Part I: Vision Screening Flashcards
What is VA Measurement Testing?
Testing EVERY structure upon which visual acuity depends, namely nearly every structure in the eye the entire neurological mechanism of vision brain structures underlying memory & cognition
Analogy for VA Measurement
If the strength of the whole chain is determined by its weakest link, the sharpness of the VA is determined by the most poorly functioning structure
We can conclude that every structure making up the pathway is working normally
This is reassuring since even one weak link in the complex machinery will cause a reduction in the VA
x
Adnexa [part of organ]
Consider the structures around the eye Lids (in contact with the eye); ptosis Quality of the tear film (lack of; debris) Inflammation of orbital structures
x
x
Optics of the Eye
- Health (clarity and shape of structures)
- Cataracts or scarring of the cornea
- Irregularity of the optical structures
Retina
- Health of the structures
- Bleeding
- Swelling or loss of cells
- Macular degeneration
Optic Nerve or Visual Brain
- Health of the structures
- Consequences of stroke (ischemia)
- Consequences of inflammation (optic neuritis)
- Functional problem
- Mis-wiring of brain neurons (amblyopia)
VA Involves Psychophysics
- Psycho involves brain machinery that enables interpretation of input and appropriate response
- Physics involves the stimulus
- Visual Psychophysics - the relationship between what someone sees, or say they see, and a particular visual stimulus
Reduced VA May Be Functional
- Vision may be reduced because the patient:
- Does not care about trying
- Cannot name the stimulus
- Misunderstands what they are being asked to do
- Is unable to respond
- Cannot pay attention or is uncooperative
Review of Visual Acuity Measurement
- Determining the VA tests EVERY structure upon which visual acuity depends, namely
- Nearly every structure in the eye
- The entire neurological mechanism of vision
- The brain structures underlying memory & cognition
Types of VA: [3]
Review of Visual Acuity Measurement
- Determining the VA tests EVERY structure upon which visual acuity depends, namely
- Nearly every structure in the eye
- The entire neurological mechanism of vision
- The brain structures underlying memory & cognition
Types of VA: [3]
- Detection
- Resolution
- Recognition
Types of VA
- Detection - can the patient see that something is there
- Resolution - can the patient distinguish the components of a complex stimulus that is detected
- Recognition - can the patient identify or name the stimulus that he can both detect and whose parts he can resolve
Screening and Clinical VA
- Visual acuity measured during a screening is resolution VA
- Detection and recognition are made very easy by making targets
- Highly familiar and bright
High contrast
•By far the most commonly used & familiar VA test is based on the one published in 1862 by Hermann SNELLEN
Snellen’s Parameters & Assumptions
- Basic target
- Standard observer
- Standard test distance
- Actual size of basic target
- Standard Snellen VA
- To understand the Snellen system, we have to distinguish the visual size from the physical size of targets
Visual Size = Angle
- Physical size of objects is normally reported in millimeters or centimeters (occasionally in inches or feet)
- Visual size is measured and reported in angular units (degrees; minutes of arc or “minarc”; or seconds of arc or “secarc”
- degree = 1/360 of a circle
- minarc = 1/60 of a degree
- secarc = 1/60 of a minarc
- For visual size, the angle starts at the center of the front of the eye
Question #1. T/F The Snellen system tries to make both detection & recognition very easy, so that the VA we measure depends upon resolution
Snellen System: Basic Target
- critical detail (visual size of the lines that make up the letters) is 1.0 minarc
- overall height (top to bottom ) = 5 times the critical detail
- Therefore, the visual size of the overall height of the basic target is 5.0 minarc
Snellen System: Standard Observer
- The standard observer can recognize letters with a visual size of
- 1 minarc critical detail and
- 5 minarc overall height
- The standard observer can recognize (therefore can detect and resolve) letters with the parameters of a “Basic Target”
The Standard Observer is “Normal”
- Quantified at
- 20 feet or
- 6 meters or
- 6000 millimeters
Snellen System: When the Standard Observer Sees “20/20”
- The standard observer is able to recognize targets with the visual size of the basic target from the standard test distance
- Restated: The standard observer is able to recognize basic targets from a viewing distance of 20 feet
What Does the Ratio 20/20 Mean?
- The numerator is the testing distance
- The denominator is the distance at which the letters subtend 5 min of arc
- What happens when the test distance is 15 feet?
- Overall height = 8.73 mm
- Critical detail is = 1/5 of overall height
But what if the patient’s visual acuity is below standard? When the VA is Reduced Below Normal What Can You Do?
- Increase the visual size by
- Moving the patient toward the chart
- Presenting larger targets
- It is always easier and more practical to increase target size than have the patient move
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When the VA is Worse than 20/20; Increase the Target’s Visual Size
- What are the rules for making and identifying larger targets in the Snellen system?
- What are the rules for making visual acuity charts in the Snellen system?
Snellen System: Rules for Making Larger Targets
- Letters go up in size in proportion
- Letter size is designated by the distance at which it would have the visual size of a basic target
- By definition, the standard observer can see a basic target; a 40 foot letter from 40 feet, a 60 foot letter from 60 feet, etc.
- The layman’s understanding of the Snellen fraction is correct
- Snellen Fraction:
(test distance at 20 feet unless otherwise indicated) / (distance a “normal” person can read the letters)
Problem #1: What is the physical size in mm of the 60 foot letter?
•The proportion process for finding the physical size of a letter works for all letters on a Snellen-type chart
In conclusion
- The proportion process described previously works only as long as the chart is calibrated for a 6 meter or 20 feet viewing distance
- If the test distance is other than 6 meters, then the physical size of the “20-foot” basic target is NOT 8.73 mm, even though it still retains the same visual size of the basic target
Determining the Physical Size of a Basic Target When Viewed at other than 20 ft
Problem #3: What is the overall height of a basic target calibrated for a 4 meter room?
- Remember a basic target
- Subtends 5 minarc overall height
- Has critical detail of 1 minarc
- To solve, set up a proportion
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