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 spectacles  For 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 made distance 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 etc  Snellen’s chart is more popular using chart  Equipments: 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.ft  Chart 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 used  The 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)

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.