Visual Acuity Flashcards
Visual acuity is the criterion for an adequate
Refraction- which is finding the optimal lenses for patients so they can see as well as they can.
Visual acuity is the foundation on which the ____ is built
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
Visual acuity is not necessarily the _____ in a diagnosis
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.
There are more than one way to measure visual acuities (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
Detection acuity
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.
Resolution Acuity
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.
Recognition Acuity
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).
Contrast Sensitivity
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
LogMAR acuity chart (different from Snellen)
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.
Difference between Snellen and LogMAR
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)
Can you interchange Snellen and logMAR?
NO. You may get different results. You can use one or the other- but stick with it.
What does the numerator and denominator mean in 20/20?
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.
Optotype
Letter or target that a patient is viewing. Ex: reading E on a chart is the optotype
Optical infinity
20/20 in english or 6/6 in metric
5’ arc =
5/60th of a degree
How can we see 1’ arc of DETAIL?
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
Visual acuity for a
- Newborn
- Three month old
- 93 year old with complete cataracts
- Pupils react to light
- Fix and follow in both eyes
- Finger counting at 1 foot.
Must have some measure of ___ on every patient. Why?
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.
Optokinetic drum purpose
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
Catford
Objective test similar to the OKN drum, but with a metal material instead.
VA sc
Visual acuity without correction
VA cc
Visual acuity with correction
Two forms of objective acuity measurement
OKN (optokinetic drum nystagmus)
or Catford.
Subjective tests vs objective tests
Subjective- requires a response from pt’s. They could lie.
Objective- Information gathered through observations.
The size of the snellen letter indicates at what distance the letter subtends ____ at the nodal point
5’ of arc
Size of letter on a snellen chart is based on the
Tangent of the angle.
Height/distance= tan theta
Height of a letter at 6m or 20 feet
8.73 mm
If a chart is designed for 3m or 10 feet, the 20/20 letter is how tall?
4.365 mm (Half of 8.73)
The relationship between Snellen letters is ____ for both size and angle
Linear
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.
10x
87.3 mm
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.
twice
17.46mm (8.73x2)
The hardest level to read on the acuity chart is __ and the easiest is ___
B
L
Photophobia
Light sensitivity. More common in light eye/pigmented people
If _____ is held constant, the visual acuity remains relatively constant for pupils ranging from 2-6mm
Illumination
Usually the ___ the pupil, the less spherical and chromatic aberration
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
Qualities of pupils smaller than 2mm
Light diffraction and therefore, cannot have maximum acuity
Average human pupil sizes in diameter
3-4mm (in bright illumination)
Pinehole acuity has a pinhole that is ___ in diameter
1mm
Stiles Crawford effect
The small 1mm pinhole is based on this.
If the VA increases with the introduction of the pinhole, the acuity can be improved/increased by refractive means. Why?
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.
What happens if you use the pinhole acuity and acuities do not change or even decrease?
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.
