compare the smallest VA line they can see with and without the cylindrical correction.

4.13.5 Recording

The results of the JCC are not recorded as the technique is just part of the subjective refraction (section 4.9).

4.13.6 Interpretation

Solsona (1975) retrospectively analysed 51 000 patients with astigmatic corrections greater than or equal to 0.75 D and found that 67% had mirror symmetry within 10°. This means that the two axes should add up to approximately 180°: both axes could be 90° or both axes 180° (i.e. 0° and 180°); one axis 175°, the other 5°; one axis 20°, the other 160°; one axis 45°, the other 135°, etc. You may wish to recheck astigmatic axes that do not follow this pattern, particularly if one axis has changed significantly from a previous examination or is significantly different from the retinoscopy result. Although astigmatism changes with age, it rarely changes significantly between eye examinations that are only a few years apart. Significant changes in astigmatism over a 1–3-year period are likely to be due to refraction error at test or retest or possibly due to ocular pathology such as keratoconus, cortical cataract, chalazion, etc. causing significant astigmatic changes (Locke 1987).

4.13.7 Most common errors

1.Using a fast presentation time in older patients whose reaction times may be slightly slower than younger patients. More reliable responses are gained if presentation times are longer and if they are repeated until a consistent response is gained. For this reason, slower presentation times in older patients actually lead to a more efficient and ultimately faster JCC process.

2.Using different presentation times for the two positions. For example, presenting the target for longer in position 2 compared to position 1. The patient should see the two presentations for the same amount of time.

3.Poor alignment of the JCC with the correcting cylindrical lens in the trial frame. This can lead to problems in determining the cylinder axis. Check that the handle of the JCC is in alignment with the axis of the correcting cylindrical lens when determining the cylinder axis.

4.Making large changes to the axis of the trial cylindrical lens in an eye with high astigmatism.

4.14 THE FAN AND BLOCK TEST

The fan and block test (Fig. 4.11) was developed to increase the sensitivity of the earlier fixed astigmatic dials, such as the ‘clock’ and ‘sunburst’ dials, and the rotary dials, such as the ‘Rotary T’. All lack sensitivity, particularly in determining the cylinder axis. The relatively large 30° gaps between adjacent sets of lines on the clock dial test, for example, significantly limit its sensitivity, and any present-day use appears to be based on tradition (Borish & Benjamin 2006). The fan and block test is only available with wall charts and includes a fixed dial with a rotating arrowhead and blocks and combines the techniques used in the earlier tests.

4.14.1 Alternative procedure for the assessment of astigmatism

The Jackson cross-cylinder (JCC) should not be the only subjective test for the determination of astigmatism that a practitioner has available. Not all patients will be able to respond accurately to the demands of the JCC and the fan and block or similar technique should be used in these cases. In patients where the subjective assessment of astigmatism is poor, it is advisable to consider multiple objective measures of astigmatism from retinoscopy, autorefraction, and (to a lesser degree and if the cylinder is not lens-induced) keratometry. The astigmatism present in the patient’s old spectacles should also be considered.

4.14.2 Advantages and disadvantages

An advantage of the test is that, unlike the JCC (where the circle of least confusion is on the retina

and one focal line behind the retina), accommodation is well controlled as the patient is fogged prior to the use of the procedure ensuring that the circle of least confusion and both focal lines are in front of the retina. Logic would suggest that the fan and block technique is more accurate than simple cylinder axis rotation (4.14.6), but there are no research studies to confirm this. The fan and block test is believed to be less accurate at determining small cylinders than the Jackson cross-cylinder (JCC), but again there is no firm evidence to support this view.

4.14.3 Procedure

The technique described assumes that you perform the test using the information gained from an objective refraction.

1.Determine the best vision sphere as described in sections 4.10 and 4.11. This ensures the circle of least confusion is on the retina.

7.Ask the patient to look at the clearer block and add 0.50 DS and ask if the block blurs. If it does blur, continue to step 8. If the block does not blur, it is possible that the retinoscopy result provided a significantly under-corrected cylinder, leaving the back focal line behind the retina after step 3. In this case, additional 0.50 DS lenses

should be added until the clearer block just blurs.

8.Set the cylinder axis in the trial frame/ phoropter at the axis indicated by the arrow. Add negative cylinder at this axis until the blurred block just becomes as clear as the other. If there is a reversal, in that the more blurred block becomes clearer, take the lower cylinder power.

9.Reduce the plus fogging sphere to determine the best sphere as previously explained.

10.Repeat steps 1 to 9 for the left eye.

2.Occlude the untested eye (i.e. LE, OS).

3.Remove the cylinder determined from retinoscopy from the right eye, and add0.50 DS to the sphere. The circle of least confusion is now in front of the retina. If the retinoscopy cylinder was correct or overestimated, then both focal lines will be in front of the retina.

4.Draw an analogy between the lines on the fan and the hours of a clock, and ask the patient if any of the lines on the fan appear clearer and darker than the other lines.

4.14.4 Recording

The results of the fan and block test are not recorded as the technique is just part of the monocular subjective refraction (section 4.9).

4.14.5 Most common errors

1.Failure to place both sets of focal lines in front of the retina before commencing the fan and block procedure.

any lines are clearer and darker. If they remain equally clear or blurred, then this suggests there is no astigmatism present.

6.If some lines are reported as clearer, point the arrow that joins the blocks towards the clearest line. Adjust the arrow until its two barbs appear equally clear. One block (with its lines running parallel with those on the fan which are clearest) should be clearer than the other.

rotation

Occasionally a patient may provide very inconsistent responses with the JCC. If you are confident of the need for cylinder power (astigmatism noted in retinoscopy and/or autorefraction; reduced VA with the best sphere result), but uncertain about the cylinder axis, simple axis rotation may be useful. The technique simply involves asking the patient to view the smallest line of VA they can see and rotating the

correcting cylinder axis clockwise until the patient reports that the letters start to blur. This point should be noted. Then rotate the axis anti-clock- wise until the patient again reports that the letters start to blur and note this point. The cylinder axis indicated by the technique is the midpoint between these two blur points. For example, if the two blur points are at 25° and 55°, the indicated cylinder axis is 40°.

4.15.1 Binocular balance of accommodation

After a monocular refraction, a binocular balance is required to ensure that accommodation is balanced in the two eyes. This test should not be performed if the patient is monocular, strabismic or has unequal visual acuity and is not necessary if they have no accommodation (patients older than approximately 60 years of age or pseudophakics). A binocular balance of accommodation is also not required after a binocular refraction.

4.14.7 Alternative technique: clock dial test

This technique is similar to the fan and block technique, but only consists of the ‘fan’ (i.e. clock dial) part of the technique (Fig. 4.15a). The technique begins in the same manner as the fan and block steps 1 to 5 (section 4.14.3). The suggested axis is determined by multiplying the smaller of the two clock dial values of the most clear lines by 30. For example, if the patient thinks that the 2 and 8 o’clock lines are the clearest, then put the cylinder axis at 60° (2 30). If the patient thinks that the 4 and 10 o’clock lines are clearest, place the cylinder axis at 120°. If the patient believes that two clock dials are clearest, then the axis should be placed midway between the two values indicated. For example, if the patient believes that the 1 o’clock and 2 o’clock lines are clearest, then place the cylinder axis at 45° (1.5 30). The cylinder power is then increased until the most blurred lines are brought to the same level of clarity as the most clear lines. The sunburst or fan dial test (Fig. 4.15b) uses a similar technique, but lines are provided at 15° intervals rather than 30°.

4.15.2 Advantages and disadvantages

There has been little comparison of the various binocular balancing techniques in the research literature (West & Somers 1984). The prism dissociated binocular balance balances accommodation between the two eyes while accommodation is relaxed in both eyes and is widely used with phoropters, where it is relatively easy to use. When using a trial frame, the large number of lens changes needed makes the test awkward and other tests are preferred (section 4.16). Clinical use suggests that the alternate occlusion test for binocular balancing is relatively crude and the prism dissociated (or polaroid dissociated) duochrome test includes all the pitfalls of the duochrome test (section 4.12).

4.15.3 Procedure

1.Occlude the left eye (or ask the patient to close their eyes; the increasing diplopia produced by the prisms can be distressing) and isolate the 20/40 (6/12, 0.5) row of letters.

2. Introduce the Risley prisms before both eyes, so that there is 3 base down before one eye and 3 base up in front of the other eye. It is important that equal prism before each eye is used to equalise any image degradation by the prisms.

3.Add 1.00 DS to the right eye and check whether the 20/40 line is blurred. They should be blurred, but readable. Add further plus power in 0.25 D steps until the 20/40 is just blurred.

4.Remove the occluder (or ask the patient to open their eyes) and ask the patient if they see two 20/40 (6/12, 0.5) lines of letters, one above the other.

5.If the patient cannot see both lines, first check that both apertures are open. If they are, cover each eye in turn, so that the patient can see the position of the line seen by each eye. The patient should then be able to see both targets at the same time. If the patient still cannot see two lines, then one eye is probably suppressing and a binocular balance is not required.

6.Add plus lenses in 0.25 DS steps to the left eye until both eyes have equally blurred images.

7.If a balance cannot be achieved, use the lenses that provide the closest match.

8.Remove 0.25 DS from both eyes, and ask whether the two images remain equally blurred (you may need to isolate the 20/30, 6/9 line for this comparison). If one image is clearer, add 0.25 DS to the eye with the clearer image until both eyes have equally blurred images. If a balance cannot be achieved, use the lenses that provide the closest match.

9.Remove the Risley prisms (ask the patient to close their eyes while this is done) and display the bottom part of the visual acuity chart. Check the visual acuity to ensure that the best acuity line has not been achieved.

10.Remove the fog in binocular 0.25 DS steps until you obtain maximum visual acuity. If the patient can read the bottom line of your chart (and this is larger than 20/10, 6/3),

you must allow extra minus/less plus that makes your bottom line of letters ‘clearer’. Ensure that the bottom line of letters is ‘definitely clearer and not just smaller and blacker’. This must be no more than 0.50 DS more than the refractive correction used to see 20/15 (6/4.5).

11.Measure monocular and binocular VAs, especially if the binocular difference is more than 0.25 D from the monocular subjective. If monocular visual acuity is reduced in one eye following this procedure recheck the results.

4.15.4 Adaptations to the standard technique

The prism-dissociation test balances the images when the eyes are blurred and this may not mean that they are balanced when vision is clear. Goodwin (1966) suggested a modified technique, which uses minimal prism dissociation and balances the images with clear vision or minimal fog (one line worse than best visual acuity).

4.15.5 Recording

The monocular subjective refraction result and the correction after prism dissociated balance can both be recorded with the accompanying VA results. Alternatively, the change in spherical power made with the binocular balance can be recorded. The binocular VA is also measured after the prism balance. For example:

Monocular subjective refraction