Differential Prism – a common exam fail

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 commonly failed station in the OSCE’s.  It is good to know the full list of options, and understand 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, and have a realistic choice ready  “for the every day folk.”

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