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Anisometropia: does this mean we have to dispense single vision lenses?

This article analyses some of the problems related to anisometropic prescriptions from a dispensing perspective and discusses potential single vision and multifocal solutions.

Optometrists (6)

Dispensing opticians (4)

1 CET POINT

Introduction

Anisometropie prescriptions require careful consideration when it comes to dispensing solutions for patients in practice, especially in relation to those with presbyopia. Practitioners may choose not to recommend multifocal lenses due to the potential problems with differential prism and unequal retinal image sizes and elect to 'play it safe' with single vision lens recommendations. This article will discuss potential issues with anisometropic prescriptions and spectacle lenses and review the possible dispensing solutions available.

The term anisometropia refers to a difference between a right and left eye's refractive error. While the majority of prescriptions will have a difference, anisometropia is often considered when there is a difference of two or more dioptres in any meridian between the eyes. For the purpose of this article we will discuss dispensing considerations for anisometropic patients with good corrected visual acuity in both eyes, whether or not the anisometropia has been longstanding or newly acquired, for example, as the result of unilateral cataract surgery. (1)

There are two main concerns that need to be addressed when dispensing anisometropic prescriptions: differential prismatic effect; and possible size differences in resultant retinal images, that is to say, aniseikonia. Firstly, prismatic effect issues will be considered, illustrated by a single vision case study to show how even these lenses require anisometropic consideration.

Case 1

A 24-year-old PhD student presents having been dispensed with the following Rx two weeks prior:

R-1.25DS 6/5 L-4.75DS [6/6.sup.-2]

Although happy with the distance vision, he felt that he couldn't study for long periods in the new spectacles without developing headaches. The prescription and optical centres of the lenses were correct and been glazed vertically in line with the patient's pupils in the primary gaze position. The patient's near visual point (NVP) when reading was measured at 10mm down and 2mm in from the optical centres of the lenses.

This scenario will be considered from an anisometropic perspective, although it is worth considering whether pantoscopic tilt is playing a role as the vertical heights may not have been compensated for in this regard. It is also important to note that this patient is not presbyopic. When viewing a distant target in the primary gaze position, the patient will be looking through the optical centres (OC) (see Figure I) and will experience no unwanted differential prism. However, when the patient is looking through the NVP then this is not the case. By applying Prentice's rule, P = cF, where P = prism, c = centration in cm, and F = power, the differential prismatic effect at the NVP can be calculated as follows:

Right eye (RE): vertically [P.sub.v] = 1 x -1.25 = [1.25.sup.[DELTA]] down;

horizontally: [P.sub.H] = 0.2 x -1.25 = [0.25.sup.[DELTA]] in

Left eye (LE): vertically [P.sub.v] = 1 x -4.75 = [4.75.sup.[DELTA]] down;

horizontally [P.sub.H] = 0.2 x -4.75 = [0.95.sup.[DELTA]] in

Thus, the differential prismatic effects for this patient at The NVP are: vertically [3.9.sup.[DELTA]] down LE; horizontally [1.2.sup.[DELTA]] IN

The smaller horizontal differential prism should not be an issue in this case as patients generally have sufficient horizontal fusionai reserves to overcome this. (2-3) However, the vertical differential prismatic effect should stand out as a potential issue. It is often considered that tolerance issues will arise when the vertical differential prism exceeds [1.sup.[DELTA]], (2) and it can be seen in case 1 that this level is exceeded by a significant margin.

Is the patient in case 1 likely to cope with the experienced differential prism at the NVP? While [1.sup.[DELTA]] of vertical differential prism is cited to be the amount at which binocular problems, such as asthenopia and possible diplopia, may be experienced, it is the author's experience that many patients can tolerate greater amounts of differential prism depending on the length of the near vision task, the individual's binocular vision status, and their habitual refractive correction. If the patient views through the NVP for short intervals, then they may remain asymptomatic. For prolonged near vision work, such as intensive studying, the differential prismatic effect is likely to have more of an impact. The patient may help offset the effects of differential prism with single vision lenses by lowering their head in order to move the NVP closer to the optical centres. However, this may create poor posture issues when looking downwards for extensive reading tasks.

Even though many practitioners may consider recommending a standard single vision pair of spectacles only for this patient, other solutions exist to help address the anisometropia. Although the patient is prepresbyopic, two single vision pairs of spectacles could be ordered: a standard distance pair of spectacles and a pair of 'reading' spectacles with the same prescription where the optical centres are repositioned to coincide with the NVP to eliminate unwanted differential prism when reading. Alternatively, the use of slab-off or biprism single vision lenses could be considered should the patient want one pair of spectacles. Such lenses involve removing the extra base-down prism at the NVP from the greater minus lens (in this case the left eye) to balance out the differential prism in the near portion only.

Should the patient wish to have only one pair of conventional single vision spectacles, glazing the optical centres slightly lower than usual will help reduce the differential prismatic effect at the NVP; this will create some differential prismatic effect when viewing in the primary position, but this may be tolerable for the patient. Indeed, it is suggested that the vertical position of the optical centres should be lowered by 1mm for every 2[degrees] of pantoscopic tilt, (2) to ensure the optical axis of the lens passes through the eye's centre of rotation, and this may help with the anisometropic cases such as the one above. If, in the case above, the pantoscopic tilt was 10[degrees], then lowering the heights by 5mm would result in the NVP being positioned at a point 5mm below the optical centres of the lenses and result in a vertical differential prism of [1.75.sup.[DELTA]] down LE at the NVP. This will certainly help reduce the strain on binocular vision at the NVP, but we will also induce a vertical differential prismatic effect of [1.75.sup.[DELTA]] up LE in the primary gaze position (5mm above the optical centres now). This case demonstrates that careful consideration of anisometropic dispensing is required, even when dispensing single vision lenses, in order to provide the most comfortable outcome for the patient.

Case 2

A 54-year-old postal worker currently wears separate distance and near vision glasses but for convenience would like to try bifocals. Their Rx is as follows:

R +1.001-4.00 x 180 Add +2.00 6/5 L+0.751-1.25x180 Add+2.00 6/5

At first glance, it is easy to overlook the potential anisometropic issues in this case when considering the sphere component only. However, if a power diagram of each lens is studied (see Figure 2, page 64), anisometropia in the vertical meridian for the distance prescription can be clearly observed.

For presbyopic patients wanting multifocal lenses, vertical anisometropia can have a greater impact on their near vision. As discussed in case 1, single vision wearers are able to modify their head position to ensure that they are viewing closer to the optical centre of the lenses, thus reducing the exposure to differential prism. On the other hand, multifocal lens wearers are forced to look downwards from the optical centre for distance in order to access the reading area/segment of the lens. In cases of anisometropia, looking downwards will induce a differential prism, and the patient may complain that reading is more difficult with their new multifocal lenses compared to their distance vision.

In case 2, the distance element of the prescription will be considered first with an assumed NVP 8mm below the distance optical centres (for the purposes of this discussion minor prismatic effects in the horizontal meridian will be ignored). Using Prentice's rule, it can be found that the vertical prism experienced at the NVP in the RE is [2.4.sup.[DELTA]] down, and the vertical prism in the LE is [0.4.sup.[DELTA]] down; this gives an undesirable differential prismatic effect of [2.sup.[DELTA]] down due to the RE. Recall that the patient wants bifocal lenses, and it is worth noting at this point that vertical differential prismatic effects can be generally worked out by consideration of the distance portion; if the segment size, heights, and additions are the same in each eye, then the vertical prismatic effects due to the segment cancel each other out. It is at this point that the author wishes to point out that there are many multifocal wearers that happily wear multifocal lenses with such levels of differential prism at the NVP, and again this can relate to how long they use their near vision portion, and their fusional reserves to adapt to the prism. (3)

A sudden shift to anisometropia, such as the result of unilateral cataract surgery, will have a greater impact on multifocal adaption. Many practitioners recommend a single vision correction while the patient is waiting for surgery in the fellow eye; however, some patients may not require surgery on the second eye (or the patient lives in an area where second eye cataract surgery is limited by NHS funding). (4) Such patients should be advised of the various multifocal solutions, as well as single vision options, as part of the dispensing consultation.

There are a few specialist multifocal lenses that can aid with anisometropic cases, but it is also worth considering the option of different round segment bifocals. Dispensing unequal round segments not only helps to balance out the differential prismatic effect at the NVP, but also presents an inexpensive alternative to specialist bifocals. The upper half of round segment bifocals generate base down prism; the larger the diameter of the round segment, the greater the induced base down prism. Considering the case above, it would be necessary to dispense the larger segment (with most base down) in the LE, and the smaller segment (with the least base down) in the RE. The difference between the segment diameters needed to eliminate differential prismatic effect at the NVP can be calculated using:

[DELTA]d = (2 x c x [DELTA]F)/Add

Where [DELTA]F is the difference in the distance power between the two lenses along the vertical meridian; c is the vertical distance from the optical centre for distance to the NVP.

For case 2:

[DELTA]d = (2 X 8 X (-3.00---0.50)) = 20mm/2.00

Thus, a difference of 20mm in the segment diameters would be required to completely eliminate any differential prismatic effect. In reality, a R24 lens for the right eye and R45 segment for the left could be ordered; although the difference is not exactly the 20mm needed, the residual differential prism should be within tolerance for most patients. While this solution controls differential prismatic effect at the NVP, it is worth noting that the patient will still be experiencing a significant amount of base down prism in both eyes, causing the images to be displaced from the stimuli's actual position. Due to the induced base down prismatic effect created by the top half of the bifocal segment, the use of different round segments tends to work better for hyperopic patients as the base down from the segment helps cancel some of the overall prismatic effect when combined with the base up of the lower half of the distance portion.

Unfortunately, the amount of differential prism that can be balanced by the use of different sized round segments is limited, and other options may need consideration. Slab-off bifocal lenses can be ordered to balance the differential prismatic effects at the NVP (as in Case 1), with the slab-off line positioned level with the segment top to improve cosmesis (see Figure 3); this solution works particularly well with myopic prescriptions as it also reduces the lens substance in the near portion. Slab-off/on processing can also be applied to progressive addition lenses; although this may create an undesirable slab-off line across one lens.

Franklin split bifocals offer another alternative solution, where the bifocal consists of two separate lenses which are cut in two and glazed together (see Figure 4). This solution provides independent positioning of the optical centres of the distance and near portions, and thus allows separate vertical and horizontal prism control for distance and near. In this case, the optical centre for the near zone can be lined up with the NVP, thereby eliminating the prismatic effect issues generated by the anisometropia. In the last few years, more modern forms of the Franklin split bifocal have been developed. (5) These cemented prism-segment lenses appear as D-segment bifocals, with the segment shape routed out of a single-vision distance lens (see Figure 5). A separate D-segment is placed within the segment space, again allowing the prism needed in the near segment to be controlled independently from the distance portion; this offers a lightweight alternative to obsolete glass prism-controlled lenses. Although the cost of such specialised lenses is higher than conventional multifocal lenses, it has been suggested that up to 60% of anisometropic patients would benefit from prism-controlled lens solutions. (3)

Case 3

A 29-year-old office worker who is monocularly aphakic presents with the following Rx:

R+12.501-0.50x45 6/9

L +6.00DS 6/6

This final case-study will focus on aniseikonia, which is another problem that can be associated with anisometropia.

From a prismatic effect viewpoint, off-centre viewing through these lenses will be an issue as the patient will experience differential prismatic effects of up to [6.50.sup.[DELTA]] when looking just 10mm away from the lens centre. As before, consideration of the patient's visual needs will need to be undertaken to determine whether these prismatic effects would cause difficulties.

In addition to anisometropia, thought should also be given to the retinal image sizes produced by such spectacle lenses when dispensing. If there is sufficient difference between the retinal image sizes (a difference of around 5% or greater may be problematic), (6) then stereoscopic fusion will be difficult and can result in poor binocular vision as a result of aniseikonia.

There are generally two elements that can affect the spectacle magnification (SM), and hence the retinal image size, when considering spectacle lenses: the power factor; and shape factor. SM can be determined as follows:

SM = power factor x shape factor = 1/ 1-[dF'.sub.v] X 1/1-t/n [F.sub.1]

Where d is the vertex distance, [F.sub.v]' is the back vertex power, t is the centre thickness of the lens, and [F.sub.1] is the front surface power.

The power factor is dictated by the prescription and vertex distance, and the shape factor is influenced by the curvature of the lens and the centre thickness. In order to balance the magnification between the two lenses, the curvature or thickness of the weaker lens can be increased to generate higher magnification. This can be achieved to some degree by just ordering a larger blank size than is needed to increase centre thickness, or by ordering a lens in a more steeply curved form. Also, the magnification of the stronger lens can be reduced by ordering a thinner, aspheric lens design. Some prescription houses offer lenses specifically designed to increase magnification while leaving the refractive power unaffected; such lenses are referred to as iseikonic or size lenses. It must be noted that these solutions will have cosmetic and weight implications for the finished spectacles, (7) and balancing of the spectacle magnification does not necessarily balance the perceived magnification. (3)

Contact lenses

While this article has concentrated on spectacle lens solutions for anisometropic patients, consideration of contact lenses should also be highlighted. Due to the contact lens position remaining relatively stable with changes in gaze, anisometropic contact lens wearers will not be troubled by off-centre differential prismatic effects, and the magnification produced by contact lenses will be significantly reduced in comparison to spectacle lenses, (6) which in turn, will help combat aniseikonia.

Conclusion

With a reported prevalence of anisometropia of up to 12% among the adult population, (3) eye care practitioners must be able to manage the needs of these patients and be aware of all dispensing solutions available in today's market. Such consideration will help enhance the eye care service for the patient and could promote growth in multifocal lens solutions and contact lens business.

Exam questions and references

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 22 February 2019. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article. CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC on the GOC website (www.optical.org) to confirm your points. Visit www.optometry.co.uk, and click on the 'Related

Mark Hickton qualified as a dispensing optician in 2006 and is currently a lecturer at Bradford College where he teaches ophthalmic lenses, optics, ophthalmic dispensing practical workshops, alongside ophthalmic business and retail skills. Prior to this he completed a degree in physics with astrophysics at the University of Kent. Alongside his teaching commitments, Mark continues to work in practice.

Mark Hickton BSc (Hons), Cert Ed, FBDO

Course code: C-61797 Deadline: 22 February 2019

Learning objectives

* Be able to dispense suitable solutions for patients with anisometropia (Group 4.1.5)

* Be able to dispense suitable solutions for patients with anisometropia (Group 4.1.2)

Caption: Figure 2 The power diagrams for case 2 showing vertical anisometropia

Caption: Figure 3 Slab-off bifocal lens

Caption: Figure 4 A Franklin split bifocal

Caption: Figure 5 A cemented prism segment
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Author:Hickton, Mark
Publication:Optometry Today
Geographic Code:4EUUK
Date:Jan 1, 2019
Words:3059
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