Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

72 Textbook of Visual Science and Clinical Optometry

•If he is able only to perceive light, his visual acuity is perception of light only (PL only).

In illiterate patients following two types of optotypes are used

to replace the alphabets in Snellen’s chart and visual acuity is interpreted on the same principle.

i.Landolt ring: It consists of a graduated series of ring, i.e. a circle with a break or gap in it. Conventionally the break is oriented in four direction—up, down, right or left in different optotypes. The subject is asked to identify the location of the break in each Landolt ring.

ii.Echart(orIlliteratechart): Itconsistsofachartwiththegraduated series of Snellen’s letter “E” oriented in various directions, which the examinee is asked to identify, i.e. the direction to which the limbs of the “E” point (up, down, right or left).

Decimal Acuity

The Snellen’s fraction can also be converted into a decimal number which is termed as decimal acuity or decimal notation. Thus, 6/ 6 (or 20/20) becomes 1.0, 6/12 (or 20/40) becomes 0.5 and 6/60 (or 20/200) becomes 0.1.

Minimum angle of resolution (MAR): It is expressed in minutes of arc. This is simply arrived at by inverting Snellen’s visual acuity and converting it into a decimal number. So, a Snellen visual acuity of 6/60 (or 20/200) corresponds to 60/6, i.e. MAR of 10 min. of arc and a Snellen visual acuity of 6/12 (or 20/40) corresponds to MAR of 2 min of arc. Therefore, MAR in min of arc is also reciprocal of the value of the decimal acuity.

LogMAR (Logarithm of MAR): It is the basis of constant geometrical progression from one letter to the next in each line and from one line to the next line. The lines progress in 0.1 logMAR steps and each letter accounts for 0.02 score of each line. So, when the visual ecuity is 6/60 (or 20/200), the MAR is 10 min of arc. So, logMAR= log10(10) = 1.0. Similarly, visual acuity of 6/6 (or 20/20) corresponds to MAR of 1 and logMAR of log10(1), i.e. 0. If the patient’s visual acuity is 6/12 (or 20/40) and he is able to read 3 letters in next line his logMAR will be = log10(2)+ (3 × 0.02) = 0.30 + 0.06 = 0.36.

Physiology of Vision 73

Bailey–Lovie visual acuity chart is the most common amongst the charts developed based on the logMAR scale. It consists of 5 letters in each row and geometric progression in the size and interval of the letters from one row to the next row, decreasing by a factor of log10(10). The letter sizes range from 6/60 (or 20/200) to 6/3 (or 20/ 10) in 14 rows (Fig. 5-5). The size of the letter in each row is 4/ 5th of the letter in upper row. Bailey-Lovie chart is more precise than a Snellen’s visual acuity chart and most useful with low vision patients.

Fig. 5-5: Bailey-Lovie chart

Near Visual Acuity Test

Near visual acuity is tested at a standard distance of 40 cm and the size of the smallest print that can be read is interpreted as the near visual acuity.

Physiology of Vision 75

•Contrast sensitivity is reduced in many diseases, e.g. cataract, optic neuritis, glaucoma, macular diseases, etc. whilst the visual acuity may be almost normal.

•Person requiring lot of contrast to see a target has lower contrast sensitivity.

Test of Contrast Sensitivity

Contrast sensitivity is assessed using a sine wave grating scale which consists of a gradual transition between alternating light and dark bands without definite edges, with the narrower bands having higher spatial frequency. The following tests are used based on sine wave grating:

a.Visitech chart

i.It is tested at a distance of 3 metres for distance system and 40 cm for near system.

ii.It contains circles of photographic plates arranged in 5 rows and 8 columns (total of 40 circles). Each circular plate contains a sine wave grating and the spatial frequency is different in each row with decreasing contrast sensitivity along the column. The sine wave gratings are either vertical or tilted 15° to the right or the left.

iii.The patients are asked to identify the orientation of each grating.

b.Bailey-Lovie visual acuity chart

i.It is tested at a distance of 4 metre

ii.It contains 5 letters on each line and a logarithmic progression in size from one row to the other row (a total of 14 rows).

c.Pelli-Robson contrast test chart

i.It is tested at a distance of 1 metre.

ii.It consists of 8 lines of letters with 16 triplets of letters with a decreasing level of contrast in next letters to their background. Each line contains two triplets.

iii.Letters of the triplets, i.e. of the same group have same contrast level.

Physiology of Vision 77

receptor level, while, the opponent theory results from the receptor outputs.

Colour Blindness

It is defined as significant departure of an individual’s colour perception from that of an normal individual.

Aetiology and types: a. Congenital

•Congenital defect in colour vision occurs due to absence of a cone pigment or a shift in spectral sensitivity of the cone pigment (Table 5-1).

•It accounts for majority of cases of colour blindness and is genetically inherited (Table 5-2) as sex linked disorder in which the defective gene is either on the X-chromosome carrying genes encoding for red and green pigments or chromosome 7 carrying genes encoding for the blue pigment.

•It affects mainly males who inherit the defective gene from their mother.

•It is very rare in females since they have to carry the defective genes on both the X-chromosomes.

•Defective colour vision occurs in 8% of male population and 0.5% of female population.

•It is stationary and stable in congenital type and affects both the eyes symmetrically.

•It is found in following genetic disorders:

i.Klinefelter’s syndrome

ii.Turner’s syndrome

iii.Glucose-6-phosphate dehydrogenase deficiency

iv.Haemophilia, etc.

Deuteranomaly is commonest and occurs in 5% of male and 0.3% of female population.

People with a colour defect of congenital origin are classified as in Tables 5-1 and 5-2.

b. Acquired

•This is relatively rare and mostly tritanopic, i.e. the blue colour sensation is missing.

•The defect may progress or regress and may be unilateral.

Physiology of Vision 79

•It occurs in following situations:

i.Diseases of the cone, e.g. retinitis pigmentosa

ii.Diabetic retinopathy

iii.Glaucoma

iv.Retinal/Macular disease

v.Optic atrophy/optic neuritis/retrobulbar neuritis

vi.Brunescent cataract

vii.Drug induced, e.g. ethambutol, chloroquine, digitalis, oral contraceptives, chlorpromazine, streptomycin, etc.

•Red green colour defects are more common in diseases of the optic nerve whereas, yellow blue defect are more common in retinal diseases. So, acquired defect of colour vision is a sensitive early indicator of serious ocular and systemic diseases. Hence, monitoring of changes in status of colour vision often helps the clinician to assess improvement of the causative disorder. Distinguishing features between congenital and acquired colour defect are discussed in Table 5-3.

Table 5-3: Distinguishing features between congenital and acquired colour blindness

•Acquired colour defects may be classified as (Köllner’s rule— 1912);

i.Red-Green defect—It is due to involvement of optic nerve and more proximal part of the visual pathway.

80 Textbook of Visual Science and Clinical Optometry

ii.Blue-yellow defects—It is due to involvement of ocular media, choroids and retina.

•A tritan colour defect is as a rule acquired, since inherited tritan defect is extremely rare.

Tests for Colour Blindness: Colour blindness is a disqualification for many occupations specially for drivers, pilots, physicians, etc. Colour vision testing should be undertaken for all at young age. Each eye should be tested separately for the colour vision.

i.ISHIHARA’s pseudoisochromatic test plates: It is a rapid and the most popular test and consists of lithographic plates on which dots of various sizes, colours, saturation and brightness are arranged so that dots of similar colour form a figure, e.g. a numeral or winding path among a background of dots of another colour (Figs 5-6A and B). An anomalous trichromat or a dichromat has difficulty in identifying the definite figure since it is indistinguishable from the background only by its difference in hue. The test also includes some plates in which the numbers can be identified by colour blinds (Fig. 5-6C), but not by the normal trichromats. These test plates mainly give an assessment of inherited red-green blindness (Figs 5-6A to C). The plates are held at 75 cm from the patient. The subject is allowed to observe each chart for a period of maximum 4 to 5 seconds. Care should be taken to avoid undue exposure of the plates to light to prevent fading of the colours.

ii.Farnsworth Munsell 100 hue test: It is a colour vision test consisting of 85 small discs made up of Munsell colours of equal chroma and value. The saturation of the colour is called “chroma” and brightness is termed “value” in this system. The patient is asked to arrange the small discs in a continuous and smooth series order. Errors are scored to arrive at a diagnosis of the type of colour defect. This test is useful for testing both inherited and acquired colour defect quantitatively.

iii.Farnsworth D–15 test: It consist of only 15 small discs of Munsell colour. It is a more rapid test and miniature version of the Farnsworth Munsell 100 hue test.