Astigmatic fan

Astigmatic fan details

1. ASTIGMATIC FAN By: JESLIN JOSE

2. ASTIGMATIC FAN 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. PROCEDURE

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. 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.

7. 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.

8. 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.

9. 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.

10. 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”.

11. 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.

12. 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.

13. 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.

14. The three possible endpoints are:

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. ADVANTAGES DISADVANTAGES

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