Continuing education CET

Routine Part 7 – Subjective refraction Bill Harvey and Andrew Franklin continue their series looking at the eye examination. Here they describe the subjective refraction of the patient. Module C9038, two general CET points

etinoscopy completed plus sphere, and tied to convergence the acuity enough, add more plus until (see Optician, April ● No separate binocular balancing is it does. 11), we can proceed to needed. This saves quite a lot of time the subjective stage of ● Where latent nystagmus is present, The ‘Hack Humphriss’ technique refraction. In an ideal it will be reduced This is one of those terribly useful and world, what you now ● Rotational phorias (cyclophorias), if commonly employed techniques which Rhave in the trial frame is the patient’s full present, will not reduce the final binoc- never seem to find their way into the spectacle correction plus your working ular VA. This is now known to be an textbooks. However, it works so well lens. important factor in refractive surgery that many practitioners use it as their The accurate correction of the astig- patients, and binocular refraction is basic technique. Essentially, this is a matic error requires that the circle of least required in the clinical protocols for pre- mixture between binocular and monoc- confusion (CLC) is placed on the retina treatment refractions in some refractive ular refraction. and kept there while the cross-cylinder surgery clinics. ● To refract the right eye, occlude the is in use. If this is not achieved, both the left eye axis and power found will be wrong. Limitations ● Refract the right eye monocularly The range of spherical powers which Binocular refraction should not be used ● Apply the +1.00 blur test. If the VA allow maximum acuity can be several where acuities are markedly unequal, or is reduced to 6/12 or so leave the +1.00 dioptres in patients with small pupils one eye is strongly ‘dominant’. If you do in. If not, adjust the fogging lens until and low acuities, though it is usually start to apply it to an unsuitable patient, it does smaller. There may be a unique value for they will usually tell you fairly quickly ● Refract the left eye with the right eye the correction which gives maximum (‘Should I be seeing double?’). fogged, that is binocularly contrast, although where accommoda- In such cases occlude the better eye, ● Apply the +1.00 blur test to the left tion is active this may also be a range. refract the worst eye. Then refract the eye In this latter case the most positive lens better eye with the worst eye fogged, if ● Check the final sphere balance of the that gives maximum contrast is usually necessary, or even unfogged if the acuity right eye binocularly with the +1.00 still in the optimum correction. is considerably worse that the eye you place before the left eye. The recommended routine includes are going to test. If the worst eye is 6/9 If you need to add a substantial a rather elaborate sequence of checks or less, it may be unnecessary to fog it. amount of plus because your retin- on the sphere power before, during and If the difference is less, it is often possi- oscopy result was some way out, it is after the cross-cylinder is used. This is ble to refract binocularly, but the final worth checking the fogged eye periodi- to ensure that: decision has to be taken on an individual cally to ensure that it is still fogged. ● The patient has not been under- case basis. Sometimes, the addition of plus to the plussed. This is particularly easy with eye being refracted will cause both eyes young patients, especially hyperopes Humphriss immediate contrast to relax accommodation. ● The initial sphere will be within the technique (HIC) effective range of the duochrome test. This method employs a fogging lens To place the CLC near the retina before the eye which is not being Initially, we must determine the ‘best Binocular versus monocular refracted. Humphriss recommended a vision sphere’ (BVS), which can be refraction +0.75DS lens, though most practition- defined as the most plus or least minus It seems remarkable that more students ers use a +1.00DS and some have used lens with which the patient can enjoy of enter their pre-registration their retinoscope working lens (namely, maximum . In order that we year performing monocular refractions +1.50) and claim it works as well. The do not under-plus, a ‘fogging’ technique than binocular ones. Some university idea is to reduce the acuity in this eye is used. For each eye: clinics appear to take the view that to about 6/12 at which point central ● Check the visual acuities with the monocular refraction is easier to learn, vision is inhibited and the ‘physiologi- working distance lenses still in place. which seems a little patronising when cal septum’ is established. The first eye The visual acuity should be around 6/24 applied to honours degree students. is then refracted. When the end point for a working distance of 66cms and a Binocular refraction has no disadvan- is reached, the eye which was fogged working lens of +1.50DS. If the pupils tages when compared to monocular is occluded, and the eye which was are small the acuity may be considerably refraction, though there are patients who refracted is fogged by +1.00. Provided better than this. cannot be refracted binocularly. that the +1.00 blur test reduces the acuity ● Reduce the plus to give 6/18. At to a satisfactory level, it can be left in as this point you should be about +1.00 Advantages of binocular refraction the fogging lens while the second eye is overcorrected. ● Accommodation is suspended with a refracted. If the +1.00 does not reduce ● Reduce the plus until the VA stops

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improving (as opposed to going smaller vision sphere as it is and proceed to and darker). investigate the cylinder. However, there is a measurable depth of focus even in To refine the best vision young patients with big pupils. On sphere (BVS) older patients with smaller pupils, and The sphere may be refined by use of the on the very astigmatic, whose principal duochrome or ± spherical twirls. The foci will be widely separated, this depth two methods give statistically identical of focus will be larger. Theoretically, results, though this does not mean that the best way to ensure that the CLC they will always agree on a particular is placed precisely on the retina is to patient. Using one method to validate allow the patient to put it there with the other is generally a waste of time. accommodation, assuming they have Each method has some limitations and any. There is actually no real scientific the techniques must be applied correctly. proof that patients do this, and the total It is important that you are in control of amount of blur is higher at the CLC the patient’s accommodation and don’t than at other points, but proceeding on stimulate it unnecessarily. Change plus Figure 1 Cross cylinder this assumption seems to work. We aim lenses by inserting the replacement therefore to allow the patient to accom- before removing the original; change offered if the patient cannot differenti- modate minimally by slightly under- minus lenses by removing the original ate between the first two. plussing them. before inserting the replacement. This If the first lens is clearer or they are For patients who are likely to goes for cylinders as well as spheres. both the same, add +0.25. If the second have a small depth of focus (young lens is clearer (as opposed to just smaller patients with low degrees of astigma- Duochrome and darker), add -0.25 to the eye being tism) modifying the BVS by -0.25 is ● If the error is over 1.00D, or the tested. appropriate vision 6/9 or worse, the results may be The rapidity with which the negative For patients with higher degrees of unreliable. The red and green only have lens must be withdrawn can cause or larger depths of focus a 0.50D difference in focus problems when a patient is slow to a larger initial modification may be ● Over 55 years of age, the chromatic react. For this reason many practitioners required. aberration of the eye drops markedly, so have modified the simultan technique the dioptric interval of the red and green to eliminate this phase. Cross-cylinder technique reduces, especially with a small pupil The cross-cylinder is a lens that has a ● Yellowing of the lens causes a red Adding plus only positive cylinder worked on one surface shift, leading to under-plussing After initially determining that the and a numerically equal negative cylin- ● It is important that the patient under- sphere is a little (not more than 0.50DS) der on the other. The axes of the two stands that they must compare the under-plussed by duochrome or simul- cylinders are at right angles to each rings on the duochrome rather than tan, +0.25 is introduced and the question other. Thus the actual power of a ±0.25 the colours themselves. Some patients asked: ‘Is it clearer with the lens, without cross-cylinder is equivalent to a sphero- concentrate more on the brightness of it…or just the same?” OR ‘Is it just the cylindrical lens of +0.25D spherical and the rings, others on the basis of favourite same with the lens, or worse?’ –0.50D cylindrical power. In general, colours The first variant has the disadvantage the axes are marked with + and – signs, ● If the rings on the green are clearer of being a multiple question. The second and usually the plus axis marked in red the answer is unambiguous regardless may confuse because the +0.25 often is and the minus axis in white. of the patient’s age. If they are clearer clearer (especially in presbyopes). You Once you have refined the sphere, on the red, the patient may be myopic or must, as always, pick the question to suit there is nothing to stop you checking the they might be accommodating. To avoid the current patient and it is sometimes VA, as this might indicate how much overdoing the minus, patients who see necessary to change the question once cylinder remains to be corrected. In the rings on the red as clearer more often you have got used to the patient. If in general, you will probably have close to than seems right should also be checked doubt, try both variations in succession the right correction (the ‘working cylin- with the other methods below. and see which one the patient responds der’) in place, but where the astigmatic to best. error appears small on retinoscopy, it Simultan technique (using ± twirls) If the patient finds the vision clearer may save time to leave out the cylinder ● The plus lens must be presented or identical with the plus, add +0.25 entirely and check initially with a cross- first for at least a second to relax to the sphere and repeat. If the patient cylinder of a power roughly equal to the accommodation. rejects the plus, add -0.25 to the sphere estimated astigmatic error over a purely ● The negative lens should not be held in the trial frame, then repeat. spherical correction. for more than 1 second, which is the With this method we may induce Once the working cylinder has been reaction time plus response time for accommodation when we add minus established it can be refined as follows: accommodation. power to the sphere in the trial frame, The patient is asked ‘Is it clearer with but we are always adding plus, and What should the patient look at? the first lens, the second lens…or are therefore relaxing accommodation, Generally, the target should be circu- they both the same?’ immediately before the comparison is lar and a little bigger than the small- It is useful to split the two halves of made. est letters that can be seen, as targets this question to avoid asking a multi- containing linear elements may preju- ple question. The initial comparison Placing the CLC on the retina dice the result if the circle of least should be between more plus and more If the human visual system had no confusion is not quite on the retina. minus. The third option should only be depth of focus, we could use the best Some practitioners have advocated

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using several letters at a time, but this ● Check the visual acuity can confuse the patient (not to mention ● Add + until this drops by one line the practitioner) when ‘some letters are and the circles on the duochrome are better with position one, and some with clearer on the red position two’. Circles or Landolt Cs of ● The patient then looks at the fan the appropriate size, or targets consist- and is asked to report which line(s) are ing of a pattern of dots, are preferable clearest. Initial selection can be aided by to single letters (other than ‘O’s). asking, ‘If this was a clock face, what time would the clearest line be pointing Axis to?’ The clearest line, or the centre line ● The cross-cylinder is presented so that of a group of clear lines, indicates the its plus and minus axes lie at 45 degrees negative cylinder axis to the axis of the working cylinder ● The axis can be refined by using the ● The cross-cylinder should be presented arrowhead. Rotate the arrow until it for at least one second in each meridian, points towards the chosen clearest line and spun as quickly as possible between and adjust the axis until the two arms of meridians. Some cross-cylinders have the arrowhead appear equally clear flat areas on the handle to assist this Figure 2 using is of insufficient power to make ● At this stage one of the blocks should ● Initially, a cross-cylinder of similar or Suitable a discernible difference to the target. be clear, while the other one, which has slightly less power than the estimated target for lines at right angles to the clear one, will required cylinder power is ideal. The cross- What if the patient always says be blurred. Add negative cylinder until axis is further refined after determin- cylinder position one is clearer? both blocks are equally clear ing the power. For large errors of axis a technique ‘Perseveration’ can be defined as a ● If all of the lines in the fan seem ±0.50 gives a larger difference in image. tendency of organismic activity to recur equally clear to the patient, increase As the error gets smaller, the ±0.25 gives without apparent associative stimulus. the fogging lens by a further +0.50D similar differences but the overall blur This is the one track mind; the patient and check again. If the lines are still all and distortion are smaller and less who keeps telling you that the first equally clear, no significant astigmatism distracting. lens is clearer when you know it can’t is present be. Perseverators may also give lower ● Reduce the fogging lens to find the Power scores on some stereotests due to their best sphere, then repeat for the left ● In this case the axes of the cross-cylin- inability to adapt. eye. der are aligned with that of the working To counteract perseveration the cylinder following strategies may be useful: Final sphere check ● To check the result, use a bracketing ● Try to arrange it that the answer to ● If your spherical power was correct technique by using the cross-cylinder on any subjective test that you do is not when using the cross-cylinder, the lenses which are + and –0.25DC from the same too many times in succession. patient should be slightly under-plussed. your end point The use of bracketing techniques is Therefore there is no logical reason to ● Remember to modify the sphere when beneficial offer more minus to the patient. If the you change the power of the cylinder ● In some cases, vary the question or patient needs more minus at this stage, to keep the circle of least confusion on the way you phrase it, for example, ‘Is your cross-cylinder has probably got the the retina. As a general rule, every cylin- it clearer with three or four/five or six/ cylinder wrong, so you would need to der change should be matched with a seven or eight’ and so on check it again change of sphere by half the amount and ● A contrived interruption, such ● If using the duochrome pre-presby- with the opposite sign (that is for each as dropping a pen, may break the opes are often best left on the green, -0.50DC, change the sphere +0.25DS). sequence. A regular rhythm of question presbyopes on the red, but there Alternatively, you can check with the and answer, while often useful with are exceptions. If in doubt, balance duochrome or sphere twirls at intervals. other patients, may only reinforce equally Automated refractor heads modify the perseveration. ● Young myopes are often used to sphere automatically. Trial frames rarely being slightly overcorrected, and young remember to change the sphere for you, Fan and block technique hyperopes under-corrected so that will be your job. Most, though not all, tests charts have ● Exophores may be happier on the a fan and block at the top. It is a useful green, as accommodative convergence Final axis determination test to have, especially for those patients may help to compensate their phoria. ● Recheck the axis with a ±0.25 cross- who do not respond well to cross-cylin- Esophores may be better with more cyl. Bracket the end point by checking at der. It may actually control accommo- plus 5 degrees either side of the axis found. dation better than the cross-cylinder ● Use a +1.00 blur test. With the other technique as a fogging technique is eye occluded, the VA should be 6/12 or What if the patient can’t make up employed. However, it is thought to 6/18, with an average pupil. If it is, you their mind? be less accurate than the cross-cylinder have the physiological septum in place This may be because they don’t under- technique for small astigmatic errors, for the other eye if you wish to check the stand what they are supposed to be doing. and it is a monocular method. The sphere balance binocularly. If it is better, This may be because your instructions starting point is the best vision sphere the patient is probably under-plussed, are not very explicit, or simply because as with cross-cylinder, but then the unless they have small pupils. they can’t hear you. Repeat the instruc- techniques diverge. The above points are valid for a 6m tions slowly and clearly. If they can ● Occlude the LE (usually the RE is chart. In practice, you may come across both hear and understand you it may tested first, for no particular reason) projection charts which may present be because the cross-cylinder you are ● Remove the cylinder from the RE an image at 3m, or at infinity. When opticianonline.net 09.05.08 | Optician | 31 Continuing education CET

working with infinity charts you need no adjustment of the sphere for Multiple-choice questions the testing distance. Direct 6m charts should in theory cause over-plussing of Which of the following statements about What should be the acuity for a 6/6 0.167D, which is why pre-presbyopes 1binocular refraction is not true? 7patient through a +1.00DS lens and an like to be left on the green. Presbyopes A It is better for patients with latent nystagmus average pupil size? usually have smaller pupils, giving B It reduces errors related to cyclophoria A 6/9 greater tolerance to blur, and appreci- C It is better with strongly eye dominant B 6/12 - 6/18 ate the slight boost to their mid-distance patients C 6/24- 6/36 vision that the extra quarter-dioptre D It suspends accommodation well D 6/60 gives. Projection charts with a 3m distance will cause over-plussing of What is the dioptric difference between Which of the following is true about 0.33D, so it is customary to add -0.25DS 2the red and green targets on a 8exophoria? to the distance portion of the subjec- duochrome target? A Exophores prefer more plus to esophores tive findings before prescribing. In the A 0.25DS B Exophores are best left on the red case of a presbyope, any extra minus B 0.50DS C End sphere for exophores should tend to minus on the distance prescription should be C 1.00DS D Exophores and esophores should not be offset by adding +0.25DS to the reading D 2.00DS treated differently addition. Which of the following statements about For a cylinder change of -1.50DC during ‘Binocular balancing’ 3duochrome is true? 9cross cylinder, by how much should the In older textbooks methods are A Smaller pupils enhance the accuracy of the sphere change? described whereby the eyes are fogged test A +0.75DS and the sphere adjusted to give equal B The brightness of the colours is more B -0.75DS acuities in the two eyes, but this is rather important than the distinctness of the rings C +1.50DS missing the point of it all. Many patients C Cataract will tend towards favouring the rings D -1.50DS have a ‘better’ eye, and if we artificially on the red background equalise the acuities the patient may be D Accommodation will favour the rings on the  What fogging lens was recommended uncomfortable. It may only be possible green 10by Humphriss for his immediate to equalise the two by compromising contrast technique? the acuity of the better eye. What is the actual power equivalent of a A +0.50 The idea of binocular balancing is to 4+\- 0.50D cross-cylinder lens? B +0.75 balance the accommodative effort in A +0.25DS/-0.50DC C +1.00 the two eyes by uncovering any extra B +0.50DS/-0.50DC D +1.25 hyperopia which becomes manifest C +0.50DS/-1.00DC when the patient is binocular. If the D +1.00DS/-1.00DC  Why might the +1.00DS blur test patient has no accommodation there is 11give less than reliable results in an little point in trying to balance it. Some Which of the following statements about elderly patient? practitioners have advocated trying to 5fan and block technique is true? A Elderly patients will not understand the test balance the depths of focus of the two A It is more accurate than but slower than cross- B Light scatter is enhanced by the fogging lens eyes, but this seems a little eccentric, cylinder technique so giving worse acuity than predicted given that the depth of focus is largely B It is a monocular method C The small pupil increases depth of focus and pupil-dependent. C It is better for larger cylinder powers makes end acuity better than predicted Various methods have been devel- D It is not suitable for presbyopes D Macular changes reduce blur sensitivity oped to give each eye a separate target without causing complete dissociation, Assuming cross-cylinder has been What is the aim of introducing using either a physical septum placed on 6performed accurately, what should be the 12-0.25DS prior to cross-cylinder the chart or mirror, or polarisation. state of the mean sphere? assessment? The actual balancing can be done A Slightly under-plussed A To ensure patient appreciates the duochrome either using the duochrome, or by B Slightly over-plussed B To improve the acuity finding the most plus lens consistent C Exact C To stimulate accommodation with best acuity for each eye. D Heavily over-plussed D To allow the patient to place the circle of least Where you have performed a binocu- confusion on the retina by accommodation lar refraction, no separate balancing is required. In those cases where binocular To take part in this module go to opticianonline.net and click on the Continuing Education refraction is inappropriate, it is unlikely section. Successful participation in each module of this series counts as one credit towards that any of these techniques will give the GOC CET scheme administered by Vantage and one towards the Association of you any worthwhile information. Optometrists Ireland’s scheme. The deadline for responses is June 5 If all else fails, and the patient is happy, keep the balance the same as the Binocular plus last correction. A change of balance With the patient fixating binocularly, plus, but practitioners who use monoc- often requires some adaptation on the check if +0.25DS is accepted binocu- ular techniques may benefit from this part of the patient, so it should not be larly. If it is, you may incorporate it step. ● undertaken casually. Some manage- into your final Rx, but allow for testing ment of the patient’s expectations is distance, binocular balance and so on. ● Andrew Franklin works in private also required in such circumstances, to Using binocular refraction techniques, practice in Gloucestershire. Bill Harvey is avoid unnecessary re-tests. it is rarely necessary to add binocular clinical editor of Optician

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