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Clinical Conundrums: Case History 3 - Differential Prism
A 78-year-old patient presented for refraction following a cataract extraction on the Right Eye. A nice easy start after lunch I thought. During history and symptoms she explained there may have been some complications during the surgery but was unsure exactly what those complications had been.
Refractive Status, Pre-operative:
R +6.00/-0.50 x 75 6/18 Add +2.75
L +5.50/-1.25 x 60 6/12 Add +2.75
Refractive status, Post-operative:
R Plano / -0.75 x 115 6/12 Add +2.75 N6
L+5.50/-1.25 x 60 6/12 Add +2.75 N6
The surgery, as mentioned, had been on the right eye, the IOL was sitting centrally and with minimal post capsular thickening. The left crystalline lens showed Grade 3 nuclear sclerosis with mild cortical opacities. The patient informed me that the hospital had not discussed surgery to Left eye.
This patient had been wearing D28 bifocals before the surgery and was now desperate to update them so that she could “see��? again. Little did she know that the bond between her and the cherished bifocals may now been broken.
So in my most diplomatic voice I discussed the prospect of separate pairs. But as the look of horror came over her face, I realised I had given her the worse possible news;
“I don’t want two pairs! I want my bifocals!��?
I’m sure we have all sat there at one point trying to explain to patients some finer point of optometry realising that it was making no difference at all.
“So when can I pick up my new bifocals then?��?
Well, if that’s what the patient wants then that’s we would try to give her. So, after a brief conversation with our dispensing optician, Mrs Bifocals left the practice awaiting our phone call informing her as to when her glasses would be ready.
Differential prism can be solved in many ways including,
• Two separate pairs (inconvenient)
• Franklin split bifocals (cosmetically unappealing)
• Prism controlled bifocal – Slab off (heavy + expensive)
• Different size round segments (cosmetically less appealing)
In this case, our choice was to use a cut Fresnel prism on one lens to correct for the induced prism. Hopefully it would work and would be a short-term measure until the hospital chose to operate on the left eye.
Pre-Reg Tip – Differential prism is a common question in the Final Assessment examinations. It is good to know the full list of options, and be able to discuss the advantages and disadvantages of each. Be prepared for simple calculations. If you can think outside the hypothetical box and give a valid but alternative method, this is important as it shows thought with problem solving. Also, remember to do the best for the patient, with cost being of no consequence, but have a realistic choice ready in case the Examiner says “and for the every day folk?��?
Jane Macnaughton // Mar 21, 2007 at 7:05 am
Peter,
Did you cut a Fresnel to the size of the seg or like a Franklin split?
And how on earth did you calculate it?
I may be deep into your Dispensing grey matter here but this really is a common exam question.
Hope you can help!
Peter Chapman // Mar 23, 2007 at 3:22 am
Hi Jane.
The fresnel prism was cut to the size of the bifocal segment using a segment template.
To calculate the power of the prism, the lens needs to be thought of as a prism. In this case when looking through the inferior portion of the plus lens, base up prism is crated.
When the bifocal add is the same in the right and left eye, no extra calculations are needed as the differential effect in both eyes is the same.
So, Prentice’s law is used
P = cF
where P is the prism value
c is the centration away from the optical centre of the lens in cm.
F is the power of the lens
For the ease of the calculation, it is assumed that the disctance from the optical centre of the lens to the near visual point is 10mm (1cm).
Now the prism power for each lens is calculated, and the difference between the values is the differential prismatic effect at the near visual point.
This is also the equivalent to the difference in lens powers!
The average person can tolerate a differential prism of between 1.0 - 1.5 prism dioptres. This can be more though is the anisometropia is loing standing.
Using a fresnel prism is a great way to overcome differential prism but should only be used for temproy cases as can cause a reduction in visual acuity by a line or 2.
gary // Mar 27, 2007 at 4:15 am
Whilst the prismatic effects of the anisometropia have been taken into account - the px is being subjected to a 6.00 D induced alteration to spectacle magnification ie from the R sph exceeding the L by 0.50 to the L exceeding the R by 5.50 giving about a 9% relative inc in L mag .Is this going to be tolerated ?
gary // Mar 27, 2007 at 5:07 am
Correction - last post reads better as 9% relative reduction magnification R, although the jist is the same.
Peter Chapman // Apr 2, 2007 at 1:58 am
Gary
Spectacle magnification is something that crossed my mind but still left me in a difficult conundrum. Spec mag occurs with single vision lenses as well as bifocals and so would be an irritation for the patient in any case. Its control is far easier with lens forms in single vision design, but this still would have deprived the patient of their bifocals.
In the end I decided just to correct the differential prism and see how the patient tolerated this. So far, we have not heard form her with any problems!
Donna Parsons // Nov 22, 2008 at 11:17 pm
My mother has (I believe) a similar situation. She also has had cataract surgery (on both eyes initially) but the lens in her right eye has had to be removed and they are not sure if they are going to be able to replace it or not. She just recently got some new glasses with for correction for her good eye balanced for her other eye cosmetically initially with the thought at the time she would be having the new IOL implantation. However, she is very much complaining about these new glasses. I don’t know if she is just still trying to use both eyes, if there are other problems, or if we should try something like what you did - however finding anyone to do that seems to be a problem. Any help you could give would be appreciated. Thanks!
Peter Chapman // Nov 26, 2008 at 7:00 am
Donna,
Firstly, it is obviously difficult to comment on any case without examining the eyes and knowing the prescription. However, from what you have said i do not feel that differential prism is a viable option for your mother. The reason is that by not having a IOL in one eye means that the prescription needed to see clearly will be very high indeed (around +15.00). Although differential prism has been shown to help relieve problems with near vision caused by induced prism, in this case i feel that there will not be any benefit due to such an extreme difference in prescriptions causing other optical effects. I agree that the best option would be to use a balance lens for cosmetic and weight reasons.
As to why your mother is having troubles with her spectacles, I cannot answer, but by having a balance lens the brain should suppress the eye without the IOL and in fact your mother will find it easier than having the correct prescription there.
If you have concerns or worries regarding the spectacles or the health of the eyes, then you should visit your Optometrist again and discuss the matter further.
Peter
rahul optometrist // Dec 9, 2008 at 11:54 pm
dear mr. chapman,
i agree with your answer but you didnt tell about base of prism .
regards,
rahul
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