Visual acuity

Visual acuity

Visual acuity is a measure of the spatial resolution of the eye or, in other words, an estimation of its ability to discriminate between two points.

If two objects are so close that two adjacent cones are stimulated, the patient would appreciate them as a single target. Therefore, there must be an unstimulated cone between stimulated ones to allow for the resolution of two targets or edges.

In real sense acuity of vision is a retinal function (to be more precise of the macular area) concerned with the appreciation of form sense.

Distant and near visual acuity should be tested separately.

The measurement of VA is done to monitor change in vision with progression of disease and/or treatment plan.

Visual acuity is dependent on: The refractive error of the eye The health and the integrity of the eye The test targets used The test conditions

Distant visual acuity

The distant central visual acuity is usually tested by Snellen’s test types.

PRINCIPLE The fact that two distant points can be visible as separate

only when they subtend an angle of 1 minute at the nodal point of the eye, forms the basis of Snellen’s test-types.

It consists of a series of black capital letters on a white board, arranged in lines, each progressively diminishing in size. The lines comprising the letters have such a breadth that they will subtend an angle of 1 min at the nodal point.

Each letter of the chart is so designed that it fits in a square, the sides of which are five times the breadth of the constituent lines. Thus, at the given distance, each letter subtends an angle of 5 min at the nodal point of the eye.

PRINCIPLE OF SNELLEN’S TEST TYPE

SNELLEN’S CHART

The letters of the top line of Snellen’s chart should be read clearly at a distance of 60 m. Similarly, the letters in the subsequent lines should be read from a distance of 36, 24, 18, 12, 9, 6, 5 and 4m, respectively.

Procedure of testing: For testing distant visual acuity, the patient is seated at a

distance of 6m from the Snellen’s chart, so that the rays of light are practically parallel and the patient exerts minimal accommodation. The chart should be properly illuminated (not less than 20 ft candles).

The patient is asked to read the chart with each eye separately and the visual acuity is recorded as a fraction, the numerator being the distance of the patient from the letters, and the denominator being the smallest letters accurately read. When the patient is able to read up to 6 m line, the visual acuity is recorded as 6/6, which is normal.

Similarly, depending upon the smallest line which the patient can read from the distance of 6 m, his vision is recorded as 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60, respectively. If he cannot see the top line from 6 m, he is asked to slowly walk towards the chart till he can read the top line. Depending upon the distance at which he can read the top line, his vision is recorded as 5/60, 4/60, 3/60, 2/60 and 1/60, respectively.

If the patient is unable to read the top line even from 1 m, he is asked to count fingers (CF) of the examiner. His vision is recorded as CF-3’, CF-2’, CF-1’ or CF close to face, depending upon the distance at which the patient is able to count fingers.

When the patient fails to count fingers, the examiner moves his hand close to the patient’s face. If he can appreciate the hand movements (HM), visual acuity is recorded as HM positive.

When the patient cannot distinguish the hand movements, he is taken into the dark room and a light is focused on his eye and he is asked to say when the light is on and when it is off. If he succeeds in doing this, V= PL (the examiner notes whether the patient can perceive light (PL) or not). If yes, vision is recorded as PL +ve and if not it is recorded as PL –ve.

Also when examiner throws light, he may be able to give some indication of the four directions from which the light is directed—up, down, right and left. This is recorded as projection of light, accurate or inaccurate in each quadrant. If he fails to see the light the vision is recorded as V=no Projection of light.

Minimum angle of resolution

The denominator in Snellen grading is an indirect measure of the size of the letters read and the angle they subtend.

The classic Snellen fraction is the reciprocal of the minimum angle of resolution (MAR)

LOGMAR SCALE

A notation of visual acuity that has the same clinically significant difference between each line and allows easy recording of every letter read is the log minimum angle of resolution (MAR) scale. The MAR is arrived at by dividing the denominator by the distance at which the letters were read, i.e. the Snellen fraction is inverted and reduced. A Snellen acuity of 6/12 or 20/40 therefore corresponds to a MAR of 2 minutes of arc.

Allows for constant geometric progression over each step.

This derivation has been used in the construction of charts such as the Bailey-Lovie chart.

Bailey-Lovie Chart

Scales of Visual Acuity

Minimum angle of resolution

Acuity decimal

Acuity Snellen

LogMAR

0.50 2.00 6/3 -0.30

0.67 1.50 6/4 -0.18

0.83 1.20 6/5 -0.08

1.00 1.00 6/6 0.00

1.50 0.67 6/9 0.18

2.00 0.50 6/12 0.30

3.00 0.33 6/18 0.48

4.00 0.25 6/24 0.60

6.00 0.17 6/36 0.78

10.00 0.10 6/60 1.00

If the vision is subnormal, the visual acuity is again determined by asking the patient to read the letters through a pinhole.

BCVA To determine the function of the macula in

the best optical conditions,the refraction of the eye must be determined and the visual acuity assessed again in the same way with the correcting glasses in place.

Pinhole Visual Acuity

Purpose To determine if a decrease in vision is

correctable by lenses PINHOLE EFFECT

DENSE CATARACT The laser interferometer forms a diffraction

pattern of parallel lines on the retina even through a moderate cataract. The patient is asked to identify the orientation of progressively finer lines, to establish the visual acuity likely to be regained after surgery.

Potential acuity meter projects a tiny Snellen chart onto the retina

around a lens opacity and the patient is required to read the alphabets.

Young children Maturation of infant visual function

has been studied by two techniques, the pattern visual evoked potential and preferential looking behaviour.

In children younger than 2 years the VEP test proves more successful .

Other tests include the Keeler-Elliot and Kay picture test, Cardiff acuity cards and Ffooks symbols,the Teller acuity test.

Given a choice, an infant prefers to look at patterned rather than unpatterned stimuli. The infant's preference may be quantified by incorporating patterns which vary in stripe width.

Preferential Looking (Keeler / Teller Cards)

Cardiff Acuity Cards

Linear Kay Pictures

E-TEST

With toddlers and slightly older 'verbal' children, simple pictures constructed on Snellen's principles may be used. A very effective test is the 'E-test' in which the examiner holds cards on which the letter E is printed, in various positions and in various sizes.

If the test is treated as a game, the child standing 6 m away will readily respond on request by indicating the direction of the letter with his hand or by holding a similar card in the same position so long as he sees it.

A similar test is the Landolt C chart. This is the chart containing a series of broken rings, with each

gap subtending an angle of 1 minute at nodal point at a given distance.

Tumbling E Chart

Landolt C Chart

PICTURE CARDS

OPTOKINETC NYSTAGMUS

PHOTOSTRESS TEST

The extent of involvement can be assessed clinically by recording vision before and after exposure to a bright light or photostress.

The photostress test is performed by covering one eye and asking the patient to read the smallest possible line on the near chart. A bright light is shone into the eye for 15 seconds, following which the patient is asked to read the same line of print and the recovery time noted. The test is repeated with the other eye.

In normal people and those with optic nerve disease there is no significant difference in the time taken for the two eyes to recover from the photo stress.

In a subject with macular disease the recovery time is prolonged.

The test is useful in early macular disease, particularly central serous retinopathy, where there may be minimal deterioration in visual acuity and yet an easily detectable decrease in photoreceptor reserve capacity.

Visual acuity for near vision

Visual acuity for near Near vision is tested by asking the patient to read the near vision chart, kept at a distance of 35 cm in good illumination, with each eye separately. In near vision charts, a series of different sizes of printer type are arranged in increasing order and marked accordingly.

Near Visual Acuity

Testing the VA at close range (usually 40cm)

The purpose is to detect people with near vision difficulties (e.g., uncorrected high hyperopia, accommodative dysfunction)

In patients over 40 years old, the reduced near visual acuity is one of the symptoms of presbyopia

Commonly used near vision charts are as follows:

1. Jaeger’s chart. In this chart, prints are marked from 1 to 7 and accordingly patient’s acuity is labelled as J1 to J7 depending upon the print he can read.

2. Roman test types. According to this chart, the near vision is recorded as N5, N8, N10, N12 and N18 (Printer’s point system).

3. Snellen’s near vision test types.