Facility of accommodation in myopia

Daniel J. O'Leary and Peter M. Allen

Department of Optometry and Ophthalmic Dispensing, Anglia Polytechnic University, East Road,Cambridge CB1 1PT, UK

Summary

Purpose: Myopes have been shown to have abnormal accommodative characteristics. This studyinvestigated the characteristics of accommodation facility in myopic and emmetropic students.

Methods: Distance and near positive and negative accommodation response time componentsof facility were measured over a 1 min period using a 22.00 D/zero lens pair for distance responsesand a ^2.00 D lens pair for near responses. 79 students (37 myopes and 42 emmetropes) aged18±27 years acted as subjects. Subjects were masked, and the results were analysed in a maskedfashion.

Results: Mean distance facility was signi®cantly lower (9.7 cycles per minute (cpm)) in themyopic group compared with the mean distance facility in the emmetropic group (15.6 cpm;p , 0.005). There was no signi®cant difference in the near facilities of the two groups (11.5 cpmin myopes vs 12.9 cpm in emmetropes). Positive accommodation response time for distance visionwas greater than 4 s in 45% of myopes and in 9% of emmetropes.

Conclusions: Our ®ndings con®rm that myopes tend to have abnormal accommodationresponses to blur. Distance facility, but not near facility of accommodation is morefrequently reduced in myopes than in emmetropes. q 2001 The College of Optometrists.Published by Elsevier Science Ltd. All rights reserved.

Myopes have been found to have abnormal amplitudes of

accommodation (McBrien and Millodot, 1986), and an

abnormal accommodative response to blur (Gwiazda et al.,

1993). The major ®nding in the latter study was that myopic

children showed a depressed accommodative response to

minus lenses when viewing a chart placed at 4 m, but not

to plus lenses when viewing a chart at 0.25 m. Accommoda-

tion to real targets placed at various distances between 4 and

0.25 m was essentially normal when lenses were not intro-

duced (Gwiazda et al., 1993). There is some evidence indi-

cating that abnormal accommodative responses are present

at the onset of myopia development (Gwiazda et al., 1995),

and are rarely found in children that do not develop myopia.

A clear mechanism relating the accommodation ®nding to

the development of myopia is lacking, however the strength

of the correlation between the two ®ndings indicates that

such a link may exist.

A simple clinical test which could predict the onset of

myopia would be valuable in investigating the ef®cacy of

potential therapies in preventing myopia from developing

The purpose of the present study, as a ®rst step to develop-

ing a predictive test, is to see whether clinical tests of

accommodative response to blur give different results in

myopes compared to emmetropes.

Because Gwiazda et al. (1993) found a reduced ability to

accommodate correctly through negative lenses, it seems

possible that accommodative facility will be reduced, at

least for part of the facility cycle. Our hypothesis was there-

fore that myopes would show a reduced facility of accom-

modation.

Materials and methods

The subjects for the present study, 37 myopes and

42 emmetropes (20.25 D , Refractive Error , 10.75 D),

were students at APU aged between 18 and 27 years. Subjects

were screened for VA (Grosvenor, 1982) and wore their

352

Ophthal. Physiol. Opt. Vol. 21, No. 5, pp. 352±355, 2001q 2001 The College of Optometrists. Published by Elsevier Science Ltd

All rights reserved. Printed in Great Britain0275-5408/01/$20.00

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PII: S0275-5408(01)00002-3

Received: 17 April 2000

Revised form: 18 July 2000

Accepted: 4 January 2001

Correspondence and reprint requests to: Daniel J. O'Leary.

E-mail address: D.O'leary@anglia.ac.uk (D. J. O'Leary).

current prescription. The mean spherical equivalent refractive

error for the myopes was 22.30 D, S.D. 1.62, range 20.75 D

to 26.25 D. Subjects were judged suitable for inclusion in the

study if VA was 6/6 or better in at least one eye, with correc-

tion; a 11.00 D blur-test which reduced acuity to less than 6/

9 screened for hypermetropia. The tests were carried out on

the right eye unless acuity was more than two letters better in

the left eye.

After recording VA, amplitude of accommodation, pupil

size and age, monocular accommodative facility in the

distance was measured using a 22.00 D lens, with the

subject viewing a Snellen chart placed 6 m away. The eye

not being used was covered with a large occluder. Subjects

were instructed as follows:

You should look at the [6/6] line of letters and try to

keep it clear. I am going to put a lens in front of your

eye and the letters will blur for a short time and then

become clear again. As soon as they are clear again

say `clear'. I will then remove [or change] the lens and

the letters might be blurred again; say `clear' as soon

as you can see the letters clearly again. We will go on

repeating this procedure to see how often you can

clear the lens in a 1 min period.

A tape recording of the responses was used to record

results for later masked analysis. The test was then carried

out.

Next near monocular accommodative facility was

measured at 40 cm. The target was the N5 print on an

RAF rule. The instruction set was similar to the distance

instructions, except this time two lenses (one 12.00 D, the

other 22.00 D, mounted on a ¯ipper bar) were inter-

changed, rather than using a single lens/no lens. The results

were again tape recorded for later analysis.

Two observers carried out the measurements. Partial

cycles were counted as a half. Where no change in lens

was obtained in the minute allowed, a facility of zero was

recorded. Where response time was analysed, no response

after 1 min was counted as a 60 s response-time. No recov-

ery time was included for these subjects. Although a true

response time would be greater than 60 s, this treatment

minimised the problem of including subjects who were

unable to respond in the allotted time. Analysis of results

was carried out by observers who were masked as to the

refractive error of the subject. Subjects were masked to the

purpose of the experiment.

Results

Our results are summarised in Figures 1 and 2.

Figure 1 shows the distribution of distance facilities for

emmetropes and myopes. A reduced accommodative

facility (i.e. below 15 cpm) was found in 27/37 myopes in

comparison with 16/42 emmetropes, and is signi®cantly more

common in myopes than in emmetropes (x2� 12.07, df� 1,

p , 0.01). The mean facility for myopes was 9.7 (S.D. 6.3)

cpm, whilst for emmetropes it was 15.6 (S.D. 6.8) cpm. The

difference between these groups was signi®cant (two-tailed t-

test, p , 0.005). Six myopes and three emmetropes failed to

clear the 22.00 D lens at all during the 60 s test period.

Figure 2 shows the distribution of near facilities in

myopes and emmetropes. There is less difference between

myopes and emmetropes for near than was found for

distance. Mean facility at near for myopes was 11.4 (S.D.

5.1) cpm, and for emmetropes the mean facility was

Facility of accommodation in myopia: D. J. O'Leary and P. M. Allen 353

Figure 1. Frequency distribution for distance facility ofaccommodation in myopes B (37 subjects) and emmetropesA (42 subjects). Facility was measured over a 1 min period.

Figure 2. Frequency distribution for near facility of accom-modation in myopes B (37 subjects) and emmetropes A(42 subjects). Facility was measured over a 1 min period.

12.9 (S.D. 6.4) cpm. The difference between these two

groups is not signi®cant (two-tailed t-test, p . 0.05).

We examined the correlation between distance facility

and near facility in both myopes and emmetropes. The

results are summarised in Figure 3.

Correlation coef®cient for myopes is 0.69, (p ! 0.001)

and the regression equation of near facility (Ym) on distance

facility (Xm) is:

Ym � 6:66 1 0:50 Xm �1�For emmetropes there is a weaker relationship between

distance and near facility. The correlation coef®cient is 0.38,

(p , 0.005), and the regression equation of near facility (Ye)

on distance facility (Xe) is:

Ye � 7:89 1 0:34 Xe �2�We analysed the results from distance facility further by

breaking the responses down into time to respond to the

minus lens (positive accommodation response time), and

time to recover acuity when the lens was withdrawn (nega-

tive accommodation response time). Positive accommoda-

tive response time for distant stimuli was on average 13.1 s

(S.D. 21.2) seconds in myopia, vs 6.49 (S.D. 15.0) seconds

in emmetropes (one-tailed t-test, p� 0.05). Mean distance

vision negative accommodative response time was 1.9 (S.D.

1.6) seconds in myopes and 1.4 (S.D. 0.6) seconds in emme-

tropes (two-tailed t-test p , 0.05). The distribution of

distance positive accommodation response times is shown

in Figure 4. Over 45% of myopes have a mean response

time greater than 4 s, compared with 9% of normals.

Near accommodation response times were only slightly

slower in myopes than in emmetropes for both positive

and negative accommodation. Positive accommodation

response times averaged 4.4 (S.D. 5.2) seconds in

myopes, and 4.2 (S.D. 4.4) seconds in emmetropes.

Negative accommodation response times averaged 2.6

(S.D. 1.7) seconds in myopes and 2.1 (S.D. 0.7) seconds

in emmetropes. The near results for emmetropes and

myopes were not signi®cantly different, although for

both groups the mean positive accommodation responses

were signi®cantly slower than mean negative accommo-

dation responses.

Discussion

Our results show that there is a highly signi®cant depres-

sion of the distance accommodative response in myopic

adults in comparison to the response of emmetropes. The

response is evident whether the facility, response time or

recovery time elicited with a 22.00 D lens is used. This

provides support for the ®ndings of Gwiazda et al. (1993,

1995) who found that accommodation responses to negative

lenses by myopic children, viewing targets at 4 m, were

lower than for non-myopic children.

Accommodation facility testing is normally carried out at

40 cm, and a ^2.00 D lens combination is often used.

Levine et al. (1985) found an average facility of 21 cycles

per 80 s for an asymptomatic urban population. Our emme-

tropes had a mean facility of about 15 cpm; although facility

is in¯uenced by the duration of measurement (Levine et al.,

1985) this would be roughly equivalent to a facility of 20

cycles per 80 s, suggesting that our control group is compar-

able with Levine et al.'s asymptomatic group.

Positive accommodation response times were on average

signi®cantly slower than negative accommodation response

354 Ophthal. Physiol. Opt. 2001 21: No 5

Figure 3. Correlation between distance facility and near facil-ity for 42 emmetropic subjects (A) and 37 myopic subjects(O). The regression equations for emmetropic (dashed line)and myopic (solid line) subjects are given in the text.

Figure 4. Distance positive accommodation response timesto a 22.00 D lens stimulus for 42 emmetropic subjects A and37 myopic subjects B. Each column gives the percentage ofthe subjects with a response time in the range indicated alongthe x axis.

times at both distance and near for both emmetropes and

myopes. This is in line with the ®ndings of Bobier and Sivak

(1983). Our results show that the distance positive accom-

modative response time is most severely affected in myopia,

being on average over twice the response time in emme-

tropes. This reinforces the ®nding of Gwiazda et al. (1993,

1995) that it is the response to negative lenses which is

particularly abnormal in myopes, however unlike Gwiazda

et al. (1993, 1995), who examined children, we were unable

to demonstrate a signi®cant effect in all members of our

adult population. Although the test is not suf®ciently discri-

minating between non-myopes and myopes to be used in its

present form as a diagnostic test, it shows promise. The

response times appear to be bimodal for myopes, rather

than following the normal distribution seen for emmetropes.

This may be due to an age effect, where some myopic

people recover their ability to respond to negative lenses

as they get older. We were additionally unable to demon-

strate a signi®cant difference in the near responses of emme-

tropes and myopes. This could be due to the absence of a

difference between the two groups, or alternately due to our

experimental design. Since we carried out near facility

measurements after measuring distance facility there may

have been a training effect which normalised the near

response.

We note that some emmetropes had reduced accommo-

dation responses more akin to the responses of myopes than

other emmetropes. We will be following as many of these

subjects as we can to see if they develop a myopic refractive

error.

In conclusion we have shown that distance accommo-

dative facility is signi®cantly lower in a group of myopes

than in a group of emmetropes of the same age. Distance

positive accommodative response seems particularly

affected.

References

Bobier, W. R. and Sivak, J. G. (1983). Orthoptic treatment ofsubjects showing slow accommodative responses. Amer. J.Optom. Physiol. Opt. 60, 678±687.

Grosvenor, T. P. (1982). Primary Care Optometry, ProfessionalPress, Chicago, IL, p. 97.

Gwiazda, J., Thorn, F., Bauer, J. and Held, R. (1993). Myopicchildren show insuf®cient accommodative response to blur.Invest. Ophthalmol. Vis. Sci. 34, 690±694.

Gwiazda, J., Thorn, F., Bauer, J. and Held, R. (1995). A dynamicrelationship between myopia and blur-driven accommodation inschool-aged children. Vision Res. 35, 1299±1304.

Levine, S., Ciuffreda, K. J., Selenow, A. and Flax, N. (1985).Clinical assessment of accommodative facility in symptomaticand asymptomatic individuals. J. Amer. Optom. Assoc. 56, 286±290.

McBrien, N. and Millodot, M. (1986). The effect of refractive erroron the accommodative response gradient. Ophthal. Physiol. Opt.6, 145±149.

Facility of accommodation in myopia: D. J. O'Leary and P. M. Allen 355