The horizontal dashed line indi-

cates the mean difference, and the area between the 2 solid horizontal

lines represents the 95% limits of agreement between the 2 values.

632 Optometry, Vol 80, No 11, November 2009


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a lead of accommodation (AR . 2.50 D) based on the sub-

jective findings. However, only one of these subjects demon-

strated a lead of accommodation objectively. Of the 8

subjects who showed a lag of accommodation (AR , 2.50

D) subjectively, all also demonstrated a lag objectively.

For the 8 subjects with a lag of accommodation based on

the subjective response, plus lenses were introduced to

achieve subjective equality, i.e., so that the 2 sets of lines

appeared equally clear. Objective measurements using the

infrared optometer verified that the lenses produced a

decline in the AR, as shown in Figure 4. Friedmans non-

parametric test indicated that the decline in AR after the in-troduction of the plus lenses was significant (c2 5 16.05;

P 5 0.000).

Discussion

The findings of the current study clearly indicate that the

subjective DCC test using the methodology adopted here

does not provide veridical measurements of the AR in young

subjects with active accommodation. The mean results

obtained with this test are significantly higher than those

obtained with an objective infrared optometer. In addition,the range of differences between the 2 measurements of AR

exceeded 2 D, while the DCC test overestimated the number

of subjects having a lead of accommodation.

In comparing the findings of the subjective and objectivetechniques for assessing the accommodative response, dif-

ferences between the procedures should be noted. For

example, with the DCC technique adopted here, a single

measurement was taken. Had either a bracketing technique

been used, i.e., going beyond the first neutral response and

then changing lenses in the opposite direction, or a high

neutral procedure, whereby additional plus power is added

initially, and then reduced until an equal response is

obtained, then different (and possibly superior) findings

might have been found. In addition, 10 readings of the

accommodative state were obtained with the infrared op-

tometer over approximately 15 seconds and subsequentlyaveraged. Accordingly, depending on the number of lens

changes required, the objective determination typically took

1 to 2 minutes to complete. In contrast, the single subjective

assessment generally took only 20 to 30 seconds. Further, it

is unclear whether subjects can be trained to respond better

with the DCC, i.e., to become more sensitive to the

differences in clarity of the rectilinear lines. However, our

personal experience when teaching this procedure to op-

tometry students is that they quickly become capable of

achieving any response, i.e., altering their AR so that either

the horizontal lines appear clearer, the vertical lines appear

clearer, or both sets of lines appear equal. Finally, to ourknowledge, no studies have been performed to determine the

sensitivity of the DCC, i.e., its ability to identify accurately

those individuals with accommodative anomalies.

It is clear that the subjective procedure frequently

overestimates the AR in young subjects. One possible

explanation for this difference is that subjects are volun-

tarily changing their accommodation to the target. It is

presumed at the outset that a subject who is accommodating

accurately on the target (AR 5 2.50 D) will maintain the

COLC on the retina, with the horizontal and vertical lines

remaining equally blurred. However, subjects could in-

crease their AR, thereby making the vertical lines appearclearer, or alternatively reduce the AR to make the

0

0.5

1

1.5

2

0 0.25 0.5 0.75

Lens power (D)

ObjectiveAR

(D)

Figure 4 Mean AR measured objectively through plus lenses using theinfrared optometer in 8 subjects. These subjects all exhibited a lag of ac-

commodation when tested subjectively with the DCC test (i.e., horizontal

lines appeared clearer) through their habitual refractive correction (0 D).

Accordingly, supplementary plus lenses were introduced to produce an

equal subjective response and the AR measured objectively though these

lenses. It is apparent that the AR changed significantly when the lens

power was modified. Error bars indicate 1 SEM.

Table 1 Subjective and objective measurements of the AR

for each of the 25 subjects tested

Subjective AR,

[D]

Objective AR,

[D] (SD)

Difference

(subjective-objective)

2.75 2.20 (0.87) 0.55

3.00 1.74 (0.13) 1.26

3.25 2.06 (0.33) 1.192.62 2.33 (0.12) 0.29

3.00 1.82 (0.35) 1.18

3.25 2.89 (0.20) 0.36

2.50 1.90 (0.19) 0.60

2.50 1.71 (0.24) 0.79

1.75 1.11 (0.27) 0.64

2.25 1.98 (0.30) 0.27

2.75 1.23 (0.28) 1.52

2.62 1.62 (0.73) 1.00

2.62 1.33 (0.24) 1.29

1.75 1.13 (0.16) 0.62

2.50 1.67 (0.18) 0.83

1.75 0.43 (0.57) 1.321.25 1.09 (0.17) 0.16

1.75 1.93 (0.25) 20.18

1.75 1.36 (0.15) 0.39

2.50 1.88 (0.17) 0.62

2.50 1.72 (0.47) 0.78

2.50 1.56 (0.12) 0.94

2.50 1.78 (0.14) 0.72

2.50 1.85 (0.24) 0.65

0.75 1.30 (0.19) 20.55

Mean 2.35 1.66 0.69

SD 0.60 0.49 0.48

Benzoni et al Clinical Research 633


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horizontal lines clear. Interestingly, Bannon and Walsh22

stated that for the recognition of letters, the vertical strokes

are most important, so that a patient may prefer to keep the

vertical strokes clear. If a patient habitually accommodates

to make the vertical portion of the target clear, then this

would explain the overaccommodation seen with the sub-

jective test. Certainly, use of a rectilinear target is not con-

ducive to maintaining the COLC on the retina. It has alsobeen proposed that fogging lenses (e.g., an additional

11.00 sphere) be introduced over the distance refractive

correction so that both the horizontal and vertical focal

lines lie in front of the retina.17 This is also the high neu-

tral procedure advocated by the Optometric Extension

Program and others.2,11,17 The termfoggingis inappropriate

here, because it refers to making a patient myopic when de-

termining the distance refractive error so as to minimize the

AR. During the DCC test, introduction of an additional

11.00 sphere will reduce the accommodative stimulus

from 2.50 D to 1.50 D but not make the patient myopic

or blur their visual acuity.The introduction of any lens during the test procedure

will modify the accommodative stimulus and therefore alter

the resulting AR. If the goal is to determine the response for

a particular stimulus level (e.g., 2.50 D), then that stimulus

must be maintained throughout the test. Once plus lenses

are introduced, if the patient reduces the AR by an amount

equal to the magnitude of the lens power, then the

subjective response will remain unchanged. Under binoc-

ular test conditions, a change in subjective response may be

obtained only when the patient is no longer able to reduce

accommodation while maintaining accurate vergence on

the target, i.e., exert negative relative accommodation.1

Therefore, the DCC procedure is not a direct quantification

of the AR for a 2.50 D stimulus, but rather uses subjective

responses to determine the lens that yields zero accommo-

dative error.

Accordingly, the DCC test as performed here does not

provide an accurate measure of the AR under normal

viewing conditions. When assessing the AR on a young

patient, an ideal test should use a naturalistic stimulus

placed at the patients habitual working distance to simulate

normal near-work conditions. Additionally, it should avoid

the use of supplementary lenses over the refractive correc-

tion because they will alter the accommodative stimulus(and resulting AR) from the habitual state. The use of either

an objective, open-fieldoptometer (if available) or Cross-

Nott retinoscopy,1,23-25 whereby the point conjugate with

the retina during active accommodation is determined by

altering the retinoscopy working distance seem to be the

optimal procedures to determine the within-task AR.

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