AVPS - Measuring Distance Vision/VA - Topic 3

Question 1

What is Vision?

Answer 1

The faculty or state of being able to see.

Question 2

Define vision in optometry.

Answer 2

The smallest line of letters than can be seen BEFORE new refractive error (the optical prescription) is determine.

Question 3

Define Visual Acuity. (VA)

Answer 3

The smallest line of letters that can be seen with the BEST possible refractive correction in place

Question 4

What’s distance vision/VA measured for?

Answer 4

• Measured for optical infinity

- IRL we use 6m

Question 5

What’s near vision/VA measured for?

Answer 5

• Measured for reading distance

- Around 40cm,but is measured at the point where a patient carries out near tasks

Question 6

What’s Intermediate vision/VA measured for?

Answer 6

-About arms length

◦Typically a consideration for patients who use computers, read music etc

Question 7

Why Measure vision/VA?

Answer 7

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

Question 8

What are we actually measuring in the VA test?

Answer 8

Minimum Angle of Resolution (MAR) = The ability to see detail

Question 9

When can two distinct points be recognised as separate?

Answer 9

-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.

Question 10

What can an average eye see a gap of for the MAR?

Answer 10

-An “average” eye can see a gap of 1min of arc (1”)

Question 11

Using the ‘letter chart’,i.e the snellen chart, how can we find out the MAR of that patient?

Answer 11

Each line is numbered, and the smallest line read tells us the MAR of that patient

Question 12

How did Snellen define ‘standard vision’ ?

Answer 12

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’)

Question 13

What’s Snellen’s Fraction?

Answer 13

Snellen’s fraction = Testing distance (usually 6m) / Distance at which optotype subtends an angle of

Question 14

Using the Snellen Chart, what is the numerator and denominator in the snellen fraction?

Answer 14

Numerator= Testing distance(Usually 6m)

Denominator= the distance(x) at which the critical details of this letter subtend 1min of arc

so 6/x

Question 15

Advantages of using Snellen?

Answer 15

Advantages:
Commonly used and recognised
Easy recording
Can be portable

Question 16

Disadvantages of using Snellen?

Answer 16

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

Question 17

Advantages of using LogMAR?

Answer 17

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

Question 18

Disadvantages of using LogMAR?

Answer 18

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

Question 19

What’s the procedure of measuring distance vision/VA?

Answer 19

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)

Question 20

What if my patient reads less than half the line?

Answer 20

– 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

Question 21

What if my patient reads more than half the line?

Answer 21

– 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

Question 22

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?

Answer 22

6/24 +2 or 6/18 -2

Both are correct

Question 23

What if my patient can’t see any of the letters at 6m, and cant use pinhole?

Answer 23

• 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.

Question 24

What if my patient can’t see any letters on the chart even at 1m?

Answer 24

1. 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
2. Hand Movements (HM) – measure the maximum distance at which the patient can see your hand moving. Record as HM @ ? cm
3. 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)

Question 25

Why is lighting important?

Answer 25

• 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).

Question 26

When is the Pinhole useful to us when measuring VA?

Answer 26

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.

Question 27

What does a pinhole do and how does it help us?

Answer 27

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

Question 28

What are the Limitations of measuring distance vision/VA ?

Answer 28

◦ 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)