1.4 - Using a snellen chart Flashcards

1
Q

What is the standard observer

A

The standard observer has a visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc

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2
Q

Why do we need a standard observer

A

We need a baseline measure for comparison
Develop a standard for comparison
To know how well we should do

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3
Q

What is form vision

A

The ability to recognise an object from its “form”/shape

e.g. A - easy to recognise due to its form - triangular shape - unusual

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4
Q

What is resolution

A

The ability to detect fine detail e.g. gaps

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5
Q

What does recognition of letters require

A

Recognition of letters as they get progressively smaller requires both form vision and resolution

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6
Q

What is visual acuity

A

Smallest line of letters read with spectacles or contact lenses worn (provided they correct all defects of focus)

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7
Q

What is vision

A

Smallest line of letters read with the unaided eye ( no refraction correction I.e. no glasses/CL’s)

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8
Q

When is VA and vision same for a px and when is it different

A
  • If you don’t wear glasses/CL’s i.e. no refractive correction = unaided vision and va are. same
    BUT
  • If you have refractive error i.e. wear glasses/CL’s = VA different to unaided vision where they don’t wear refractive correction
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9
Q

How can vision be measured

A

Both eyes = binocular

One eye = monocular

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10
Q

Which one is better - monocular or binocular

A

Binocular - gives advantage when using both eyes

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11
Q

How can vision/va be recorded

A

Snellen Fraction V ( most common )
Minimum Angle of Resolution (MAR)
Decimal V

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12
Q

What is the snellen fraction

A

see equations in notebook

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13
Q

What does V = 6/18 mean

A

6: Testing distance – 6m is standard in the UK
(20 feet in the USA)

18: Distance at which the limbs of the last line of letters correctly identified subtend1’ arc. Equivalently the angular height of the last line of letters correctly identified is 5’ arc

= all letters on 18m line correctly identified

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14
Q

What is minimum angle of resolution

A
  • Angular separation of two points or lines at the resolution acuity limit
  • Eye’s ability to see fine detail
  • Angle subtended by limb of letter at observer for last line of letters correctly identified
  • Angle A
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15
Q

What is MAR for standard observer

A

1’ arc

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16
Q

How to convert snellen to decimal

A

Decimal V = 1/MAR

MAR is in minute of arc
e.g. 6/5 = 1.2

17
Q

Recording Snellen V/VA

A
  • If all letters are correctly identified on the final line read, record as the Snellen fraction e.g. 6/5 if the 5m line was completely read
  • If only 2 letters are correctly identified on the 6/6 line, record as 6/9 +2. This tells us all letters on the ‘9m’ line were correctly identified + 2 letters on the next (6/6) line
  • If, for example, 3 letters are incorrectly identified on the 6/5 line then record 6/5 -3

The +/- number should be as small as possible so 6/5 - 3 rather than 6/6 + 5

18
Q

What to do if VA below 6/60

A
  • Reduce distance to chart until Px can read top letter. If at 3m then record 3/60 ( numerator = testing distance )
  • Counting Fingers. Record CF @25cm if Px could count fingers at 25cm ( 25cm = distance from subject ). Counting fingers should be at less than 1m.
  • Hand movements. Record HM if Px can detect your hand moving at 1/2m
  • Perception of Light. Record PL if Px can detect your pen torch. (Can also record if they can accurately detect which quadrant the light is projected into) - can px accurately project ie. they know where the light source is - quadrant perception = projecting light into different parts of eye
  • No Perception of LightRecord NPL if Px cannot detect light
19
Q

Practical Matters

A
  • Do not ask someone with vision/VA below 6/60 to jump out of the chair. They are sight impaired and may trip in a dark consulting room!
  • Increase the letter size above ‘60m’ on computer charts
  • Use a single ‘60m’ letter E, which you hold at different distances. Rotate it to test if it is resolved.
  • Counting fingers should be at less than 1m. If 1m or more then use increased letter sizes or the ‘60m’ E described above