c l i n i c a l 34 Will different sized segments work for anisometropia? In the next in her regular series looking at -related calculations, Janet Carlton looks at one of the options for managing anisometropia

Ask any student what they would Power F1 Power F2 do for a patient with anisometropia – a difference of greater than 2 of vertical power between the two eyes – and they will almost certainly offer different sized segments. Unfortunately Optical centre of this solution rarely works, and cosmeti- distance portion cally, it looks poor. Different sized segments are a potentially Χ in good way of removing the differential prism. cms This was seen as a practical solution in the r1 Seg top r2 days of fused bifocals. A now discontinued position lens was the R5/R9. It was a deep ‘B’ style fused Univis lens. Placed one way up the seg top was 5mm from the geometrical Smaller segment 1 centre of the seg and, upside down, 9mm Larger segment 2 from the seg top to the geometric centre (Figure 1). This means it could contribute F equals the vertical distance power of each lens and A is the addition of segment radius r

a different amount of base-down prism, (Χ F1) - (r1 A) = (Χ F2) - (r2 A) dependent on its orientation. With the 9mm where (Χ F1) = prism induced by the distance portion of the lens seg top uppermost, it will exert more base- (r1 A) = prism induced by segment 1 down prism than if it was the other way up. (Χ F2) = prism induced by the distance portion of the lens But the amount of prism removed by using (r2 A) = prism induced by segment 2 R5/R9 is quite small, and this solution is no This becomes F - F ) = A (r - r ) longer used. Χ ( 1 2 1 2 where F1 + F2 = the vertical differential power 5mm 9mm 9mm 5mm As we need to think about diameters of the segment, the equation now becomes

Χ (F1 - F2) = A {2(r1- r2)} therefore diameter1 - diameter2 = 2(D F/A) cms where DF is the differential vertical power

Figure 3. The relationship between different segments, radii and powers

Figure 1. R5/R9 positioning effect. If this solution is attempted, calcula- then would be 23mm. You can see from tions will be needed on how different the the table that this maximum difference in Today resin bifocals and round segments segments will need to be, to achieve the segment size will have limited use. There are more commonly used. The larger required effect. Using P=cF, an examina- is a new lens marketed by Norville for use segment will exert more base down at tion of the prism induced by the distance by golfers that is 15mm in diameter. If this the near vision point than the smaller one portion of the lens (the anisometropic is used with a 45 round, this will mean a (Figure 2). So obviously the larger segment part) should take place to determine what difference in diameters of 30mm, which (more base down) goes in the eye with the difference in segment size will eliminate will correct more anisometropia. However, least base down to ‘balance’ the prismatic the difference in vertical power (Figure 3). this combination also demonstrates a Table 1 shows the difference in very good reason for not using different segment diameter for adds between 1D sized segments – they look awful. This and 4D. So, if the smallest resin round solution is only workable for low amounts segment of 22mm and the largest of of anisometropia, for older presbyopic 45mm is used, the difference between patients only. With a lower adds, other

Table 1. The difference in segment diameter for adds between 1D and 4D

Differential power (∆F) Add (A) 2D 3D 4D 5D 6D +1.00+ 40mm 60mm 80mm 100mm 120mm +2.00 20mm 30mm 40mm 50mm 60mm +3.00 13.33mm 20mm 26.67mm 33.33mm 40mm Figure 2. Larger segment (right) offers more +4.00 10mm 15mm 20mm 25mm 30mm base-down prism

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Table 2. Solutions for anisometropia

Separate SV and DV independently Good for static vision only (for both pre- and post- centred ) Decentration to split the differential Good for dynamic vision in pre-presbyopes. More prism between the two eyes complicated and less successful for multifocal wearers Bi-prism lens (‘slab off’), minimum Guaranteed success in straight-top bifocal 2∆ Limited success in a progressive Different sized segments Poor cosmesis and low amount of differential prism removed Closing one eye Good for ‘survival reading’ Occluding near portion on one lens Works but not practical Cemented bifocal Optically good but cosmetically poor Figure 4. Slab off ‘Franklyn’ split bifocal Optically good but cosmetically poor solutions could be used and, if successful, Prism and centre control bifocals Optically good but cosmetically poor there should be little reason to change Invasive patients to different sized segments when Contact lenses Good for pre-presbyopes. Presbyopes can use the add increases. ‘normal’ reading spectacles over their contact Now we know this may not work as lenses a solution for anisometropia, we need to Do nothing Many patients may not notice even with look briefly at those that do work for the high levels of differential prism presbyope. The first and easiest solution is to dispense a pair of single-vision reading spectacles, remembering to drop the optical a ‘slab off’ technique. This produces a bi- limbus coincident with seg top position, centre by 8-10mm, so the patient views prism lens, with two optical centres, and a where the line won’t get in the way. Table through the optical centre when they read. visible dividing line (Figure 4). 2 shows a list of alternative solutions for Most patients prefer the convenience of To ‘slab off’ means remove base down anisometropia; many (such as different dynamic vision, so practitioners are likely to from the bottom half of the lens so is always sized segments) are more theoretical than dispense a multifocal. For most multifocal done on the most minus lens. It is better practical. prescriptions, the physical removal or to use ‘slab off’ only for differential prism addition of base-down prism from one of the greater than 2∆ as below this amount it is ◆ Janet Carlton is dispensing manager at lenses is needed. One option is to remove difficult keep the dividing line straight. the Fight For Sight Clinic, City baseAd E7down 20/3/06 from the most11:13 minus am eye,Page using 1 This line needs to be placed at the lower University

An ever increasing number of practices are relying on The Outside Clinic to solve their patient requests for domiciliary eye tests. Interrupting the routine of a busy High Street Practice to attend to domiciliary eye care, can be a problem. Not to mention spectacle delivery and fitting, and after care visits. If your practice has difficulties in adapting to the needs of those patients that can no longer visit you, then domiciliary eye testing could be a means of complying with the Disability Discrimination Act, and extending your practice services. Refer your patients to The Outside Clinic and they will be seen in a few days and be grateful to you for the service. Friends and relatives will reward you with their continued support and goodwill. www.outsideclinic.com National Freephone view our range of products at www.outsideclinicdirect.com 0500 295 245

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