9.2 the fan and block test

Question 1

when and why is the fan and block test used ?

Answer 1

. subjective test used to determine astigmatism

. used if patient’s can’t respond accurately to JCC test

Question 2

what does the fan and block test require ?

Answer 2

the test requires a knowledge of types of astigmatism and the blurring present to predict what the appearance of fan and block chart will be at different stages of correction .

Question 3

what is the structure of the fan and block chart ?

Answer 3

. fan 
. blocks
. maddox V 
. fan lines every 10 deg 
. lines same as limb width of 18 m letter ( can differ on computer charts ) - less than 18m

Question 4

how would the fan lines appear when astigmatism is high/low ?

Answer 4

. high astigmatism = blurred line

. low astigmatism = hard to detect slight blurring on 18 m letter limb width .

Question 5

what are the labels on the radial lines on the fan chart ?

Answer 5

. labels give the negative cyl correcting axis

Question 6

what were the labels on the lines on the old fan chart ?

Answer 6

. some old charts have two sets of numbers by the lines and it was the larger outer numbers that indicated the negative cyl correcting axis and small numbers used to indicate the positive cyl axis

Question 7

how can the fan chart be viewed ?

Answer 7

. direct vision chart ( with out mirror ) - on computer based chart
. mirror view - charts for use with mirrors have axis labelling mirrored

Question 8

explain the appearance of the fan chart for a subject with SMA 0.00/-3.00x45 ?

Answer 8

. the axis will be switched for trial frame but this
is is because this is the subject’s view whose looking from behind the trial frame , the examiner’s view would be different.
. the axis is the direction along which there is no power acting and therefore there is no blurring along this direction and the blurring is therefore happening along the 135 direction
. we get a long ellipse along 135 direction - very close to lines
. the lines along 135 appear clearest and lines at right angle to that appear most blurred

Question 9

when are the blocks used ?

Answer 9

. the blocks are used when we’re correcting astigmatism using CYLS and what we try to do is equate the blocks that means make them equally clear now the blocks will be parallel to the clearer set of lines on the fan chart and the other set of blocks will be out of focus

Question 10

what do the blocks on the fan chart rotate with ?

Answer 10

. the block’s rotate with the Maddox v that’s the Arrowhead the inverted v Arrowhead that you see underneath the fan chart . you should point the Maddox v to the clearer set of lines and the blocks will automatically rotate with it you’ll see that happening if you use the wall mounted charts but if you switch from the fan chart to the blocks on a computer based chart you’ll see that the orientation of the blocks has already been set for the negative correcting cylinder axis is perpendicular to the clearest blocks

Question 11

what would the axis be if the clearest line is along 135 ?

Answer 11

on the fan chart it was the lines at 135 that were clearest but that meant that the negative CYL correcting axis needed to be at 45 so although it was the lines at 135 that were clearest it would be labelled 45 on the fan sharp telling us that that was the negative still correcting axis

Question 12

what is the modified maddox V and what is it’s function ?

Answer 12

. The type Of pointer on our computer based test charts is what’s known as a modified Maddox v
. it has an Arrowhead with two different angles on it and narrower angle between the lines at the top of the Arrowhead and a wide angle at the bottom
. you are aligning this Arrowhead with the clearest direction on the fan chart you’re trying to make both sides of the Arrowhead equally clear or equally blurred now

Question 13

how does maddox V look in low cyls ?

Answer 13

wider part of V is best

Question 14

how does maddox V look in high cyls ?

Answer 14

there is a greater differential in the narrow part of V

Question 15

what are the steps in a fan and block routine ?

Answer 15

1. Determine BVS
2. Add +0.50DS and see if clearer direction on fan chart. Rotate the Maddox V (the blocks rotate with it) to point at the clearest direction in the fan
   3.Keep adding +0.50DS until this direction starts to blur
   its blurred but still clearer then start , induce compound myopic astigmatism
   4.Add negative cyls at the axis indicated by the Maddox V until the blocks appear equally clear/blurred. Don’t overshoot.
   5.Refine the sphere to get acuity as good as possible

Question 16

how to determine BVS ?

Answer 16

. the disc of least confusion on retina
. circular blur patch os blurring equal in all meridians
. all lines in the fan chart equally blurred
. you get mixed astigmatism - one foci is in front the retina and other behind the retina with DLC ON retina

Question 17

what happens when in step 2 when you Add +0.50DS and see if clearer direction on fan chart?

Answer 17

. posterior ( most positive / least negative ) meridian focus closest to retina - should become a clearer direction on fan chart
. you get simple/compound myopic astigmatism
. strong elliptical blur on retina

Question 18

what happens in step 3 when you add +0.50DS until this direction starts to blur ?

Answer 18

. posterior ( most positive / least negative ) meridian focus just anterior to retina
. induce compound myopic astigmatism
. clearest line not quite clear as in step 2
. elliptical blur on retina

Question 19

what happens in step 4 when you Add negative cyls at the axis indicated by the Maddox V until the blocks appear equally clear/blurred. ?

Answer 19

. when you switch to the blocks we will find that they are orientated parallel to and perpendicular to the direction of the maddox V
. we add negative cyls to make these two sets of lines on the two blocks equally blurred or equally clear
. compound myopic astigmatism reduced with negative cyls
. we expects to see - final appearance of blocks equally clear with astigmatism corrected .

Question 20

what happens in the last step 5 ?

Answer 20

. refine sphere to get best VA
. fan and blocks equally clear in all directions
. good VA