fan and block stenopaeic slit practical 5

Question 1

What was the first thing asked to do?

Answer 1

achieve the BVS - best vision sphere

Question 2

What does the BVS do ?

Answer 2

this places the DLC on the retina - since there is circular blur patch on the retina all lines on the fan chart should be equally blurred

Question 3

What happens when you ask the patient to actually look at the fan chart ?

Answer 3

• they report there is a clearer direction for the lines - meaning the DLC is not on the retina

Question 4

How do you actually calculate the BVS?

Answer 4

Clincally not this way but using results of RETINOSCOPE

Question 5

Why are all lines not equally blurred ?

Answer 5

• hard to get DLC on retina
• Slight shift will show a preference for one set of lines

-Snellen chart has clear orietntattions in letter- verticals and horizontals which will give better elgibility when one of the astigmatic foci is on the retina.- clear preference for vertical lines foxed on retina a as will allow you to focus- explain why DLC not on retina using this method

Question 6

Why do we add positive lenses?

Answer 6

• To place the rear focus close to the retina .

i. e the subject will perceive one or several lines on fan chart to be sharp

Question 7

Why do we add +0.25 or +0.50?

Answer 7

• to move the rear focal line just infront of the retina- creating compound myopic astigmatism

Question 8

What is the effect of adding fogging lenses?

Answer 8

• moves foci forward- places back focus near retina
• the lines in the fan chart - have a. width equivlanent to a 18m letter limb width.
• 18m letters are good in showing changes in focus when there is de-focus, not good at picking up small changes in the clarity or fogging of letters when refractive error is small
• Add

Question 9

What does fogging do for this experiment?

Answer 9

by adding the fogging lens you are adding a little bit of refractive error to then making the ability to detect the blur on the fan and lines on the chart easier.

Question 10

How do you know its the back focus - which astigmatic focal line is closer to the retina/ how do you know which one it is?

Answer 10

• if rear focus is closer to retina, adding positive lenses would blur more but keep the clearest lines in the same orientation
• However a negative lens can move them ( the astigmatic foci back in the eye) back and possible to move from rear focus closest to retina to front focus closest to retina and switch the clearest lines to the other principal meridian. - clear lines in other direction.

Question 11

How is the astigmatic focus close to the retina ?

Answer 11

• when positive lens been added to BVS , to move astigmatic foci infront of the retina-
• give clearer lines on fan chart

Question 12

How is the astigmatic foci back in the eye/ behind retina ?

Answer 12

• Negative lens added
• now front focus of group closest to retina, that orientation is at right angles to the rear focus therefore the clear lines on the fan chart switch 90 degrees

Question 13

What are the difficulties of the old Maddox V fan chart?

Answer 13

• Only works well when astigmatism is small
• as when pointed towards a clear direction the limbs are offset from that oil terms of angle and become blurred quite quickly

Question 14

What does the modified Maddox V allow?

Answer 14

for both weak and strong astigmatism

• The 2 arrows which have a small angle between them at the top of the arrow head that are more likely to indicate slight changes in focus and therefore indicate which direction to move the arrow to point at the astigmatic axis.
• allows for different level of astigmatism

Question 15

Why are the wider parts of the arrow head less useful ?

Answer 15

-they are already so blurred so the ability to detect small changes in blur between them isn’t good.

Question 16

What is the design of the fan chart?

Answer 16

• Based on limb width of 18m letters
• compromise between weak and strong astigmatism
• if lines too fine blurring is too much to detect difference easily
• if weak astigmatism can blur chart a bit more with a higher fogging lens power- smaller changes to greater perception of blur on the lines which can be picked up more easily.

Question 17

What are some points to why we got results that were not the same as the dummy lens?

Answer 17

• By creating CMA using a fogging lens it is easier to correct the astigmatism.
• Must use subjective refinement at end
• Note accommodation in young- they can cope with inexact correction
• may swap sphere and cyl requires accurate cylinder

Question 18

What is the basis of the steneopaeic slit?

Answer 18

• A pinhole that acts in one meridian
• in a case where you have astigmatism - one clear direction on Snellen chart - which means there is one astigmatic focus close to retina
• the other meridian is causing significant blurring
• so it is possible to orientate the stenopaweic slit to reduce the blurring in that meridian and make the Snellen chart appear equally clear in all directions
• Axis for best vision direction only possible blue if astigmatism dominates vision or one astigmatic focus close to retina

Question 19

What is the problem in the stenopaeic slit ?

Answer 19

• is achieving the one clear direction at the beginning
• so requires using spheres to reduce the spherical component and get a clear direction on retina in first place.
• so if have dominant spheres they must be reduced to SMA/CMA first

Question 20

How do we carry out this technique ?

Answer 20

-subjective refraction using spheres along each meridian isolated by stenopaeic slit.

Question 21

What is the most common error in this experiment ?

Answer 21

not placing both focal lines in front of the retina- make sure you add enough positive power that the car lines started to fog and they wouldn’t flip over to other direction which would indicate the front focus being closer to the retina to start with

Question 22

Why is the fan and block chart supposed to be less accurate ?

Answer 22

for smaller cylinder , but there is no research however there is plenty of evidence
-for example the width of the limbs in fan chart and how they may not be sensitive to smaller levels of astigmatism

Question 23

Why is stenopaeic slit limited in accuracy ?

Answer 23

• to do with subjective refinement that is carried out using Snellen chart
• snellen chart has clear line orientations which will favour certain astigmatic axis one others
• could argue that you could carry out that subjective correction using the fan chart however the problem is that the fan chart has fixed limb widths equivalent to 6/18 letter and won’t be sensitive to small areas
• so little bit hard to see how stenopaeic slit be used pracitcally- could be used in high levels of astigmatism - difficultt to use fan and block and JCC for this.