4 dispensingoptics March 2012

Refraction and prescribing by Claire McDonnell FAOI

Competencies covered: Refractive management for dispensing opticians, visual function for optometrists, binocular vision for optometrists Target groups: Dispensing opticians, optometrists

Refraction and prescribing are two the most commonly used forms of front of the retina and therefore the different concepts and often a objective refraction is the patient is myopic and vice versa for prescribed correction is not the same autorefractor. Many studies have hyperopia (see Figure 1). By adjusting as the error found in the testing room. shown that autorefractor results are the position of the object eventually a This article explores the reasons why generally very similar to subjective single focus of light will be seen by the the results of a refraction may not results but it is widely accepted that patient and the patient’s far point and necessarily be identical to the the main use of an autorefractor is to therefore can be correction the patient needs to wear. give the optometrist a starting point for determined. In the autorefractor two subjective1. (usually infra-red) diodes illuminate the Refraction can be divided into two retina through a small aperture and types: subjective - where input is Autorefractors are based on the their images are reflected onto a required from the patient and Scheiner double pinhole principle. In photodetector. By moving the objective which requires patient the Scheiner experiment a double aperture the image can be brought cooperation but little or no input. pinhole is placed in front of the eye into focus and its position then gives a Objective refraction is particularly and the patient is asked to look at a measurement of refractive error. useful where a patient may have spotlight which they perceive as communication difficulties, eg, young double. If the top pinhole is covered Autorefractor results themselves are children, mental handicap, Alzheimers and the patient reports that the upper not usually prescribed because they etc and indeed often in these cases spotlight disappears (remember don’t control/relax accommodation the objective results are the only images on the retina are inverted) as well as can be achieved during a power that can be prescribed. One of then the light must be focussing in subjective refraction. Proximal

This article has been approved for 2 CET points by the GOC. It is open to all FBDO members, including associate member optometrists. Insert your answers to the twelve multiple choice questions (MCQs) online at www.abdo.org.uk, or on the answer sheet inserted in this issue and return by 12 April 2012 to ABDO CET, 5 Kingsford Business Centre, Layer Road, Kingsford, Colchester CO2 0HT OR fax to 01206 734156. If you complete online, please ensure that your email address and GOC number are up-to-date. The pass mark is 60 per cent. The answers will appear in our May 2012 issue. C-17823 Continuing Education and Training

Figure 1: Scheiner’s double pinhole Figure 2: Shows a letter viewed with a correctly aligned cylinder on the right and a 2.50DC five degrees off-axis on the left accommodation from an awareness movement is observed centrally and a to over-minus the patient to achieve of being in an enclosed environment different movement observed in the the same effect. This works well with (propinquity), may be evoked by periphery. This can make it difficult to young patients with lots of some autorefractors2. Although a decide on an exact end point. accommodation and may be cycloplegic drop can be used to relax considered in favour of prisms, which accommodation, this is still not ideal. Even with subjective refraction there can have a tendency to be Autorefractor results are affected by can be errors. In fact in one study absorbed, thus requiring increasing corneal refractive surgery and certain comparing subjective refraction with amounts. ocular pathologies, eg, asteroid autorefraction, VA was better with hyalosis (a form of vitreous autorefraction than subjective To help correct a convergent degeneration in which calcium soaps refraction in 15% of cases1. Subjective deviation with an accommodative aggregate in the vitreous body). refractions are usually carried out at a element, the patient is usually given Autorefractor results can lack testing distance of six metres. (This their full cycloplegic prescription. This accuracy in high degrees of distance usually being achieved by relaxes accommodation and reduces due to off-axis aberrations3. use of a mirror). The hypothesis is that convergence helping to straighten the at six metres the effects of eyes. Sometimes a patient cannot The other common objective method accommodation will be negligible. accept the full prescription as there is of refraction is . (Although in fact the eye must ciliary muscle spasm preventing Retinoscopy results are prescribed for accommodate by 0.167D to see an complete relaxation of children, as subjective refraction of the object at six metres). When test charts accommodation. In these situations under fives is meaningless, from ages are used at four metres (as with many the patient is often given 75% of the five to seven subjective responses can logMAR charts) or at three metres (as full prescription initially. In the case of only be used as a rough guideline and is the case with some computerised children under the age of seven, 100% it is only really from age eight and and projector test charts), the amount of the prescription must be given upwards that the subjective refraction by which the patient is within six months or the convergent can be used for prescribing. accommodating is 0.25D and 0.33D deviation may never be fully Retinoscopy can also suffer from respectively. This can lead to the corrected. problems of accommodation. The patient being under-plussed or over- technique must be carried out with minussed in the refraction and some Prescriptions may also have to be the practitioner’s eye exactly aligned optometrists will make an allowance modified in cases of anisometropia. with the patient’s eye, as being off-axis for this in their final prescription. This can be divided into two types: can result in over or under estimation early onset - which has been present of both the sphere and the cylinder Optometrists may wish to modify since childhood and late onset or components. Patients with a manifest prescriptions to correct binocular vision secondary anisometropia which may strabismus may have to have their problems. If a patient has a divergent occur as a result of unilateral surgery fixing eye occluded during deviation (a tendency for an eye to or injury, eg, cataract operation or retinoscopy (to straighten the eye that turn outwards) the optometrist may laser refractive surgery to one eye. turns) which increases the likelihood of prescribe the lowest plus prescription Patients with early onset anisometropia unwanted accommodation. compatible with comfortable VA. By normally have some suppression of the Obtaining an accurate result with leaving the patient under-plussed, the eye with the higher prescription but retinoscopy can be difficult with small eyes are obliged to use this suppression may be mild and pupils, media opacities and accommodation. An increase in usually only applies to central vision. keratoconus. Even large pupils can accommodation causes the eyes to The depth (or density) and extent of cause problems as sometimes a split converge and this can correct the the suppression will depend on a reflex can be seen, where one divergent deviation. It is also possible number of factors such as whether or Continued overleaf 6 dispensingoptics March 2012 not there is also a turn in the eye, should be reduced to a point where it modified sphere (b), then the whether the patient ever wore a is within 1.25 - 2.12D of the prescription worse eye dos not need any patch, how much anisometropia is in the good eye. (Usually an inter-eye cylinder. present etc. Some optometrists will not difference of no more than 1.50D is • If (a) is numerically less than even bother to refract the worse eye if the aim). That is straightforward the proposed modified sphere the corrected VA in this eye is poor (ie, enough if the prescription is spherical (b), then the worse eye will the eye is amblyopic) and will simply but what if the prescription has a need some cylinder in most write “balance” in the prescription. cylindrical component? cases. Others may refract the amblyopic eye The modified prescription for the worse but still just prescribe “balance”, If cannot be fully eye never requires the full cylinder as leaving it up to the dispensing optician corrected, then the best case established in refraction. or even the technician to decide scenario would be to have the circle what power lens should be placed in of least confusion on the retina, the Example: Hyperopic anisometropia front of the amblyopic eye. Normally next best would be to have one focal A patient has the following the balance lens is a lens line on the retina (preferably one that prescription at the end of refraction: approximately equal in power to the is close to vertical). RE: +4.00/-1.00x90 LE: +0.50 prescription in the fellow eye (usually (a) Add the sphere and the cylinder equivalent to the mean spherical From personal experience a useful rule (from the worse eye) together: equivalent). This can be frustrating for of thumb for modifying prescriptions +4+(-1) = +3D a patient with good peripheral vision with astigmatism is as follows: (b) Add 1.50 to the prescription in the in their amblyopic eye. It is more 1. Hyperopic anisometropia with good eye: appropriate to prescribe a specific negative cylinder: +0.50+1.50 = +2.00D. This is the power in the balance lens, but what (a)Add the sphere and the cylinder proposed modified sphere. should that power be? (from the worse eye) together (a) is greater than the proposed (b)Add 1.50D to the prescription in modified sphere (b) and therefore the Anisometropia causes two problems: the good eye, this will be the poorer eye does not need any differential prismatic effect and proposed modified sphere for the cylinder correction. aniseikonia. A patient with a small worse eye The new modified prescription will be: amount of central suppression could • If (a) is greater than or equal to RE: +2.00DS LE: +0.50DS probably tolerate up to one prism the proposed modified sphere dioptre of vertical prismatic effect. (b), then the worse eye does This new prescription for the RE leaves Assuming that most people look 8mm not need any cylinder. both focal lines behind the retina but if below the optical centres of their • If (a) is less than the proposed any cylinder component was to be spectacles to read, then an inter-eye modified sphere (b), then the incorporated into the prescription one difference of 1.25D could probably be worse eye will need some focal line would lie even further tolerated by most amblyopes (using cylinder in most cases. behind the retina. Prentice’s rule: P =cF = 0.8x1.25 = 1 2. Myopic anisometropia with prism dioptre). Aniseikonia seems to negative cylinder: convert the Example: Myopic Anisometropia become clinically significant at values prescription to plus cylinder form A patient has the following of 3-5%4,5,6,7,8. (See note 1 on how to (a)Add the sphere and the cylinder prescription at the end of refraction: calculate spectacle magnification as (from the worse eye) together RE: -4.00/+3.00 x 90 LE: -0.50 a percentage). (b)Subtract 1.50D from the (a) Add the sphere and the cylinder prescription in the good eye, this will (from the worse eye) together: This translates as an inter-eye be the proposed modified sphere -4+3 = -1D difference of 2.12D (assuming a for the worse eye vertex distance of 13.75mm). Therefore • If (a) is numerically greater (b) Subtract 1.50 from the prescription the prescription in the worse eye than or equal to the proposed in the good eye: -0.50-1.50 = -2.00D. This is the proposed modified sphere. (a) is numerically less than the Spectacle magnification as a percentage proposed modified sphere (b) and RE: Plano LE:+2.12D therefore the poorer eye will need Vertex distance = 13.75mm cylinder correction. K (ocular refraction) = +2.18D The new modified prescription will be: Spectacle magnification (SM) for the LE = K/Fsp = 2.18/2.12 = 1.03 RE: -2.00/+1.00 x 90 LE: -0.50 Spectacle magnification as a percentage: 100(SM-1) = 100(0.03) = 3% Note: in the case of a thick lens, the form of the lens will affect the This new prescription for the RE leaves spectacle magnification, in which case the thick lens formula should be one of the focal lines on the retina. used.

Note 1 It is also possible for the optometrist to modify the refraction results in the trial frame to come up with a modified Continued overleaf 8 dispensingoptics March 2012

Trial lens power for near Correction value for the stated add opticians would not know the +2.00 +2.50 +3.00 thickness and front surface power of +8.00 the proposed spectacle lens and so it +8.50 is simpler just to use a table (see Table +9.00 +0.25 1). It is not necessary to calculate +9.50 NVEE for minus lenses as these tend to +10.00 +0.25 be quite flat and thin and are +10.50 therefore similar in form to trial lenses. +11.00 +0.25 +11.50 Even for distance prescriptions the BVP +12.00 of a combination of trial lenses in the +12.50 +0.50 trial frame is not necessarily its +13.00 algebraic sum. It depends on the +13.50 power, thickness, form and position of +14.00 +0.50 the lenses used. This is particularly so +14.50 with high refractive errors. After +15.00 refracting a patient with high +15.50 +0.50 ametropia in a trial frame, the BVP +16.00 +0.75 should be measured using a focimeter and the subsequent reading should Table 1 be ordered for the patient’s spectacles. prescription that the patient is Modification of a prescription may comfortable with and most dispensing also be required to correct for near It is well known that a change in vertex opticians will look at the patient’s vision effectivity error (NVEE). The back distance between that read from the previous balance lens and ask if the vertex power (BVP) of a lens represents trial frame during refraction and that patient was happy with that lens. In the vergence leaving the lens when which the patient will eventually wear cases where the poorer eye has a the light originates from a distant in their new spectacles can cause very reduced VA, the prescription in object. For near vision the light problems, but at what power does this the balance lens is largely irrelevant. originates from a point which is a finite become an issue? According to the Where the anisometropia is late onset distance from the front of the lens. In relevant standards, the vertex or secondary the optometrist may this case the vergence of light leaving distance is only required to be want to attempt to give the full the back surface depends on the BVP, included on written prescriptions prescription (particularly if it is the the form and thickness of the lens and above ± 5.00D. Assuming that the patient’s dominant eye that has the the patient’s working distance. When closest a frame (trial frame or higher prescription). In this case the patients are tested for a reading spectacle) might be worn is 8mm and dispensing optician can look at prescription, trial lenses made in flat the furthest it might be worn is 15mm dispensing bicentric lenses or form are used. However when the (giving a change in vertex distance of attempting to order different lens lenses are made up into spectacles, 7mm) then the lowest prescription that forms to minimise the inter-eye they are in a more curved form. In the could have a change of difference in retinal image size. case of lenses greater than +8D, this approximately 0.25D is ±5.50D. difference in form means that the Realistically however the change in If the patient with late onset patient gets less power from their vertex distance between the testing anisometropia cannot tolerate or spectacles (compared to the trial room and the dispense is not normally afford specialist lenses or does not like lenses) and so they have to make up that large. For example nearly all their cosmetic appearance, the the shortfall with accommodation. The phoropters have a vertex distance of prescription should be modified by difference between the vergence 13.75mm assuming they are used either changing the prescription in the leaving a lens when the light originates correctly. If we say that a change of eye with the worse VA to bring it to from a near object and the vergence 5mm is more likely then the lowest within 1.50D of the other eye or, if the for a distant object (the sum of the prescription with an approximately VAs are equal, changing the incident vergence + BVP) is known as 0.25D change becomes ± 6.50D. The prescription in the non-dominant eye. the NVEE. It is possible to calculate simplest way to avoid having to make There are many different ways of what the error will be using the adjustments for vertex distance is to testing for ocular dominance but for formula: NVEE = [(t/n)L1(L1+2F1)] help the patient to choose a frame the purposes of prescription where t is the centre thickness of the prior to the test, measure the back modification, the simplest way is to put spectacle lens, n is the refractive vertex distance of the chosen frame the prescription in a trial frame with index, L1 is the incident vergence (for and then set the trial frame up at this the RE modified and then with the LE a reading distance of a third of a distance for the actual refraction itself. modified and check which one metre, this would be -3D) and F1 is the causes the patient the least amount of front surface power of the spectacle Unequal reading additions are noticeable blur. lens. In reality, most dispensing uncommon in prescriptions but there Continuing Education and Training

may be reasons why they are unable to take their full cylinder static model of accommodation and sometimes required. The additions prescription. In this case their spheres vergence. Ophthal Physiol Opt 16, 31- may be different where the should remain the same and their 41 accommodation is not equal in both cylinders reduced. 3. Collins MJ, Wildsoet CF, Aitchison eyes. There can be a number of The new modified prescription will DA. (1995). Monochromatic reasons why this might occur. If a depend on their VA requirements but it aberrations and myopia. Vision Res. patient has unilateral pseudophakia could be: 35, 1157-1163 the eye with the intraocular lens RE: Plano/-2.00x35 4. Jiménez JR, Ponce A, del Barco LJ, implant will have little or no LE: +0.50/-1.75x155 Díaz JA, Pérez-Ocón F. Impact of accommodation while the other eye induced aniseikonia on stereopsis with still has accommodation normal for Example 2 random-dot stereogram. Optom Vis the patient’s age. Third nerve palsies A patient has already had spectacles Sci. 2002;79:121-125. can cause paralysis of made up to their full refracted 5. Jiménez JR, Ponce A, Anera RG. accommodation. Horner’s syndrome prescription which is: Induced aniseikonia diminishes causes an increase in RE: -2.00/-1.00x70 binocular contrast sensitivity and accommodation and Adie syndrome LE: -2.50/-1.50x65 binocular summation. Optom Vis Sci. causes a reduction in but they are noticing distortion that 2004;81:559-562. accommodation but this is usually they never had with their previous 6. Katsumi O, Tanino T, Hirose T. Effect short-lived. spectacles (no astigmatic correction). of aniseikonia on binocular function. To place the CLCs on the retina, the Invest Ophthalmol Vis Sci. 1986;27:601- Some patients who are prescribed prescription can be modified to: 604. astigmatic spectacle prescriptions for RE: -2.50 7. Crone RA, Leuridan OM. Tolerance the first time will not tolerate their full LE: -3.25 for aniseikonia. I. Diplopia thresholds in cylinder (or indeed any cylinder). A the vertical and horizontal meridians of patient is increasingly likely to struggle Another rare problem that can occur the visual field. Albrecht Von Graefes to adapt to astigmatism, the older with astigmatic prescriptions is Arch Klin Exp Ophthalmol. 1973;188:1- they are when they get their first cyclotorsion. This is where the eyes 16 spectacles, the higher the cylinder, the rotate as well as converging when 8. Crone RA, Leuridan OM. Tolerance more oblique the axis and the less looking at a near object. For most for aniseikonia. II. Determination based they wear their spectacles. Normally if patients this is not a problem but a on the amplitude of cyclofusion. an astigmatic prescription has to be patient with high cylinders and Albrecht Von Graefes Arch Klin Exp modified the aim would be to put the incyclotorsion at near may find that Ophthalmol. 1973;188:17-22 circle of least confusion (CLC) on the they need two separate pairs of retina. However if the spherical spectacles for distance and reading element of the patient’s prescription is with different axes in each. Figure two plano or mildly hyperopic then they shows the effect that five degrees of will be accustomed to having one cyclotorsion can have on a 2.50D focal line on the retina so it should be cylinder. checked to see if they prefer a modified prescription that puts the It can be seen then that there are CLC or a focal line on the retina. quite a number of scenarios in which a Once the cylinder has been reduced prescription may differ from that found to a level that the patient can adapt in the refraction. It is important to to, the CLC can be placed on the remember that if it is necessary to retina by adding half the modified modify a prescription it should be cylinder to the sphere and this is the noted on the patient’s copy (as well new sphere. If the aim is to keep one as the practice’s record card) that a focal line on the retina, leave the modification has been made so that sphere as found on refraction. If the other optometrists will be aware of it patient cannot tolerate any cylinder and will not query what appear to be simply add their sphere and half the large changes in prescription. It is also cylinder together and prescribe this as necessary to ensure that patients still Claire McDonnell FAOI is a lecturer in a sphere. Again this will place the CLC meet certain visual standards (e.g. the the Department of at the on the retina. driving standard) when their Dublin Institute of Technology where prescription is modified. she teaches advanced clinical Example 1 techniques to optometry A patient has the following References undergraduates and qualified prescription at the end of refraction: 1. Wood IC. A review of autorefractors. practitioners. She has worked in RE: Plano/-3.50x35 Eye 1987;1(Pt 4):529-35 private practice, refractive surgery LE: +0.50/-3.25x155 2. Hung GK, Ciuffreda KJ, Rosenfeld M. and education in Ireland, the UK and The patient has a history of being (1996). Proximal contribution to a linear New Zealand. n MCQs overleaf 10 dispensingoptics March 2012 Multiple choice questions (MCQs): Refraction and prescribing

1. What principle is used as the basis of design for an auto 7. Which of the following parameters is not a required refractor? known factor when calculating NVEE for a prescription? a. The Fundamental Paraxial equation a. The lens thickness, t b. The Airy Disc b. The refractive index of the lens material, n c. The Scheiner double pinhole c. The back surface power of the lens, F2 d. Snell’s Law d. The front surface power of the lens, F1

2. How much would an eye have to accommodate as a 8. What kind of power modification is most likely to be result of the testing distance if a logMAR chart was used at selected for a patient with a divergent deviation, in order to a distance of 4 metres from the subject? assist their binocular vision? a. 0.167D a. A small increase in plus power b. 0.25D b. A small decrease in plus power c. 0.33D c. The lowest plus prescription that will allow a comfortable d. 0.50D V/A d. The highest plus prescription that will allow a 3. Given that a patient is able to tolerate 1 prism dioptre comfortable V/A differential between their two eyes, what degree of anisometropia is likely to be the most that a patient could 9. A patient has the following prescription: RE +1.00DS LE tolerate when looking downwards to read through a point +4.25/-0.75 x 90. In this case of this being a troublesome 8mm below the distance optical centres? anisometropic prescription requiring alteration what is the a. 0.50D most likely combination to be prescribed? b. 0.75D a. RE +1.00DS LE +2.50DS c. 1.00D b. RE +2.50DS LE +4.25/-0.75 X 90 d. 1.25D c. RE +1.00DS LE +3.50DS d. RE +2.50DS LE +3.50DS 4. Aniseikonia is a condition caused by the two lenses of a pair of spectacles giving different levels of spectacle 10.What is/are likely to cause secondary anisometropia in magnification from one another. At what degree of an adult? difference between the image sizes does aniseikonia a. A congenital condition appear to become significant? b. Laser refractive surgery to one eye a. 2 - 4% c. An injury to one eye in infancy b. 3 - 5% d. All of the above c. 4 - 6% d. 5 - 7% 11.When dealing with a convergent deviation, what refractive result would usually be prescribed? 5. In the case of an anisometropic astigmatic prescription a. 75% of the full prescription where the astigmatism cannot be fully corrected, which of b. 100% of the prescription within six months the following options would be the best case scenario? c. The full cycloplegic prescription a. Place the circle of least confusion on the retina d. 50% of the full prescription b. Place the focal line closest to the horizontal on the retina 12.Spectacle lenses for near vision are being dispensed for c. Place the focal line closest to the vertical on the retina a patient whose prescription is: d. None of the above would make any significant RE +9.50/+0.50 x 90 LE +9.25/+0.75 x90 difference Reading Addition +3.00 R & L Vertex Dist. 12mm. 6. NVEE only has an impact when considering a certain If this prescription is dispensed at 12mm, what lens powers prescription range. Which is this prescription range? would be ordered? a. Over -8.00D a. RE +12.00/+0.50 x 90 LE +11.75/+0.75 x 90 b. -8.00D to -0.25D b. RE +9.50/+0.50 x 90 LE +9.25/+0.75 x 90 c. +0.25D to +8.00D c. RE +12.50/+0.50 x 90 LE +12.25/+0.75 x 90 d. Over +8.00D d. RE +13.00/+0.50 x90 LE +12.75/+0.75 x 90

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