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
what does the Pellie-Robson letter chart use to measure contrast sensitivity consist of
letters of a constant size but progressively lower contrast
what does the Mars letter contrast sensitivity test use to measure contrast sensitivity consist of
similar to Pellie-Robson but smaller and used at closer distance of 50cm
what does the Regan and Bailey-Lovie contrast acuity charts use to measure contrast sensitivity consist of
- it is similar to conventional visual acuity charts
- letters on the Bailey-Lovie are at 100% and 10% contrast
and - letters on the Regan are 96%, 50%, 25%, 11% and 4%
- there is a separate chart at each contrast
why is there a separate chart at each contrast when measuring with the Bailey-Lovie and Regan contrast charts
as the idea is to measure VA at each level of contrast
why would you want to measure VA at each level of contrast using a Bailey-Lovie contrast chart or a Regan contrast chart
because for most people, VA is high at high contrast, but at low contrast VA drops by a couple of lines.
for people with VI, the drop from high to low contrast is much steeper e.g. at 96% contrast VA can be 6/12 and then at 50% contrast VA can drop to as low as less than 6/60, so then you know they have a problem with their CS
what does A1-A9 stand for in the functional acuity contrast test (FACT)
low SF’s as black and white bars = thick
what does E1-E9 stand for in the functional acuity contrast test (FACT)
high SF’s as black and white bars = thin
what does 1-9 stand for in the functional acuity contrast test (FACT)
contrast reduces from high to low
what do the results of a functional acuity contrast test (FACT) give
the patients CSF compared to a normal CS curve and this is observed to see if the patients CS is normal or abnormal
how far is the Pellie-Robson chart held and what correction should be used
- held at 1 m
- so use correction for 1 metre this can be distance rx for pre-presbyope or a +0.75D/+1.00D add for a presbyope
what 2 ways can you score contrast sensitivity with the Pellie-Robson chart
- letter by letter scoring where each letter = 0.05
2. look at the line, if the px scored 2/3 on that line, then take that score of the line
name 5 reasons why we should measure CS
- its a comprehensive evaluation of function in low vision patients (somebody can have good va’s but have lots of complaints, and this is where you measure cs)
- may be used to indicate what everyday tasks the patient is likely to have trouble with
- correlates better than va with performance on daily living tasks such as reading and mobility
- predict which patients will not benefit from optical aids (poor cs is more likely to benefit from electrical aid)
- used to determine whether to prescribe a monocular or binocular low vision aid (depending on cs in each eye)
what targets do reading charts use
words
rather than letters
which reading chart uses unrelated words
Bailey Lovie reading test
which reading chart uses sentences
MNREAD acuity charts
what things do reading charts measure
- reading (near) acuity
- reading speed
- low vision reading comprehension assessment measures comprehension as well
what 5 ways can someones near acuity be scored
- point system
- N notation
- Sloan M notation
- Keeler A system
- Jaeger notation
how does the point system work when scoring near acuity
- one point is 1/72 inch
- size of the letters is measures from the top of an ascending limb to the bottom of a descending limb and measurement is converted into points. e.g. 10 print point is, 10/72 of an inch
how does the N notation work when scoring near acuity
- N indicates that the measurement is at near
- a number following the N is the point size of the print
how does the Sloan M notation work when scoring near acuity
- commonly used in the US
- XM refers to the overall size of a lower case letter that subtend 5 min of arc at ‘X’ m
- e.g. 5M = size of the lower case letter that subtends 5 min of arc at 5 metres
how does the Keeler A system work when scoring near acuity
- used at 25cm
- letter labelled A1 subtends an angle of 5 minutes of an arc at this distance and each following size is 0.1 log units (1.25x) larger
- it is a uniform progression as you go from the top to the bottom of the chart
how does the Jaeger notation work when scoring near acuity
it is popular among ophthalmologists, but it is not standardised
J5 on one chart may be a different size to J5 on another
why is MNREAD acuity charts good for measuring reading speed
because each sentence has the same amount of 60 characters
what va range can MNREAD acuity charts measure
1.3 - 0.4/0.5 LogMAR
for what reason is measuring reading ability important
- reading is cited as the primary rehabilitation goal by most low vision individuals
- lv has sometimes been defined as the inability to read the newspaper with best optical correction at a normal reading distance
name 3 factors affecting reading
- acuity reserve
- contrast reserve
- field of view
what is acuity reserve and how is it calculated
- the print size relative to the acuity threshold
- a patients va is N6 but reads N12, then the reserve is 12/6 = 2:1
what is contrast reserve and how is it calculated
- the print contrast relative to contrast threshold
- a patients contrast is 30%, they read a print at contrast 90%, then the reserve is 90/30 = 3:1
what is field of view
the number of simultaneously visible characters
what 2 things can lower reading speed
- poor FOV
- Magnifier - as print gets larger
how many words per minute is optimum reading
300 wpm (no lv px will achieve this)
how many words per minute is fluent reading
160 wpm (most likely with a lv px)
how many words per minute is spot reading
40 wpm
what is the, acuity reserve, contrast reserve, scotoma diameter and FOV in characters, for someone who has optimal reading (300 wpm)
acuity reserve: 6:1
contrast reserve: 30:1
scotoma diameter: 0 degrees/none
FOV: 4-6 characters
what is the, acuity reserve, contrast reserve, scotoma diameter and FOV in characters, for someone who has fluent reading (160 wpm)
acuity reserve: 3:1
contrast reserve: 10:1
scotoma diameter: 4 degrees/small
FOV: 4-6 characters
what is the, acuity reserve, contrast reserve, scotoma diameter and FOV in characters, for someone who has spot reading (40 wpm)
acuity reserve: 1:1/none
contrast reserve: 3:1/limited
scotoma diameter: 30 degrees
FOV: 1 character
what can you use to measure central visual fields
amsler grid or tangent screen
what can you use to measure peripheral visual fields
arc perimeter
what tool to measure visual fields is usually done is research
scanning laser ophthalmoscope
what is the advantage and disadvantage of the microperimetry (Nidek MP1)
- good for people who don’t have good fixation e.f. AMD px with a central scotoma, their fixation will keep varying so the whole screen of the Nidek MP1 will move if a patients fixation moves
- but it is expensive
why is automated perimetry not used on LV patients
as it is unreliable in LV patients particularly AMD as it is difficult to monitor their fixation
name 4 reasons why you want to measure visual fields on a LV patient
- diagnostic importance
- provide functional assessment of the extent, location and quality of areas of best vision
- can help the clinician determine what low vision aids and rehabilitation strategies the patient might benefit from
- provide information about how well an individual performs in real life conditions
what 2 tests will you use to evaluate binocular vision on an LV patient
- cover test (only done if LV patient complains of diplopia)
- stereopsis e.g. titmus test, frisby test, TNO test
which stereopsis test is best used on a LV px and why
frisby
because with the other tests, px has to use glasses which reduces their contrast sensitivity
what 3 subjective VA tests could you carry out on a patient to find out their VA behind mild to moderate media opacities e.g. cataract
- potential acuity meter (PAM)
- laser interforemeter
- vernier acuity
what test will you carry out to differentiate between early macular disease and optic nerve disease and how it is carried out
photostress recovery time
px is to read the snellen chart, shine a bright light into their eye, the px reads the snellen chart again, most patients should read the same as they did before and after but if someone has macular disease, it takes them longer to recover and read the chart or may not reach the same line
how is a potential acuity meter used to find out a patients va behind their media opacity
the meter is placed on the slit lamp, and you focus it so the snellen chart comes into focus on the back of the patients retina, you get the patient to read down the snellen chart and record it. once the cataract surgery is performed the results should be the level of vision they can get to
how is a laser interferometer used to find out a patients va behind their media opacity
it is similar to the potential acuity meter, but gratings are used instead of a snellen chart and px tells you the orientation, this is then converted into a visual acuity
how is a vernier acuity used to find out a patients va behind their media opacity
there are two lines one above the other, the patient is to align them, the better the patient can align them then the better their prognosis is likely to be following cataract surgery
how will you do glare testing as a subjective test of patients with a media opacity and retinal loss
by using the brightness acuity test
- a reduction of VA will be in presence of glare when a media opacity is present
when will you do colour vision testing as a subjective test of patients with a media opacity and retinal loss and what will you use
- on someone who has optic atrophy or damage to ONH
- R/G colour vision loss will be noticed and the red will appear faded
- dont do all colour vision tests in LV clinic but if decide to do then do a jumbo D15 test
list the objective testing done on patients with media and retinal loss
- B-scan ultrasonography
- Electro-diagnostic testing e.g. ERG, VEP, EOG
this gives info about a patients visual function
which Electro-diagnostic test will you do on a patient with multiple sclerosis
VEP
which will show a delayed response
Electro-diagnostic test will you do on a patient with bests disease
EOG
what 3 things do quality of life questionnaires QOLs assess
functional capacity, social intersections and relationships, and wellbeing
i.e. how well a patient is able to cope in the environment they find themselves in
what type of questionnaires are less used in LV clinics and more as a research tool
- low vision quality of life questionnaire LVQOL
- national eye institute visual function questionnaire NEIVFQ
