Perimetry Flashcards
Meaning and purpose of perimeter
- the study of the visual field
- the VF is the area in which objects can be seen in the peripheral vision while the eye is focused on a central point
Do we test VF monocularly or binocularly
Monocularly
Purpose of perimetry
Provides a more standardized assessment of the visual field
-assessment of visual function throughout the visual field for detection and analysis of damage along the visual pathway
Superior boundaries of the monocular field
60 degrees from the point of fixation
Nasal boundary of monocular field
60 degrees from the point of fixation
The inferior boundary of the monocular field
75 degrees from fixation point
Temporal boundary of the monocular field
100 degrees from the point of fixation
Monocular fields of vision in both eyes
Overlap
Temporal crescent
Most temporal portion of vision from approximately 70-100 degrees which is seen only by one
Why do glaucoma patients not realized they are losing vision
Because the VF in each are overlap, so the better eye can make up for the worse eyes loss
Blind spot
- corresponds to the ONH
- located 10-15 degrees temporally
The eye has a small and stable preferred retinal locus of fixation
Most people it is the macula, but low vision patients can have it elsewhere
The center of ifixation correpsonds to
The anatomical center of the fovea where cone density is highest
Dipalsment of the fovea and the center of fixation associated with
Vision loss
Nasal retina projects to
Temporal VF
Temporal retina projects
Nasal VF
Inferior retina projects
Superior VF
Superior retina projects into the
Inferior VF
Limit of VF: nose
May limit or affect the nasal and inferior visual field
What are the things limiting the VF
Nose
Overhanging brow
Small pupils
Overhanging brow
Limits superior field
Small pupil and VF limitation
A pupil less than 2.5mm in diameter can decrease the amount of light reacting the retina and can increase diffraction leading to visual distortions
What can cause small pupils
Cocaine, opioids, diabetes, old people
Sensitivity
The ability to respond to physical stimuli or to register small physical amounts or differences
Threshold
The Manila intensity of light at which a stimulus is perceived by the visual system at a specific location in the field of vision
Brightness
The attribute of visual sensation according to which an area appears to emit more or less light
Humphrey visual field
Static perimetery
Goldmann perimetry
Kinetic perimetry
Two perimetry techniques used to determine the shape of the hill of vision
Static perimetry: Humphrey
Kinetic: Goldmann
Gold standard for VF
Humphrey visual field
Which is more sensitive, static or kinetic
Static
Why is Humphrey gold standard
Because it is static and it is more sensitive than kinetic
Set size and brightness, Moving from non-seeing to seeing
Kinetic
Involves moving target of constant brightness horizontally, from wher its is not visible, toward the center until it intersects with the HOV
Kinetic
Advantages of static
Best strategy for assessing the central VF
Reproducible conditions
Sensitive to VF defects
Disadvantage of static
- does not directly plot borders of vision as single points are chosen for strategies
- tedious
- expensive instrumentation
Advantages of kinetic
- rapidly evaluate the peripheral field
- rapidly plot deep defects
- quick and accurate for steep-bordered defects
Disadvantages of kinetic
- difficult to recognize early field defects
- examiner has much influence on field; must be well trained or will affect the test
The weakest stimulus which is just barely visible (50% of the time) for a particular retinal point being tested
Threshold
Infra-threshold (subliminal)
Below threshold and not visible
Worst disadvantage of kinetic
Examiner has a lot of influence on the field
Supra-threshold (supraliminal)
Above threshold and visible
Factors affecting visibility (threshold) of the stimulus
Size
Luminance
Background luminance
Duration of stimulus presentation
How does stimulus size affecting threshold
Increased in stimulus size with decrease threshold
Sensitivity equals
Height of HOV
Luminance and threshold
The average luminance os the dimmest test object that cab be seen is determined
Background luminance and threshold
Webers law
- when the background luminance decreases, cones adapt more quickly
- controls adaptation over time
- controls sensitivity profile
- some VF defects are more pronounced with difference background levels
- should keep both stimulus and background constant and reproducible
Background luminance: what todo to keep it reproducible
Should keep both the stimulus and background constant and reproducible
Increase in stimulus size
Decrease threshold secondary to spatial summation
