THE POSSIBLE INFLUENCE OF CONTACT LENSES ON MYOPIAt

Abstract

JANET STONE, FBOA,HD, DCLP

London Refraction Hospital (Received February 1977)

This is the third published report on the research project being carried out at the London Refraction Hospital.

Introduction For just over five years I have been working at the London Refraction Hospital on a research project to try to establish whether or not hard corneal lens wear has any retarding effect on the rate of progress of myopia in children.

For many years now contact lens practitioners have noticed that when they fit young my opes with corneal lenses, they rarely need to change the power of the lenses, and it has seemed as if the contact lenses control the myopia. Frank Dickinson2 was the first to report this finding after fitting his daughter with corneal lensrs and finding that as she got older it was not necessary to alter their power. Then many other well-known practitioners in the U.S.A., among them Robert Morrison6, Jack Nei!J7 and John Nolans, all reported similar findings.

Obviously the reporting of isolated cases proves nothing and a properly mounted research project, using as many subjects as possible, is necessary to establish whether or not these findings are significant. Such research must be carried on over a number of years and preferably a group of contact lens wearers should be compared with a group of spectacle wearers.

Several such projects have been undertaken, for example by Kelly and Butler4 in England, by Baldwin and his co-workersl in the U.S.A. and, more recently, bymyselfl8 in London.

Before considering any of the findings, we should look again at the causes of myopia and also the effects on eyes of wearing corneal lenses.

EXPLANATION OF MYOPIA AND ITS EMERGENCE AS THE EYE GROWS It can be seen from Fig. I that during growth the eye enlarges. If it does so uniformly there is an increase in axial length which would tend to produce myopia, but this is offset by the flattening of the cornea and crystalline lens.

In Fig. 2, reproduced from Professor Sorsby's16 work, it can be seen how this compensation of the axial increase by the lens and cornea keeps the refraction stable.

If the growth of the eye is not uniform, as shown diagrammatically by Spooner17 in Fig. 3, the volume of the eye may increase to the same extent as before but the axial length may increase while the equatorial diameter remains the same. Such an effect leads to very little flattening of the cornea and crystalline lens: in fact they may steepen; so that all three components tend to produce myopia.

Provided that there is partial compensation by the cornea and crystalline lens the way in which the myopic refractive error develops can be seen from this diagrammatic illustration from Sorsby's work16 (see Fig. 4).

t Paper read at the Scandinavian Contact Lens Congress, Oslo, Norway, June 1975

89

So in wishing to prevent myopia from developing it is desirable to prevent the eye from elongating more than the amount by which it can be compensated by the flattening of the cornea and crystalline lens.

Fig. 1. Comparison of the adult and neonatal eye (after Spooner) (reproduced by kind permission of the British Optical Association)

Iii 1

0 Change in axial length 1:;;;:;:;1 Change in lens power

~~~ Chonge in·corneal power

[] .

I iii.

Stationary refmction: three case; with different degrees of axial clongaiion showing full comriensation. from change:; in the cornea and lens.

(i) (ii) '(iii) Period between examinations 2 yrs. 11 mths. 4 yrs. 8·mths. 2 yrs: 4 rnths. Initial refraction + 1·3 D + 1·0 D +0·8 D Axial elongation 0·2 rnm 0·7 mm 1·1 rnm Refraction at 2nd examination + 1·3 D + 1·0 D +0·8 D

Fig. 2. Three different increases in axial length, all compensated by reductions in power oft he crystalline lens and cornea to keep the refraction constant (reproduced by kind permission of A. Sorsby16 and HMSO, London)

90

Fig. 3.

~

1///~f!!J \l\\\\\\~

·~ l?:j ~~

~ -v

~ l "' i\ fj-~ 'L

1-' ~

Co-ordinate distortion of the globe. In the lower figure the horizontal scale is increased by 1.25 times as compared with the upper figure (after Spooner11-reproduced by kind permission of the Hatton Press, London)

4·0

3·5 3·0

!' 2·5 n.

2·0 0 0

1·5

1·0

0·5

0 I' I . I' I jj) (iii}

The p~rsistenc~ of hyp~:rm~tropia and the emergence of emmetropia and of myopia: three cases showing,similar (and rather marked) degrees of axial elongation but different degrees of partial compensation-fairly full in the first, moderate in the second, and rather poor in the third.

Period between examinations Jni tia I refraction Axial elongation Refraction at 2nd examination

(i)

4 yrs. 8 mths. +l·OD.

l·J mrn +0·7D

(ii) 4 yrs. 8 mths. +1·3D

1·6mm O·OD

(iii)

5 yrs. 3 mth +1·5 D

1·7mm -l·OD

Fig. 4. The emergence of myopia due to insufficient compensation for axial increase by the cornea and crystalline lens (reproduced by kind permission of A. Sorsbyl6 and HMSO, London)

91

This is obviously very difficult to do. Tomlinson and Phillips19 and others, during investigations into intra-ocular pressure and glaucoma have implied that raised intra-ocular pressure or a pressure slightly higher than normal can lead to myopia.

Others have suggested that excessive close work may bring about myopia and very convincing evidence that this is so has been produced from Japan9, and from the U.S.A20• Whether or not the excessive close work causes slightly increased intra-ocular pressure is uncertain.

THE ROLE OF CORNEAL CONTACT LENSES IN ALTERING THE REFRACTIVE STATUS OF THE EYE

If contact lenses are going to influence myopia, how are they likely to do so? This is a question which is very difficult to answer, for there are so many avenues to be explored. Do contact lenses prevent people from doing so much close work, do they affect the intra-ocular pressure, do they alter the accommodation/convergence ratio, do they affect ciliary tonus etc., etc? We are only just beginning to find out the answers to some of these questions.

Obviously myopia can be reduced by just flattening the cornea, which at the same time may slightly shorten the eye.

If contact lenses are doing this, they are not overcoming the intrinsic cause of myopia. Orthokeratology is a technique whereby myopia is overcome by purposely flattening the cornea, but this is a remedy rather than a cure.

In my own research project we have tried to fit lenses which would not flatten the cornea. However, all corneal lenses tend to upset the metabolism of the cornea somewhat, so that its curvature, thickness and refractive index are altered, and this affects the eye's refractive error.

Rengstorff has done extensive work on this subject in the U.S.A. He and Arnerl5 have shown how altering the cornea affects the refractive status.

Fig. 5 shows the corneal constants used by Gullstrand. When a contact lens is

n = 1.00 n' = 1.376 nl = 1.336 r, = 7.7mm rt = 6.8mm d = 0.5 mm

De= 01+02 - dD1 02

= 1000[n'-n + n1. n' _B...(n'-n .~)] [ r, r1 n r1 r:a,

= 48,83- 5.88 + 0,10 = 43.05

Fig. 5. The refractive power of the cornea according to Gullstrand (reproduced by kind permission of Rengstorff and Arner)

92

worn, the only values which are likely to remain unchanged are the refractive indices of the air and aqueous. All the other "constants" are usually altered to a greater or lesser extent.

The greatest change likely to be brought about is in the curvature of the front surface of the cornea because of the significant refractive index change between it and air. Fig. 6 shows that a 2.4 mm radius change gives 16 D of power change. Thus if the cornea is flattened by 0.1 mm the eye will become approximately 0.67o less myopic .

., co: w .... 0.. 0 0

S2

50

AS

A6

AA

A2

40

38

• \ \

6.4

\ \ \ \ \

\ \

• \

6.8

\ \

\ \

\ \

\ .. \

7.2

\ \

\

( anterior )

r • corneal curvature

\ \

\

\

7.6

\

MM

\ \

\ \

\

" ' ' ' ' ' ' ''" '

8.0 8.4

' ' ' ' ...

8.8

Fig. 6. Relationship between the refractive power of the cornea and its front surface radius (reproduced by kind permission of Rengstorff and Arner)

Similar but less marked changes occur when the back of the cornea alters curvature as shown in the upper part of Fig. 7. A 2.4 mm radius change now gives rise to only a 2o power change, and if the back surface is flattened by 0.1 mm the eye will become more myopic by almost O.lo. This small increase in myopia is of course because the back surface is a concave surface in contact with the aqueous, as compared with the front surface which is convex to air. Thus if both surfaces alter together and flatten by 0.1 mm then the eye becomes less myopic by just under 0.6o.

What is very interesting is that a thickness change alone (see lower graph in Fig. 7) has little effect on corneal power, but of course, any thickness increase of the cornea, for example, due to oedema or corneal swelling, means that the cornea has taken in water. Water has a refractive index of 1.3333, so that when it enters the cornea the corneal refractive index falls. In theory if it were to fall as low as that of the aqueous, i.e. 1.336, then the back surface of the cornea would have zero power. This means that the corneal front surface would no longer be partly neutralized by the back surface. So, as shown by the central graph of Fig. 7, as the refractive index of the cornea falls, so its power increases. Oedematous corneae therefore lead to an increase in myopia whether or not there is any associated corneal steepening. This is shown in Fig. 8.

93

Ill o=: w 1-c. 0 c

45

44

43

42

41

45

44

43

42

41

45

44

·--5.8

1.30

--.. -----__ ... -6.2 6.6

... ..... ......

rl corneal curvature (. posterior )

_ .. ----· .. ---.. ----

70 74 7:8 82

n' (cornea! 'index)

........ ..... -- ... ...... ----- .......... -... , 1.34 1.38 1.42 1.46 1.50 1.54

d (corneal thickness)

43 ·----------------------------··

42

41

.2 3 .4 .5 .6 .7 .e Fig. 7. Relationship between the refractive power of the cornea and (top) its back surface radius,

(middle) its refractive index, (bottom) its thickness (reproduced by kind permission of Rengstorff and Arner)

~ 1·00

Fig. 8. Increase in converging power of the cornea by lowering its refractive index

94

::t .. < c ::a ..a ~ 0 z Col

• "' Cl'

~----FLATTER----- ---STEEPER---~

'

HHHHHH~~++++++'~~~~·H· HH~H~~·r , HH~++~~+++1 MH! ~·+l~~~ri+~\~

! ; j i I li I - I HH-~++H14~1 ~1 ~++1 H+~i+lr.H\! I ~++++H1 ~+;+rri,Hi~f++1 ~~H+'+++~~~.~~H-~++~HH++~~~ HHHH-H~~

1+

1

++1 ++++~HH-H~~++II~~ I I

I -,

0 I

I

Fig. 9. Curvature changes due to hydration and dehydration of PMMA cornea/lenses. The corneal curvature undergoes similar hydration and dehydration changes to a negative corneal lens (reproduced by kind permission of A . J. Phillips and the British Optical Association)

95

The sequence of evems during the regular wear of corneal contact lenses appears to be as follows:

When the lens is put on the eye at the beginning of the day, instead of the cornea thinning slightly as it would normally, it remains the same thickness or gets slightly thicker, depending on how good the fit of the lens is. Mandell and Polses have gone so far as to suggest that measurements of corneal thickness can be used as an index for determining the fit of the lens. This is because the presence of the lens upsets the corneal metabolism slightly which leads to water retention in the cornea. If this is pronounced we describe it as oedema and the familiar appearance of central circular clouding can be seen with the slit lamp using sclerotic scatter illumination. The onset of oedema leads to steepening of the curvature of the cornea, and the result both of the steepening and of the water intake is to make the patient more myopic.

In the presence of oedema, after a certain length of time the cornea will begin to flatten again and so the myopia may reduce a little.

The mechanism of this curvature change with oedema is not fully understood, but it is very similar to the curvature change which takes place in a negative plastic corneal lens-see Fig. 9. Gordon3 first described how a dry minus corneal lens, when put in soak, at first becomes steeper and then flattens. PhillipslO has attributed this change in curvature to the mechanical stress which the contact lens undergoes as it takes in water. First of all the water enters the surfaces and gradually penetrates to the centre of the lens substance. As a minus lens is thinnest centrally it becomes fully hydrated in the centre sooner than at the edges and this leads to initial steepening. As the thicker periphery gradually becomes fully hydrated the lens flattens again.

Now the cornea can be compared to a negative corneal lens, being thinnest in the centre. In its normal state it is also relatively dehydrated, so that if its metabolism is upset water enters the cornea via the epithelium and endothelium. This waterlogging effect, or oedema, is greatest centrally under the centre of the contact lens. By analogy with the negative contact lens, then, we should expect the cornea to steepen and then flatten; which is in fact what Rengstorff12 has found. This is shown in terms of the change in myopia in Fig. I 0.

1.25

"' oc ... ··---... ./ ------. a.. 1.00 0 R qe 0

~

~ 0.50 .., C) z

When a corneal lens is removed and left out the cornea gets thinner. This is because it loses its oedema, and in fact it appears to over-react as Polsell has shown, so that it gets thinner than normal, and then gradually returns to normal again.

Now this can be compared to taking a negative corneal lens out of soak when, as we know, it dehydrates, and at first it flattens and then it steepens again eventually return-ing in curvature to its original state. The cornea, over a much longer time-scale does the same thing: it flattens and then steepens, returning approximately to its original curvature. Thus on removing contact lenses and leaving them out we can expect a reduction in myopia followed by an increase.

That a definite and fairly rapid reduction in myopia occurs has been shown by Rengstorff13 from measurements made on removal of lenses at night and again before their insertion the following morning. A drop of lo in myopia overnight is not un-common, as shown in Fig. II. Leaving lenses out for a long period of time shows that

R.M. J, c.

I I I I

/~' 3.

I I I

:\ I

.,

~,' ~I .J Ill: I I IU ...

I \t' I II. I 0 I 2. I 0 I I I I

c 1.50 I II. 0 I >-~ 1.00 I

o.s -.cJ OYIE.NIOHT, NO CONTACT LINS D· .. DAY, CONTACT LINS

o.oo 1 2 3 4

TIME (DAYS)

Fig. 11. Myopia drops overnight when no contact lens is worn, and increases during daytime wear (reproduced by kind permission of R. H. Rengstorff)

myopia drops for about three days and after three weeks has returned to approxi-mately its original value as shown in Fig. 12.

In more detail, showing the vertical and horizontal meridians separately, Fig. 13 illustrates Rengstorff's13,14 findings on over 100 young myopic army recruits when their contact lenses were withdrawn for over six weeks. The solid lines show the change in myopia and the dotted lines the change in corneal power.

97

O.IX)

w o.2s ., • .. o.so .. u ... A o.7S

"" 1-0 .. 1.00

0

>< a

l.l$

.11 A Y 5

Fig. 12. Mean changes in myopia during seven weeks following the removal of contact lenses (repro-duced by kind permission of R. H. Rengstorff)

Two points of interest emerge here. One is that the vertical meridian finishes up more myopic than the horizontal-an increase in with-the-rule astigmatism. This is probably due to the presence of a contact lens intensifying the normal eyelid action on the cornea. It is well known that in young people the vertical corneal radius is usually steeper than the horizontal, whereas in the elderly when the lids become more flaccid and have less muscle tonus, the corneal with-the-rule astigmatism reduces and often becomes against-the-rule. Probably the presence of a contact lens increases the activity of the eyelids and thereby tends to steepen the vertical meridian and flatten the horizontal.

The other point is that Rengstorff found his corneal power change paralleled the refractive change, but was not so great, as Fig. 13 shows. Now he used a Bausch and Lomb keratometer which utilizes areas of cornea about 3.2 mm apart on the cornea. Also it is calibrated for a refractive index of 1.3375. But the maximum change in curvature of the cornea probably occurs right at the apex or in the centre of any oedematous area which occurs. This oedematous region is usually situated over the pupil area, through which, of course, the refraction is measured. The keratometer reflection-areas are, relatively speaking, well apart, being situated fairly close to the edges of the pupil and near the edges of the oedematous region. Therefore they do not allow the maximum curvature change to be recorded, nor do they reflect the entire change in the refractive error. Also by using 1.3375 as the index of calibration, the Bausch and Lomb keratometer assumes that the curvature change of the back surface of the cornea follows that of the front surface.

In my own research project I have used a Gambs keratometer which measures from areas of cornea only about 2.3 mm apart (see Fig. 14) and I have converted the radii to power readings using the true corneal refractive index of 1.376. I made some similar measurements to.Rengstorff's by recording refraction and front surface corneal power both at the time of contact lens removal and then two days later. The change in

98

---- MEAN CHANGE IN OIOPTE RS --~ r ~ :t - ~ ~ 0 0 6 0 0 ::!1 "' ~ "' 0 :g :li! ~ 0 n Ul VI .•. -C... •.• 0 ?' ...

I I

0. w

' '\ :r: '\ 'a_ 0 ::0

\ -\ N \ 0

\

'q_ 2 - ..., ~

.fl. \ )> ~

\ "" r ~

N .... 2 tJ )'

I -< (/'\

t 9 I I

I ~

I "11

Ei E

10·0

9·5

9·0

~ 8·5 = = > a: = ... ... 8·0 c::l

~ E c a:

2·0 2·2 2·6 2·8 3·0 3·2 3·6 3·8

SEPARATION OF MIRE REFLECTION AREAS ( mm:) Fig. 14. Separation on the cornea of the mire reflection areas of various keratometers-after Mandell

(1965) and Lehmann (1967) (reproduced by kind permission of Barrie and Jenkins)

refraction which occurred was then found to be exactly the same as the change in front surface corneal power, over this two-day period. This suggests that the fluctua-tions of refraction which occur both during daily wear of contact lenses and for some three weeks after their removal can be attributed to fluctuations in front surface corneal power alone.

Thus in my own research, I have had to allow for these changes and interpret my findings accordingly.

What should be noted particularly at this stage is that the refraction and corneal power are both approximately stable three weeks after contact lens removal as Fig. 13 shows. The refraction of the horizontal meridian stabilizes at about 0.12 D less myopia than that found at the time of removal of contact lenses, and the vertical meridian about 0.25D more myopic, i.e. about 0.37D more with-the-rule astigmatism. Thus if we record the refraction at the time of contact lens removal it can be expected to be within 0.25D of the value at which it will eventually stabilize. Using my own kerato-metry technique which gives corneal power changes exactly parallel to the refractive findings, the same assumption can be made for corneal power. What must be remem-bered is that these graphs represent the average findings of a large number of subjects

100

%

and that not all corneae and refractive errors alter in this fashion. Generally speaking, as Polsell has shown, the smaller the upset to the corneal metabolism caused by the contact lens, the smaller is the refractive fluctuation which the eye undergoes.

From the point of view of my own research, I have benefited by knowing that the refraction stabilizes within a very little of the value found at the time of contact lens removal: it was not practical to get my subjects to leave out their lenses for long periods of time, as it would defeat the objects of the research.

My own findings follow, although these are still incomplete for the project is not yet finished and the statistical significance of all the findings has not yet been established.

THE LONDON REFRACTION HOSPITAL PROJECT At the London Refraction Hospital we have been following up over 1 50 children of whom 120 have continued to attend regularly for four years. Of these, 40 children are wearing spectacles and 80 have been fitted with corneal lenses.

The contact lenses fitted were multicurve, with the BCOR just steeper than the flattest keratometer reading. The BCOD was 7.00 mm, os 9.2 mm and the peripheral curves were chosen sufficiently flat to prevent visible oedema after the adaptation period. Where necessary the os and BCOD were made smaller to get rid of any oedema or "3 and 9 o'clock" staining.

At the beginning of the investigation the- children's ages ranged from approximately St years to 16! years in the experimental group, and from 6t years to 16 years in the control group. At the 50 per cent frequency level there was only a seven-month age difference between the two groups as Fig. 15 shows, so, for statistical purposes, they can be considered as two similar age groups.

AGE Dt~TR.I&UTlON

x---X C.or-~TIIC.T' Le.!i!l. GrA.ou.P c~o)

C.uMLtL-ATI\1£

F u:Aue.Nc. 'J

(4o)

A c.e R. A-Na. e:

S·42 _/{:,·58llfUIU

!2·19 b·S8 -16·0S

roo ,~1l

~ . 71'110t1ths ~~y./ Cl/.

-----_ -----------¥-~~1 ---_-------So% u;vE"L / . ~

!1.0 ,/'::/' ,;Y ,, . --lC--- ..... ~~ 0~-r--A-~~--r----.----.-----r----.,-----r----r---~------~

13 14 IS l(o 7 (0 l.:t 17

Alire. IN !:JEA~':.

Fig. 15. Cumulative frequency curves showing the age distribution of the experimental and control groups at the commencement of the project (reproduced by kind permission of the British Optical Association)

101

The distribution of refractive errors in the two groups was also similar as shown by the scatter plot in Fig. 16 which also indicates the age range of the two groups. Another scatter plot (Fig. 17) showing corneal radius and cycloplegic refraction at the first visit again shows the two groups to be fairly similar in distribution. In fact more subjects have been added since the last two plots were made.

Besides measuring refractive error both with cycloplegia and without, we have measured corneal curvature with a Gambs keratometer, corneal thickness with a Haag-Streit pachometer, anterior chamber depth with a Haag-Streit gauge and intra-ocular pressure with a Goldmann tonometer.

The accuracy of measurement is as follows:

Keratometry Pachometry A.C. depth Tonometry

±0.015 mm ±0.015 mm ±0.05 mm ±1 mmHg

We also measure heterophoria, distance and near; we record the number of hours of close work done daily, and family history of myopia. Subjective refraction is carried out following retinoscopy, and the Humphriss binocular refractive technique is used with crossed cylinder and duochrome.

So far it has been impossible to process all the data accumulated. At present I am concentrating on the changes in keratometry and in refraction.

All subjects are seen at six-monthly intervals. The contact lens wearers are asked to remove their contact lenses two days before measurements are made. This was decided on so as to allow any small corneal irregularities, caused by corneal lens wear, to subside. However, it does mean that we make measurements of myopia when they are at about their lowest value-for Rengstorff found myopia dropped for about three days after lens removal (see Fig. 12). Previously we made an allowance for this, based on Rengstorff's work which we called the "Rengstorff correction factor". Then we found sufficient measurements at the time of lens removal to calculate a correction factor of our own. But now we always take measurements on the contact lens wearers at the twice-yearly aftercare checks which we also carry out, and so we are able to obtain values actually at the time of removal of contact lenses. These checks are normally carried out during the afternoon when the subjects can be expected to be at their most myopic.

I should point out here that we measure spectacle refraction without cycloplegia at these aftercare checks, for the convenience of our patients, whereas when the lenses have been out for two days spectacle refraction is measured both with and without cycloplegia.

However, this does not invalidate our findings. For what we are interested in is the change in refraction between one occasion and the next, and the change between two consecutive measurements of cycloplegic refraction is the same as between two con-secutive measurements of non-cycloplegic refraction. For example:

Cycloplegic refraction Non-cycloplegic refraction

At the start of the project -2.00 -2.25

Ajier one year -2.50 -2.75

Each has changed by 0.50n.

102

Difference -0.50 -0.50

.. • •

• •

•

• • .. • •

• •

Ill l!l q: ><

..J X X

lJ') (

i: ~ !(

X

••

Fig. 17.

1/)

" Ul 1/) IX .J < 0 11.1 0: ~ .... z: .J 0 u u X • • • •

• • • • X

• X •• • X • • X • .X

• • I • . ' • • • • • • • • • .. • •

• • • -~ • • •

X~ • • •

• • X X • • • • ••• • • •• • • •

• • • • • • • • • • • • ••

• •

• •

• •

• •• • • X

• • ~ • I x• • • • • • •

~· • • X. X ••• • • • • • • • • • • • •

X X

• X •

I •• X X

•

• • •

• •

•

X •

• ·~X

X.

X

X X X

•

•

•

E E ~

co t-<

Ul z IX 0 (J

II. 0

Ul \Do r:.~

0: ~

f/1 1-z 0 It ~

'11'11. ,:.o

Ill :::1

c < 0:

Distribution of mean cycloplegic spectacle refraction against mean front surface corneal radius at the initial examination (reproduced by kind permission of the British Optical Association)

104

r ~ Q.

~ E w

" 0 l:

-2·

-1·75

-1·50

-1·2$

-1·00

-0·1$

-CI·$"0

-0·2.5

C.HJ'.NuE: IN C. YC.L.OPLEGriC

SPEC.TAc.&...E

11

.S4

,, St

R~:FAACTLON

20

Fig. 18. Changes found in the control group: spectacle wearers (mean values: number of eyes as shown)

FINDINGS AFTER FIVE YEARS FOLLOW UP Looking at the spectacle wearers first, we can see from Fig. 18 that their myopia has increased by an average of -1. 75D over five years, and there has been a small increase in with-the-rule astigmatism. Once the project is complete the number of children followed up will be greater and the general slope of the graph may alter a little The standard deviations are not shown on this graph as they would complicate its appearance, but they are shown in the table given in Fig. 19. As with any biological variable the standard deviations are fairly wide, but this is to be expected and can be taken into account in the final statistical analysis.

TIME : MONTHS 6 12 18 24 30 36 42 48 54 60 MEAN (MEAN -0·38 -0·46 -0·73 -0·76 -0·83 -0·79 -1·24 -1·58 -1·88 -1·84

OF ~ S.D. 0·48 0·68 0·65 0·89 0·98 1·00 HI 1·33 1·52 1·42 H&Vl No. 124 108 104 96 84 56 40 40 40 28

r MEAN -0·34 -0·39 -0·67 -0·70 -0·65 -0·75 -1·13 -1·40 -1·61 -1·73 I H ~ S.D. 0·37 0·64 0·52 0·80 0·89 0·89 1·06 1-31 1-44 1·23

L No. 62 54 52 48 42 28 20 20 20 14

r MEAN -0·41 -0·53 -0·80 -0·82 -1·01 -0·82 -1·36 -1·76 -2-14 -1·95 v ~ S.D. 0·56 0·72 0·76 0·96 1·03 HO 1-14 1·33 1·55 1·58 l No. 62 54 52 48 42 28 20 20 20 14

N.B. + = changes giving less myopia - = changes giving more myopia

Fig. 19. Changes in cycloplegic spectacle refraction as compared with original values: spectacle wearers

105

~

Fig. 20 shows the change in corneal power of the same group--the control group--over the five-year period. It can be seen that the•average finding is a flattening of only 0.12o. The table in Fig. 21 shows the figures from which the graph was drawn as well as the standard deviations.

% -0·50 CHANGE 1N F~NT 5uRFAC.E CoRNEAL Po~A>E'A. (n.' .. 1·37b) )o. l:

Ill

~ -2!> r.

1--~~~~~--~----~~~~--i "''.-..:-:.:.; ... ...,..

(, l:l. Iff 2.4 ~0 J"

"2 $4 S2 5'0 ~2. .18 4-2 48 $4-

.20 ;l.o :l.O

HORIZ ON'TFIL

.,MEliN VEit'I'IUII..

c..o T1me..in mo11ths 14 NWI!bu of r~es

Fig. 20. Changes found in the control group: spectacle wearers (mean values: number of eyes as shown)

TIME : MONTHS 6 12 18 24 30 36 42 48 54 60 MEAN {MEAN -0·03 +0·03 +0·03 +0·07 +0·03 +0·08 +0·08 +0·11 +0·07 +0·13

OF S.D. 0·24 0·23 0·27 0·23 0·26 0·25 0·27 0·29 0·27 0·29 H & V No. 124 108 104 100 84 56 40 40 40 28

r MEAN -0·04 +0·02 +0·05 +0·03 +0·05 +O·ll +0·14 +0·26 +0·19 +0·07 H ~ S.D. 0·19 0·16 0·24 0·19 0·21 0·21 0·24 0·15 0·22 0·32

l No. 62 54 52 50 42 28 20 20 20 14

r MEAN -0·03 +0·05 0·00 +O·ll +0·01 +0·06 +0·02 -0·05 -0·05 +0·19 v ~ S.D. 0·29 0·29 0·30 0·27 0·30 0·28 0·28 0·31 0·25 0·24 l No. 62 54 52 50 42 28 20 20 20 14

N.B. + = changes giving less myopia - = changes giving more myopia

Fig. 21. Changes in front surface corneal power as compared with original values (control group: spectacle wearers)

Now if we compare these graphs with the findings on the contact lens wearers, Fig. 22 gives the changes in cycloplegic spectacle refraction which can be seen to follow a varied course over the five years. In fact the mean values are about 0.12o less myopic after four and a half years than at the outset of the project. However, we must bear in mind that two days after contact lens removal the myopia is a good deal less than the value at the actual time of lens removal which itself is the value at which the refraction subsequently stabilizes. The results also show a fairly large increase in with-the-rule astigmatism, of 0.87o. The mean and standard deviations are shown in Fig. 23.

106

-/·00

~ -0·75 Q.

~

l: -O·!>CI w « 0 :r

-D·2S

i

v l~0·50

:r ~+0·75

~ +I ·fiO

JZ .24

f:J.O

30

/07

!!>"

80

42

7'6

48

....

VEa.TICAI.-

HoRt20NiAI..

4 Ttlfte.. iA 1'1\0nth~

~ NLU~~berof~~

Fig. 22. Changes found two days after removal of contact lenses (mean values: number of eyes as shown)

TIME : MONTHS 6 12 18 24 30 36 42 48 54 60 MEAN {MEAN +0·36 +0·28 -0·03 -0·02 +0·13 -0·09 -0·01 +0·34 +0·09 -0·99

OF S.D. 0·50 0·88 1·05 0·96 1·00 1·30 1·14 H9 1·41 1·02 H & V No. 88 244 244 240 214 160 156 96 68 22

{ MEAN +0·46 +0·39 +0·16 +0·21 +0·51 +0·31 +0·28 +0·88 +0·54 -0·73 H S.D. 0·54 0·88 1·05 0·97 1·01 1·30 1·14 1·19 1·52 1·09 No. 44 122 122 120 107 80 78 48 34 11 f MEAN +0·26 +0·16 -0·22 -0·24 -0·25 -0·50 -0·30 -0·20 -0·35 -1·25 v S.D. 0·44 0·86 1·01 0·90 0·84 1·15 1·05 1·02 H2 0·86 l No. 44 122 122 120 107 80 78 48 34 11

N.B. + = changes giving less myopia - = changes giving more myopia

Fig. 23. Changes in cycloplegic spectacle refraction measured two days after contact lens removal, as compared with original values

Changes in corneal front surface power, measured two days after contact lens removal on each occasion, show the same increase in with-the-rule astigmatism as Fig. 24 indicates. This is obviously caused by contact lens wear, and as expected the values show the corneae to have flattened by an average value of about one dioptre after four and a half years of contact lens wear. Again, mean and standard deviations for this graph are shown in the table in Fig. 25.

If we now look at the findings obtained at the time of removal of contact lenses, which give approximately the values at which the eye can be expected to stabilize, we see from Fig. 26 that the subjects have definitely increased in myopia by an average of about -0.37o over four and a half years, and there is an increase in with-the-rule

107

u ~

~ I: -O·SO Ul

CHANG& ,,., FRONT SuRFACE C~N~AL. Po""EA. (n' = 1·37b) It

0

I: -0·.25

i !H>·.25

+1·00

-t-I·ZS

+ 1·5"0

6 46

12 ,,.~

,. {~

30 IO'l

.... 48

VERTICAL

MEAN

HORIZONTAL

Fig. 24. Changes found two days after removal of contact lenses (mean values: number of eyes as shown)

TIME : MONTHS 6 12 18 24 30 36 42 48 54 60 MEAN f MEAN +0·39 +0·49 +0·50 +0·49 +0·69 +0·67 +0·70 +0·83 +1·01 +0.86

OF S.D. 0·53 0·66 0·68 0·59 0·68 0·77 0·73 0·72 0·85 0·60 H&Vl No. 92 244 244 240 218 160 163 96 67 22

r MEAN +0·53 +0·61 +0·65 +0·68 +0·97 +0·99 +0·97 +1·21 +1·37 +1·15 H ~ S.D. 0·44 0·66 0·66 0·62 0·65 0·72 0·70 0·58 0·79 0·58

l No. 46 122 122 120 109 80 82 48 34 11

r MEAN +0·25 +0·37 +0·35 +0·30 +0·42 +0·35 +0·42 +0·45 +0·64 +0·57 v ~ S.D. 0·58 0·64 0·66 0·49 0·60 0·67 0·64 0·64 0·75 0·47 L No. 46 122 122 120 109 80 81 48 33 11

N.B. + = changes giving less myopia - - changes giving more myopia

Fig. 25. Changes in front surface corneal power measured two days after contact lens removal as compared with original values

astigmatism of about 0.5D. Also the contact lens wear over four and a half years has obviously caused corneal flattening of about 0.5 D. Strangely, very little change in corneal astigmatism is apparent from these results. The mean and standard deviations are shown in Fig. 27.

By comparison, again, the findings for both refractive and corneal changes in the control group are shown in Fig. 28.

It is interesting to study the findings for the contact lens wearers by comparing results made at the time of contact lens removal with those made two days later. We

108

v 0: 0

-1·00

-0·7&

VERTICAL

l: -0·50 MEAN

~ ~ -o·.Z5 HORIZONTAL.

1 J

-

HOilllONTAL MEA"' 'VICII.TIGI\L

I:

~ +0·1~ ~

CHANG-E IN Fa~»JT 5oa.FACE. C..O.U.EAL fbwe~

(n' .. l·37b)

L-~,_---,12~---:,:o::,:----:l::r4:----:30~-~3;o;&---:c-----:4r.;8----:$~4;---;o.,oc:::- Time in monri.11

30 liS 119 "" Num!Mr of~~~~

Fig. 26. Changes found at the time of removal of contact lenses (mean values: number of eyes as shown)

SPECTACLE REFRACTION TIME :MONTHS 18-24 30-36 42--48 54--60 MEAN ( MEAN -0·40 -0·25 -0·30 -0·41

OF ~ S.D. 0·99 1·01 1·18 1·23 H & V L No. 56 226 238 122

'MEAN -0·45 -0·13 -0·12 -0·23 H ~ S.D. 0·84 0·99 1-13 1-36

L No. 28 113 119 61

r MEAN -0·36 -0·37 -0·47 -0·59 v ~ S.D. 1·12 1·02 1·20 1·06

L No. 28 113 119 61

N.B. + changes giving less myopia changes giving more myopia

FRONT SURFACE CORNEAL POWER 18-24 30-36 42--48 54-60 -0·11 +0·22 +0·30 +0·45

0·47 0·73 0·66 0·63 60 230 238 120

-0·16 +0·19 +0·28 +0·42 0·43 0·73 0·61 0·58

30 115 119 60

-0·08 +0·26 +0·32 +0·49 0·51 0·72 0·70 0·67

30 115 119 60

Fig. 27. Changes found at the time of removal of contact lenses as compared with original values

can see from the slopes of the graphs in Fig. 29 that the front surface corneal power change and the refractive change diverge slightly. This divergence of the two slopes is an indication that axial elongation of the eye or else steepening of the crystalline lens is taking place. As expected, both pairs of slopes diverge at a similar rate.

Now if the contact lens wearers (measured at the time of removal of lenses) are compared with the spectacle wearers, we can see that after four and a half to five years, there is a 2o gap between the slopes for refraction and for corneal power change in the spectacle wearers, whereas there is only a 1 o gap between the two slopes for the contact lens wearers as shown in Fig. 30. This suggests that axial elongation or crystalline lens power increase is proceeding faster in the spectacle wearers than in the contact lens wearers. A preliminary statistical analysis suggests that this result is significant.

109

-2·00

-1·75

-1·50

-1·25

(KANGE IN

( YCLOPLE(;IC

SPECTACLE

REFRACTION

VERTICAL

MEAN

HORIZONTAL

t -1·00

" a: 0 - ATT\il!: TIM.£: oF- Ftel1ovfH ..

OF C.ON1l'IC-,. L.Jtfii5E 5

M&:AN OF Plt!NCIPfH. M~RIIllft~S

~-0· C.HI\NG:~IN

Ill ot 0

Sf>!!:.C.TIICL.E REfjllfiC.TlON

I: -025

i l \

\ 'il~--

.... ..... ..... ""'-------, ' ' ,'- _____ _

/ /

............... .....

' .....

/ (Hf'INI!I£ IN (~"r~L.£IRN'!>

12 18 30 31. 48

Fig. 29. Changes in the experimental group of contact lens wearers

A one-sided test was applied, the hypothesis being that the contact lens wearers have a refraction and a corneal power which is less myopic than the control group (a less myopic cornea meaning a less powerful cornea, as might result if a contact lens led to corneal flattening).

The results obtained are as shown in the following tables:

Changes in spectacle refraction Follow-up periods in months 18-24 30-36 42-48 54-60

{ Contact XI -0.40 -0.25 -0.30 -0.4I lens si 0.99 l.OI l.l8 1.23 wearers ni 56 226 238 I22

{ Inte

-.2·0

- I·?S

-I·SD

____ Chongn found ATTtti:TIMii Of «EMOVAL OF CONTACT LEN'!.ES

(M~on ot Pr&neiptal Meridians)

___ C.honge.~ .found \n the. C.ONT"-0\. "O\IP (Mun of Pr.nclfOtFAU< CORNii:AL fOWl!~

FRONT 't>UR.FIIC~ C.OjtNe#'L P 01.410A.

Fig. 30. Differences between contact lens wearers and spectacle wearers followed up for five years

Changes in front surface corneal power Follow-up periods in months 18-24 30-36 42-48 54-60

1 Contact XI -0.11 * 0.22 0.30 0.45 lens SI 0.47 0.73 0.66 0.63 wearers ni 60 230 238 120

{ Inte'PolatOO x2 0.05 0.06 0.10 0.09 values for s2 0.25 0.25 0.28 0.28 control group n2 102 70 40 34

s2 0.12 0.42 0.39 0.33 t -2.84* 1.81 1.87 3.23

--n1 + n2 - 2 160 298 276 152

Significant at 5% level (1.64) No* Yes Yes Yes

Significant at I% level (2.33) No* No No Yes

• The 18-24 month result is anomalous, because the corneal power of the contact lens wearing group became more powerful, or more myopic, than that of the control group.

112

0 Col 0 .a> 0

I

Ill 0

I I I I I I I I I I I U1 0 ~ J;; ~ r-

..., 0

» 0

0 0

I

Fig. 31. Cumulative frequency curves ~howing rates of change per year in myopia in the control group and contact lens group

Here, the fact that the difference between the two groups only becomes significant at the 1 per cent level after a follow-up period of 54-60 months shows that the differ-ence between the two groups, as far as change in front surface corneal power is con-cerned, is hardly conclusive. The mechanism of front surface corneal power change being the prime cause of change in spectacle refraction has therefore not been significantly demonstrated. Therefore we must look elsewhere than the front surface of the cornea to explain the reduction in the rate of progress of myopia in the contact lens wearers as compared to the spectacle wearers.

113

(The above two tables were compiled using:

and

t=

(n1 - 1) si + (n2 - 1) s~ n1 + n2 - 2

where s1 and s2 were computed with n -1).

Conclusions Preliminary statistical results on this incomplete project show that corneal lenses definitely slow down the rate of progress of myopia slightly, but that this is partly due to flattening of the cornea. As it is not entirely due to the corneal flattening it is suggested that there is some retarding effect on axial elongation, but the mechanism for this is not known and requires further study.

Acknowledgements I thank the General Optical Council, the British Optical Association, the Worshipful Company of Spectacle Makers and the Max Wiseman Memorial Fund for their financial support for this project. I am also indebted to the subjects themselves and to many of the staff and my colleagues at the London Refraction Hospital for their help and encouragement during the last five years.

References 1 BALDWIN, W. R., WFST, D., JoLLEY, J. and REID, W. (1969), "Effects of contact lenses on

refractive, corneal and axial length changes in young myopes", Am. J. Optom., 46,903-11. 2 DICKINSON, F. (1957), "The value of microlenses in progressive myopia", Optician, 133, 263-4. 3 GORDON, S. (1965), "Contact lens hydration: a study of the wetting-drying cycle", Optom.

Wkly., 56, 55-62. 4 I