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.
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.
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
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.