term 2 content Flashcards
how do you check eyes
Right and left eye- find the best vision sphere, check the cyl with a fan and block or JCC and then recheck the sphere. BVS- most plus or least minus spherical lens that provides the best va- with BVS you move the circle of least confusion onto the retina. Clc= midpoint between two focal lines. You can do subjective refraction (finding bvs, check cyl and recheck sphere) after or before retinoscopy.
plus or minus technique
occlude left eye, measure unaided vision direct px attention to their best va line and offer +ve lenses first. clearer with without or same. if va blurs w it dont add it. stop at the most plus or least minus lens that does not blur the va= clc is on the retina
if they see a blur first that means the clc is moving even furtehr away. we give negative tentttively bc neg stimulates accomodation and positive relaxes it. we want to give the least negative possible
maximum plus to maxima visual acuity BVS
we normally use this when patients are young so we aim to relax accomodation by overcorrecting with positive lenses and then bringing this down
occlude one eye estimate unaided vision and put this lens in the trial frame and add +1.00DS to the estimated spherical lens and the va should be reduced by about 4 lines with it and this is to ensure the eye is fogged. continue to reduce the amount of fog by 0.25 and say are the letters clearer with 1 or 2, continue till no improvement in ca. by adding that +1 you are moving the clc further away from the retina (should blur by 1 snellen line per 0.25DDS)
When removing positive lenses make sure you put one in then take it out because otherwise it can stimulate accom
when is fan and block done and describe it
once we have calculated BVS we need to determine the axis and amount of astigmatism. very useful when jcc cannot be done, useful with high cylindrical errors.
internally illuminated black lines. fan lines are fixed at 10 degree intervals and they are marked. maddox v and blocks can be rotated to any setting, the maddox v is used to determine the meridian of the posterior focal line in the astigmatic eye.
once the bvs is in place the clc will either be on the retina or close to it
how do we carry out fan and block
measure the va again, estimate cylinder using table and add half of the cyl as positive sphere which brings the posterior focal line back onto or near to the retina.
now the patient looks at the fan chart and indicates the axis. maddox v is rotated to clear line ton determine the principle meridian. when the limbs of v are equally blurred, the point of v= principle meridian. if one limb is clearer than the other rotate the v in the direction of the more blurred line to equalise the appearance of the limbs. now we have the axis we need to find out the power. (if prescription axis is 180 the patient will report maddox v needs to be moved to an orientation of 90)
before using the blocks to determine the power we add +0.50DS sphere to move the posterior focal line in front of the retina (fogging sphere) then place your estimated cyl power into the trial frame and look at blocks, if blocks awre same cyl power is correct, check by adding +0.25 and the block perpendicular to axis should become darker
block parallel to arrow direction is darker= add cyl power. power is too low
block perpendicular to arrow direction is darker= remove cyl power, power is too high
then what do we do
refine the sphere, plus minus or duochrome
and check tests
+1.00DS for sphere- prevents overcorrection in sphere w negative lenses, va should blur by 4 lines
+0.50DS for cyl= blocks should be blurred= prevents overcorrection of astigmatism, when you add this the blocks should be equally blurred as both focal lines should move in front of the retina
explain bvs and fan and block with regards to clc
find the bvs, this brings the clc onto the retina
when you place the bvs the position of the focal lines have moved and the fan chart will appear equally blurry
estimate the amount of astigmatism and add half of the estimated cyl power as positive sphere which brings back the focal lines onto the retina then determine axis
then add +0.50 to ensure the back focal line is just in front of the eye. focal lines have moved to the left by 0.50
then with the cylinder in place both lines image on the retina, once the sphere has been refined w the correct sphere and cyl both focal lines should be on the retina.
jackson cross cyl
Once we have calculated BVS we need to determine the axis and amount of astigmatism ie the cylinder. Jcc is the most common way to do that. A typical JCC lens is a spherocylindrical lens having a spherical power in meridian and another in the meridian.
Find bvs, now we are using verhoeff rings or a collection of dots. Adjust sphere for accommodation using duochrome patient should see equal or slightly better on the green. Choose the appropriate cross cyl, if 6/12 or better select the +-0.25D crossed cyl, if 6/18 or worse use the +-0.50 cylinder.
Place the estimated negative cylinder power in the trial frame and remember to add half of the cyl power as positive sphere to the best sphere. Then twirl the cross cyl first w handle presented along 45 or 135 and 90 or 180. Ask which ones provide a clearer and blacker image and the estimated cyl axis is halfway between them.
Determine cylinder axis by presenting the handle parallel to the cylinder axis. Rotate the trial cylinder towards the negative axis of the JCC cylinder. When the patients are unable to detect differences in clarity between both positions= correct cylinder axis.
Then determine the power by now aligning the handle parallel with the trial cylinder and if they prefer negative then make it more minus so add more cyl and if they like the positive then make it positive by decreasing the cyl power. Remember to add 0.25DS to your sphere for every -0.50DC increase in cyl and vice versa. When the patient is unable to detect a difference in clarity between 2 positions= correct cylinder power.
for a myope, hyperope where is the focal point and what lens fixes this
For a myope= focal point is in front of the retina
Positive lens- makes the eye even more positive so moves the focal point further away (moves it left)
Negative lens- moves the focal point onto the retina (moves it right)
myope= negative
hyperope= focal point is behind the retina
Positive lens- moves the focal point in front of it so on the retina now (left)
Negative lens- moves it further away
hyper= pos
Hyperopia- dont leave the eye without lens otherwise you get accommodation so if your using positive lenses put it in and then remove previous lens.
if the patient answers same or no difference at first presentation
you might have the right lens already, vision may be too poor due to inaccurate retinoscopy result or pathology might be present so increase lens power and repeat. Or small pupils increase power and repeat, use a pinhole.
duochrome test
whatever colour is focused nearest to the retina will be seen as clearest.
emmetrope= equal
myope= sees the red clearer
hyperope= sees green clearer
Increase power till both circles of colour are on the ret= equal. Leave a young patient on the green.
Red clear= minus (we have overplussed them) (too much left so in front of ret)
Green clear= add (we have overminused them) (too much right so it is behind the ret, this is why we like to leave the px slightly on green if young)
If they are the same- confirm by adding 0.25DS and now red should be the clearest. bc its moving fl right so in front of retina so myope so red
limitations of duochrome
can turn the lights off so pupils larger so it will work better
If px answers same or no difference- balanced confirm by using +0.25 red should be clearest.
or rx too far out, small pupils or vision too poor due to pathology.
Will not work if vision is less than 6/12 The difference in focal position due to chromatic aberration is 0.50DS Will not work if prescription is significantly incorrect
Small pupil will reduce size of blur circles
Difference between the clarity of red and green is reduced Reduce room lighting for older patients
Peak wavelengths may vary from test to test
Relative brightness of red and green may affect reliability Always be aware of the alternative tests!
Patient for cataracts= not good
Chromatic aberration of eye decreases with age
▪ Crystalline lens – yellow
▪ Blue-green light is partially absorbed, red bias to test
▪ Colour defectiveness – can still do the test as sharpness of focus not affected only the appearance of the colour
so rlly use +1.00 w old people
why does the +1.00 check test reduce vision
if vision is better or worse with +1.00 what does that mean
overplussing should induce a blur circle on the retina hence reducing vision and it should blur by 1 snellen line per 0.25DS. Should blur by 4 lines.
If vision with +1.00 is better than 4 lines the blur circle must be smaller than expected ( focal point behind the retina) which means you have either not added enough plus or added too much minus and vice versa. you need to add more plus to bring it onto the ret. or more minus to push it that way.
If its worse the blur circle must be bigger (focal point in front of the retina)so you have not added too much plus or not enough minus.
when might results be unusual
if ocular pathology When in doubt, use another test to confirm
*Pupil size is important * Reduced pupil size can also reduce the size of the blur circle * Large pupil has opposite effect; will blur back too quickly
before doing jcc what do we leave the pre presbyopic patient as
hyperopia and myopia
leave pre presbyopic patient slightly over minused
Jcc is to determine the astigmatic component of the rx
Leave the pre presbyopic patient slightly over minused so leave them on the red. If its clear offer them -0.25
Near vision blurring with good distance vision suggests hyperopia
Distance vision blur with good near vision= myopia
Blur at all distance can indicate astigmatism
6/12
6/12
d= test distance in metres
D is 12 which is the letter height that subtends minutes or arc.
ret- what if you dont see an against
- if you dont see an against reflex when looking at the second meridian do not panic, simply add plus to neutralise the with movement. And then return to the first meridian that will be against.
far point
Far point= space at which light from the patients retina comes into focus
Far points for myopes in front of px for hyperopes behind px and
emmetrope their far point is at infinity so you still need to bring this onto the ret so even emmetropes have like a with or against movement.
how do you relax accomodation in ret and then what do you do
when is it a spherical refractive error and astigmatism
Relax accommodation by fogging the other eye to get against movement in all meridians, not necessary in px over 60 years.
Spherical rx if reflex same in all meridians, astigmatism rx if reflex differs in each meridian. With streak rotate it.
Correct slowest with first if theres 2 withs but always try to correct with. And if all against correct the fastest against first bc think about it your trying to leave an against in the second meridian. Use bracketing to neutralise reflex, now check neutral point by moving forward slightly= with. backwards= against.
if you see an against movement what does this mean and what does a with movement mean
If you see against myope= myope. You need to add negative lenses. Far point is between the patient and the examiner.
But for with movement its not only a hyperope it could also be: emmetrope or low myope bc far point is behind the examiner or could be a hyperope bc far point is behind the patient.with movement is not always an hyperope= could be emmetrope or low myope as well.
