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Reading Cards for Selection of Optical Aids for the Partially Sighted*
CONGENITAL MYOPIA
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REFERENCES
1. Klien, B. A.: Dysgenesis mesodermalis der Hornhaut und Iris, mit gleichzeitigem Vorkommen von neuroekodermalen Entwicklungstörungen. Arch. f. Ophth., 164:12, 1961. 2. Axenfeld, T.: Embryontoxon corneae posterius. Ber. deutsch ophth. Gesell., 42:301, 1920. 3. Burian, H. M.: Chamber angle studies in developmental glaucoma. Missouri State M. A., 55:1088, 1958. 4. Von Hippel, E. : Ueber die angeborenen zentralen Defekte der Hornhauthinterfläche. Arch. f. Ophth., 95:184, 1918. 5. Burian, H. M., v. Noorden, G., and Ponset, J. V.: Chamber angle anomalies in systemic connective tissue disorders. AMA Arch. Ophth., 64:671, 1960. 6. Badke, G. : Kritische Auswertung der modernen Theorien der Myopiegenese und der Versuch einer einheitlichen Betrachtung. Arch. f. Ophth., 153:231, 1952. 7. Collier, M. : La dysplasie marginale postérieure de la cornée dans le cadre des anomalies squelettiques et ectodermiques. Ann. ocul., 195 :512, 1962. 8. Klien, B. A.: The ciliary margin of the dilator muscle of the pupil. Arch. Ophth., 15:958, 1936. 9. Friedman, A. G., and Epstein, E.: Familial retinal detachment. Am. J. Ophth., 61:33, 1961. 10. Jaeger, W. : Beitrag zur Ätiologie der Oraabrisse im temporalen unteren Quadranten. Klin. Monatsbl. Augenh., 126:423, 1955. 11. Cameron, M.: Nontraumatic dyalysis in the young. Brit. J. Ophth., 44:541, 1960. 12. Huerkamp, B.: Häufigkeit und ätiologische Bedeutung des Hornhautastigmatismus bei Ablatio retinae. Arch. f. Ophth., 175:592, 1956. 13. Franceschetti, A., and Carones, A. V.: A case of familial keratoconus with disseminated neurodermatitis, cataract and ectodermal dysplasia. G. ital. of tal., 13:143, 1960. 14. Perkins, E. S.: Open angle glaucoma in young patients. AMA Arch. Ophth., 64:882, 1960. 15. Franceschetti, A., and Linder, A.: Osservazioni sul lavoro di Santino "Esiste efïetivamente una correlazione tra cheratocono e retinite pigmentosa" G. ital. of tal., 13:3, 1960. 16. Cullin, J. F., and Butler, H. G.: Mongolism and its ocular manifestations with special reference to keratoconus. Wilmer Residents Meeting, 1962. (Quoted by Zimmerman, Ophth. Path. Club, 1962.) 17. Mier, M., Schwartz, S. O., and Boshes, B.: Acanthrocytosis, pigmentary degeneration of the retina and ataxic neuropathy: A genetically determined syndrome with associated metabolic disorder. Blood, 16:1586, 1960.
READING CARDS FOR SELECTION O F OPTICAL AIDS FOR T H E PARTIALLY SIGHTED* LOUISE L.
SLOAN, P H . D . , AND DARLENE J.
BROWN
Baltimore, Maryland INTRODUCTION Reading aids for the patient with subnormal vision cover a wide range of dioptric powers, ranging from a conventional reading addition of 3.0 to 4.0 diopters to an aspheric or compound lens whose equivalent power may be 50 diopters or more. In selecting the approximate power needed by each patient to achieve useful reading vision it is desirable to have some better method than a mere trial of lenses of gradually increasing 1 power
T h e purposes of this paper are ( a ) to review the procedures used by others to determine the required power of the reading aid, ( b ) t 0 describe a set of reading cards especiall y devised to meet this need, (c) to present data showing that the size of print rec uired for useful l reading cannot be predicted accurately from a conventional measure of acuitv ' ( d ) t 0 analyze the causes for the discrepancies between acuities measured on a Snellen chart and reading ability measured with continuous text, ( e ) to illustrate the use of the reading cards in selection of a suitable
*From the Wilmer Ophthalmological Institute of The Johns Hopkins Medical School and Hos pital. This research was supported by Grant B-810 from the National Institute of Neurological Diseases and Blindness, Public Health Service, Bethesda, Maryland.
reading aid. M E T H O D S USED BY OTHERS TO SELECT DIOPTRIC POWER OF READING AID . , . - , , . , . Several workers in this field utilize con-
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LOUISE L. SLOAN AND DARLENE J. BROWN
ventional measures of distant acuity for capi tal letters as the basis for selecting the dioptic power of the optical aid needed for reading. Kestenbaum1 was probably the first to make systematic use of such data. His basic formula states that the dioptric power of the addition required for reading ordinary newsprint (JS) is numerically equal to the reciprocal of the visual acuity (that is, to the threshold visual angle). If the reading ma terial differs significantly in size from J5 the dioptric values are multiplied by an appropri ate factor. Colenbrander's formula,2 when reduced to its simplest form, states that the power of the reading addition in diopters should equal the threshold visual angle plus 1.50 diopters. This minor modification of Kestenbaum's formula is of significance only in the case of low-power reading aids. Zettel3 published a table giving, for each level of acuity measured at 20 feet, "calcu lated" and "suggested" reading additions. The calculated dioptric values, like Kesten baum's, are directly proportional to the threshold visual angle, but are 25-percent higher. His suggested reading additions, how ever, are significantly less than his calculated values in the higher powers. For example, a reading addition of 12D. to 16D. is suggested when the calculated power is 25D. ; 32D. is suggested when the calculated power is 50D. These reductions are probably explained by his statement that the "suggested addition"
refers to the minimum "which should be given to the patient at the beginning of his care." According to Gettes,4 Volk recommends reading aids which are twice the power of those computed from the Kestenbaum for mula. Thus, although these investigators all assume a direct relationship between re quired dioptric power and threshold visual angle measured with capital letters at dis tance, they do not agree closely as to the ac tual amounts they prescribe for any given level of vision. For example, for the patient with 20/400 vision (threshold visual angle 20 minutes), Kestenbaum would prescribe 20D., Colenbrander 21.5D., Zettel 12 to 16D., Volk 40D. Visual acuity measurements can also be made at any standard near distance instead of at 5, 10 or 20 feet, if the patient is pro vided with a reading addition when it is needed to obtain a clearly focussed retinal image. The well-known Lebensohn chart, first described in 1936,5 is an example of such a test. It is designed for use at 35 cm. and requires therefore a +2.75D. reading addition, unless the patient supplies some or all of this power by accommodation. In addi tion to capital letters it provides numbers, illiterate symbols (o's and x's) and isolated words in lower case letters. The largest test letter corresponds to a visual acuity of 20/800. The gradation of sizes correspond to those on a conventional Snellen 20 ft. chart. Other letter charts for measuring acuity at a
TABLE 1 L E T T E R CHARTS FOR MEASURING ACUITY AT NEAR
Lebensohn
Keeler
Stimson
Testing distance
14 in. (2.75D)
25 cm. (4D)
14 in. (2.75D)
40 cm. (2.5D)
Test material
Capital letters, illiterate symbols, isolated words
Capital letters, Landolt rings, iso lated words
Capital letters, iso lated words
Capital letters (special selection of ten)
Equivalent size of largest test characters
5/200
2.9/200
4.5/200
12.5/200
Gradation of sizes
Conventional
Logarithmic
Logarithmic
Logarithmic
Good-Lite (Sloan)
SELECTION OF AIDS FOR PARTIALLY SIGHTED
close distance are those of Keeler,6 Stimson7 and Sloan.8 Pertinent data regarding these are given in Table 1. Some low-vision clinics9'10 use samples of reading material to assist in selecting a suitable reading aid. No fixed testing dis tance is employed. The procedure is a trialand-error one in which, with each increase in power of the reading lens, the patient finds the distance at which the print is in focus, and indicates the smallest print which he can read at this distance. Feinbloom,9 who uses this procedure, says that the power of the reading addition should be increased "as long as there is an improvement in near acuity or facility in reading." With each in crease in power and consequent decrease in reading distance the patient should be able to read smaller print. The process could therefore be continued indefinitely to permit reading of any desired size of print were it not for the following difficulties associated with the use of strong lenses in a trial frame: 1. With decrease in distance it becomes difficult to illuminate the reading page. 2. Aberrations become noticeable unless the high power lenses used in testing are es pecially designed to minimize such defects. 3. The depth of focus decreases rapidly with increasing lens power. As a conse quence it becomes increasingly difficult to maintain the reading page at the proper dis tance from the lens. The use of successively stronger lenses in a trial frame to select a suitable reading aid takes no account of the fact that the patient might accept a significantly higher dioptric power in some other form of magnifier which provides adequate illumination, re duces aberrations, and maintains the reading page at the correct distance. The difficulties inherent in this procedure can be avoided if, prior to the trial of any actual magnifier, a test of reading vision is made at a fixed dis tance using graded sizes of print that are accurately specified in some convenient unit. Tf continuous text is used, rather than iso lated letters or words, the testing conditions
1189
simulate the actual task confronting the pa tient when he uses an optical aid to read ordinary books, typed material and news papers. The Fink Near-Vision Test 11 is one of the few which provides continuous sentences in quantitatively specified sizes. It is calibrated for use at 14 inches and at this distance measures acuities from 14/14 to 14/98 (20/20 to 20/140). The near vision card devised by Keeney and Duerson,12 though using con tinuous text, is not a close duplication of nor mal reading material because it employs only capital letters. It measures at 14 inches acui ties ranging from 14/14 to 14/89 (20/20 to 20/127). READING CARDS USED IN PRESENT INVESTIGATION
A series of test cards with continuous text, developed by the senior author in 1956, have been described briefly in several pub lications.13-16 These reading cards, devised specifically for prescribing for patients with subnormal vision, provide a direct measure of the magnification required to read print approximating in size ordinary typewritten material and newsprint. The letter form is a reproduction of that used on a standard typewriter. The smallest print is designated 1.0M because the overall dimensions of its lower case letters subtend a visual angle of five minutes at a distance of 1 meter. To read 1.0M print at 40 cm. requires therefore an acuity of 40/100 in Snellen notation. At 33 and 25 cm. the corresponding acuities are 33/100 and 25/100. The complete series of sizes are, in M units, 1.0, 1.5, 2, 2.5, 3, 4, 5, 7, and 10M. Sizes equivalent to 14M and 20M at the standard distance are obtained by viewing the 7M and 10M cards from one-half this distance. Figure 1 shows the nine test cards. Since the 10M print at 20 cm. corresponds to a Snellen acuity of 20/1000 (4/200) the range is ample for testing the lowest level of subnormal vision at which useful reading
LOUISE L. SLOAN AND DARLENE J. BROWN
1190
vision can be attained. T h e sizes from I M to 20M, forming approximately equal steps on a logarithmic scale, were adopted after preliminary experience with more finely graded steps. These reading cards can be used at 40, 35, or 25 cm. or at any other distance that the examiner prefers. A standard distance of 40 cm. has several advantages. 1. T h e resulting Snellen notations, 40/100, 40/150, 40/200, and so forth, can be con verted to the corresponding visual acuities measured at 20 feet, that is, 20/50, 20/75, 20/100, etc., simply by dividing numerator and denominator by 2.* 2. T h e presbyopic patient who can read with ease I M print at a distance of 40 cm. will ordinarily not require more than a con ventional reading addition of 2.50 diopters. * This simple computation gives the Snellen no tation of the letter subtending the same visual angle at 20 feet. The threshold visual angle for recogni tion of isolated capital letters and for easy reading of continuous text are, however, seldom the same. Experimental evidence on this point is given later in this paper.
It ie veryhot in the
This is because I M print happens to be of about the same size as ordinary newsprint, and because most adults prefer a reading distance of 40 cm. or more unless subnormal vision forces them to read at a closer dis tance. 17 3. If ability to read, with ease, print of I M size is accepted as a satisfactory solution of the patient's reading problem, then a measure in M units of the print he reads at 40 cm. gives a direct measure of the re quired dioptric power of the reading addi tion. T h e reading ability in M units times 2.5 diopters is equal to the power of the required reading addition. F o r example, if the patient reads 10M print at 40 cm. he can read I M print at 4.0 cm. and will require a reading addition of 25 diopters. ( I n apply ing this formula a suitable allowance is made for any dioptric power which the patient contributes by accommodation). A previous publication shows how this system can be applied not only to reading additions in spectacle form but to all other types of mag nifier. 18
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Fig. 1 (Sloan and Brown). Cards used to measure reading ability in M units at 40 cm.
SELECTION OF AIDS FOR PARTIALLY SIGHTED
1191
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Fig. 2 (Sloan and Brown). Chart used to provide precise measures of distant acuity for Sloan letters. RELATIONSHIP BETWEEN ACUITY MEASURED WITH CAPITAL LETTERS AND READING ABILITY MEASURED WITH CONTINUOUS TEXT
The degree of relationship between con ventional measures of subnormal acuity and the angular size of print required for easy reading has never before been determined experimentally. One difficulty in obtaining such data is that most Snellen charts do not provide accurate measures of acuities of 20/100 and less because of the small num ber of test characters. In this study a spe cial chart composed of Sloan letters19 was used having five letters on the 20/200 line and 10 on each of the lines of smaller letters (figure 2). To obtain precise measures of acuities be low 20/100, determinations were made of the distances at which the patient could just read three of the five letters on the top (20/200)
line and seven of the 10 letters on the second (20/100) line. In this way, two in dependent measures were obtained, for ex ample, 16/200 and 8/100 if the first line was read at 16 feet, the second at eight feet. All measures of acuity were made with correction of the error of refraction. The final determination of the refractive error was based on thorough subjective testing with special techniques suitable for low levels of visual acuity.3 In verifying the spherical error, tests were made with high plus and minus spheres when lesser powers did not produce any change in acuity. When a preliminary retinoscopic examination is impossible, such tests are necessary to insure that a very high spherical error will not be overlooked. The astigmatic error was meas ured with a cross-cylinder. The standard procedure was modified by the use of a series of O's in graduated sizes instead of
LOUISE L. SLOAN AND DARLENE J. BROWN
1192
letters as test objects, and by the use of crosscylinders of high power ( + 0 . 5 0 and + 1 . 0 0 diopter).
Reading ability was measured at 40 cm. with the series of cards described previously. These were presented on an illuminated
ACUITY CHART FOR NEAR SLOAN LITTERS Thi» chart should ba hald 16 inch« (40cm) from t h · e y « , at right angles to th« lin· of viiion, and illuminated with not l e u than 10 or more than 25 foot canai·· of light.
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Fig. 3 (Sloan and Brown). Chart used to measure near acuity for Sloan letters.
SELECTION OF AIDS FOR PARTIALLY SIGHTED 54 >20
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1193
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Vlaual Acuity for Capital Letter· S l i e of Text In M unit» Subtending Same Visual Angle
Fig. 4 (Sloan and Brown). Frequency diagram showing relationship between conventional measures of visual acuity using capital letters, and reading ability measured with continuous text. stand with a chin rest to maintain the correct distance. T h e refractive correction required at 20 feet was automatically increased by 2.50 diopters in the case of elderly patients. In other cases the reading addition giving maximal acuity at 40 cm. was determined by actual test. ( D u r i n g the later stages of the study the Good-Lite Near Vision Chart composed of Sloan letters 8 (fig. 3) was used in determining the reading addition for 40 c m ) . T h e reading test was started with one of the larger sizes of print. The patient was asked to read aloud successively smaller sizes until there was a noticeable decrease in speed or in accuracy. T h e reading vision in " M units" was taken as the smallest print which could be read with ease. A great deal of information, both qualita tive and quantitative, is provided by this test of reading ability. Most patients show an
abrupt transition from easy rapid reading at one size to difficult slow reading with errors at the next smaller size and complete in ability to read any smaller print. Such pa tients can almost always be given a magnifier which will provide easy reading. A few pa tients read slowly all print above a certain size and never achieve rapid easy reading. This type of response is noted particularly in children and in other unskilled readers. W i t h a suitable magnifier, the patient in this category can usually be given reading ability commensurate with his natural rate of read ing. A few patients show either no improve ment or even a definite decrease in reading ability with increase in size of print beyond a certain level. This type of response occurs for example in patients who are using a central field of vision reduced to a patch only a few degrees in diameter. Unless bet-
1194
LOUISE L. SLOAN AND DARLENE J. BROWN
ter reading ability is achieved with excentric fixation, the prospects of improving the reading vision of such patients by the use of a magnifier are limited. Illustrative cases in this group will be presented later. Figure 4 is a frequency diagram showing for 443 eyes the observed relationships be tween reading ability measured with con tinuous text and acuity measured with capi tal letters. T h e measured values of reading vision in M units are shown on the vertical ordinate. Capital letter acuities are shown on the horizontal ordinate, where the intervals are chosen to agree with the sizes of con tinuous text in M units subtending the same visual angles. If there were perfect agree ment in the threshold visual angles measured in each way, patients having, for example, acuities of 16/100 or better but less than 20/100 should read 2.5M print but not 2M print at 40 cm (16/100 equals 4 0 / 2 5 0 ; 20/100 equals 4 0 / 2 0 0 ) . T h e figures enclosed by the heavy lines show the number of eyes in which there is complete agreement in the angular size of the capital letters read at distance and of the print read with ease at 40 cm. It is obvious to inspection that there is not a close rela tionship between the two sets of measure ments. There are very few instances in which the reading ability is significantly bet ter than would be expected from the acuity measured with capital letters, but many in stances in which reading ability is markedly poorer. It is obvious therefore that if we wish to know the increase in size of print needed to provide easy reading, a conven tional measurement of visual acuity is of little use except in setting a lower limit to the required magnification. R E A S O N S FOR LACK OF AGREEMENT BETWEEN CONVENTIONAL MEASURES OF ACUITY AND MEASURES OF READING ABILITY
Differences in the two types of measure ment are not surprising when we consider the several possible sources of disagreement. One which probably plays an important role in all cases is the higher standard of per
formance that was required in rating the pa tient's reading vision than in measuring the acuity with capital letters. In the latter tests the patient was given credit for ability to read a certain size of letter if seven of 10 were identified correctly, even though they were read very slowly and in the wrong order. This corresponds to conventional methods of rating visual acuity. In the tests of reading ability, on the other hand, the pa tient's score in M units was determined by a more realistic standard which required rapid and essentially correct reading of the text. T h e smaller separation of the letters on the reading cards than on the capital letter charts undoubtedly accounts in some cases for a significantly higher threshold visual angle for the former test material. The im portance of letter separation as a variable in measurements of subnormal acuity has been the subject of several recent studies. "Separation difficulty" has for example been demonstrated in the impaired vision associ ated with strabismic amblyopia. 20-22 A de crease in reading ability with decrease in letter spacing has also been demonstrated in impaired vision caused by uncorrected error of refraction. 23 The findings of the present study indicate that when subnormal vision is the result of ocular disease there may be a similar increase in the threshold visual angle with decreased separation of the test letters. The data summarized in Figure 4 show a number of instances in which a relatively good acuity on the letter chart was associ ated with a very low level of reading vision. I n several of these cases it was possible to obtain reliable and reproducible data on the location, extent, and density of the scotomas responsible for the low vision.* These find* The central fields were examined at 33 cm. on the Ferree-Rand Perimeter by means of a gray tangent screen equipped with a pericentral target to insure foveal fixation. This target consists of a white circle five degrees in radius combined with four radial lines which extend inward from the circle to within one degree of its center. The pa tient is instructed to fixate the center of the circle, that is, the point at which the four radial lines would meet if extended inward.
SELECTION OF AIDS FOR PARTIALLY SIGHTED
Case A.W. Test object 3 / 3 3 0 White
Case N. DTest object
1/330
White
Case F. 0· Test object
Case D. CTest object
1195
3/330White
6 / 3 3 0 White
Fig. 5 (Sloan and Brown). Central visual fields of four patients showing marked discrepancies in acuity for isolated capital letters and for continuous text.
ings indicated that unusual features of the central visual fields, such as are illustrated in Figure 5, account for the marked dis crepancies in the two measures of acuity. CASE REPORTS CASE A. W.
A patient with optic atrophy had a cecocentral scotoma for a 3/330 white test object with a tiny oval area bordering on the fovea in which this test object could be seen (fig. 5-A). This patient could distinguish between foveal fixation using this spared area and fixation with a paracentral area outside the scotoma, and could shift from one to the other at will. On the letter chart she could
read the 20/70 line with foveal but not with excentric fixation. On the reading cards, on the other hand, she had difficulty in reading the 10M print with foveal fixation although with excentric fixa tion she could read 7M print with ease. CASE F. O.
A patient with macular degeneration had a some what similar field defect consisting of a central scotoma for a 3/330 white test object with a small spared area at the fovea (fig. 5-B). With foveal fixation his letter acuity was 20/50. At 40 cm. he was able, using foveal fixation, to read with diffi culty print ranging in size from 1.5M to 4M. Be cause of the small extent of the spared central field 5M print was read less well than smaller sizes. When the patient was instructed to use excentric
LOUISE L. SLOAN AND DARLENE J. BROWN
1196
fixation he was able, after a little practice, to read 7M print rapidly. He was therefore given a mag nifier of strength appropriate for this level of reading vision. A few weeks later he reported that reading with parafoveal fixation was far easier than with a weaker magnifier which required foveal fixation. CASE N.
D.
A patient, with diabetic retinopathy, had a visual field for 1/330 white reduced to a small foveal patch about four degrees in diameter and a second larger patch in the nasal field (fig. S-C). On the letter chart he was able to read the 20/40 line. In spite of this relatively good acuity, reading was very difficult with foveal fixation because he could see only a few letters at a time. Increase in the size of the print made matters worse rather than better. When, however, he viewed the print excentrically, using the larger patch of functioning field, he was able to read 10M print rapidly and without error. CASE D. C.
A patient with hereditary macular degeneration was found to have two large scotomas for a 6/330 white test object, separated by a narrow horizontal strip which appeared to include the fovea (fig. 5-D). In this strip he was able to see a 1/330 white but not a 6/330 red or blue test object. His visual acuity, tested with capital letter charts, was 20/30 at 20 feet, 40/80 at 40 cm. In spite of this relatively high acuity for capital letters at both distance and near, 7M print was the smallest that could be read with ease at 40 cm. It seems likely that this patient also used central fixation when reading the capital letter charts, but shifted to a paracentral fixation in order to obtain a wider field when reading continuous text. T h e most marked discrepancy noted in the two measures of visual acuity was observed in a patient who could read 20/30 letters when they were shown one at a time. W h e n an entire line of letters was shown, the re sponses were so variable that it was impos sible to arrive at any definite rating of acuity. On the reading cards no size of print, either large or small, enabled this patient to do more than recognize an occasional letter. There was a large central scotoma for a 3/330 white test object. Within this scotoma it was possible to demonstrate at times a single point at which the white test object could be seen. A supplementary test meas uring the largest letter which could be seen as a whole suggests that the functioning area
at the fovea was not more than about 30 minutes in diameter. R E L A T I O N S H I P BETWEEN MEASURES OF READING ABILITY AND POWER OF READING AID ACTUALLY PRESCRIBED
Because patients with subnormal vision do not show a close relationship between the threshold visual angle for isolated capital letters and for continuous text, conventional tests of acuity are of little use in predicting the magnification the patient needs to read with reasonable speed and accuracy. T h e reading cards, on the other hand, duplicate closely the enlarged image of the reading material provided by a simple magnifier. They should therefore provide a valid meas ure of the magnification required for easy reading. In a later paper we plan to present data to demonstrate how these reading cards assist in prescribing magnifiers of various types for all ages and for various visual tasks. In this paper we will illustrate the useful ness of the reading cards in prescribing for a group of 100 patients selected in accord ance with the following criteria: 1. Those under 45 years of age were ex cluded unless aphakic. Because of this re striction the accommodation which the pa tient can contribute need not be considered in determing the required dioptric power of the reading aid. 2. Patients were excluded if their visual requirements deviated significantly from ability to read ordinary books and news papers. Those interested only in reading large headlines for example were not in cluded because the relationship between reading ability measured in " M units" and the required dioptric power differs in these atypical cases. 3. Only those patients were included who were given headborne reading aids. F o r this type of magnifier the dioptric power, which must be added to the correction for distance should be, for the complete presbyope, di-
SELECTION OF AIDS FOR PARTIALLY SIGHTED 16
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Reeding A b i l i t y in M Units Fig. 6 (Sloan and Brown). Frequency diagram showing relationship between size of print in M units read at 40 cm and M rating of prescribed reading addition. rectly proportional to the reading ability in M units. T h e headborne reading aids comprised conventional spectacles, either bifocal or unifocal (single vision) ; Continental, Keeler and Policoff bifocals ; Feinbloom, Hyperocular and Keeler unifocal lenses ; and loupes of various types (Aloe, Ary, Bausch & Lomb, Behr, Selsi). T h e true dioptric powers of most of these headborne mag nifiers have been listed in a previous pub lication. 18 Table 2 shows the expected relationship between reading ability, measured in M units at 40 cm., and the range of dioptric powers of the reading addition required to read I M print. By reference to this table any
headborne magnifier whose dioptric power is known can be classified according to the TABLE 2 CLASSIFICATION OF READING ADDITIONS ACCORDING TO PREDICTED LEVEL OF READING ABILITY IN M UNITS FOR WHICH EACH IS SUITED
Dioptric Power*
M Rating
3.00 or less 3.25 to 4.75 5.00 to 6.00 6.25 to 7.25 7.50 to 9.75 10.00 to 12.00 12.50 to 17.00 17.50 to 24.50 25.00 to 34.00 35.00 to 49.00 50 or greater
1.0 1.5 2.0 2.5 3.0 4.0 5.0 7.0 10.0 14.0 20.0
* When the true dioptric power differs significantly from the vertex power the former is used.
1198
LOUISE L. SLOAN AND DARLENE J. BROWN
level of reading ability in M units for which it is suited.* Figure 6 is a frequency diagram showing the relationship between size of print in M units read with ease at 40 cm. and the M rating of the prescribed reading addition (defined as indicated in Table 2 ) . In 57 of the 100 cases (figures enclosed by heavy lines) there was perfect agreement in the M rating of reading ability and the correspond ing M rating of the prescribed reading aid. In 20 cases the reading aid was slightly stronger, in 23 cases slightly weaker, than would be expected on the basis of the pa tient's reading ability. T h e close agreement between the two measures shows that a de termination of the reading ability in M units at a standard distance provides a valuable indication of the required dioptric power of the reading aid. S U M M A R Y AND CONCLUSIONS
In 443 eyes with subnormal vision result ing from ocular disease or anomaly, com parisons were made of the threshold visual angle for capital letters on a Snellen chart
and that for reading continuous text at a useful level of speed and accuracy. T h e minimum visual angle required for easy reading was almost always equal to or greater than that for reading capital letters. At any given level of acuity for capital letters a wide range of reading abilities was observed in different patients. Generally ap plicable reasons for the discrepancies in the two types of measurement include ( 1 ) the higher standard of performance required for reading continuous text and ( 2 ) the smaller separation of the individual letters of the reading material. In some few patients with unusual defects in the central fields the use of foveal fixation to identify the capital letters, with a shift to parafoveal fixation to secure a wider field for reading, was associ ated with a markedly higher threshold visual angle for reading. Measures of reading abil ity for continuous text, using standard test cards at a standard near distance, were shown to provide a reliable and accurate means of determining in advance the diop tric power required in a headborne magnifier for reading ordinary books and newspapers.
Johns Hopkins Hospital * The values listed in the table are based on the assumption that the patient's power of accommoda tion is negligible. It should be possible to prescribe a weaker magnifier when the patient can contribute a significant amount of accommodation.
(5).
The authors will be glad to supply on request laminated plastic reading cards as shown in Fig ure 1.
REFERENCES
1. Kestenbaum, A., and Sturman, R. M.: Reading glasses for patients with very poor vision. A.M.A. Arch. Ophth., 56:451-469, 1956. 2. Colenbrander, M. C : Optical aids in nonoptical abnormalities of the eye. Ophthalmologica, 134:359363, 1957. 3. Zettel, J.: The use of high-add bifocals in cases of low visual acuity. Am. J. Optom., 36:237-244, 1959. 4. Gettes, B. C. : Optical aids for low vision. Sight Saving Rev., 28:81-83 (Summer) 1958. 5. Lebensohn, J. E. : Scientific and practical considerations involved in the near-vision test with presen tation of a practical and informative near-vision chart. Am. J. Ophth., 19:110-117, 1936. 6. Manufactured by Keeler Optical Products, Inc., 5536 Baltimore Ave., Philadelphia 43, Pa., 39 Wigmore St., London Wl, England. 7. Stimson, R. L. : Optical aids for low acuity. Los Angeles, Braille Institute of America, 1957. 8. Sloan, L. L. : New test charts for the measurement of visual acuity at far and near distances. Am. J. Ophth., 48:807-813 (Dec.) 1959. (These charts are manufactured by Good-Lite Mfg. Co., 7638 W. Madison St., Forest Park, 111. ) 9. Feinbloom, W. : Outline of the technique of examination of the partially blind patient with the clear image lens. Bull. Optometric Soc. City N.Y., 1953, pp. 3-8. 10. I.H.B. Optical Aids Service Survey: First 500 Cases. The Industrial Home for the Blind, Brooklyn, 1957.
SELECTION OF AIDS FOR PARTIALLY SIGHTED
1199
11. Manufactured by Benson Optical Co., Minneapolis, Minn. 12. Keeney, A. EL, and Duerson: Collated Near-Vision Test Card. Am. J. Ophth., 46:592-594, 1958. 13. Sloan, L. L., and Habel, A.: Reading aids for the partially blind. Am. J. Ophth., 42:863-872, 1956. 14. Tillett, C. W.: Visual aids in office practice. Am. J. Ophth., 46:186-194, 1958. 15. Kasik, M : Evaluation of the A.O. projection magnifier and its use with partially sighted children. Optometric Weekly, 49:1145-1148, 1958. 16. Bier, N. : Correction of Subnormal Vision. London, Butterworth & Co., 1960, pp. 148-153. 17. Cannon, R. L.: A survey of the reading and working distance of presbyopes. 111. M. J., 115:4-6 (Jan.) 1959. 18. Sloan, L. L., and Jablonski, M. D. : Reading aids for the partially blind : Classification and meas urement of more than two hundred devices. AMA Arch. Ophth., 62:465-484, 1959. 19. Sloan, L. L., Rowland, W. M., and Altman, A.: Comparison of three types of test target for the measurement of visual acuity. Quart. Rev. Ophth., 8 :4-16, 1952. 20. Müller, P.: Ueber das Sehen der Amblyopen. Ophthalmologica, 121:143-149, 1951. 21. Stuart, J. A., and Burian, H. M.: A study of separation difficulty: Its relationship to visual acuity in normal and amblyopic eyes. Am. J. Ophth., 53:471-477 (Mar.) 1962. 22. Flom, M. C, Weymouth, F. W., and Kahneman, D.: Visual resolution and contour interaction. J. Optic Soc. Am., 52:598, 1962. 23. Prince, J. H.: Completion of project 650 (special print for subnormal vision). Am. J. Ophth., 48: 122-124 (July Pt. II) 1959.
SOME POINTS I N T H E SURGICAL T E C H N I Q U E O F KERATOPLASTY* W.
R.
J.
M A R T I N , M.D.,
A N D E R I C L.
SMITH,
M.D.
Vancouver, British Columbia P A R T I. A N ANALYSIS OF CASES
During the five-year period from 195761, 117 corneal grafts in 100 eyes were per formed at the Vancouver General Hospital. We have attempted to assess the results of these corneal transplantations both with re spect to clarity of the grafts and the final visual improvement. Because of certain gaps in the available data, we have chosen to limit discussion of the refraction results to a con secutive series of 60 grafts from the records of one of us. The length of hospital stay varied greatly, from a maximum of 49 days to a minimum of four days. The average length of time graft cases were in hospital was 21 days ; and they were confined to bed an average of 4.7 days. Length of patching of the operated eye varied greatly. In our own series the average was four days but in the whole group the average was eight days. Suture removal varied, as would be expected, but averaged to the 19th postoperative day. A * From the Department of Surgery, Division of Ophthalmology, University of British Columbia, and the Vancouver General Hospital.
TABLE 1 PREOPERATIVE DIAGNOSIS
(100 eyes) Diagnosis Keratoconus Lattice dystrophy Fuch's dystrophy Dystrophy unclassified Bullous keratitis Interstitial keratitis (leuetic) Chronic herpetic ulcer Acute herpetic ulcer Perforating central corneal ulcers Central corneal leukoma (old ulcer scar—etiology uncertain) Traumatic leukoma (excluding burns and blast) Lime burns Alkaline burns Blast injury (explosive) Adherent leukoma Planned 2nd stage penetrating graft
No. of No. of Eyes Grafts 18 2 3 2 4 12 18 5 6
18 4 4 2 4 13 22 5 6
10
14
7 7 1 4 0 1
7 10 1 4 2 1
100
117
trend, however, toward leaving the sutures in for longer periods of time is apparent. In Table 1 we have listed the diagnoses, the number of operated eyes and the number of grafts done. Table 2 shows that of these 117 grafts, 92 were primarily for optical
1187
REFERENCES
1. Klien, B. A.: Dysgenesis mesodermalis der Hornhaut und Iris, mit gleichzeitigem Vorkommen von neuroekodermalen Entwicklungstörungen. Arch. f. Ophth., 164:12, 1961. 2. Axenfeld, T.: Embryontoxon corneae posterius. Ber. deutsch ophth. Gesell., 42:301, 1920. 3. Burian, H. M.: Chamber angle studies in developmental glaucoma. Missouri State M. A., 55:1088, 1958. 4. Von Hippel, E. : Ueber die angeborenen zentralen Defekte der Hornhauthinterfläche. Arch. f. Ophth., 95:184, 1918. 5. Burian, H. M., v. Noorden, G., and Ponset, J. V.: Chamber angle anomalies in systemic connective tissue disorders. AMA Arch. Ophth., 64:671, 1960. 6. Badke, G. : Kritische Auswertung der modernen Theorien der Myopiegenese und der Versuch einer einheitlichen Betrachtung. Arch. f. Ophth., 153:231, 1952. 7. Collier, M. : La dysplasie marginale postérieure de la cornée dans le cadre des anomalies squelettiques et ectodermiques. Ann. ocul., 195 :512, 1962. 8. Klien, B. A.: The ciliary margin of the dilator muscle of the pupil. Arch. Ophth., 15:958, 1936. 9. Friedman, A. G., and Epstein, E.: Familial retinal detachment. Am. J. Ophth., 61:33, 1961. 10. Jaeger, W. : Beitrag zur Ätiologie der Oraabrisse im temporalen unteren Quadranten. Klin. Monatsbl. Augenh., 126:423, 1955. 11. Cameron, M.: Nontraumatic dyalysis in the young. Brit. J. Ophth., 44:541, 1960. 12. Huerkamp, B.: Häufigkeit und ätiologische Bedeutung des Hornhautastigmatismus bei Ablatio retinae. Arch. f. Ophth., 175:592, 1956. 13. Franceschetti, A., and Carones, A. V.: A case of familial keratoconus with disseminated neurodermatitis, cataract and ectodermal dysplasia. G. ital. of tal., 13:143, 1960. 14. Perkins, E. S.: Open angle glaucoma in young patients. AMA Arch. Ophth., 64:882, 1960. 15. Franceschetti, A., and Linder, A.: Osservazioni sul lavoro di Santino "Esiste efïetivamente una correlazione tra cheratocono e retinite pigmentosa" G. ital. of tal., 13:3, 1960. 16. Cullin, J. F., and Butler, H. G.: Mongolism and its ocular manifestations with special reference to keratoconus. Wilmer Residents Meeting, 1962. (Quoted by Zimmerman, Ophth. Path. Club, 1962.) 17. Mier, M., Schwartz, S. O., and Boshes, B.: Acanthrocytosis, pigmentary degeneration of the retina and ataxic neuropathy: A genetically determined syndrome with associated metabolic disorder. Blood, 16:1586, 1960.
READING CARDS FOR SELECTION O F OPTICAL AIDS FOR T H E PARTIALLY SIGHTED* LOUISE L.
SLOAN, P H . D . , AND DARLENE J.
BROWN
Baltimore, Maryland INTRODUCTION Reading aids for the patient with subnormal vision cover a wide range of dioptric powers, ranging from a conventional reading addition of 3.0 to 4.0 diopters to an aspheric or compound lens whose equivalent power may be 50 diopters or more. In selecting the approximate power needed by each patient to achieve useful reading vision it is desirable to have some better method than a mere trial of lenses of gradually increasing 1 power
T h e purposes of this paper are ( a ) to review the procedures used by others to determine the required power of the reading aid, ( b ) t 0 describe a set of reading cards especiall y devised to meet this need, (c) to present data showing that the size of print rec uired for useful l reading cannot be predicted accurately from a conventional measure of acuitv ' ( d ) t 0 analyze the causes for the discrepancies between acuities measured on a Snellen chart and reading ability measured with continuous text, ( e ) to illustrate the use of the reading cards in selection of a suitable
*From the Wilmer Ophthalmological Institute of The Johns Hopkins Medical School and Hos pital. This research was supported by Grant B-810 from the National Institute of Neurological Diseases and Blindness, Public Health Service, Bethesda, Maryland.
reading aid. M E T H O D S USED BY OTHERS TO SELECT DIOPTRIC POWER OF READING AID . , . - , , . , . Several workers in this field utilize con-
1188
LOUISE L. SLOAN AND DARLENE J. BROWN
ventional measures of distant acuity for capi tal letters as the basis for selecting the dioptic power of the optical aid needed for reading. Kestenbaum1 was probably the first to make systematic use of such data. His basic formula states that the dioptric power of the addition required for reading ordinary newsprint (JS) is numerically equal to the reciprocal of the visual acuity (that is, to the threshold visual angle). If the reading ma terial differs significantly in size from J5 the dioptric values are multiplied by an appropri ate factor. Colenbrander's formula,2 when reduced to its simplest form, states that the power of the reading addition in diopters should equal the threshold visual angle plus 1.50 diopters. This minor modification of Kestenbaum's formula is of significance only in the case of low-power reading aids. Zettel3 published a table giving, for each level of acuity measured at 20 feet, "calcu lated" and "suggested" reading additions. The calculated dioptric values, like Kesten baum's, are directly proportional to the threshold visual angle, but are 25-percent higher. His suggested reading additions, how ever, are significantly less than his calculated values in the higher powers. For example, a reading addition of 12D. to 16D. is suggested when the calculated power is 25D. ; 32D. is suggested when the calculated power is 50D. These reductions are probably explained by his statement that the "suggested addition"
refers to the minimum "which should be given to the patient at the beginning of his care." According to Gettes,4 Volk recommends reading aids which are twice the power of those computed from the Kestenbaum for mula. Thus, although these investigators all assume a direct relationship between re quired dioptric power and threshold visual angle measured with capital letters at dis tance, they do not agree closely as to the ac tual amounts they prescribe for any given level of vision. For example, for the patient with 20/400 vision (threshold visual angle 20 minutes), Kestenbaum would prescribe 20D., Colenbrander 21.5D., Zettel 12 to 16D., Volk 40D. Visual acuity measurements can also be made at any standard near distance instead of at 5, 10 or 20 feet, if the patient is pro vided with a reading addition when it is needed to obtain a clearly focussed retinal image. The well-known Lebensohn chart, first described in 1936,5 is an example of such a test. It is designed for use at 35 cm. and requires therefore a +2.75D. reading addition, unless the patient supplies some or all of this power by accommodation. In addi tion to capital letters it provides numbers, illiterate symbols (o's and x's) and isolated words in lower case letters. The largest test letter corresponds to a visual acuity of 20/800. The gradation of sizes correspond to those on a conventional Snellen 20 ft. chart. Other letter charts for measuring acuity at a
TABLE 1 L E T T E R CHARTS FOR MEASURING ACUITY AT NEAR
Lebensohn
Keeler
Stimson
Testing distance
14 in. (2.75D)
25 cm. (4D)
14 in. (2.75D)
40 cm. (2.5D)
Test material
Capital letters, illiterate symbols, isolated words
Capital letters, Landolt rings, iso lated words
Capital letters, iso lated words
Capital letters (special selection of ten)
Equivalent size of largest test characters
5/200
2.9/200
4.5/200
12.5/200
Gradation of sizes
Conventional
Logarithmic
Logarithmic
Logarithmic
Good-Lite (Sloan)
SELECTION OF AIDS FOR PARTIALLY SIGHTED
close distance are those of Keeler,6 Stimson7 and Sloan.8 Pertinent data regarding these are given in Table 1. Some low-vision clinics9'10 use samples of reading material to assist in selecting a suitable reading aid. No fixed testing dis tance is employed. The procedure is a trialand-error one in which, with each increase in power of the reading lens, the patient finds the distance at which the print is in focus, and indicates the smallest print which he can read at this distance. Feinbloom,9 who uses this procedure, says that the power of the reading addition should be increased "as long as there is an improvement in near acuity or facility in reading." With each in crease in power and consequent decrease in reading distance the patient should be able to read smaller print. The process could therefore be continued indefinitely to permit reading of any desired size of print were it not for the following difficulties associated with the use of strong lenses in a trial frame: 1. With decrease in distance it becomes difficult to illuminate the reading page. 2. Aberrations become noticeable unless the high power lenses used in testing are es pecially designed to minimize such defects. 3. The depth of focus decreases rapidly with increasing lens power. As a conse quence it becomes increasingly difficult to maintain the reading page at the proper dis tance from the lens. The use of successively stronger lenses in a trial frame to select a suitable reading aid takes no account of the fact that the patient might accept a significantly higher dioptric power in some other form of magnifier which provides adequate illumination, re duces aberrations, and maintains the reading page at the correct distance. The difficulties inherent in this procedure can be avoided if, prior to the trial of any actual magnifier, a test of reading vision is made at a fixed dis tance using graded sizes of print that are accurately specified in some convenient unit. Tf continuous text is used, rather than iso lated letters or words, the testing conditions
1189
simulate the actual task confronting the pa tient when he uses an optical aid to read ordinary books, typed material and news papers. The Fink Near-Vision Test 11 is one of the few which provides continuous sentences in quantitatively specified sizes. It is calibrated for use at 14 inches and at this distance measures acuities from 14/14 to 14/98 (20/20 to 20/140). The near vision card devised by Keeney and Duerson,12 though using con tinuous text, is not a close duplication of nor mal reading material because it employs only capital letters. It measures at 14 inches acui ties ranging from 14/14 to 14/89 (20/20 to 20/127). READING CARDS USED IN PRESENT INVESTIGATION
A series of test cards with continuous text, developed by the senior author in 1956, have been described briefly in several pub lications.13-16 These reading cards, devised specifically for prescribing for patients with subnormal vision, provide a direct measure of the magnification required to read print approximating in size ordinary typewritten material and newsprint. The letter form is a reproduction of that used on a standard typewriter. The smallest print is designated 1.0M because the overall dimensions of its lower case letters subtend a visual angle of five minutes at a distance of 1 meter. To read 1.0M print at 40 cm. requires therefore an acuity of 40/100 in Snellen notation. At 33 and 25 cm. the corresponding acuities are 33/100 and 25/100. The complete series of sizes are, in M units, 1.0, 1.5, 2, 2.5, 3, 4, 5, 7, and 10M. Sizes equivalent to 14M and 20M at the standard distance are obtained by viewing the 7M and 10M cards from one-half this distance. Figure 1 shows the nine test cards. Since the 10M print at 20 cm. corresponds to a Snellen acuity of 20/1000 (4/200) the range is ample for testing the lowest level of subnormal vision at which useful reading
LOUISE L. SLOAN AND DARLENE J. BROWN
1190
vision can be attained. T h e sizes from I M to 20M, forming approximately equal steps on a logarithmic scale, were adopted after preliminary experience with more finely graded steps. These reading cards can be used at 40, 35, or 25 cm. or at any other distance that the examiner prefers. A standard distance of 40 cm. has several advantages. 1. T h e resulting Snellen notations, 40/100, 40/150, 40/200, and so forth, can be con verted to the corresponding visual acuities measured at 20 feet, that is, 20/50, 20/75, 20/100, etc., simply by dividing numerator and denominator by 2.* 2. T h e presbyopic patient who can read with ease I M print at a distance of 40 cm. will ordinarily not require more than a con ventional reading addition of 2.50 diopters. * This simple computation gives the Snellen no tation of the letter subtending the same visual angle at 20 feet. The threshold visual angle for recogni tion of isolated capital letters and for easy reading of continuous text are, however, seldom the same. Experimental evidence on this point is given later in this paper.
It ie veryhot in the
This is because I M print happens to be of about the same size as ordinary newsprint, and because most adults prefer a reading distance of 40 cm. or more unless subnormal vision forces them to read at a closer dis tance. 17 3. If ability to read, with ease, print of I M size is accepted as a satisfactory solution of the patient's reading problem, then a measure in M units of the print he reads at 40 cm. gives a direct measure of the re quired dioptric power of the reading addi tion. T h e reading ability in M units times 2.5 diopters is equal to the power of the required reading addition. F o r example, if the patient reads 10M print at 40 cm. he can read I M print at 4.0 cm. and will require a reading addition of 25 diopters. ( I n apply ing this formula a suitable allowance is made for any dioptric power which the patient contributes by accommodation). A previous publication shows how this system can be applied not only to reading additions in spectacle form but to all other types of mag nifier. 18
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SELECTION OF AIDS FOR PARTIALLY SIGHTED
1191
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Fig. 2 (Sloan and Brown). Chart used to provide precise measures of distant acuity for Sloan letters. RELATIONSHIP BETWEEN ACUITY MEASURED WITH CAPITAL LETTERS AND READING ABILITY MEASURED WITH CONTINUOUS TEXT
The degree of relationship between con ventional measures of subnormal acuity and the angular size of print required for easy reading has never before been determined experimentally. One difficulty in obtaining such data is that most Snellen charts do not provide accurate measures of acuities of 20/100 and less because of the small num ber of test characters. In this study a spe cial chart composed of Sloan letters19 was used having five letters on the 20/200 line and 10 on each of the lines of smaller letters (figure 2). To obtain precise measures of acuities be low 20/100, determinations were made of the distances at which the patient could just read three of the five letters on the top (20/200)
line and seven of the 10 letters on the second (20/100) line. In this way, two in dependent measures were obtained, for ex ample, 16/200 and 8/100 if the first line was read at 16 feet, the second at eight feet. All measures of acuity were made with correction of the error of refraction. The final determination of the refractive error was based on thorough subjective testing with special techniques suitable for low levels of visual acuity.3 In verifying the spherical error, tests were made with high plus and minus spheres when lesser powers did not produce any change in acuity. When a preliminary retinoscopic examination is impossible, such tests are necessary to insure that a very high spherical error will not be overlooked. The astigmatic error was meas ured with a cross-cylinder. The standard procedure was modified by the use of a series of O's in graduated sizes instead of
LOUISE L. SLOAN AND DARLENE J. BROWN
1192
letters as test objects, and by the use of crosscylinders of high power ( + 0 . 5 0 and + 1 . 0 0 diopter).
Reading ability was measured at 40 cm. with the series of cards described previously. These were presented on an illuminated
ACUITY CHART FOR NEAR SLOAN LITTERS Thi» chart should ba hald 16 inch« (40cm) from t h · e y « , at right angles to th« lin· of viiion, and illuminated with not l e u than 10 or more than 25 foot canai·· of light.
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SELECTION OF AIDS FOR PARTIALLY SIGHTED 54 >20
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Vlaual Acuity for Capital Letter· S l i e of Text In M unit» Subtending Same Visual Angle
Fig. 4 (Sloan and Brown). Frequency diagram showing relationship between conventional measures of visual acuity using capital letters, and reading ability measured with continuous text. stand with a chin rest to maintain the correct distance. T h e refractive correction required at 20 feet was automatically increased by 2.50 diopters in the case of elderly patients. In other cases the reading addition giving maximal acuity at 40 cm. was determined by actual test. ( D u r i n g the later stages of the study the Good-Lite Near Vision Chart composed of Sloan letters 8 (fig. 3) was used in determining the reading addition for 40 c m ) . T h e reading test was started with one of the larger sizes of print. The patient was asked to read aloud successively smaller sizes until there was a noticeable decrease in speed or in accuracy. T h e reading vision in " M units" was taken as the smallest print which could be read with ease. A great deal of information, both qualita tive and quantitative, is provided by this test of reading ability. Most patients show an
abrupt transition from easy rapid reading at one size to difficult slow reading with errors at the next smaller size and complete in ability to read any smaller print. Such pa tients can almost always be given a magnifier which will provide easy reading. A few pa tients read slowly all print above a certain size and never achieve rapid easy reading. This type of response is noted particularly in children and in other unskilled readers. W i t h a suitable magnifier, the patient in this category can usually be given reading ability commensurate with his natural rate of read ing. A few patients show either no improve ment or even a definite decrease in reading ability with increase in size of print beyond a certain level. This type of response occurs for example in patients who are using a central field of vision reduced to a patch only a few degrees in diameter. Unless bet-
1194
LOUISE L. SLOAN AND DARLENE J. BROWN
ter reading ability is achieved with excentric fixation, the prospects of improving the reading vision of such patients by the use of a magnifier are limited. Illustrative cases in this group will be presented later. Figure 4 is a frequency diagram showing for 443 eyes the observed relationships be tween reading ability measured with con tinuous text and acuity measured with capi tal letters. T h e measured values of reading vision in M units are shown on the vertical ordinate. Capital letter acuities are shown on the horizontal ordinate, where the intervals are chosen to agree with the sizes of con tinuous text in M units subtending the same visual angles. If there were perfect agree ment in the threshold visual angles measured in each way, patients having, for example, acuities of 16/100 or better but less than 20/100 should read 2.5M print but not 2M print at 40 cm (16/100 equals 4 0 / 2 5 0 ; 20/100 equals 4 0 / 2 0 0 ) . T h e figures enclosed by the heavy lines show the number of eyes in which there is complete agreement in the angular size of the capital letters read at distance and of the print read with ease at 40 cm. It is obvious to inspection that there is not a close rela tionship between the two sets of measure ments. There are very few instances in which the reading ability is significantly bet ter than would be expected from the acuity measured with capital letters, but many in stances in which reading ability is markedly poorer. It is obvious therefore that if we wish to know the increase in size of print needed to provide easy reading, a conven tional measurement of visual acuity is of little use except in setting a lower limit to the required magnification. R E A S O N S FOR LACK OF AGREEMENT BETWEEN CONVENTIONAL MEASURES OF ACUITY AND MEASURES OF READING ABILITY
Differences in the two types of measure ment are not surprising when we consider the several possible sources of disagreement. One which probably plays an important role in all cases is the higher standard of per
formance that was required in rating the pa tient's reading vision than in measuring the acuity with capital letters. In the latter tests the patient was given credit for ability to read a certain size of letter if seven of 10 were identified correctly, even though they were read very slowly and in the wrong order. This corresponds to conventional methods of rating visual acuity. In the tests of reading ability, on the other hand, the pa tient's score in M units was determined by a more realistic standard which required rapid and essentially correct reading of the text. T h e smaller separation of the letters on the reading cards than on the capital letter charts undoubtedly accounts in some cases for a significantly higher threshold visual angle for the former test material. The im portance of letter separation as a variable in measurements of subnormal acuity has been the subject of several recent studies. "Separation difficulty" has for example been demonstrated in the impaired vision associ ated with strabismic amblyopia. 20-22 A de crease in reading ability with decrease in letter spacing has also been demonstrated in impaired vision caused by uncorrected error of refraction. 23 The findings of the present study indicate that when subnormal vision is the result of ocular disease there may be a similar increase in the threshold visual angle with decreased separation of the test letters. The data summarized in Figure 4 show a number of instances in which a relatively good acuity on the letter chart was associ ated with a very low level of reading vision. I n several of these cases it was possible to obtain reliable and reproducible data on the location, extent, and density of the scotomas responsible for the low vision.* These find* The central fields were examined at 33 cm. on the Ferree-Rand Perimeter by means of a gray tangent screen equipped with a pericentral target to insure foveal fixation. This target consists of a white circle five degrees in radius combined with four radial lines which extend inward from the circle to within one degree of its center. The pa tient is instructed to fixate the center of the circle, that is, the point at which the four radial lines would meet if extended inward.
SELECTION OF AIDS FOR PARTIALLY SIGHTED
Case A.W. Test object 3 / 3 3 0 White
Case N. DTest object
1/330
White
Case F. 0· Test object
Case D. CTest object
1195
3/330White
6 / 3 3 0 White
Fig. 5 (Sloan and Brown). Central visual fields of four patients showing marked discrepancies in acuity for isolated capital letters and for continuous text.
ings indicated that unusual features of the central visual fields, such as are illustrated in Figure 5, account for the marked dis crepancies in the two measures of acuity. CASE REPORTS CASE A. W.
A patient with optic atrophy had a cecocentral scotoma for a 3/330 white test object with a tiny oval area bordering on the fovea in which this test object could be seen (fig. 5-A). This patient could distinguish between foveal fixation using this spared area and fixation with a paracentral area outside the scotoma, and could shift from one to the other at will. On the letter chart she could
read the 20/70 line with foveal but not with excentric fixation. On the reading cards, on the other hand, she had difficulty in reading the 10M print with foveal fixation although with excentric fixa tion she could read 7M print with ease. CASE F. O.
A patient with macular degeneration had a some what similar field defect consisting of a central scotoma for a 3/330 white test object with a small spared area at the fovea (fig. 5-B). With foveal fixation his letter acuity was 20/50. At 40 cm. he was able, using foveal fixation, to read with diffi culty print ranging in size from 1.5M to 4M. Be cause of the small extent of the spared central field 5M print was read less well than smaller sizes. When the patient was instructed to use excentric
LOUISE L. SLOAN AND DARLENE J. BROWN
1196
fixation he was able, after a little practice, to read 7M print rapidly. He was therefore given a mag nifier of strength appropriate for this level of reading vision. A few weeks later he reported that reading with parafoveal fixation was far easier than with a weaker magnifier which required foveal fixation. CASE N.
D.
A patient, with diabetic retinopathy, had a visual field for 1/330 white reduced to a small foveal patch about four degrees in diameter and a second larger patch in the nasal field (fig. S-C). On the letter chart he was able to read the 20/40 line. In spite of this relatively good acuity, reading was very difficult with foveal fixation because he could see only a few letters at a time. Increase in the size of the print made matters worse rather than better. When, however, he viewed the print excentrically, using the larger patch of functioning field, he was able to read 10M print rapidly and without error. CASE D. C.
A patient with hereditary macular degeneration was found to have two large scotomas for a 6/330 white test object, separated by a narrow horizontal strip which appeared to include the fovea (fig. 5-D). In this strip he was able to see a 1/330 white but not a 6/330 red or blue test object. His visual acuity, tested with capital letter charts, was 20/30 at 20 feet, 40/80 at 40 cm. In spite of this relatively high acuity for capital letters at both distance and near, 7M print was the smallest that could be read with ease at 40 cm. It seems likely that this patient also used central fixation when reading the capital letter charts, but shifted to a paracentral fixation in order to obtain a wider field when reading continuous text. T h e most marked discrepancy noted in the two measures of visual acuity was observed in a patient who could read 20/30 letters when they were shown one at a time. W h e n an entire line of letters was shown, the re sponses were so variable that it was impos sible to arrive at any definite rating of acuity. On the reading cards no size of print, either large or small, enabled this patient to do more than recognize an occasional letter. There was a large central scotoma for a 3/330 white test object. Within this scotoma it was possible to demonstrate at times a single point at which the white test object could be seen. A supplementary test meas uring the largest letter which could be seen as a whole suggests that the functioning area
at the fovea was not more than about 30 minutes in diameter. R E L A T I O N S H I P BETWEEN MEASURES OF READING ABILITY AND POWER OF READING AID ACTUALLY PRESCRIBED
Because patients with subnormal vision do not show a close relationship between the threshold visual angle for isolated capital letters and for continuous text, conventional tests of acuity are of little use in predicting the magnification the patient needs to read with reasonable speed and accuracy. T h e reading cards, on the other hand, duplicate closely the enlarged image of the reading material provided by a simple magnifier. They should therefore provide a valid meas ure of the magnification required for easy reading. In a later paper we plan to present data to demonstrate how these reading cards assist in prescribing magnifiers of various types for all ages and for various visual tasks. In this paper we will illustrate the useful ness of the reading cards in prescribing for a group of 100 patients selected in accord ance with the following criteria: 1. Those under 45 years of age were ex cluded unless aphakic. Because of this re striction the accommodation which the pa tient can contribute need not be considered in determing the required dioptric power of the reading aid. 2. Patients were excluded if their visual requirements deviated significantly from ability to read ordinary books and news papers. Those interested only in reading large headlines for example were not in cluded because the relationship between reading ability measured in " M units" and the required dioptric power differs in these atypical cases. 3. Only those patients were included who were given headborne reading aids. F o r this type of magnifier the dioptric power, which must be added to the correction for distance should be, for the complete presbyope, di-
SELECTION OF AIDS FOR PARTIALLY SIGHTED 16
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Reeding A b i l i t y in M Units Fig. 6 (Sloan and Brown). Frequency diagram showing relationship between size of print in M units read at 40 cm and M rating of prescribed reading addition. rectly proportional to the reading ability in M units. T h e headborne reading aids comprised conventional spectacles, either bifocal or unifocal (single vision) ; Continental, Keeler and Policoff bifocals ; Feinbloom, Hyperocular and Keeler unifocal lenses ; and loupes of various types (Aloe, Ary, Bausch & Lomb, Behr, Selsi). T h e true dioptric powers of most of these headborne mag nifiers have been listed in a previous pub lication. 18 Table 2 shows the expected relationship between reading ability, measured in M units at 40 cm., and the range of dioptric powers of the reading addition required to read I M print. By reference to this table any
headborne magnifier whose dioptric power is known can be classified according to the TABLE 2 CLASSIFICATION OF READING ADDITIONS ACCORDING TO PREDICTED LEVEL OF READING ABILITY IN M UNITS FOR WHICH EACH IS SUITED
Dioptric Power*
M Rating
3.00 or less 3.25 to 4.75 5.00 to 6.00 6.25 to 7.25 7.50 to 9.75 10.00 to 12.00 12.50 to 17.00 17.50 to 24.50 25.00 to 34.00 35.00 to 49.00 50 or greater
1.0 1.5 2.0 2.5 3.0 4.0 5.0 7.0 10.0 14.0 20.0
* When the true dioptric power differs significantly from the vertex power the former is used.
1198
LOUISE L. SLOAN AND DARLENE J. BROWN
level of reading ability in M units for which it is suited.* Figure 6 is a frequency diagram showing the relationship between size of print in M units read with ease at 40 cm. and the M rating of the prescribed reading addition (defined as indicated in Table 2 ) . In 57 of the 100 cases (figures enclosed by heavy lines) there was perfect agreement in the M rating of reading ability and the correspond ing M rating of the prescribed reading aid. In 20 cases the reading aid was slightly stronger, in 23 cases slightly weaker, than would be expected on the basis of the pa tient's reading ability. T h e close agreement between the two measures shows that a de termination of the reading ability in M units at a standard distance provides a valuable indication of the required dioptric power of the reading aid. S U M M A R Y AND CONCLUSIONS
In 443 eyes with subnormal vision result ing from ocular disease or anomaly, com parisons were made of the threshold visual angle for capital letters on a Snellen chart
and that for reading continuous text at a useful level of speed and accuracy. T h e minimum visual angle required for easy reading was almost always equal to or greater than that for reading capital letters. At any given level of acuity for capital letters a wide range of reading abilities was observed in different patients. Generally ap plicable reasons for the discrepancies in the two types of measurement include ( 1 ) the higher standard of performance required for reading continuous text and ( 2 ) the smaller separation of the individual letters of the reading material. In some few patients with unusual defects in the central fields the use of foveal fixation to identify the capital letters, with a shift to parafoveal fixation to secure a wider field for reading, was associ ated with a markedly higher threshold visual angle for reading. Measures of reading abil ity for continuous text, using standard test cards at a standard near distance, were shown to provide a reliable and accurate means of determining in advance the diop tric power required in a headborne magnifier for reading ordinary books and newspapers.
Johns Hopkins Hospital * The values listed in the table are based on the assumption that the patient's power of accommoda tion is negligible. It should be possible to prescribe a weaker magnifier when the patient can contribute a significant amount of accommodation.
(5).
The authors will be glad to supply on request laminated plastic reading cards as shown in Fig ure 1.
REFERENCES
1. Kestenbaum, A., and Sturman, R. M.: Reading glasses for patients with very poor vision. A.M.A. Arch. Ophth., 56:451-469, 1956. 2. Colenbrander, M. C : Optical aids in nonoptical abnormalities of the eye. Ophthalmologica, 134:359363, 1957. 3. Zettel, J.: The use of high-add bifocals in cases of low visual acuity. Am. J. Optom., 36:237-244, 1959. 4. Gettes, B. C. : Optical aids for low vision. Sight Saving Rev., 28:81-83 (Summer) 1958. 5. Lebensohn, J. E. : Scientific and practical considerations involved in the near-vision test with presen tation of a practical and informative near-vision chart. Am. J. Ophth., 19:110-117, 1936. 6. Manufactured by Keeler Optical Products, Inc., 5536 Baltimore Ave., Philadelphia 43, Pa., 39 Wigmore St., London Wl, England. 7. Stimson, R. L. : Optical aids for low acuity. Los Angeles, Braille Institute of America, 1957. 8. Sloan, L. L. : New test charts for the measurement of visual acuity at far and near distances. Am. J. Ophth., 48:807-813 (Dec.) 1959. (These charts are manufactured by Good-Lite Mfg. Co., 7638 W. Madison St., Forest Park, 111. ) 9. Feinbloom, W. : Outline of the technique of examination of the partially blind patient with the clear image lens. Bull. Optometric Soc. City N.Y., 1953, pp. 3-8. 10. I.H.B. Optical Aids Service Survey: First 500 Cases. The Industrial Home for the Blind, Brooklyn, 1957.
SELECTION OF AIDS FOR PARTIALLY SIGHTED
1199
11. Manufactured by Benson Optical Co., Minneapolis, Minn. 12. Keeney, A. EL, and Duerson: Collated Near-Vision Test Card. Am. J. Ophth., 46:592-594, 1958. 13. Sloan, L. L., and Habel, A.: Reading aids for the partially blind. Am. J. Ophth., 42:863-872, 1956. 14. Tillett, C. W.: Visual aids in office practice. Am. J. Ophth., 46:186-194, 1958. 15. Kasik, M : Evaluation of the A.O. projection magnifier and its use with partially sighted children. Optometric Weekly, 49:1145-1148, 1958. 16. Bier, N. : Correction of Subnormal Vision. London, Butterworth & Co., 1960, pp. 148-153. 17. Cannon, R. L.: A survey of the reading and working distance of presbyopes. 111. M. J., 115:4-6 (Jan.) 1959. 18. Sloan, L. L., and Jablonski, M. D. : Reading aids for the partially blind : Classification and meas urement of more than two hundred devices. AMA Arch. Ophth., 62:465-484, 1959. 19. Sloan, L. L., Rowland, W. M., and Altman, A.: Comparison of three types of test target for the measurement of visual acuity. Quart. Rev. Ophth., 8 :4-16, 1952. 20. Müller, P.: Ueber das Sehen der Amblyopen. Ophthalmologica, 121:143-149, 1951. 21. Stuart, J. A., and Burian, H. M.: A study of separation difficulty: Its relationship to visual acuity in normal and amblyopic eyes. Am. J. Ophth., 53:471-477 (Mar.) 1962. 22. Flom, M. C, Weymouth, F. W., and Kahneman, D.: Visual resolution and contour interaction. J. Optic Soc. Am., 52:598, 1962. 23. Prince, J. H.: Completion of project 650 (special print for subnormal vision). Am. J. Ophth., 48: 122-124 (July Pt. II) 1959.
SOME POINTS I N T H E SURGICAL T E C H N I Q U E O F KERATOPLASTY* W.
R.
J.
M A R T I N , M.D.,
A N D E R I C L.
SMITH,
M.D.
Vancouver, British Columbia P A R T I. A N ANALYSIS OF CASES
During the five-year period from 195761, 117 corneal grafts in 100 eyes were per formed at the Vancouver General Hospital. We have attempted to assess the results of these corneal transplantations both with re spect to clarity of the grafts and the final visual improvement. Because of certain gaps in the available data, we have chosen to limit discussion of the refraction results to a con secutive series of 60 grafts from the records of one of us. The length of hospital stay varied greatly, from a maximum of 49 days to a minimum of four days. The average length of time graft cases were in hospital was 21 days ; and they were confined to bed an average of 4.7 days. Length of patching of the operated eye varied greatly. In our own series the average was four days but in the whole group the average was eight days. Suture removal varied, as would be expected, but averaged to the 19th postoperative day. A * From the Department of Surgery, Division of Ophthalmology, University of British Columbia, and the Vancouver General Hospital.
TABLE 1 PREOPERATIVE DIAGNOSIS
(100 eyes) Diagnosis Keratoconus Lattice dystrophy Fuch's dystrophy Dystrophy unclassified Bullous keratitis Interstitial keratitis (leuetic) Chronic herpetic ulcer Acute herpetic ulcer Perforating central corneal ulcers Central corneal leukoma (old ulcer scar—etiology uncertain) Traumatic leukoma (excluding burns and blast) Lime burns Alkaline burns Blast injury (explosive) Adherent leukoma Planned 2nd stage penetrating graft
No. of No. of Eyes Grafts 18 2 3 2 4 12 18 5 6
18 4 4 2 4 13 22 5 6
10
14
7 7 1 4 0 1
7 10 1 4 2 1
100
117
trend, however, toward leaving the sutures in for longer periods of time is apparent. In Table 1 we have listed the diagnoses, the number of operated eyes and the number of grafts done. Table 2 shows that of these 117 grafts, 92 were primarily for optical