VISUAL ACUITY

PRESENTED BY

T.Muthuramalingam

Is an ability of eye to discriminate two stimuli separated in space.

Is the resolving power of eye.

DEFINITION

PURPOSE

For refraction and prescribing spectaclesFor monitoring ocular health

To see an object it must subtend 1min angle at the nodal point.

That means to produce an image of the minimum size of 0.004mm

PRINCIPLE

NOTATION

Visual acuity is noted in terms of Snellen’s fraction.

Near visual acuity is noted in terms of N notation

V/A = distance at which test is madedistance at which the smallest

letter read subtend angle of 5min.

E.g. the distance between pt and test object is 6m and he read the letter of size for 9m for normal person, then V/A=6/9

Print size of the letter is near visual acuity E.g letter size is 8point the V/A= N8

Equipments for measurement

V/A is measured by using different charts.Like snellen’s chart, ETDRS chart,logmar chart

etcSnellen’s chart is more popular using chartEquipments:

• Snellen’s chart • Pointer• Occluder• Pinhole• Near vision chart

Construction of snellen’s chart

Principle: Each letter is designed in a

(Fig:)square with sides 5 times the width of letter strokes.

The breadth of black (red) strokes and spaces are equal.

The breadth of line and spaces produce 1° of angle at nodal point when viewed at certain distance.

Each letter subtends an angle of 5° at the nodal point when seen at a certain distance.

Construction

The first line of the type is so constructed that the angle is formed at a distance of 60 meters, the sixth at 9, the seventh at 6,while additional lines are usually inserted which subtend the same angle at 5 and 4 meters .

These letters should thus be read by a person with standard vision at these distance away.

Contd.

Meters Feet

6/6 20/20

6/9 20/30

6/12 20/40

6/18 20/60

6/24 20/80

6/36 20/120

6/60 20/200

Types of chart and room setup

Paper chart, projection chart, E-chart, alphabet chart, picture chart

Room’s length should be 6m as light rays are parallel for practical purpose.

If it is not possible then required distance is achieved by using mirror in 3m.(for reflection of reversed test types)

Illumination should not less then 50lumen/sq.ftChart should be more or less at the eye level of

the patient.

Distance visual acuity

This measure the patient’s distance vision by testing the ability to read distance characters at a standard distance.

Snellen’s chart is used with different test types.For literate number chart and alphabet chart and

for illiterate E-chart,broken C-chart are usedThe normal V/A is 6/6 in meter or 20/20 in feet

Near visual acuity

It is measured at distance with in arm’s length usually it is 33to 40cm.

Test material is in the style of book or newspaper but in series of unrelated words.

The size label ‘N8’indicates that the size of test font is 8points and distance is specified.

E.g. N8 at 40cm.

Pinhole acuity

A below – normal visual acuity recording may be result of refractive error which can be conformed with pinhole acuity

If use of pinhole improves a patients poor uncorrected V/A means pt’s has refractive errors.

If not improved then it is any other problem other than refractive error.

Procedure of recording V/A

Contd.

Position the Patients 20 feet( 6 meters ) from an illuminated snellen’s chart.

Have the patient cover the left eye with an Occluder. Ask the patient to read the letters from left to right up to

last line. Note the smallest line in which more then half the

characters are read correctly. Record the corresponding acuity fraction as well as the

number of letters missed. Ex,6/18 –2

Repeat Steps 2 to 4 with the right covered. Record the acuity value for each eye separately, with and

without correction.

Contd.

If the subjects cannot read the largest letter he is asked to walk towards the types.For example if he sees the top letter at a distance of 2 meters,

then VA =2/60. If not possible – VA = CF(counts fingers) at 1meter If not possible – VA = HM(Hand movements). If not possible – VA = PL(perception of light) If not recorded - VA = No PL, this is total

blindness.

Vision assessment in children

Neonates - Follow a light, face object(ask mother) Optokinetic nystagmus

► 3-6 months - Fixation and following► 6-18 months – Preferential looking (keeler card)► 9-24 months - Cake decoration ► 2-3 years - Cardiff vanishing card► 21/2–5 years - picture cards & matching charts

(S.G.card)► >5 years - Snellen chart

Types of chart

Keeler Card( fig.1)

Cardiff Card( fig.2)

Sheridan Gardiner test( fig.3)

Cambridge Crowding Card( fig.4)

Picture Chart( fig.5)

Snellen’s Chart( fig.6)

1. 2.

3.

4.

5.

6.

Points to remember

Carefully observed whether the patients is completely covering the eye with cup of hand or not

Observed the position of head whether he is trying to see from side

Tell the patient to sit straight As V/A plays a vital role in eye examination it

should be recorded carefully.