Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

cs > Refraction on Human eyes pt 2 > Flashcards

Refraction on Human eyes pt 2 Flashcards

1
Q

How do we perform refraction on human eyes>?

A

-Measure PD
-Trial frame
• Measure monocular unaided vision R+L (see homework on next slide)
• Retinoscopy
• Remember to adjust for w.d.
• Monocular VA R+L
• Monocular subjective RE, then LE (we fog one eye so no accommodation in one eye)
• BVS
• Duochrome
• Check astigmatism/cyl

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the duo chrome test based on?

A

principle of axial chromatic aberration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What do the wavelengths indicate ?

A

• Shorter wavelengths of light (GREEN ~535nm) refracted more than longer wavelengths (RED ~620).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What wavelength is focused on the retina in a full corrected eye/emmetrope?

A

yellow (570nm)- approximately equidistant between green and red

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What can the duo chrome test determine?

A
  • can determine spherical component of refractive error by examining the position of red an green wavelegnths with respect to retina
    -hence it also enables determination of the Circle of Least Confusion (CLC)
    • Least blurred image attainable
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What happens after we estimate the BVS?

A
  • After BVS, ask Px to look at black circles (or dots) on the red and green background
    •
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What do we have to ensure when conducting the duo chrome test after BVS is taken?

A

ensure size of circles appropriate for VA - e.g large for poor VA and vice versa

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

When are the circles are useful?

A

• circles only useful If VA better than 6/12

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What happens when refractive error is bigger than 1.00DS or if patient VA is 6/9 or worse ?

A

and if refractive error is >1.00DS or if vision is 6/9 or worse, results may be unreliable as both colours too out of focus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What happens with older patient ?

A

• Unreliable in older Px: crystalline lens yellows and green light is absorbed and scattered resulting in red bias (ie reduction in chromatic aberration), especially in a small pupils

-duochrome test is reliable for older px then and also patients with smaller pupils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the duo chrome test not affected by ?

A

• Not affected by colour vision deficiency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What question do you ask px?

A

‘Do the circles/dots appear darker and sharper on the red, green or about the same?’

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What do we aim for when asking the patient this question\?

A

Aim for black targets to appear equally clear on red and green, or in a young Px, could be slightly clearer on the green
-This is because Places CLC on the retina with the px accommodating minimally

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Example Q : Assuming eye is emmetropic, what happens if +0.25DS placed in front of eye?

A

A: Wavelengths brought forward by 0.25 and RED is in focus- THIS IS why the red circles appear clearer when the eye has been over plussed or minused

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What would happen if you place a -0.25DS in front of an emmetrope?

A

Similarly, if -0.25DS in front of eye, wavelengths sent back and GREEN will be in focus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What happens after the BVS and duo chrome is completed?

A

Subjective correction of astigmatism

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What are the methods of Subjective correction of astigmatism?

A
  • JCC

- Fan and Block method

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the JCC lens?

A

An astigmatic lens with principal powers numerically equal but opposite in sign
+0.25 DC axis θ/-0.25 DC axis θ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How is the - and + marked on the cyl?

A
  • = is marked red

+ cyl axis marked white

20
Q

What is the common powers used for JCC?

A
  1. 25

0. 50

21
Q

where is the handle?

A

at 45 degrees to the axis -midway be§tween the 2 axis which are 90 degrees to each other

22
Q

What happens in a eye that is fully corrected/emetrope in a JCC?

A

-Placing JCC in front of emmetropic eye would separate image into 2 focal points (equidistant) in front of and behind the retina.

23
Q

What happens if there is an uncorrected cyl ?

A

-If there is uncorrected cyl, placing the JCC in one position, the astigmatic interval reduces and vision improves
JCC rotated in perpendicular position, astigmatic interval is greater and vision deteriorates

24
Q

What happens if there is uncorrected astigmatism?

A

If uncorrected astigmatism, in one orientation, this will reduce the gap between the focal points (as in diagram). In the other orientation it will increase this gap.

25
Q

What happens if there is uncorrected astigmatism?

A

If uncorrected astigmatism, in one orientation, this will reduce the gap between the focal points (as in diagram). In the other orientation it will increase this gap.

26
Q

What does the orientation tell us when gap is reduced?

A

The orientation in which the gap is reduced tells us whether we need more or less negative cylinder power to correct the astigmatism.

27
Q

What is the cross cyl procedure?

A
  • Select appropriate cross-cyl power:
  • if VA better than 0.30 logMAR (6/12), use ±0.25 JCC
  • if VA worse than 0.30 log MAR (6/12), use ±0.50 JCC
  • Ask Px to look at circles/dots on the white background
  • Appropriate size target for VA
  • First, check cyl axis
  • Then, check cyl power
28
Q

How do you start the JCC?

A
  • First start by checking the cyl axis

- Then the next step is checking the cyl power

29
Q

How do you check the cyl axis?

A
  • Handle is held parallel to trial case lens axis = powers either side of axis- after ret - trial case lens axis is 90 hence put parallel
  • Then you will ask are the circles rounder and clearer with 1?
  • Then flip the cyl so then ask or with 2?
  • powers have moved
30
Q

What happens when patient reports no difference in vision ?

A

• If Px reports no difference, cyl axis is correct

31
Q

What happens if patient prefers either position 1 or 2?

A

• If Px prefers a lens, move trial lens axis towards orientation of -ive power of the cross cyl

  • So if patient preferred the negative cyl then move the axis of the trial frame to the negative cyl
  • Then repeat this process until the patient finds the circles clear in both position 1 and 2
32
Q

What are the guidelines for refining cyl axis?

A

• Always keep X-cyl handle parallel to trial case lens axis
• Always rotate trial lens cyl axis toward minus axis of X-
cyl
• If trial lens cyl power is high, use steps of 5o until opposite is preferred, then 2-3o back
• If cyl power is low, use steps of 10o until opposite is preferred, then 5o back
• When both positions of X-cyl are reported as being the same, the endpoint has been reached
• The patient may be concerned that this lens is no good, so be prepared to explain….

33
Q

Now, How do we check for cyl power ?

A

Example
Trial case lens +3.00DS and -1.00DC x90
-Handle is held 45 degrees to trial case lens axis i.e. the powers on both lenses are now superimposed
-

34
Q

What happens after ?

A

adjust the power of the trial case cyl based on the power of the JCC that is presented in front of the px eye in the position they prefer.

35
Q

What happens with the patient preference?

A
  • Px prefers lens with the -0.25 superimposed (on the 90 degree axis - in they e.g.), give more minus (i.e. more –ive lens)
  • Px prefers lens with the +0.25 superimposed, reduce the minus
    (i. e less –ive lens)
36
Q

So how would the prescription of this example look like

Originally
Trial case lens originally +3.00/-1.00x90

A

Trial case lens now replaced with +3.00/-1.25x90

37
Q

What is the rule for checking cyl power ( so CLC is on the retina) ?

A

RULES

  1. for every TWO steps of -0.25DC, change sphere by +0.25DS
  2. for every TWO steps of +0.25DC, change sphere by -0.25DS
  3. If in doubt, give LEAST minus
38
Q

How do we bring the CLC on the retina?

A

Let’s say we started with the CLC on the retina.
Then we added minus 0.50 to the cylinder (see below).
Now the CLC is behind the retina and we need to move it forward.
To do this, we add plus 0.25 DS, bringing both focal lines forward.

39
Q

How do we refine the cyl axis power ?

A

High cyl power: steps of ~5 degrees until opposite is preferred, then ~2-3 degrees back
• Low cyl power: steps of ~10 degrees until opposite is preferred, then ~5 degrees back

40
Q

How do we refine the cyl power ?

A

steps of -0.25DC or 0.50DC until opposite is preferred

• for every -0.50DC also adjust spherical component with +0.25DS

41
Q

How do we check for Cyls when none found on Retinoscopy?

A
  • Refine the BVS
    With only the BVS lens in place, direct the Px to the cross- cyl circles/dots.
    -Hold the handle at 45 degrees so negative is shown at 90 and 180
    • Hold the x-cyl and flip the lens as before so that the – ve power is shown at 180/90 degrees
    -Ask patient the same q?
    -Then do the same with the powers at 45 and 135 degrees
  • Then place the a cyl lens at the mid point between the the px preferred orientation
42
Q

So for example?

A

You placed the JCC at 90 and 180 degrees

  • Patient found the rings rounder and clear at 90 degrees
  • Then went present at JCC of 135 and 45 and the px preferred it at 45 degrees
  • This means that your final cyl axis will be between the 90 and 45 ( half way between them - around 60 degrees)
  • Then can normally refine the cyl axis.
  • So JCC parallel to 60 degrees etc
43
Q

What happens if patient has no preference at all?

A

this means the patient has no cyl whatsoever

44
Q

What happens at the end?

A
  • After refining the cylinder, we re-check the BVS in the same way as before.
  • We check VA monocularly (cover the other eye) – is vision at an acceptable level? Is it better than the retinoscopy result?
45
Q

Subjective routine summary

A 
RE first (LE occluded)
• Check VA
• Find BVS
• Use duochrome
• Refine cyl axis and power using Cross cyl technique
• Check BVS again
• Finally recheck VA
• +1.00 blur test
• Repeat for LE (RE occluded)

cs (18 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions