2. • The fan and block test is used to determine
the axis and magnitude of astigmatism. It
determines the presence of any astigmatism
and its principal axes.
• The fan block test consists of series of
radiating lines spaced at 10°,15° or 30° interval
and arranged after the manner of rays of rising
sun. The 10° spacing gives better accuracy but
may be confusing which is why some
practitioners prefer the wider spacing.
3. There is a central panel
carrying a ‘V’ and two
sets of mutually
perpendicular lines( the
block).
The V and block
simultaneously can be
rotated through 180°.
4. Occlude one eye( preferably RE)
The patient should be fogged with
no cylinder lens in phoropter or
trial frame.
5. Testing is done under fogging and it always
results in neutralization of cylindrical error with
cylinder lens of minus power.
Fogging makes the eye a compound myopic
astigmatism. This means that both meridians are
infront of retina. One of the meridians close to
retina will always be clearer and the axis of
minus cylinder can be identified by this
meridian.
Why fogging is important?
6.
7. Adding the minus cylinder gradually will push the
other meridian closer to the previous meridian,
which is closer to the retina until they form a
point focus.
Adding a plus cylinder will bring the meridian
closer to retina further ahead inside the eye,
creating a much blurred image of fan target . This
blurred effect will not allow any response on
subjective testing.
8.
9. The estimated fog required in an astigmatic
eye after best sphere is achieved and is
calculated by a rough estimate and that is
done in two steps.
1. Estimate the approximate astigmatic
correction based on visual acuity( this is the best
vision after best vision).
2. Now this estimated astigmatic value
divided by 2 plus 0.50 spheres is the
approximate fog required.
10. Suppose it is 6/12 then astigmatic
estimation based on visual acuity formula is:
12/9 = 0.75
0.75/2=0.37
0.37+0.5=0.87
Thus +0.87 or +1.00 is the estimated value
for fogging for doing the test.
If the best vision calculation is correct the
fog lens should blur the vision by atleast 2 lines.
11. Check the fog as follows:
1. Add +0.5DS. All lines should become less
clear, but the same lines should remain clearest.
2. Add another +0.5Ds. All lines should become
even less clear. Again the same lines should remain
clearest, but depending on the degree of astigmatism,
the extra blur may make it difficult for the patient to
detect clearer lines.
If at either of these steps the “clearest”
orientation changes, then the eye is not fogged at all.
If the first +0.5DS makes all lines blurry then it is
possible that there is too much fog.
12. After the eye is fogged the patient is asked
to look at the astigmatic dial.
If astigmatism is present , one or more lines on the
chart will appear sharper than the others
It is helpful to describe the dial for the patient in
familiar terms with phrases like “ similar to face of a
clock” or “ like a wagon wheel missing its hub”.
13.
14.
15. Next ask to patient “ tell me which line is
blackest, sharpest and clearest. We can also add
“ the one which looks as if it were printed with
more ink in it.
The object is for patient to identify in terms of clock
position the meridian which is least blurred.
16. Use the pointer if you are using the
astigmatic dial slide in the projector to point out
the 12 o’clock, the 1o”clock and the rest of the
meridians in terms of their clock position.
Then point out that the lines in between are called
“one-thirty, two-thirty,” etc. doing this at a relaxed
pace allows the patient time to think about which
line or lines are blackest, sharpest.
17. When an astigmatic eye is correctly fogged,
the axis of correcting cylinder is at right angle to
clearest line.
The phoropter/trial frame cylinder is set at that axis
and cylinder power is increased until all lines are
equally clear that is patients cylinder correction.
The fog is now reduced in 0.25D steps of sphere,
monitoring vision at each step until the highest
positive sphere power which gives best vision is
obtained.
18. 1. Two
adjacent
lines are
equally
black.
2. One line is
blackest and
the two
neighboring
lines are
equally black.
3. One line is
blackest and of
the two
neighboring
lines, one of
them is
blacker than
the other.
The three possible endpoints are:
19. Distinguish which meridian would really be
blackest of all. This may require some
extrapolation but that is not always
required.
In case of endpoint in which two adjacent lines are
equally black, you know that of the 180 theoretically
possible lines, the one which would be blackest is not
on the dial, so you must extrapolate.
20. For instance , if 2:00 and 2:30 are
equally black, then you will assume that the
line representing 2:15 would be the blackest line.
Again this is not usable information. To make it usable
restate 2:15 as 2 1/4 and then multiply that by 30.
The product is 67.5°. Either 67° or 68° is equally
acceptable as the position for the introduction of
minus cylinder.
21.
22. In case of second endpoint
listed above for instance no extrapolation is
necessary. If patient selects 2:00 as the blackest
line with 1:30 and 2:30 equally black then you know
2:00 is the blackest line.
But “2:00” is not in our formal ophthalmic
lexicon it is not a usable term. In order to express this
as a useful term we must go to the next step:
multiplying the “o’clock” position by 30 to determine
the axis for introducing minus cylinder.
23. For example if you have decided 2:00 is the
blackest line , then calculate
2*30=60
• start introducing minus cylinder at axis 60
• keep doing so until all lines are equally black.
Another example if patient has identified 6:00 as
blackest line with 5:30 and 6:30 being equally
black, with 6*30=180
Then we can start introducing minus cylinder at
180.
24.
25. In case of third end point one line blackest
with one of the neighboring lines blacker than
the other. You must still extrapolate. This time it’s a
little more complicated.
For eg : 2:00 blackest and 2:30 blacker than 1:30. we
can conclude that the blackest of the 180 possible
lines would be between 2:00 and 2:30, but closer to
2:00 than 2:30( because 2:00 was blacker than 2:30).
In that case select 2:10, restate it as 21/6 and multiply
that by 30. Since 21/6*30=65, start introducing minus
cylinder at axis 65.
26.
27. If you happen to overcorrect, the patient may
announce that suddenly the blackest line is in
the opposite axis.
For example: We started with 1:00 as the blackest and
now suddenly 4:00 has become the blackest.
If that happens take out some cylinder until it goes
back to 1:00 being the blackest again slowly and
gradually add ¼ diopter at a time until the patient
selects the point at which , if the lines are not
completely uniform they come closest to being that
way.
28. A note of caution:
Once you have selected cylinder axis this way
don’t try to refine it with cross cylinder technique as
this method of selecting axis is the more precise
method. If any uncertainity prevails do the whole
process again.
We could get to know about our mistake when the
patients express that the blackest line has changed its
position. It may not because of patients error but may
be because of mistake in our calculation.
Done in intelligent and cooperative patient.
29. No special equipment.
Easy to perform for screening.
Easy to communicate with high
amount of astigmatism.
Best with minus cylinder phoropters.
Successful in patients with
amblyopia and corneal opacity where
retinoscopic findings not possible.
Difficult to discriminate
in small amounts of
astigmatism.
Difficult for axis
orientation in small
amounts of astigmatism.
Difficult to communicate
in small amounts of
astigmatism.
ADVANTAGES DISADVANTAGES
