DEC 12, 2012
This letter in the September Journal of Cataract & Refractive Surgery points out that from a basic optical view, it is incorrect to say that corrected myopic patients experience more magnification than corrected hyperopic patients for a specific working distance. This is written in response to an article in the March issue of the journal on the implantation of multifocal IOLs using a magnification strategy in cataractous eyes with AMD. This letter presents an important concept that we ophthalmologists who don't love optics don't get. I think it is critical information for anyone caring for AMD patients.
The letter's authors write that AMD patients need more positive magnification, especially for reading. If they are to have cataract surgery, aiming for myopia will give them less magnification than if they were left emmetropic or hyperopic. The only benefit for the patient will be the ability to read unaided. They recommend exercising caution before aiming for myopia in AMD patients having cataract surgery, particularly if the patient is not myopic preoperatively.
I believe that we probably should be making one eye of severe AMD patients hyperopic, although I haven't yet had the guts to do it.
The authors explain that a -4.00 myopic patient reading at 25 cm will get the benefit of proximal magnification only. An emmetropic patient will have to wear +4.00 glasses to read at 25 cm and will therefore get the benefit of the proximal magnification and the spectacle magnification. A +4.00 hyperopic patient reading at the same 25 cm (using +8.00 glasses) will get the most magnification.
They say that the situation for distance viewing is the same, with the corrected hyperopic patient getting more magnification than the myopic patient. A corrected hyperopic patient's optical system is similar to a Galilean telescope. The hyperopia acts as the negative eyepiece, and the positive spectacle lens acts as the objective lens. A corrected myopic patient's system is similar to a reverse Galilean telescope adding minification. The letter's authors say that the article's focus on multifocal IOLs does not change these basic optical principles.
No abstract is available.