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.
What happens if you over minus the subject?
-Added too much negative power so pushes image behind the retina, however the eye will accommodate and bring image back on retina- fine on looking at a young monocular situation
However with a binoculars situation- can avoid accommodation hence leads to headaches, eye strain and other asthenopic symptoms
Once you have found out the correct prescription eye , what do you do?
-Carry out a +0.25DS check to make sure you have not under + or over minused
What does the +0.25 test do?
It brings the image slightly infront of the retina
- Should blur the last line read- not make it unreadable - might lose a letter or 2
- Would expect a less clear last line
- If you haven’t reached the correct end point- which means you haven’t added enough plus or too much minus , then it will simply relieve a bit of accommodation and the letter stay just as clear- doesn’t cause any change in the final line you are reading - hence wrong correction
What is the next confirmation check test?
+1.00DS
- brings image more in-front of the retina
- makes subject 1D myopic WITH the previous prescription is accurate
- should blur 3 to 4 lines BUT ONLY IF FULLY CORRECTED.
What is another check test?
Duochrome Test
- 2 red and green panels with circular rings
- check if subject is fully corrected
What is the duochrome test?
based on light of different colours wavelength focuses on different location in the eye- longitudinal chromatic aberration
Red and green colours are chosen as they are equally blurred on each side of retina.
-Optimum wavelength in focus is yellow
-IF corrected eye, the yellow will be focused on the retina and the red and green will be slightly blurred but the same
What happens if the green screen of the duo chrome test is blurry and red is clear ?
- Havent added enough minus to correct the myope or add too much plus to hypermetrope
- as closest focus to retina is the red focus- as why they appear sharp and the green is blurred.
What happens if the red screen of the duo chrome test is blurry and green is clear ?
- Over minuses or under plused
- green closer to retina and red is behind retina
- Add plus to prescription to get rid of accommodation
- if subject accommodates and can equalise panels without adding lenses!
What are some of the tips for the duochrome test?
- if added more than +- 0.50DS on duochrome repeat subjective refraction- as suggests the subjective refraction wasn’t right
- If still duding more than +-0.50DS on duochrome abandon test- unreliable
- Some practitioners always add +0.50DS or +0.75DS before duochrome to make the red clearer then add minus to make equal
- A young subject left just on the green is OK since equates to about 0.25D of accommodation
What is the theory behind the practical?
- Go back to Snellen chart after all adjustments are made.
- Can continue adding maybe a minus lens if it makes it better to read (better VA)
- can add a plus lens- and if ti doesn’t make it worse and it stays the same then it means you have corrected it!
What do you do after looking up dummy lens powers?
-After check dummy lens power and need to change the sign before comparing against the Rx you have found.
summary
- reduced vision in young subjects most probably indicates myopia
- maximum plus, minimum minus - eliminates accommodation
- What you measure is a combination of the dummy lens and any uncorrected Rx
- +1.00DS willl blur 6/4 back to 6/9 or 6/12 but 6/6 back to 6/18 to 6/24 be CAREFUL!
- Duochrome can be unreliable
- The determining factor is best VA
