measuring visual function in low vision patients Flashcards
list 8 reasons for the importance of measuring visual acuity in LV patients
- to compare with ‘normal’ performances (to know if someone has reduced va’s)
- set a baseline to monitor changes overtime
- to quantify patients’ subjective impression of visual performance
- early detection and diagnosis of disease
- assess benefits of optical devices e.g. mag or telescope
- refraction procedures and decision making
- social-legal purposes e.g. registering as SI or SSI etc
- predicting visual function for everyday life e.g. if legal to drive
why is it useful to record va’s in order to quantify a patient’s subjective impression of visual performance
to see if it correlates with what we’ve measured
why is it useful to record va’s in order predict visual function for everyday life
to see if legal to drive etc
what does 6/60 stand for in snellen notation
at 6 metres the letter (60) will subtend 5 minutes of arc
with a 6/60 letter at __ metres the letter will subtend _ minutes of arc, so if you have somebody who has _______ vision, if they stand at __ metres they ______ _____ be able to see 6/60. But somebody who has __________ vision who can _____ see the 6/60 letter means they will need to walk to _ metres and at _ metres they will be able to see the ____ letter
with a 6/60 letter at 60 metres the letter will subtend 5 minutes of arc, so if you have somebody who has normal vision, if they stand at 60 metres they would still be able to see 6/60. But somebody who has impaired vision who can only see the 6/60 letter means they will need to walk to 6 metres and at 6 metres they will be able to see the 6/60 letter
how is VA calculated
test distance/distance the letter subtends 5 min of arc
how do you calculate to decimal notation from snellen e.g. 6/60
6/60 = 0.1
just divide 6 by 60
how do you calculate minimum angle of resolution of 6/60
60/6 = 10 min of arc
how do you calculate the logarithm of MAR (LogMAR) of 6/60
6/60 is Log of 10 which = 1.0
how do you calculate to keller A system notation from snellen
1 min arc MAR us A1 i.e. 6/6 = A1
list 4 ideal properties of a chart used for low vision
- charts should be moveable (as most px cannot see at 6m)
- variable illumination
- high contrast
- good range of sizes
why is it ideal to have a good range of sizes of a test chart for low vision
to measure a range of acuities with one chart
what are the 2 most commonly used charts to measure va in low vision patients
- snellen chart
- LogMAR charts
list the 3 types of LogMAR charts used to measure va in low vision
- Bailey-Lovie distance visual acuity chart
- Early treatment of diabetic retinopathy study chart
- keller A chats
list 2 advantages of the snellen chart
- cheap and readily available
- can be used to obtain:
baseline visual acuity measurements
predict and verify magnification for low vision aids
= easy to make calculations
list 4 disadvantages of the snellen chart
- inability to control the crowding phenomenon
- non-uniformity in letter size progression
- inability to easily score letter by letter acuity
- insufficient acuity for accurately testing low vision patients
why does the snellen chart have an inability to easily score letter by letter acuity
as theres not the same amount of letters on every line
list 4 advantages of the LogMAR chart
- equal number of letters on every line
- logarithmic progression of letter size per line
- letter by letter acuity can also be measured
- more popular in a research setting
list 2 disadvantages of the LogMAR chart
- cost is expensive
- large size of wall mounted version
what is used where it is not possible to use a letter chart to measure VA
BRVT - the berkeley rudimentary vision test
what three things is the BRVT - the berkeley rudimentary vision test composed of
3 pairs of hinge cards at 25cm
- single tumbling E card pair
- grating acuity card pair
- basic vision card pair
when using the BRVT - the berkeley rudimentary vision test
start off with the _________ _ card at _m. if can’t see any letters then show it at __cm. if you show the chart at _m the tumbling E card’s LogMAR ranges from - LogMAR, at 25cm the LogMAR ranges from _ - _ LogMAR in steps of ___ LogMAR.
If cannot see the tumbling E then go onto ______ ______ which is shown at __cm and the patient has to identify whether it is _______ or _______. this measures VA from __-__ LogMAR in steps of ___ LogMAR.
if cannot see the grating acuity test then use the _______ _______ card pair which consists of the _______ and _______ targets, the patient must say whether the targets are black or white, this is equivalent to __ or ___.
start off with the tumbling E card at 1 m. if can’t see any letters then show it at 25 cm. if you show the chart at 1 m the tumbling E card’s LogMAR ranges from 2-4 LogMAR, at 25cm the LogMAR ranges from 2.6 - 2 LogMAR in steps of 0.02 LogMAR.
If cannot see the tumbling E then go onto grating acuity which is shown at 25cm and the patient has to identify whether it is horizontal or vertical. this measures VA from 2.3-2.9 LogMAR in steps of 0.2 LogMAR.
if cannot see the grating acuity test then use the basic vision card pair which consists of the black and white targets, the patient must say whether the targets are black or white, this is equivalent to LP or NLP.
what is similar to the BRVT - the berkeley rudimentary vision test but is a computerised version
and what does it used instead
the Freiburg visual acuity test
uses Landolt C instead
the Freiburg visual acuity test
uses a ____________ system to __________ and _________ the ______ of the letter __. only an ________ letter __ is used, so no _________ is in this test. the size of the letter increases or decreases depending on what ________ the _________ gives
uses a psychophysical system to increase and decrease the size of the letter C. only an isolated letter C is used, so no crowding is in this test. the size of the letter increases or decreases depending on what response the patient gives
what is contrast threshold
the smallest difference in luminance that an observer can detect
what is contrast threshold measured with
sine wave gratings
what are sine wave gratings
alternate light and dark bars of variable spatial frequency (cycles/degree) and contrast
other than sine wave gratings, what else can be used to measure contrast threshold and how
letters on a Pellie Robson chart
what are the values from and to on a Pellie Robson chart when measuring contrast threshold and what are the results expressed as
values vary from 0 to 1
usually expressed as a % from 0 - 100%
what is contrast sensitivity
the reciprocal of contrast threshold
what are the values of contrast sensitivity from and to
vary from 1 to infinity
what is the contrast sensitivity function CSF
a plot of contrast sensitivity over a range of spatial frequencies
what is the cut off at a CSF used for
to determine someones grating va
what is the cut off frequency
the point where CSF cuts at the x-axis and this represents the grating va
how can the denominator of the snellen va be determined from the cut off frequency on a CSF
by dividing 180 by the cut-off frequency
e.g. cut off frequency of 30 cycles per degree gives you a denominator of 6 and a snellen va of 6/6
180/30 = 6 (the denominator)
list 4 methods/clinical tests for measuring contrast sensitivity
- functional acuity contrast test (FACT)
- Pellie-Robson letter chart
- Mars letter contrast sensitivity test
- Regan and; Bailey-Lovie low contrast acuity charts
what does the functional acuity contrast test (FACT) to measure contrast sensitivity consist of
sine wave gratings at different spatial frequencies