Netherl. Ophthal. Soc., 166th Meeting, Eindhoven 1971 Ophthalmologica, Basel 171: 100-108 (1975)

Visual Acuity, Visual Field and Physical Ability A Proposal of a Formula

M. C. COLENBRANDER

Introduction Often we need, for reports, a general starting point for the evaluation of disability. These general estimations do exist with regard to visual acuity. The schema of Z eeman is frequently used in the Netherlands. One is justified in wondering whether, as a principle, the visual field should also be included in the calculation. It goes without saying that this calcula­ tion can only be a starting point for the estimation of physical ability.

In principal there are various possibilities for connecting the visual acuity to the presumed validity. But for ail possibilities normal vision should corresponds with normal validity. If one takes an arithmetical progression both for vision and validity, one gets steps of unequal importance, for the difference in validity be­ tween a visual acuity of 0 and 0,1 is of course much greater than the difference in validity between a visual acuity of 0.9 and 1,0. This difference can be ruled out by taking a geometrical progression of the vision against an arithmetical progression of the validity. Then each halving of the vision coincides with the same amount of decreasing validity. But this progression gives us no mangeable figures for very low vision, because blindness comes out to be minus infinity. A better proposal is to consider the validity of vision equal to the square root of the visual acuity (C olenbrander , 1951). See table I.

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Validity of the Visual Acuity

COLENBRANDER

101

Table I. Visual acuity and physical ability Visual acuity

Validity, °/o 125

1.56 1.44

120 1.32

1.21

115 110

1.10 1.00

105 100

0.90 0.81

95 90

0.72 0.64

85 80

0.56 0.49

75 70

0.42

65 60

0.36 0.30

55 50

0.25 0.20

45 40

0.16 0.12

35 30

0.09 0.06 0.04

25 20

0.0225

15 10

0.01

5

0.0025 0.00

0

Various routes can be taken, too, for the calculation of the validity of the visual field [C olenbrander , 1947], For instance the surface of the visual field might be chosen. But this overlooks the fact that the center of the visual field is much more important than the periphery. The visual acuity decreases towards the periphery, approximately proportional to the excentricity. Up to 30° the visual acuity is: one, divided by twice the excentricity in degrees [C olenbrander , 1938]. If the visual field is divided into radially arranged sectors, like a cake into wedges, and subsequently all sectors are divided into rectangles by

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Validity of the Visual Field

102

COLENBRANDER

equidistant circles, then the surface of each part is approximately inversely proportional to the visual acuity obtained and each part, therefore, contains an equal number of image points. This seems a good starting point. But there is more to give attention to. Not all directions in the visual field are equally important. The direction to the right, to the left and down­ ward are more important than the upward and diagonal directions. This can be taken into account by making the width of the sectors dependent on the importance. Halving of the width doubles the importance. Finally, the following possibilities should also be taken into account: hemianopsias, quadrantanopsias and retinal dysfunction phenomena end­ ing in the raphe retinae. For this, the vertical and horizontal meridians of the visual field should coincide with a sector boundary. If all these demands are taken into consideration, I arrive at the follow­ ing formula: (a) divide each quadrant into four sectors, successively 15, 30, 30 and 15° wide, and (b) take for the criterion of the normal visual field the values found in figure 2 and 3. An exact value of 1,000 is then found for the normal visual field by adding up the extension of the visual field in these 16 directions.

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Fig. 1. Monocular visual field and physical ability.

Visual Acuity, Visual Field and Physical Ability

103

normal figures for the right and left eye apart 50

50

50

so

70 80 / 90 l

260

2C0 II III

I IV

330

210

90

50

so

so/

] 50

50 \

\

70 200

260 :i m

I IV 330

210 50

50

70

50

60

\ 80 y 90 90

60

----- 80 70

m axim al figures for both eyes together 10

10

10

10 80 / 90 \

110

110 II

I

HI

IV

330

330

90

\ 80

J

90

IV extra 120

10

90

70

70

/ 90

10

Fig. 2. Normal figures for the visual fields for the evaluation of the physical ability.

The validity of a given visual field in percentages is the extent of the visual field in these 16 directions, divided through ten. The summation can be simplified by taking each quadrant separately and by looking at the differences between the actual figures and 50 degrees for each direc­ tion. The sum of the figures, i.e. the validity may come out above 100 8/o, but this cannot be an objection, because visual acuity too can be more than 100 %. The separate, validity of the. visual acuity and the visual field for each eye is in the first place meant for clinical use and for the evaluation of detonation or amelioration.

The validity of the visual organ as a whole, which has a social value, is built up of various components. Because of stereoscopic vision it is important whether one sees with one eye or with two eyes. The visual

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Validity of the Visual Organ as a Whole

104

COLENBRANDER

acuity of each eyes separately should, therefore, be elaborated in the whole, not only the visual acuity of the better eye. In order to avoid in medical examinations an ability of more than 100%, the maximal value of the visual acuity is fixed at 1.0. The same holds for the visual field. The visual field of each eye separately is of importance but the visual field of both eyes together is of greater im­ portance. A complication is encountered in the visual field. While, for visual acuity, the value of both eyes together is usually equal to the visual acuity of the better eye, the extent of the visual field is usually for both eyes together much greater than for each eye separately. If we wish to maintain the calculation with four sectors per quadrant as explained for each eye separately, a deduction should be made somewhere to arrive at 1,000 again for the normal value. The best thing to do is to make this deduction for the visual field as a whole. But there are more considerations. In view of the traffic from the right that has the priority of way, the extension of the visual field to the right is more important than that to the left (in countries driving on the right of the road). For reading, too,

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Fig. 3. Binocular visual field and physical ability.

Visual Acuity, Visual Field and Physical Ability

105

Table II. Build-up of the total eye validity Vision A (separate) B (both) C (combined)

Fields

AVR

AVL

AFR

BV

AFI. BF

C

4 x 5 % = 20 % 2 x 2 0 % = 40% 1 x40% = 40%

Table III. Visual acuity and separate validity Visual acuity

20/20 18/20 16/20 14.5/20 13/20 11/20 10/20 8.5/20 7/20 6/20 5/20 4/20 3/20 2.5/20 5.4/60 3.6/60 2.4/60 1.3/60 3/300 1/300 0

S,%

A = 'AS ,%

B = 2S, %o

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

50 47.5 45 42.5 40 37.5 35 32.5 30 27.5 25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0

200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

the central part of the visual field in the right lower quadrant is more im­ portant than the other quadrant. If all these demands are taken into consideration, the best solution seems to me that given in figures 3 and 4. In the fourth quadrant a part of the visual field (IV extra) is counted double.

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1.00 0.90 0.81 0.72 0.64 0.56 0.49 0.42 0.36 0.30 0.25 0.20 0.16 0.12 0.09 0.06 0.04 0.0225 0.01 0.0025 0.00

Separate validity

COLENBRANDER

106

Table IV. Sheet for computing the physical ability Ingredients for the calculation of the eye validity Fields

Vision R

L

R

RL Quadrant

V = visual acuity SV = separate validity, °/o AV, «,¿0 = % SV

-

L

RL

I II III IV IV extra

F = sum, %o SF = 0.1 F = spearate validity, %>

-

AF, %o = 'A SF BF, %o = 2 SF

BV, %o = 2 SV C = 0.1 BV x 0.1 BF

Calculations of the eye validity Credit Max. Debet

Credit Max. Debet 50 50

50 50

AFR AFL

Sum = AV BV

100 200

Sum = AF

100

BF

200

Sum = (A+B) V

300

Sum = (A+B) F

300

Credit (A+B) V (A+B) F C Sum = A+B+C 0.1 (A+B+C)

Maximum

Debet

300 300 400 1,000 100

Validity, %o Validity, %> Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 12:26:11 AM

AVR AVL

Visual Acuity, Visual Field and Physical Ability

107

1

2

3

4

5

Debet AV BV AF BF C

50 0 50 38 76

50 0 50 30 60

0 0 30 112 224

0 0 50 88 176

0 0 59 68 136

Invalidity in %o in %

214 21

190 19

386 39

314 31

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The numerals represent the maximal values that can be brought into account. The maximum value is again 1,000 for the normal binocular visual field. To determine the validity of our eyesight as a whole, a choice can again be made between various possibilities. If the total validity of the eye were to be seen as the sum of the validity of the visual acuity and that of the visual field, the value found would be too high in those cases in which one of these components would be very low, i.e. in cases of central scotomas and in case of a very small visual field with reasonable visual acuity. The calculation should, therefore, also contain an amount for the combination of the two components, i.e. visual acuity x visual field. If, subsequently, the demand is made that the calculation be as simple as possible, we arrive at a formula as represented in table II. 5 % of the validity of the visual acuity of the right eye separately (AVR); 5% of the validity of the visual acuity of the left eye separately (AVL); 5 % of the validity of the visual field of the right eye separately (AFR), and 5 % of the validity of the visual field of the left eye separately (AFL). 20% of the validity of the visual acuity of both eyes together (BV), and 20% of the validity of the visual field of both eyes together (BF). 40% of the product of the validity of the visual acuity of both eyes together and the visual field of both eyes together. This formula seems intricate, but it is not if a schema is used in the calculation. Here follows the invalidity in some frequently occurring conditions: (1) loss of the right eye; (2) loss of the left eye; (3) right hemianopsia; (4) left hemianopsia, and (5) bitemporal hemianopsia.

108

COLENBRANDER

References C olenbrander, M. C.: Ned. T. Geneesk. 82: 154(1938). C olenbrander, M. C.: Ned. T. Geneesk. 91: 3022 (1947). C olenbrander, M. C.: Ned. T. Geneesk. 95: 1100 (1951). Request reprints from: Prof. Dr. M. C. Colenbrander, Oude Vest 105, Leiden (The Netherlands)

Discussion

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Winkelman: I have also always adopted a procedure similar to that of the speaker, using the table of Zeeman and the booklet of Hambresin of the Inter­ national Congress in 1958 in Brussels. But then I get the answer: this is not important; what matters is what this man is still able to earn with his handicap. For example, a house painter who loses an eye is no longer allowed to climb scaffoldings, but he still can do other work. He then earns, for example, 4% less. Thus, his disability is not 15 but 4°/o. The queer thing is that we have to judge how much a person with a certain handicap still can earn. C olenbrander: Every schema is, per definition, schematic. Particular circum­ stances will always have to be taken into account. But also in those cases a schema, one that also includes the visual field, is important as a starting point. J anssen C apriles: Eye-disability tables are often based on reduction of one of the functions of the eye (visual acuity = seeing of shapes). It is, therefore, highly important that you have extended this by including the visual field. Still, in practice, difficulties remain, in, for example, partial disturbance of the color sense and/or the stereoscopic sense; what to do with, for example, a patient who has become unilaterally aphakic (visual acuity and visual field good) and who cannot wear a contact glass for one reason or another? C olenbrander: In case of incorrigible unilateral aphakia the patient walks round with one eye with bad visual acuity and good visual field; this has been elaborated in the calculation.

Visual acuity, visual field and physical ability.

Netherl. Ophthal. Soc., 166th Meeting, Eindhoven 1971 Ophthalmologica, Basel 171: 100-108 (1975) Visual Acuity, Visual Field and Physical Ability A P...
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