Anisometropia: what difference does it make?

Transcription

1 Optometry in Practice 2013 Volume 14 Issue Anisometropia: what difference does it make? Paul McCarthy FBDO PG(cert)HE FHEA Anglia Ruskin University, Cambridge EV C CET point for UK optometrists This article discusses the identification and presence of anisometropia. The effects on patients with anisometropia, such as aniseikonia and diplopia, are considered, as are the issues that the correction of anisometropia presents to both optometrist and dispensing optician. Lenses that are still currently available and which help to eliminate or reduce symptoms of off-axis vision to anisometropes to levels that are within tolerance are discussed. Consider for a moment how concerned patients would feel if, when collecting their first pair of multifocal or progressive lenses, they find that, on looking down to read, the print appears to be double and reading is impossible. Maybe they would feel that the prescription is wrong or the glasses have not been made correctly. Their sudden fear of having a serious eye condition should not be discounted. Fortunately situations like this are unusual as well as avoidable. Careful analysis of the patient s prescription would very likely have identified the presence of anisometropia. Consideration of the dispensing process should then have led to an informed decision on how to prevent any non-tolerance. There is however a growing tendency today for prescriptions to be transferred directly on to a computer within the consulting room, which can sometimes lead to an order being created for a new pair of spectacles with little or no discussion with the optometrist. It is imperative therefore that both optometrist and dispensing optician first recognise an anisometropic prescription and then if necessary discuss the issues that need to be considered to ensure patient non-tolerance is avoided. To do this we need to consider the following points: What is anisometropia and how do we recognise it? What impact does it have on the patient s vision? How do we avoid the effects of anisometropia? What lenses are available? Anisometropia Anisometropia is present when a subject s right and left prescriptions are unequal: the prefix aniso means not the same. A significant difference in refractive error between the two eyes of more than 1.00D in any meridian is often given as a definition of anisometropia (Harvey and Gilmartin 2004; Weale 2002). Woodhouse (2012), when describing the rarity of anisometropia, states that a difference between the eyes of more than 1.00D should be considered abnormal. When the eye looks at a point away from the optical centre of a lens, or the axis of a plano cylinder, it encounters a prismatic effect. The amount of this prismatic effect, determined by the expression P = cf, is the product of the distance in centimetres from the optical centre and the power of the lens in the corresponding direction. Looking through a point 8mm directly below the optical centre of a lens of power will result in a prismatic effect of 2.4Δ (0.8cm +3.00D). The prism base direction which is indicated by where the thickest part of the lens is from the visual point will be up. If the right and left prescriptions are the same and both lenses are centred correctly, then each eye will encounter equal prismatic effects at the corresponding visual points, which should not present any difficulties to the wearer. All isometropic progressive lens wearers with equal prism thinning in each lens encounter this effect with rarely any difficulties. Both eyes exhibit a version or conjugate movement where they move together towards the apex of the prism. Displacement of the image is the same and so each eye will rotate by the same amount so that binocular vision is maintained. Far more problematic is when two eyes have different prescriptions, especially if the difference is in the vertical meridian. This will give rise to a differential prismatic effect Date of acceptance: 11 September Address for correspondence: P McCarthy, Anglia Ruskin University, Cambridge Campus, East Road, Cambridge CB1 1PT. paul.mccarthy@anglia.ac.uk The College of Optometrists 1

2 P McCarthy at the same right and left visual points away from the optical centres. Depending upon the degree of differential prism it may or may not be possible for one eye to exhibit a vertical vergence, either supra or infra, to achieve and maintain binocular vision. If the differential prism is too great so that fusion of the right and left images cannot be sustained, it will result in the object being seen double. It is not always possible to predict the degree of tolerance, however. Dadeya et al. (2001) have found from the use of Bagolini lenses that some binocularity can be preserved even with the presence of 3.00D of anisometropia. Holladay and Rubin (1988) agree that, whilst horizontal prism disparities rarely cause problems because of the large horizontal vergence amplitudes, this is not the case with the much smaller vertical amplitudes. A subject with just a 1.00D difference in right and left refractive errors looking through corresponding points 1cm away from the optical centres of lenses will encounter 1 of differential prismatic effect. It is generally accepted that many subjects cannot tolerate more than about 1 of differential prismatic effect for a prolonged period, especially in the vertical meridian (Jalie 1984). This is more likely to manifest itself to the multifocal or progressive lens wearer where, for close-up tasks, the eyes are required to look down vertically into the near visual zone. If we consider points at 10mm below the optical centres of the lenses R +1.00DS, L +3.00DS, a differential prismatic effect of 2 base up in the left (or down in the right) will result. Not so obvious at first glance is the effect at the same visual point with a prescription: R / , L / Figure 1 shows that for the right eye the vertical power is just +0.25D. For the left eye the cylinder power in the vertical direction can be calculated using F sin 2 θ where θ is (135 90). The cylinder therefore exerts only 0.50D in the vertical direction, giving a total power of +2.25D. The differential prismatic effect is again 2 base up in the left (or down in the right) at 10mm below the optical centres. R; +2.00D / x D Figure D -1.75D = +0.25D +2.75D L; +2.75D / x D +2.75D -1.00D = +1.75D The above example shows the importance of considering the effect of the power and direction of the cylindrical element on the meridional anisometropia and its impact on the differential prismatic effect, especially in the more critical vertical direction. It is useful to remember that the differential prism at near will always be base down for the eye requiring the more negative or less positive lens. Induced anisometropia From the author s experience it was not unusual in the earlier days of intraocular implants for meridional anisometropia to have resulted from the astigmatism induced during the operation. This often led to a notable change from the patient s original prescription. Cases do still occur but thankfully this is now less frequent. The following example is given where a patient after many years of wearing bifocals was unable to tolerate the same lens type after undergoing a cataract removal from the left eye. Prescription before cataract removal from the left eye: R: / VA 6/9 1 Add L: / VA 6/36 Add Prescription following left lens implant: R: / VA 6/9 1 Add L: / VA 6/6 1 Add Assuming a near visual point (NVP) of 10mm below the optical centre, with the old lenses the subject would have encountered a difference of only 0.25 base down in the right, which would not have caused any problems of diplopia, although, of course, vision in the left eye would have been poor. With the new prescription the spherical element of both prescriptions has not greatly altered but the new cylinder power and axis direction in the operated eye have resulted in a differential prismatic effect of about 2¼ base down in the left lens at the same NVP. In contrast to the more common amblyopic anisometrope, this subject s good acuities had been restored in both eyes. It would not have been unreasonable therefore for this subject to have expected excellent vision through new bifocals. Unfortunately the 2¼ difference at the NVP gave rise to diplopia, resulting in the patient being unable to tolerate the new lenses. Amblyopic anisometropia Amblyopia is present when the visual acuity (VA) of a corrected eye which is ophthalmoscopically normal is reduced compared to the other. Anisometropia is one of the leading causes of amblyopia, although the mechanism of anisometropic amblyopia is poorly understood. It is not clear what levels of anisometropia should be corrected in children and at what age this correction should take place to ensure the best chance of visual development (Dadeya et al. 2001). Weakley (1999) has stated that early detection 2

3 Anisometropia: what difference does it make? and treatment of anisometropia before or early in the development of amblyopia are likely to produce better visual outcomes. Weale (2002) states that a disparity between refractions of the two eyes may cause functional impairment manifesting in amblyopia. This is mainly the case with hypermetropic subjects who, if uncorrected before their early critical period, are only able through accommodation to focus with the eye requiring the lower prescription as this requires less accommodative effort. This will commonly lead to poorer VA in the more hypermetropic eye due to blur monocular deprivation (Tunnacliffe 1984). Wu and Hunter (2006) record also that anisometropia gives rise to amblyopia when one eye, typically the more hypermetropic, remains blurred. A subject with a prescription R +0.75DS, L +3.25DS (Figure 2a) will accommodate by the lower amount in both eyes, leaving +2.50D uncorrected in the left eye (Figure 2b), resulting in the likely VA of no better than 6/24. Early correction would seem essential therefore in these cases if amblyopia is to be avoided. Duane (1922) measured accommodation separately for each eye and came across cases exhibiting different amplitude in the two eyes. This he attributed to differences in the mechanical properties of the two lenses Uncorrected. With no accommodation Figure 2a. Uncorrected, with no accommodation. Corrected uncorrected Uncorrected. With accommodation Figure 2b. Uncorrected, with accommodation. Myopic anisometropia rarely causes amblyopia until it is greater than 2.00D, whereas hypermetropic amblyopia may arise with as little as 1.00D difference. Astigmatic anisometropia may cause amblyopia when the difference becomes greater than 1.50D. Attempts to improve the VA in the amblyopic eye of hypermetropes are often carried out by providing a spectacle correction together with the occlusion of the fellow eye. Levartovsky et al. (1998) in a study with infants concluded, however, that those subjects with larger amounts of hypermetropic anisometropia have a worse prognosis for successful treatment. Their study found that those with anisometropia of up to +1.50D were likely to maintain any improvement in VA after treatment whereas those with higher values showed a significant deterioration from 6/12 to 6/21 over a period of 6.4 years after treatment. Walsh et al. (2009), in a pilot study in which they compared two methods of cessation of amblyopic management, namely abrupt cessation and therapy tapering, found no difference in the rate of recurrence of amblyopia for either method. With myopes having the far points in front of the eyes, both are able to be stimulated and so amblyopia is less common. A subject with R 0.25D, L 3.00D could use the right eye for more distance viewing and the left for distances of 33.3cm or less. Where VAs are significantly different there is the likelihood of suppression of the poorer image. If the eye with the worse VA is accompanied by a significantly higher prescription, as is usually the case, the expected diplopia with off-centre viewing may or may not manifest itself to the wearer. It is possible that little advantage would be gained for the patient in providing any solution to the anisometropia in these cases and a balance lens for the worse eye may be advised. An example is of a contact lens wearer whose prescription was in the order of R 3.50, L with corresponding VAs of 6/6 and 6/36. An infection led to her being advised to change to wearing her spectacles. Years of dislike regarding the unsightly nature of the left lens in all her previous glasses caused much distress for this patient and led to reluctance in following the advice to cease contact lens wear. The simple process of neutralising the extra minus power of the left lens with her glasses being worn indicated that no difference to her vision had occurred. Lenses of 3.50D for both eyes were dispensed and the patient happily wore the glasses. It is likely in this case that even if both VAs had been good, binocular vision would have been poor due to unequal image sizes and so a balance lens may still have been the best spectacle lens option. We have seen that a differential prismatic effect will only occur when both eyes look through corresponding points away from the optical centre of lenses of different powers. It is usually the multifocal or progressive lens wearer who is affected by this requirement as the consideration is mainly focused on the differential prismatic effect at the near visual 3

4 P McCarthy zone to where the subject lowers the eyes to read. However it should not be overlooked that both the distance visual point and fitting cross position on a progressive power lens will be above the prism reference point. With a fitting cross height of 6mm which is found on many progressive lenses, anisometropia of just 1.67D would induce a differential prismatic effect of 1 at the point on the lens that is likely to be used for prolonged distance vision. Whilst anisometropic single-vision lens wearers are able to move their head to ensure the eyes view objects through the optical centres, there may be circumstances, such as prolonged or constant use for close-up work, where visual difficulties may be encountered. To overcome this issue, lowering the optical centres to coincide with the pupils when looking down to read will ensure the anisometropic subject encounters no differential prismatic effect at this point. In order to comply with manufacturers fitting criteria it is recommended that the optical centres of single-vision lenses for normal use should be lowered from the pupil centre position in relation to the pantoscopic tilt of the front of the frame. With the eyes likely to be positioned at least 5mm above the optical centres in the primary position of gaze, a 2D difference in right and left lens powers will result in a prolonged differential prismatic effect of 1. It may be worth considering a reduced pantoscopic tilt with optical centres on or close to the pupil centres in these cases. Assessing patient tolerance From the earlier discussions it could be supposed that a subject with a significant difference in right and left prescriptions would require a compensating prism at the off-centre visual point of one or both of the lenses, which is usually at the NVP. Some anisometropic subjects however are able to adapt to a degree of differential prism and exhibit no symptoms. Some will just suppress, especially at higher levels. Others, at lower levels, may have good fusional reserves and tolerate the differential prism. Subjects with marked anisometropic amblyopia will often benefit from prism compensation even though, when corrected, the VA in one eye is poor. Pouliquen de Liniere et al. (1998) studied 34 progressive lens-wearing anisometropes. Near vision fusion as well as phorias, VA and binocular vision were some of the checks carried out to support the evaluation of tolerance to progressive lenses over a 6-month period. The majority were satisfied with their lenses (62%), whilst the rest either abandoned them or preferred single-vision lenses for prolonged reading. It should be reasonably straightforward for the dispensing optician to determine whether a subject will be tolerant of the differential prismatic effect resulting from an anisometropic prescription. Subjects must be instructed to look through the near visual zone of their single-vision reading or distance lenses, depending upon whether they are first-time presbyopes or not, whilst observing print close to their resolution limit, making any problems more obvious. This would normally require subjects to lower the eyes about 10mm below the optical centre. The required compensating prism is placed before the corresponding eye and, if a better response is given, prism compensation would be beneficial (Tunnacliffe 1998). It is not uncommon for anisometropic subjects who appear to suffer no symptoms with existing lenses that have not been prism-compensated to be dispensed with the same lens type. It is simple to employ the method above to determine whether a solution to the anisometropia offers any added benefit to the wearer. Aniseikonia When a solution using spectacle lenses is provided in order to eliminate the differential prism induced by an anisometropic prescription, we may still be left with the problem of the lenses producing different image sizes as detected by the brain. Most anisometropias of greater than 2.00D result from differences in the axial length of the eye rather than the refractive power (Bradley et al. 1983). It should theoretically, according to Knapp s law, be possible to equate the retinal image sizes of axial anisometropes by placing the spectacle correction at the anterior focal plane of the eyes. Bradley et al. (1983) found, however, that with confirmed axial anisometropia and accurate lens positioning, aniseikonia was reported by subjects with equal retinal image sizes. This raised the question of either some intervention between the retinal and ocular images or changes within the eye itself. It is worth briefly reminding ourselves of the difference between retinal image size and aniseikonia. Aniseikonia is a binocular vision anomaly in which two eyes perceive images of different size and/or shape (De Witt 2007). Both aniseikonia and different retinal image sizes may occur for both corrected and uncorrected eyes. It could be possible for a subject to have different retinal image sizes but report no indication of aniseikonia. The two axial myopic eyes in Figure 3 show that the uncorrected blurred retinal image in the less myopic shorter eye is smaller than that in the more myopic eye. De Witt (2007) describes that, with a greater axial length, retinal stretching around the fovea may increase by a greater proportion; therefore in an uncorrected myopic eye with a relatively large retinal image, the number of photoreceptors that are stimulated may be smaller, giving rise to a smaller perceived image. The difference in the perceived images indicates aniseikonia. Changes in the retina may not be uniform and therefore variations in aniseikonia may occur in other parts of the visual field. 4

5 Anisometropia: what difference does it make? Figure 3. Axial myopia. Small uncorrected retinal image size Larger uncorrected retinal image size A further issue that is likely to be present in unilateral aphakics is the breakdown of binocular vision through diminution of the amplitude of sensory fusion and disturbance of motor fusion (Crone and Leuridan 1973). When dispensing anisometropes with first-time spectacles the issue of aniseikonia may need to be investigated and addressed, especially if both eyes have good acuities. There would be little point in correcting for differential prism only to find that fusion of the two images is prevented because of the degree of aniseikonia. It is worth remembering that the curvature, thickness and refractive index of a lens will affect the shape factor 1 element, given by 1 (t/n)f 1, of the magnification of a lens. Refractive ametropes with equal axial lengths are unlikely to suffer retinal stretching and so the difference in the uncorrected retinal image sizes should theoretically be the same as the perceived image sizes. Reed (2011) discusses Knapp s law at length, with mention of it falling short in clinical practice. Part of the law states that the refractive power of the eye must be equal to that of the standard emmetropic eye, implying that different retinal image sizes resulting from refractive ametropia cannot be equalised with spectacle lenses. It is not surprising that in view of the above issues contact lenses are often the favoured method of correction of both anisometropia and aniseikonia by most optometrists. Kowal et al. (2005) suggest a case for supplying contact lenses to all spectacle-wearing myopic anisometropes for at least a few hours to assess the aniseikonic response. An extreme example of refractive anisometropia is with aphakia of one eye where a difference of the right and left prescription could be over 10.00D. If a spectacle lens correction were given, this could result in at least 30% of aniseikonia, which together with the anisometropia will mean that, especially away from the optical centre, any fusion of the images will be prevented. If an anisometropic subject is currently wearing spectacles and has no symptoms, he or she will either be suppressing or will have adapted to different right and left cortical image sizes. Adaptation is unusual in subjects with more than 5% aniseikonia and symptoms may even be suffered by patients with aniseikonic values of 3% or less (De Witt 2007). Crone and Leuridan (1973) reported mean tolerances of 7%, which is the average residual aniseikonia when the example above of unilateral aphakia is corrected with contact lenses. Intraocular lens implants, which are now the preferred method of correcting aphakia, greatly reduce the problem of differing perceived image sizes. Changing the vertex distance and combining spectacles with contact lenses are also ways of altering the magnification factor of the correcting system. A further, more complex issue to assess and control is dynamic aniseikonia, which is defined as a heterophoria which varies in magnitude with the prismatic effect as the eyes deviate from the optical centre of the spectacle lens (De Witt 2007). The tolerance of this will depend upon the fusional reserves of the patient. It should be said that, for current spectacle lens wearers, provided that any changes in the above parameters are minimal, there is unlikely to be any additional problems with regard to aniseikonia. It should be apparent from the discussions so far that many factors need to be considered before deciding upon the most suitable solution for a patient with an anisometropic prescription. These factors include: the degree of anisometropia the patient s tolerance of any differential prism the likely improvement in the vision with prism compensation the degree of aniseikonia and retinal image size differences the ability to fuse the different image sizes the visual acuities any suppression of one eye whether the use of a balance lens would be preferred The above points, where possible, should be routinely used by the dispensing optician in collaboration with the optometrist as a systematic checklist to ensure that the most satisfactory outcome for anisometropic patients is achieved. 5

6 P McCarthy Solutions for anisometropia If it is established that the dispensing of lenses with no prism compensation for an anisometropic subject is likely to lead to diplopia, we need to consider the available options for eliminating or reducing the induced differential prism responsible for this problem. In nearly all cases apart from contact lenses, which will be discussed later, the critical point will be the near visual zone of the spectacle lens. Prism compensation within this area may be provided through the following methods: slab-off different round bifocal segment sizes Franklin split glass prism-controlled bifocals cemented or bonded bifocal segments Slab-off This is arguably the most common as well as cosmetically attractive method employed on both single-vision and multifocal lenses to remove or neutralise unwanted vertical prismatic effect in the near vision zone only. With progressive power lenses this may also be extended to removing the differential prism at the distance vision zone. More commonly the technique is performed on bifocal lenses, where the horizontal line, showing the apex of the slabbed-off prism, is made to coincide with the segment top (Figure 4d). a b c Base Down removed from F2. Equal Base Up removed from F1 above segment. Finished lens with required prism below segment top. When performed on an executive bifocal, the line becomes indistinguishable from the segment top. However it should be remembered that, with the optical centre of the segment now not on the dividing line, there will be a displacement of the image as the eye passes into the reading segment of the lens, so removing the no-jump property of this type of bifocal. Figure 4 shows the procedure carried out on the less plus (or more minus) bifocal, this being the lens exhibiting the more base down prismatic effect at the NVP. In Figure 4a the whole of F2 is surfaced to remove the required amount of base-down prism. The same amount of base-up prism is removed from the portion of the lens above the segment top in Figure 4b, neutralising the prism introduced in Figure 4a in this part of the lens. This leaves the required amount of prism just in the reading zone of the lens. The bifocals in Figure 4d show the right lens, which is the less positive, having been slabbed off with the line of the apex of the prism at the segment top. We can see that, although the left images are more magnified, their vertical positioning within the bifocal segment is almost the same as that of the right lens. The process shown on Figure 4 is that carried out on a glass-fused bifocal where the front of the segment has the same curvature as the main lens and so there is no ridge at the top of the bifocal. The process is more challenging with the more common plastics lenses where the protruding segment on the moulded front surface requires the entire slab-off process to be performed on the concave rear surface only (Norville Prescription Companion 2006). The slab-off technique for progressive power lenses involves removing the differential prism at the distance reference point by working prism over the whole prescription surface and then removing the required base-down prism again from the lower part of the weaker plus (or higher minus) lens. Vision will however be restricted at the height of the prism measuring point due to the slab-off edge (Carl Zeiss ND). As stated previously, most single-vision lens wearers can lower the head to maintain vision through the optical centres of their lenses when needing to look down to read. A lowering of the optical centres for those primarily involved in close-up tasks may be beneficial, provided the distance vision is not compromised. Compensation for anisometropia for the single-vision lens wearer should not be overlooked, however, if appropriate to the patient s needs and likely to provide better optical comfort. Figure 4a-d. Slab-off. Slab-off may be performed on a single-vision lens creating a bicentric lens which will have a separate optical centre in the near portion that is positioned to eliminate or reduce any differential prism within the near visual zone. A bicentric lens is made by cementing a cover on to the lens (Figure 5b) and then the required amount of prism is removed by grinding the lens at the correct angle 6

7 Anisometropia: what difference does it make? (Figure 5c). This is continued until the lower edge of the lens is at a precalculated thickness, placing the dividing line at the required distance below the optical centre of the distance zone of the lens (Figure 6). a b c d Left eye: OS NVP = 13mm. (1.3 x 1.00D = 1.3 base down) Right eye: OS NVP = 6mm. (0.6 x 1.00D = 0.6 base down) Differential prism per dioptre of add at the NVP = 0.7 base down (Alternatively; the product of the difference in segment radii and 1.00D gives us 0.7.) R; +1.00D Add L; +3.00D Add mm 4mm OD 10mm 4mm OD NVP OS 12mm NVP OS Figure 7. Different segment sizes. Figure 5a-d. Slab-off technique: single-vision lens. In this example the distance powers will induce a differential prismatic effect of 2 (base up in left) at the NVP. With a +3.00D reading add, the different segment sizes will produce 2.1 (0.7 3) base down in the left at the NVP, resulting in just 0.1 of differential prism, which will be easily tolerated by the wearer. If the difference in segment diameters is required, the 2c δf formula Add can be used, where c is the distance from the optical centre to the NVP (in mm) and δf is the difference in lens powers. Figure 6. Bicentric lens showing distance and near optical centres (OC). Different bifocal round segment sizes Although not cosmetically attractive, the correct choice of round segment size for each eye can reduce, if not always eliminate, the differential prismatic effect at the NVP to a level the wearer may tolerate. Round segments exert base-down prism at the NVP with the amount of prism dependent upon the reading add and the distance between the NVP and the optical centre of the segment (OS). From the geometry of the lenses in Figure 7, we can calculate that the round 38mm segment in the left eye will exert 0.7 base down more per 1.00D of reading add at the NVP than the round 24mm segment in the right eye. Franklin split There cannot be many 250-year-old inventions that may still be offered as a solution to a current-day problem. With correct positioning of the optical centres of the reading lens portion, the Franklin split can be used to eliminate differential prism arising from anisometropia. The bifocal, which is simply two lenses cemented together, may be used to overcome other issues apart from differential prism. These include prism in either the near or distance portions only and different cylinder axes in the near prescription. Although it is indeed a very versatile option, the lens is somewhat cosmetically unattractive (Figure 8). Interestingly, it is often the first solution given by many dispensing examination candidates to these more challenging problems but whether the number of recommendations in practice reflects this choice as an examination answer is doubtful. Despite its versatility, it is unlikely that Franklin in the 1760s would have considered the potential of the lens where the optical centres of each separate lens could be positioned to create a desired prismatic effect. The lenses would most certainly have been just for distance and near vision, although with his close association with many famous artists of the day, eg Reynolds and West, it is not 7

8 P McCarthy beyond doubt that, to help them with their work, an intermediate portion could have been used, thus creating the world s first occupational lens! Bonded segments As with the Franklin split, when one lens is bonded on to another, both optical centres can be positioned independently to provide a desired prismatic effect at a given point. When removing differential prism the reading segment, with its compensating prism, is bonded on to the main lens at the near-vision zone. It is preferential to use this prism compensation on the main lens that requires base down in order to prevent a thick ridge at its upper edge. Other options A Fresnel prism is not a permanent option because of the poor cosmetics and durability and the reduction in VA. It is mainly used to assess the wearer s acceptance of the compensating prism before a more permanent solution is decided upon. Figure 8. Franklin split. Prism-controlled solid bifocal Although only produced in glass material, another versatile lens is the 30mm round segment prism-controlled bifocal, which has been available for many years. It can produce prisms in any direction within the segment of up to 6, although for the control of anisometropia-induced differential prism, we are primarily concerned with vertical compensation. For cosmetic reasons the normal procedure would be to split the differential prism compensation between the two lenses. Where an unwanted prism of 3 base down in the left needs to be compensated, for example, the right segment would incorporate 1½Δ base down with the left having 1½Δ base up. If one lens only were to be used for prism compensation, then all of the 3 base up would be incorporated in the left lens to avoid the ridge at the dividing line (Figure 9). The right eye would be matched with a standard solid R30 bifocal lens. Single-vision distance and reading spectacles remove the need for the eyes to look away from the optical centres. However, the benefits of avoiding the issues of anisometropia are offset by the inconvenience of needing to change spectacles for different viewing distances. Contact lenses offer the most natural and obvious solution to any anisometropic prescription in that they remove the issue of any unwanted prismatic effect that spectacle lenses create when the eyes look away from the optical centres. Contact lenses also give more natural vision in that the differences in retinal image sizes compared to those of spectacle lenses are far less and so images produced by eyes needing different ocular corrections can be more easily integrated. There are a number of variables to consider but, as an example, a +4.00D contact lens will give a magnification of about 1.7% compared to about 5% with a spectacle lens fitted at, say, 12mm from the eye. A subject with anisometropia of 4.00D is therefore unlikely to tolerate uncompensated spectacle lenses but should suffer no symptoms when fitted with contact lenses. It would be expected therefore that, where suitable, either single-vision or multifocal contact lenses should always be strongly recommended to patients with these types of prescriptions. Different reading additions A point that is rarely considered when providing a bifocal solution to anisometropia, but one that may affect the wearer s comfort, is that accommodation will vary depending on the given prescription. A difference in prescription of about 4.00D will lead to the eyes needing to accommodate by a difference of about 0.25D. We can determine from Figure 10 that, to see at 0.3m away, the left eye with a prescription of 1.00 will need to accommodate by nearly 0.25D more than the right eye and therefore may benefit from an additional 0.25D in the reading prescription. Figure 9. Prism-controlled solid bifocal. 8

9 Anisometropia: what difference does it make? R L Figure cms -5.00DS 12mm K = 1000/-212mm = D L 1 = -3.00D, L 1 ' = -8.00D L 2 = 1000/ -137mm = -7.30D A = (-7.30) = 2.583D 33.33cms -1.00DS 12mm K = 1000/-1012mm = D L 1 = -3.00D, L 1 ' = -4.00D L 2 = 1000/ -262mm = D A = (-3.817) = 2.828D Conclusion Differences in right and left prescriptions in corresponding meridians signify the presence of anisometropia. We have considered the fundamental aspect of identifying these differences and the likely effects the resultant differential prismatic effects can have on our patients if they are not compensated for. The anisometropic prescription is, however, just one of a number of associated factors that need to be considered when deciding whether any value will gained for the patient in providing a correcting lens with prism compensation. The key to the final decision will often lie in the discussion between the dispensing optician and the optometrist concerning the patient s likely acceptance of the prescription in either its prescribed or modified form. The solutions to the issues of anisometropia in terms of correcting lenses have been discussed in detail. These lenses are still available and so offer dispensing opticians the means of providing their anisometropic patients with the best possible vision. Summary Anisometropia frequently raises the question in the mind of the optometrist or dispensing optician: To correct or not to correct. When the eye with the higher prescription frequently has noticeably worse visual acuity, especially with hypermetropes, a balance lens may often be the most suitable solution. The decision to provide the correct prescription may well lead to difficulties for the wearer: Retinal and/or cortical image sizes may differ Differential prism at off-axis points is likely to lead to diplopia Monocular vision due to suppression may still occur The correcting lenses are likely to be cosmetically unacceptable Contact lenses remove many of the issues faced by the anisometropic subject but for those unwilling or unable to wear them a number of spectacle lens solutions are still available. References Bradley A, Rabin J, Freeman RD (1983) Non-optical determinants of anisometropia. Invest Ophthalmol Vis Sci 24, Carl Zeiss (ND) Worth Looking Through, p.g29 Crone RA, Leuridan OMA (1973) Tolerance for aniseikonia. Graefes Arch Klin Exp Ophthalmol 188, 1 16 Dadeya S, Kamlesh, Shibal F (2001) The effect of anisometropia on binocular visual function. India J Ophthalmol 49, 261 De Witt GC (2007) Retinally-induced aniseikonia, optical diagnosis. Eikvaren 19, 3 5 Duane A (1922) Studies in monocular and binocular accommodation. Am J Ophthalmol 5, Harvey W, Gilmartin B (2004) Paediatric Optometry. Edinburgh: Elsevier, p. 8 Holladay JT, Rubin ML (1988) Avoiding refractive problems in cataract surgery. Surv Ophthalmol 32, 358 Jalie M (1984) The Principles of Ophthalmic Lenses, 4th edn. London: ABDO, p107 Kowal L, Battu R, Kushner B (2005) Refractive surgery and strabismus. Clin Exp Ophthalmol 33, 93 Levartovsky S, Oliver M, Gottesman N et al. (1998) Long term effect of hypermetropic anisometropia on the visual acuity of treated amblyopic eyes. Br J Ophthalmol 82, 55 8 Norville Prescription Companion (2006) Lenses for the relief of anisometropia induced prism. p.38 Pouliquen de Liniere M, Hervault C, Meillon JP et al. (1998) Anisometropia and presbyopia: prescription of progressive lenses, a new approach. J Fr Ophthalmol 5,

10 P McCarthy Reed B (2011) Aniseikonia: A Case Series and Literary Review. Pacific University, College of Optometry, paper 2. yourself (improved knowledge, performance, confidence)? Tunnacliffe AH (1984) Introduction to Visual Optics. London: ABDO, p.462 Tunnacliffe AH (1998) Essentials of Dispensing. London: ABDO, p.84 Walsh LA, Hahn EK, LaRoche GR (2009) The method of treatment cessation and recurrence rate of amblyopia. Strabismus 17, Weakley DR (1999) Anisometropia, amblyopia and binocularity. Trans Am Ophthalmol 97, 1017 Weale RA (2002) Age-related prevalence of anisometropia. Ophthal Res 34, your colleagues? Woodhouse M (2012) Development of visual functions II. Optom Today Feb, 41 Wu C, Hunter DG (2006) Amblyopia: diagnostic and therapeutic options. Am J Ophthalmol 141, CET Multiple Choice Questions This article has been approved for one non-interactive point under the GOC s Enhanced CET Scheme. The reference and relevant competencies are stated at the head of the article. To gain your point visit the College s website and complete the multiple choice questions online. 2. How might you assess/measure this impact? CPD Exercise After reading this article can you identify areas in which your knowledge of anisometropia has been enhanced? How do you feel you can use this knowledge to offer better patient advice? Are there any areas you still feel you need to study and how might you do this? Which areas outlined in this article would you benefit from reading in more depth, and why? To access CPD Information please click on the following link: college-optometrists.org/cpd Reflection 1. What impact has your learning had, or might it have, on: your patients or other service users (eg those who refer patients to you, members of staff whom you supervise)? 10