4: predicitng unaided distance vision in eyes with different types and amounts of uncorrected refractive error

impacts of uncorrected refractive error on distance vision

accom doesnt help myopes see more clearly in distance

uncorrected hyperopia may or may not reduce distance vision, depends on accom which depends on the age of the px . if theres enough accom distance vision will be good

does astigmatism reduce distance vision

always reduces distance vision but to a lesser extent than myopia and hyperopia that cant overcome by accom

accom helps distance vision but doesnt overcome astigmatism

eg of when / why we need to predict uncorreccted refractive error from distance vision

• we conduct a refraction and find the eye to be myopic by -1.00 DS ubt when do an unaided vision check we find that itd 6/4.8 VAR 105

• eye is myopic so there shoud be a reduction in VA

• or when we conduct a refraction and find the eye hyperopc by +1.00DS by a px aged 10 and when we do unaided va it is 6/12 VAR 85. shouldnt be so low as theyre young so should be accomodating

predicting unaided distance vision from uncorrected refractive error

for spherical ametropia eg -3.00DS myopia

can predict the unaided vision as every 0.25 dioptres is a reduction of 1 line

so 0.25 × 3 = 12 lines reduction

5 letters is 1 line

12 lines is 60 VAR reduction

so 110 ( starting VAR) -60 = 40 VAR

predicting unaided vision from sph/cyl refractive error

S + C/2

sphere equivalent refractive error

or second method (pic)

what is the sphere equiv and vector length of -3.00/-1.50 × 180

• sphere equivalent

-3.00 + -0.75 = -3.75D

• vector length = -3.82D

2 approaches for relating unaided vision and refractive error

• spherocylindrical refractive errors and visual acuity. Thomas W. Raasch

• relation between spherical refractive error and visual acuity . George Smith

Raaschs approach for predicting VA from refractive error

unaided vision when we plug values into the equation

plugging in the vector length

if refractive error was -2.00/-1.00 × 180 the vector length would be 2.55D

when this entered the raaschs equation we get a predicted unaided vision of logMAR 0.99

VAR= 100-50 . LogMAR

VAR= 50 × 0.99 = 50

vector must be positive in equation egif vector length is -3.00, it would be entered as +3 in equation

for emmetropia vector value is 0.1

this gives a LogMAR score of -0.13

equivalent to a VAR of

( 100-50 ) x 0.13 = 106.5

data points if Raaschs approach

0.1 where eyes is emmetropic, expect the eye to achieve high VA score 20/20

when vector length gets bigger, refractive error increases

eg a vector length of 8 gives an unaided VA of 20/1000

each data point represents data from at least 50 points

hyperopia and hyperopic astigmatism were not plotted as accom is an unknown

myopia, myopic astigmatism and mixed astig plotted

Smiths approach for predicting unaided vision from refractive error

it is in MAR minimum angle of resolution

A= refractive error

E= power in dioptres

Dmm= pupil diameter

k= constant estimated from theory 0.83

E could be the spherical equivalent refractive error or the vector refractive error

advantage of smiths is that is includes pupiil diameter

another version of smiths approach

only used when the refractive error is very close to 0

comparing raasch and smiths

less defocus means better VA or slightly better VA : not directly proportional

with a cylinder prescription, use the vector length not spherical equivalent

spherical defocus

smiths: -1D myopia predicts a var reduced to 80 compared to 75 with Raasch

what can we conclude from the approaches with the n0 lines lost

astig affects va

said that every 0.50D cylinder reduces A by 1 line

what would you expect the unaided vision to be in an eye with -1.00 /-0.50 × 80

4 lines lost since there is 1D of myopia

extra 1 line lost as theres 0.50D astigmatism

so 5 lines lost in total approx

5 × 5 = 25

105-25 = 80

predict the unaided vision with eye of -1.50 /-1.00 × 45

6 line reduction for the -1.50 myopia

2 line reduction for the cylinder

8 lines x 5 letters on each line = 40

105-40 = 65 VAR