Visual Acuity

Question 1

Visual acuity is the criterion for an adequate

Answer 1

Refraction- which is finding the optimal lenses for patients so they can see as well as they can.

Question 2

Visual acuity is the foundation on which the ____ is built

Answer 2

Vision exam (some people may not have the same potential acuity in one or both eyes compared to other people). It is built on vision exam, but there are cases when 20/20 cannot be met. Ex: corneal scar preventing clear vision.

USE VISUAL ACUITIES TO GUIDE THE EXAM

Question 3

Visual acuity is not necessarily the _____ in a diagnosis

Answer 3

Final endpoint. This is the art to the science. Yes there is a science to finding the prescription, but the art is fitting the prescription to the patient and giving them what they will be happiest in.
Ex: Give the pt the least amount of minus and most amount of plus to make them happy.

Question 4

There are more than one way to measure visual acuities (4)

Answer 4

Detection acuity - is an object present?
Resolution Acuity - Detect small misalignment
Recognition Acuity - Snellen/LogMAR/broken C/Tumbling E/LEA symbols, HVOT. Detect, recognize and identify.
Contrast Sensitivity - Distinguish bar pattern rather than a uniform grey. 30 cycles= 20/20

Question 5

Detection acuity

Answer 5

The patient reporting whether they are looking at a blank field or if something is there. They do not have to identify the object. Just recognize something is there.

Question 6

Resolution Acuity

Answer 6

What we’re best at. The ability of the human eye to be able to detect a very small misalignment of a linear object or dot. The angle of displacement determines the most accurate type of acuity.

Question 7

Recognition Acuity

Answer 7

Snellen Acuity Chart. The ability to see a target, recognize what it is, and identify it. The determination of a threshold of recognition of two or more targets or objects on a contrasting background.

For patients who are non-verbal/illiterate: Use the tumbling E (table top), broken wheel/C (top, bottom, left or right), HVOT, or LEA symbols (only for kids).

Question 8

Contrast Sensitivity

Answer 8

Minimum contrast which is required to distinguish that there is a bar pattern rather than a uniform grey. This is measured by test gratings of equal width; bright and dark bars in a sine-wave pattern. 30 cycles per degree = 20/20

2 mins of arc in 1 cycle of the sine wave … there are 60 minutes in 1 degree… that’s 30 cycles per degree

Question 9

LogMAR acuity chart (different from Snellen)

Answer 9

Logarithm of the minimum angle of resolution
More common in research type acuity measurements.
Relative to the size of the letter, LogMAR keeps the space between the letters consistent - The ratio to size to space between the adjacent letter.

Question 10

Difference between Snellen and LogMAR

Answer 10

LogMAR keeps the space between the letters consistent - The ratio to size to space between the adjacent letter.

Snellen does not keep the spacing consistent, relatively. Larger spaces between low level (20/200) acuities and high level acuities (20/20)

Question 11

Can you interchange Snellen and logMAR?

Answer 11

NO. You may get different results. You can use one or the other- but stick with it.

Question 12

What does the numerator and denominator mean in 20/20?

Answer 12

Numerator- The distance of testing situation or where the patient stands. Can mirror chart or calibrate if you cannot test patient at 20 feet.

Denominator- Distance at which the letters subtend 5’ of arc at the nodal point of the eye or the letter the patient reads. AKA 5 minutes of 1 degree of the 360 degree theoretical visual perception.

20/20 the size of the letter at 20 feet away will take up (subtend) a 5’ portion of the visual field.

Question 13

Optotype

Answer 13

Letter or target that a patient is viewing. Ex: reading E on a chart is the optotype

Question 14

Optical infinity

Answer 14

20/20 in english or 6/6 in metric

Question 15

5’ arc =

Answer 15

5/60th of a degree

Question 16

How can we see 1’ arc of DETAIL?

Answer 16

Cones densely packed in the center of the retina and 1 photoreceptor will give you varying information from the one next to it. Very precise and detailed.

Limiting factors: cone density and neural connection

Question 17

Visual acuity for a

1. Newborn
2. Three month old
3. 93 year old with complete cataracts

Answer 17

1. Pupils react to light
2. Fix and follow in both eyes
3. Finger counting at 1 foot.

Question 18

Must have some measure of ___ on every patient. Why?

Answer 18

Visual acuity

Say someone comes in with something in their eye and then you add a numbing drop and then they say the drop caused their vision to worsen and it’s my fault….. Legal reasons.

Question 19

Optokinetic drum purpose

Answer 19

Form of objective test based on principle of optokinetic nystagmus (eyes moving back and forth to follow the drum or to follow trees while in a car driving)

Without purposely trying, the eyes of a seeing patient will follow the drum. Called an OKN response

Question 20

Catford

Answer 20

Objective test similar to the OKN drum, but with a metal material instead.

Question 21

VA sc

Answer 21

Visual acuity without correction

Question 22

VA cc

Answer 22

Visual acuity with correction

Question 23

Two forms of objective acuity measurement

Answer 23

OKN (optokinetic drum nystagmus)

or Catford.

Question 24

Subjective tests vs objective tests

Answer 24

Subjective- requires a response from pt’s. They could lie.

Objective- Information gathered through observations.

Question 25

The size of the snellen letter indicates at what distance the letter subtends ____ at the nodal point

Answer 25

5’ of arc

Question 26

Size of letter on a snellen chart is based on the

Answer 26

Tangent of the angle.

Height/distance= tan theta

Question 27

Height of a letter at 6m or 20 feet

Answer 27

8.73 mm

Question 28

If a chart is designed for 3m or 10 feet, the 20/20 letter is how tall?

Answer 28

4.365 mm (Half of 8.73)

Question 29

The relationship between Snellen letters is ____ for both size and angle

Answer 29

Linear

Question 30

If a chart is designed for 6m or 20 feet, the 20/200 letter is ___times bigger than the 20/20. So the 20/200 letter would be ___mm.

Answer 30

10x

87.3 mm

Question 31

If a chart is designed for 6m or 20 feet, the 20/40 letter is ___ as big as the 20/20 letter, so the letter size would be ___ mm.

Answer 31

twice

17.46mm (8.73x2)

Question 32

The hardest level to read on the acuity chart is __ and the easiest is ___

Answer 32

B

L

Question 33

Photophobia

Answer 33

Light sensitivity. More common in light eye/pigmented people

Question 34

If _____ is held constant, the visual acuity remains relatively constant for pupils ranging from 2-6mm

Answer 34

Illumination

Question 35

Usually the ___ the pupil, the less spherical and chromatic aberration

Answer 35

smaller- this helps because the small pupil cuts out peripheral rays of lights. However, you don’t want the pupil to be smaller than 2mm. That may cause problems with too MUCH light diffraction

Question 36

Qualities of pupils smaller than 2mm

Answer 36

Light diffraction and therefore, cannot have maximum acuity

Question 37

Average human pupil sizes in diameter

Answer 37

3-4mm (in bright illumination)

Question 38

Pinehole acuity has a pinhole that is ___ in diameter

Answer 38

1mm

Question 39

Stiles Crawford effect

Answer 39

The small 1mm pinhole is based on this.

Question 40

If the VA increases with the introduction of the pinhole, the acuity can be improved/increased by refractive means. Why?

Answer 40

This indicates that the pt’s eyes are working properly and that it is simply a refractive fix that will make vision sharper. Indicates that the poor vision is not caused due to something like a corneal scar.

Question 41

What happens if you use the pinhole acuity and acuities do not change or even decrease?

Answer 41

This indicates that the reduced VA is not refractive, but organic or pathological. For example, if acuities decrease, this could indicate a cataract that is worse where the pinhole is located.