3: factors affecting visual acuity .

what does VA measure

assesses how well an eye or an individual sees

va taken for distance viewing in every eye exam

factors that affect VA are divided into external and internal factors

external: relate to our equipment and methdology

internal: to the px, eye the optical system of px eye or retina, cognitive ability, cooperation

external factors affecting va

includes chart type, scoring method and chart luminance

full control over external factors eg we can choose what chart to use and how bright it is

internal factors affecting VA

pathology, pupil diameter , cooperation

we have less control over internal factors but still have some control, eg we can take steps to increase the chances of getting a child to co-operate with VA testing

some internal and external factors interact eg pupil dimater and chart luminance interact to determine how much light falls on the retina

more external factors

chart type

scoring methof

chart luminance

chart contrast

termination rule

general lighting conditions

our approach to taking VA eg wat we do to encourage the px avoiding cherry picking

more internal factors

retinal eccentricity- part of retina used to view letters

pupil diamter

refractive error

pathology

age

px cooperation

can change external factors in certain px to increase chances of getting better measurement

eg if texting a toddler, will change the chart to something more age appropriate

eg if px has high refractive error, we increase the letter size

accuracy

reflects how close a measurement is to the true value

precision

reflects how reproducible measurements are even if they are far from the accepted value

repeatability

variation that occurs when repeated measusremnts are made of the same itwm under absolutlely identical conditions

when you measure VA of the same px re and le on separate occasions, in the same room using the same techniwue, lighting and equipmet

gap must be short enough that there has been no actual change to the px VA

so an individuals optometrists VA measures should be repeatabe

same lighting, same practitioner, same room ect

reproductibility

variation that results when diffeent conditions are used to make the measurements, eg diffeent operators, different et ups and measurement systems

when another optometrist or severral take VA measurements in the same px but different room, different charts , the measures should be similar, ideally identical to our measurres

if they are identical or very siimilar then VA measures are reproducible

internal: retinal eccentricity

VA is best when the image falls on the centre of the fovea

the fall in VA with eccentricity depends on where image falls on other than centre

its steep if plotted at a ratio, less steep if plotted in MAR

we turn our eyes to recieve the image on the foveal when we eant to see small details

some eyes have poor acuity due to eccentric fixation

some eyes have a damaged fovea and so have to use a non foveal retinal location for fixation

in most px we dont have to worry about whether they are fixating with their fovea as they do this naturally

internal: old age

in adults va is constant up to the age of 50

on average VA is considerably better than 6/6 in young adults

VA doesnt approach 6/6 until around the age of 65

after 65 the decline is more rapid

internal: young age

VA in early life is not good eg 6/24 in 6 month old

hard to measure VA in very young and young children

combbination of acuitty that is still developing and an inability to co operate with VA measurement makes VA measures imprecise and innacurate

by age 3 va approaches 6/6

internal; refractive error

uncorrected myopia reduces distance va

uncorrected hyperopia may or may not reduce distance va

uncorrected astigmatism always reducses distance VA

internal: pupil size

the pupil controls the amount of light entering the eye

it dilates in low light and constricts in bright light

when we read the eyes converge and accomodate and the pupil constricts

older px generally have smaller pupils

pupil size interaction with refractive error

egif theres -5.00 DS of myopia the VA is worse than 6/96 when te pupil diameter is 7mm, but VA i much better if the pupil size is 0.5mm

diffraction and aberrations - interaction with pupil size

these are imperfections of an optical system which mean that the image of a point object is not itself a point

diffraction: deviation in the direction of propgation of a beam of light which occurs when the light passes the edge of an obstacle such as the pupil of the eye. only an issue when pupil is small

aberrations: optical defect in whcih rays from a point object do not form a perfect point passing through an optical system . proble when pupil large

impact on VA of pupil size

line spread function LSF is the image of a thin line formed by an optical system

the system has better optical quality when the LSF is thin , so VA will be better

lsf is thinnest when pupil sizes between 3.4 and 3.8]

if theres a gap between the LSF and diffractionlimit curve this is due to aberraions

when pupil small LSF wide due to diffraction

internal: pathology

many diseases of the beye and visual sstem cause reduced VA

figure shows results from a study about the treatment of diabetic eye disease

each triangle in the 2 panels on the L represents a single eye

the 2 panels on the R show average pre and post treatment VA

va redced in the pre treatment group

internal: px co operation and other cogntive

if px cant eg too young or wont co operate with VA testing it impacts

have to rely on objective means for VA

communication issues can also arise eg px cant speak or dont speak same language

eg by using the umbling e chart rathe than letter chart where the px indicates with their fingers whether the bars of the es are facing up down left or right

external factors: our approach to VA testing

eg dont say what can u read, ask can you read the top line, instruct pc where you want them to start

as they reach their limit point to letters you want them to attempt

no cherry picking

give lots of encouragement

need to reach termination

termination rule

1. let px stop reading when they want - will intorduce variablility based on how confident the px is

2. ask every px to attempt every letter on the chart - hugely time efficient anf dispiriting for many px

most common termination rule is stopping when they get 4/5 letters wrong

external factors: chart type

crowing: P H N H D or PHNHD

different chart types used to asses VA

on average Regan chart VAs were better than VAs measured with the 3 other charts

external: scoring method

could score by line eg 6/5 or using per letter scoring system eg VAR

eg

on 0.0 line all letters identified so cooridinates as (0,100)

on 0.3 line, no letters correct so coordinates (-0,3.0)

determine % correct for lines between -0.2 and -0.1

can work out va from this curve

determine logMARA letter size corresponding to 50% correct

external factors : chart luminance and general lighting condtions

retina l luminanceby the pupil size and chart luminance

retinal luminance is measured in trolands where 1 troland is the retinal illuminance for a pupil area of 1mm² viewing a surface that as a luminance of 1 candela per square metra

to ensure adequate retinal illuminance we must ensure chart luminance is high enough

for luminance the permissible range is 80 to 320 cd/m²

internally illuminated charts should have a luminance of not less than 120 cd/m²

to avoid glare the test chart surroundings should be illuminated to a smiliar level

external; chart contrast

like poor luminance, VA is affected by poor contrast

if L1 denotes the luminance of the white background and L2 the luminance of the black letters , the luminance contrast is given by:

( L1- L2 / L1)

VA may be resistance to reduction in contrast to 20% but does not apply to px with opacities of the ocular media