Visual fields || Flashcards
What is the threshold sensitivity ?
- every point in the retina has a certain threshold sensitivity.
- the higher it is the higher the number you will see in the visual field spots.
What does it mean if your threshold sensitivity is higher ?
means because you can see more , you can see dimmer light.
What happens if you see in a visual field plot 0 dB?
this means that is the maximum brightness of that perimeter/that visual field machine can show
-for each machine it is different
How can we record threshold sensitivity ?
- just when the px sees barely a stimulus- but that happens very rarely so we dont choose that
- where the px can see the stimulus 50% of the time
- if the perimeter showed a light 4x at a certain brightness and size the px would say they could see it twice out of the 4 types
What does it mean when the macula has the highest density of cones?
it also has the highest threshold sensitivity .
What happens as we move across the retina ?
the number of cones greatly reduces
=therefore so does px threshold sensivity
What is at the optic disc. ?
no cones at all
therefore the px will not be able to see anything
-even under 0dB the px will still be unable to see the light
-therefore at optic disc you are expected to see less than 0dB
What are the 2 ways you can assess threshold sensitivity as we cross over the retina and the number of cones start to reduce ?
by increasing the brightness of light or size of the light.
What is a threshold type of visual field test ?
at each location across the retina / visual field we are going to measure/quantify the threshold sensivity
-used in hospittals- for check ups as they have already been diagnosed etc
What is a supra threshold screening type of visual field test ?
- screening to see if the px is normal or abnormal (pathology)
- show a stimulus that is a few db brighter than what the px can see
- used in hughstreet practices
How is the supra threshold 2 zone test look like when results are shown ?
when the px has missed the point twice the point comes up as black.
- it is called a 2 zone because they could either see the stimulus (white point) or not (black point)
- every location where px can see would be marked as a circle white and not seen marked black
- if average px is shown 76 points at a time- chances are they are shown only once and not twice
- chances are that the px might of missed a point from being distracted or got bored.
- every single point in this test - is however shown twice.
What is the problem in seeing a scotoma in a px marked on the supra threshold two zone map test ?
-is that we would be able to see the scotoma but not how deep it is
So, how can we see a scotoma ?
using a suprathreshold three zone.
What is the suprathreshold three zone?
there are 3 choices
- px can see stimulus- circle white
- px can not see- black mark
- if px cant see , they are then shown a stimulus at the maximum brightness of the perimeter and if they can see it then- it is then marked as an X- not what an average person sees at but can see at the maximum possible brightness
How does practitioner know when to show a stimulus that is of higher db to a px ?
- 2 ways of knowing:
- based on px age and actually measuring what the px should see
What does age-related supra-threshold strategy test do?
-stimuli set at a certain number of db brighter than age-matched normal.
all people are compared against the average normal person of the same age .
How can we measure threshold related supra threshold strategy test?
- in 4 separate locations which is the normal way
- show px a stimulus outside the normal and can make it brighter so px can see
- do it in 4 different locatioin
- choose the most 2nd most sensitive point as the starting threshold sensitivity of what they should see
- a few decibels brighter than the 2nd most sensitive point- showing it all over the visual field- adjusting it brighter and brighter as it goes towards the periphery as that’s where there are less cones
What method would be the quickest?
quicker to plug in someones age therefore would be the age related suprathreshold strategy method
-on highstreet
What can supra threshold screening be broken down into ?
- With quantification
- no quantification
What is no quantification ?
- going to do supra threshold test- use db few brighter than what they see for their age or what you have measured
- show suprathrehsold stimulus- then can see - but then have scotoma and then cant see the stimulus
- continue across the visual field
- explore which areas they can or cant see with no number
What supra threshold screening technique has no quantification ?
two zone and three zone- in both strategies they have no numbers , haven’t quantified the visual field loss in any way
- we just say whether they can see it with the circle, ta relative defect with the x, or they got an absolute defect where there is black.
- have no idea how deep the scotoma is
What is the supra threshold technique with quantification ?
- most recommened
- show a suprathreshold stimulus-few db higher than what they should see
- now we reach their scotomoa - marked with red
- those locations where there is a scotoma where the px cant see the stimulus -the perimeter goes back and now starts to measure how deep the defect is
- to eventually when they are able to see the stimulus in that particular scotoma.
- have numbers- the higher the number the higher the threshold sensitivity (the more they can see), the dimmer the light they can see.
What happens when measuring central visual field?
it is inside the 30 degrees
