Evaluation of contact lenses for central

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Brit. j. phthal. (I 974) 58, I 41 Evaluation of ntact lenses for central vision in high myopia GERALD FDA ew Jersey, U.S.A. ntact lenses do not significantly improve central vision as mpared spectacle lenses.

1 Brit. j. phthal. (I 974) 58, I 41 Evaluation of ntact lenses for central vision in high myopia GERALD FDA ew Jersey, U.S.A. ntact lenses do not significantly improve central vision as mpared spectacle lenses. I have never rehabilitated a highly myopic patient with subnormal vision by prescribing ntact lenses as an optical aid, nor have I ever seen a patient so rehabilitated. My clinical experience nfirms theoretical calculations that the effective magnification is not greater than 27 per cent. for a spectacle rrection of -2 D. This study presents data supporting this statement based upon my own clinical tests. I suspect that what are believed be dramatic improvements made by ntact lenses can in many cases be attributed an inrrect spectacle rrection or inaccurate vision testing. In I958, I was enthusiastic about the use of ntact lenses for improving the vision of patients with subnormal vision associated with high myopia, because at that time I believed that there was a 25 per cent. increase in magnification for every Io D of myopia with the spectacle rrection. I became disenchanted by my clinical experience since I uld never obtain this degree of improvement. Later, a better understanding of the theory nvinced me that I should not expect so great a degree of magnification. The graph published by Boeder (I938) showing a magnification of 46 per cent. for a -2 D myope has been misinterpreted refer the spectacle rrection. At a vertex distance of 12 mm. from the rnea the spectacle rrection rresponding -2 D on the rnea is D, and it is this that the 46 per cent. magnification refers. Boeder's figures do not represent a mparison between ntact lenses and spectacle lenses but give the retinal image size in the myope rrected with a ntact lens as mpared with that of the standard emmetropic eye. Theory of lens rrection of ametropia and magnification A lens must form an image of the distant object at the far point of the eye in order rrect the ametropia. For any given eye the size of the retinal image is directly proportional the size of the image formed at the far point of the eye. This discussion will be limited the rrection of myopia because it is our specific interest. Fig. I shows that the send principal focus (F2) of the lens must incide with the far point (FP) of the eye in order form the image of the distant object at the far point. The image at the far point determines the size of the retinal image (RI). The power of the lens which is needed make the send principal focus incide with the far point of the eye will depend upon the degree of myopia of the eye and the distance between the lens and the rnea. Fig. 2 represents the effect of the difference in distance of the rrecting lenses from the rnea, i.e. the vertex distance, which produces the magnification. Received for publication December 5, 1972 Address for reprints: G. Fonda, M.D., 551 Millburn Avenue, Short Hills, ew Jersey 778, U.S.A. Paper presented at a meeting of the ntact Lens Association of phthalmology at Las Vegas, evada, on September 19, 1971 Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

2 142 Gerald Fonda RI - Retinal imoge FP-For point of eye FIG. i Lens rrection of myopia 6(o- Focal distance of ntact lens fz(spj - Focal distance of spectacle lens Ici -Image of ntact lens Isp -Image of spectacle lens M 62 1=.24 24% FIG. 2 F2 ~ ~ f(c) (SP) m m 12 62mm Magnification of ntact lens in mparison with spectacle lens Fig. 2 illustrates magnification at the far point (FP) of the eye produced by mparing the size of the image formed by thie ntact lens (Icl) the image formed by the spectacle lens (Isp). f the two images, the larger is formed by the weaker ntact lens (Icl) with the longer focal length and the smaller by the stronger spectacle lens (Isp) with the shorter focal length. From this we may nclude that the size of the retinal image is directly proportional the focal lengths and indirectly the power of the rrecting lenses in dioptres. Since the method of calculating the degree of magnification is based upon this illustration, an example is presented. GIVE Spectacle rrection Vertex distance SLVE FR: -2 D 12 mm. (1) Focal length of -2 D spectacle lens =.2~ = 5 mm. 2 (2) Magnification =f2 (cl) _62 =_-4pe et f2(sp) - 54peet (3) Power of ntact lens 1, ~ D Fig. 3 (opposite) summarizes the magnifications calculated by the above method for spectacle rrections ranging from D at a vertex distance of i12 mm. Fig. 3 shows that the retinal image is increased (magnified) in myopia but decreased (diminished) in hyperopia. ote that the magnification for ntact lenses in mparison Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

$iz 2 t o1 ntact lenses for central vision in high myopia -lo CZI\ o-1 X-2. -4-35 -3-25 -2-15 -1-5 5 t1.*1.2o SPECTACLE CRRECTI Magnification produced FIG.$

3 iz 2 t o1 ntact lenses for central vision in high myopia -lo CZI\ o-1 X t1.*1.2o SPECTACLE CRRECTI Magnification produced FIG. 3 by ntact lenses in mparison with spectacle lenses a -2 D spectacle rrection is 24 per cent. and for a - io D spectacle lens 1I2 per cent. An additional 3 per cent. magnification is produced when the form and shape of lenses are nsidered. The data were calculated byj. L. Francis and published by Bennett (I963). The assumption that the magnification referred the spectacle rrection rather than that on the rneal surface has left the belief that a magnification of 5 per cent. is produced by a -2 D spectacle lens as ordinarily undersod. Boeder (1938) intended the magnification of 5 per cent. refer a ntact lens rrection, which is equivalent a spectacle rrection of D at a vertex distance of I2 mm. The relation of the degree of effective magnification the Snellen test chart is shown in Fig. 4 (overleaf). Each line of letters on the charts using Sloan letters differs in steps of 26 per cent. A magnification of 26 per cent. enables the patient read the next smaller line which is the degree of improvement made by fitting a patient wearing a spectacle rrection of -2 D with a ntact lens. This amounts an improvement of vision from 6/18 6/I5 (2/6 2/5).* Clinically, I have rarely ever made any greater improvement than this. The diagnosis of seven patients in the Table (overleaf) was pathological myopia, in six ngenital myopia, and in seven myopia sendary the primary pathology such as retrolental fibroplasia. I differentiated ngenital from pathological myopia when the patient manifested nystagmus. The significant data as far as this presentation is ncerned are the visual acuities with spectacles and ntact lenses. The visual improvement was frequently less than 26 per cent. even in very strong rrections. This Table shows that the clinical visual improvement was less than that calculated in most cases. For testing vision the Sloan letters were usually used. It is the best chart, because it includes 6/48 (2/I6), 6/37.5 (2/I25) and two lines for 6/3 (2/ioo) and 6/24 (2/8). These letters are about as equal in legibility as can be obtained. The rresponding spectacle and ntact lens rrections agree reasonably well. However, the first case was either overrrected with spectacles or wore the spectacle rrection at a vertex distance of I9 mm. Minus 25 D ntact lenses were prescribed first, and these were improved -2 I D by refracting over the first pair. f the eighteen patients who were followed, seven were judged be failures and eleven be successes. The criteria for success was whether the patient ntinued wear the ntact lenses. The principal cause of failure was inability read with the ntact lenses; this was the case in five patients. Highly myopic patients with subnormal vision * The rerded measurements of acuity are based on measurements in feet, and for the nvenience of readers using the metric system the equivalent at 6 metres is also given. o attempt has been made equate the Snellen distance types. 143 Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

4 144 Gerald Fonda %S _~, ~. v 3~~~~~~ c; ~~~~~~~~~ - L o _ - d _c4 d o ci _ C4I,Z + s e S s +L Z c i4c.( C + c g o ci C C Ci t o 9 t of ~ C cl ) ci 1.4 d It II II II I II L t LL o: -d d- o CC t C o.l i 1- c d4 -d '~t'~t~ 99 k tn C4 --~~~ C cci' ~c +~~~~~~~~~~~~~1 tt (~c ci IC CI4Ci (Q I c+ C C ci ci ci CiCi LL oo c i C( i i ) ) t~ LL L, t tl IX 4 CC - - I- C- Ci ' I q Ij tl -Cen C ) ~ tl C It Cf 'E X i cl cl X + o o 4;' CC 4 CC -d Ci CiC CiC ic PI n q iiii I I II II II I I P. $L U~~~~~~~~~~~1- - U U U U U.1 '4-4 -~~~~~~~.. 4 g:i a I tte) L t t 1 L t( (r- 1r- ~~~o *~~~~b *o *o bb Cb -4 -C4-- -v- t " " ".~~~~~ ~ ~4-1-1 ~ bo~~~~~c~~ X C P.. k~~~~~~ ~~~~ ~ ) L* - ( Ci L (.- ) 4 Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

5 ( I 1 44 ( ) ) 4 -._'. G; v U (A U) 4.) 4.),.,.I.) (12 C1 ) (1 44i C'l Csl C 4 C4 - C _4 - bi) ci 1^ 1 44 L le * o ( C ") 44 )) ( 4-". - C. S a ( o - 44i _ ~o oc Lo C "4 l 4 ( Lo L2 *-4 1 V C. - CC t-~ in - m) o - 44 L 2 L L t- 4-J Cd ntact lensesfor central vision in high myopia V. C. Ci ( C1-4 4 ;o + so 1.YC (a.d c) o; (1 ~o 4 ( (I ce &4- P. = A, Z-4 -o Z4-,. V Vz.o L a- 2 Ut Cd o. so. (a Ci) so C4) I r Y IIC-) $. >1 r- Is( 4d ( o IL) Su V: 12 + CA V: C. 4 $s WZ (in ( o. 44 _1 I ++I._ ce) -o 5 C. a L '45 + Cl b, C. t... 1 c. I + l LtLe) tt c ci l o o _- - i- * 5 I C 4 Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

6 146 Gerald Fonda FIG * I J VISUAL ACUITY CHART FR 2 FEET LAM tittles x 2 _ ~~~3 R 2 K H S T Snellen test chart with Sloan letters 2 D C VISUAL ALUITY CHART fr 2 FEET SI&A Ilits V K ZS H V C D R K C S Z H V D S D K H H K D V R H V R H Z H R R C Z H C V R Z C Z R D S V C R H Z C S D C S K C Z S V D K o V S D I are fortunate because they have a built-in magnifier by just taking their glasses off read at a distance of their far point or a little closer. For distance, the ntact lenses were better lerated, and only two patients abandoned their use: one because of glare on lights and the other because his vision was not improved. The strong minus ntact lenses proved be as mfortable as weaker ntact lenses. The higher the myopia the greater is the smetic advantage of ntact lenses. The subjective improvement of more natural or better peripheral vision was not impressive. My experience leads me nclude that the principal advantage of ntact lenses in myopic patients with subnormal vision is essentially smetic. Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

7 Summary and nclusions ntact lenses for central vision in high myopia The magnification of ntact lenses in mparison spectacle lenses is only half the amount which is the accepted belief. The magnification is 27 per cent. rather than 5 per cent. for a ntact lens in mparison a-2 D spectacle rrection fitted at a vertex distance of 12 mm. This is a magnification equivalent improving vision from 6/I 5 (2/5) 6/12 (2/4) on the Snellen chart. I have never rehabilitated a patient with subnormal vision associated with high myopia by prescribing ntact lenses, nor have I ever seen one rehabilitated by anyone else. The theory of lens rrection of ametropia and magnification is presented. A graph illustrates the percentage of magnification by ntact lenses in mparison spectacle lenses. The data of twenty cases of high myopia fitted with ntact lenses are summarized. The clinically determined visual acuities with ntact lenses in mparison spectacle lenses are less than those calculated in many cases. The calculated and clinically determined magnification nvinces me that ntact lenses do not make a significant improvement in vision. The advantage of ntact lenses for high myopia is essentially smetic. f the eighteen cases evaluated, eleven were judged be successful and seven be failures. Five of the failures were attributed inadequate near vision, because the patient uld no longer read at his unrrected near point, which is easily done by lifting up his spectacles. I wish acknowledge the expert ntributions of Dr. Donald Fonda. References BEETT, A. G. (I963) "ptics of ntact Lenses", pp. 6o, 6i, 66. Association of Dispensing pticians, London BEDER, P. (1938) Arch. phthal. (Chicago), 19, 54 I147 Br J phthalmol: first published as /bjo on 1 February Downloaded from on 24 September 218 by guest. Protected by

Tips for selecting a useful stand magnifier

Tips for selecting a useful stand magnifier William L. Brown, O.D., Ph.D., F.A.A.O. Mayo Clinic Rochester, MN Ph: (507)284-4946 brown.william2@mayo.edu Learning Objectives Following the lecture the participant