AVPS - Measuring Distance Vision/VA - Topic 3 Flashcards
What is Vision?
The faculty or state of being able to see.
Define vision in optometry.
The smallest line of letters than can be seen BEFORE new refractive error (the optical prescription) is determine.
Define Visual Acuity. (VA)
The smallest line of letters that can be seen with the BEST possible refractive correction in place
What’s distance vision/VA measured for?
- Measured for optical infinity
- IRL we use 6m
What’s near vision/VA measured for?
- Measured for reading distance
- Around 40cm,but is measured at the point where a patient carries out near tasks
What’s Intermediate vision/VA measured for?
-About arms length
◦Typically a consideration for patients who use computers, read music etc
Why Measure vision/VA?
1) Legal Requirement
2) Baseline Data – To provide a baseline recording.
3) Diagnostic Tool - To aid the examination and diagnosis of eye disease or refractive error (can demonstrate the need for glasses/contact lenses).
4) Measures progression of disease - To assess any changes in VA over time.
5) Evaluates treatment - To measure the outcomes of therapeutic intervention, e.g. cataract surgery.
6) Patient Expectation – Many people still think this IS the eye test
What are we actually measuring in the VA test?
Minimum Angle of Resolution (MAR) = The ability to see detail
When can two distinct points be recognised as separate?
-Two distinct points can only be recognized as separate when they subtend an angle of one minute of arc at the nodal point of the eye.
What can an average eye see a gap of for the MAR?
-An “average” eye can see a gap of 1min of arc (1”)
Using the ‘letter chart’,i.e the snellen chart, how can we find out the MAR of that patient?
Each line is numbered, and the smallest line read tells us the MAR of that patient
How did Snellen define ‘standard vision’ ?
Snellen defined “standard vision” as the ability to recognize an optotype (a letter on the chart) when it subtends an angle of 5 minutes of arc (5’)
What’s Snellen’s Fraction?
Snellen’s fraction = Testing distance (usually 6m) / Distance at which optotype subtends an angle of
Using the Snellen Chart, what is the numerator and denominator in the snellen fraction?
Numerator= Testing distance(Usually 6m)
Denominator= the distance(x) at which the critical details of this letter subtend 1min of arc
so 6/x
Advantages of using Snellen?
Advantages:
Commonly used and recognised
Easy recording
Can be portable
Disadvantages of using Snellen?
Disadvantages:
X Different number of letters on each line
X Unequal size progression per line (no 6/7.5 or 6/15 line)
X Less repeatable than LogMAR
X 1/3 less sensitive than LogMAR to interocular differences in vision
Advantages of using LogMAR?
Advantages:
1) Same number of letters on each line
2) Even line spacing and crowding
3) Standardised recording – easier to compare; letters scored individually
4) Logarithmic progression – 0.1 steps
5) More repeatable than Snellen
6) More sensitive to interocular differences than Snellen
Disadvantages of using LogMAR?
Disadvantages:
X Large, not portable
X Less commonly used, so less recognisable
X Recording can be trickier initially to interpret – minus numbers mean better vision
What’s the procedure of measuring distance vision/VA?
Step 1• Ensure chart at appropriate distance from patient, and that patient is seated comfortably
Step 2• Make sure the room lights are on
Step 3• Explain the test to patient and gain their verbal consent to carry out the procedure
Step 4• Cover one eye with your occluder
Step 5•Ask the patient to read the smallest line on the chart that they can (push them to read as small as possible)
Step 6•Record your result, then repeat the procedure for the other eye, and then again with both eyes open (BEO)
What if my patient reads less than half the line?
– record as the line above, + how many letters are seen from that line
E.g. Patient reads 6/12 line fully, and then 2 letters from the 6/9 line, record as:
6/12 +2
What if my patient reads more than half the line?
– record as that line, minus how many letters are missed from that line
E.g. Patient reads 6/12 line fully, and then 4 letters from the 6/9 line, record as:
6/9 - 2
What if my patient reads the whole of the 6/24 line, but then only 2 letters of the 6/18 line, which has 4 letters in total?
6/24 +2 or 6/18 -2
Both are correct
What if my patient can’t see any of the letters at 6m, and cant use pinhole?
- Bring them closer to the chart/bring the chart closer to them (if possible)
- Start by halving the distance from 6m to 3m. IF STILL NOT, then 2m and then 1m if necessary.
- So instead at line ‘6/24’, it will be 3/24 which is written as 6/48 to make comparison easier.
What if my patient can’t see any letters on the chart even at 1m?
- Count Fingers (CF) – measure the maximum distance at which the patient can count the number of fingers you are holding up. Record as CF@ ? cm
- Hand Movements (HM) – measure the maximum distance at which the patient can see your hand moving. Record as HM @ ? cm
- Light Perception (LP) – hold a pentorch 50cm away from the patient and ask if they can tell you when you turn the light on or off. If they can, record as LP. If they cannot, record as NLP (no light perception)
Why is lighting important?
- Bright light causes the pupil to constrict, which can result in a better vision measurement than expected.
- British regulations are “normal room lighting” to simulate natural viewing conditions (between 80-320 lux).
When is the Pinhole useful to us when measuring VA?
We can use this to our advantage to get an idea of a patient’s expected distance VA.
Use a pinhole whenever VA is not 6/6. Follow the same procedure as before, but with the pinhole in front of the eye you are testing.
What does a pinhole do and how does it help us?
The pinhole reduces the amount of refractive blur (smaller blur circle on retina; the same effect as reducing the pupil size) and thus helps to distinguishes between refractive blur and disease.
If vision doesn’t improve through a pinhole, suspect disease
What are the Limitations of measuring distance vision/VA ?
◦ Only measures central vision.
◦ Only measures ability to see high contrast letters (doesn’t test contrast sensitivity).
◦ 6m is not the same as optical infinity (vergence of light at 6m is -0.167D)
