practical 2 summary Flashcards
What are the baseline measurements ?
Start with
- Vision- reduced vision can indicate optical blurring or pathology
- Vision with Pinhole-
What does Pin hole do?
reduces retinal image blur and improves vision if the cause is a blurred image
What happens when you use pinhole when vision is good?
will make vision worse if already a sharp image on the retina
What happens if reduced vision does not improve with Pinhole?
Then it is known that the cause is not due to optical blur
-Could be pathology?
What can pinhole help in a myope?
When simulated myopia in the eye and then tested vision without PH- REALLY BAD
-WITH PH- really improved the vision of a myope
What can pinhole do in a hypermetrope?
Simulated hypermetropia in the eye
- vision didn’t change that much from without pinhole to with ph - not much improvement
- With PH- accommodation attempts to compensate hypermetropia- helps to maintain vision
What happens if accommodation is exceeding ?
- It is in a hypermetrope without PH - (look a tree line on graph)
- It means the vision gets worse
What does a pinhole do in hypermterope (refer To diagram)?
small difference in red and blue lines
The vision starts slowly as getting worse WITH PH- as image is already sharp on retina
-thats because PH is not an actual pupil size, it reduces light levels, increases diffraction and isn’t a natural pupil size that maximises the VA
-no significant retinal image blur
What can we do with all this information?
Estimate refractive error (PRESCRIPTION)
What can you assess from a younger age group?
below 6/6 vision most probably indicated myopia - 3 to 4 lines lost per Dioptre
- If PH improves vision- shows there is a correctable refractive error/ prescription(Rx)- best guess is MYOPIA
- If no/worse PH- there is no significant retinal blur or pathology ?
- If hypermxntropic - they will accommodate to give good vision
What do we do before we start subject refraction?
determine whether you have myope of hypermetrope.
-2 tests
So, Whats the first test to determine whether you have myope of hypermetrope.?
- Do +1.00DS test to confirm type of refractive error
- it moves image anteriorly in eye ( image moves to left)
- If myopic- the vision becomes more myopic and becomes so much worse- moves
- If no change or slightly better - you estimate hypermetropia of at least +1D
- If weak hypemetrope (less than 0.50DS) then +1.00DS could make vision worse - potentially indicating myopia
Whats the second test to determine whether you have myope of hypermetrope.?
-Add +0.50DS to confirm weak hypermetrope
-IF have myope- +0.50 lens makes vision worse
-IF weak hypermetrope - +0.50 can reduce to emmetropia - accommodation may compensate hence not likely to see change
so if the vision does not change after adding 0.50DS onto of the +1.00DS then it confirms a weak hypermetropia - if it gets worse then means myopia
How are the trial lenses put in to get vision better for a myope?
- they must be able to read more letter in order for you to add a more negative lens in prescription until they are at 0 refractive error and thats where you stop.
- dont over interpret the letter chart- as not significant - may get fluctuation as letters are not all equal - range in legibility - factor of letter chart.
How are the trial lenses put in to get vision better for a hypermetrope?
- Dont know the end point
- So add further plus lens until you get to a point with a clear decrease in vision to know the end point hence choose the lens prescription which was the last lens with the best vision.
