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Medical Forum / General / Vision / October 20052 questions to ODs...
Folks, What is the difference between Rx and CRx? Just that one is a cyclo? I was wondering why my military OD prescribed my glasses according to autorefractor instead of the cyclo results... Would cyclo prescription be an overprescription? Because I have both results and I am on my own to make a decision when buying glasses, I am not getting military issue glasses. Second, since you see hundreds of patients every year, from your observations, in average what refraction level produces 20/20 or 20/15 acuity? Are they usually emmetropic or they would almost always have some slight refraction after cyclo? I am more interested in myopic values, as I am thinking hyperopic patients, even they have moderate-high refraction errors, should see 20/20. The reason I am asking this, I was comparing the military flight school entrance requirements in USA and Turkey. Turkey will only allow a candidate when he is 18 or 19 years old and HE HAS TO BE EMMETROPIC, (-/+0.00 right?) given today's technology, information around us and the amount of reading and close work we do, I though about it, how common is to have 0.00 anymore? A candidate is able to see 20/20 but is eliminated because of the -.25 or -.50 there, this was brought up as very demanding compared to US standards of -1.50 in myopia. What do you think? Thanks... John Let's try thousands of patients, not hundreds. Patients with hyperopia can compensate with accommodation. Myopic patients cannot. Patients who typically present with the best unaided visual acuity are often slightly hyperopic within the limits of their accommodation. The cycloplegic refraction, because it reduces or eliminates accommodation, can result in a refraction that is more hyperopic than the manifest. Occasionally it turns out to be very slightly more myopic for reasons that are not entirely clear to me, although I surmise it has to do with an increase in positive spherical aberration from the edges of the cornea. DrG >Occasionally it turns out to be very slightly more >myopic for reasons that are not entirely clear to me, although I >surmise it has to do with an increase in positive spherical aberration >from the edges of the cornea. > >DrG This might be the case for me in my last cyclo, where myopia was 1/4 more than the autorefractor. I was surprised as I expected that cyclo would produce result towards positive. Neither your auto-refraction nor your cycloplegic refraction is a "true" wearable Rx for most people. When we look into a dark box, we tend to accomodate for no good reason. We theorize that this "black box myopia" is an innate fear response. So, when you look into an autorefractor, you tend to "suck up minus" because of the accidental accomodation that the machine reads as myopia. Every manufacturer has a different nomogram or theory to "cancel" black box myopia, and some are better than others. Still, the quick point here is that the cycloplegic Rx has too much plus, and the autorefractor Rx has too much minus. In general, of course. You can factor in age and ability to accomodate as a big variable here. That's what your optometrist is there for. > Folks, > [quoted text clipped - 23 lines] > > John >Neither your auto-refraction nor your cycloplegic refraction is a >"true" wearable Rx for most people. [quoted text clipped - 9 lines] >factor in age and ability to accomodate as a big variable here. That's >what your optometrist is there for. Thanks, interesting info here. in average what refraction level produces 20/20 or 20/15 > acuity? Are they usually emmetropic or they would almost always have > some slight refraction after cyclo? I am more interested in myopic > values, as I am thinking hyperopic patients, even they have > moderate-high refraction errors, should see 20/20. Ignoring astigmatism, which is often a factor, most healthy young people with -.5 can get 20/20 unaided, and will have trouble with 20/15. Most with -.25 can get 20/15. Most with -.75 will have trouble with 20/20. The reason I am > asking this, I was comparing the military flight school entrance > requirements in USA and Turkey. Turkey will only allow a candidate when > he is 18 or 19 years old and HE HAS TO BE EMMETROPIC, (-/+0.00 right?) Right. I'm sure they have tolerances, unless they are real turkeys. and have very few candidates. Maybe they don't even know what emmetropia is. > given today's technology, information around us and the amount of > reading and close work we do, I though about it, how common is to have > 0.00 anymore? I'd guess I see fewer than 1% bilateral emmetropes, if you use the strict definition with no tolerances. But then most of my patients come in because they have a problem with vision. General population, probably 2-5%. A candidate is able to see 20/20 but is eliminated because > of the -.25 or -.50 there, this was brought up as very demanding > compared to US standards of -1.50 in myopia. What do you think? Read their complete requirements, and how they define emmetropia. You should find some tolerances... w.stacy, o.d. > Ignoring astigmatism, which is often a factor, most healthy young > people with -.5 can get 20/20 unaided, and will have trouble with > 20/15. Most with -.25 can get 20/15. Most with -.75 will have > trouble with 20/20. Thanks Bill this is what I was assuming. > Right. I'm sure they have tolerances, unless they are real turkeys. > and have very few candidates. Maybe they don't even know what > emmetropia is. They are very anal about it. US is indeed a country of freedoms as I have always thought. The information is always published for candidates. I have checked their (Turkish) website. No definitive information on any standards. The only commissioning program is through their AF academy. They require excellent academic record and only accept 125 people a year out of millions who take the preliminary SAT type test.. Only 25 or so make it to flight school, maybe 10 to fighter track every year. 2 chances after high school. The only reference to eyesight requirements in their application procedures is that; one brochure sasys "NO EYE PROBLEMS" the other says "not using Rx glasses or contacts for *any* degree of eye/vision problems" ... When I found out I had -.5 after high school, I realized the BS in their system, and research what US was asking from its candidates for military flight programs. Not to forget they disqualified a friend of mine because he had crooked teeth. So here I am ... > I'd guess I see fewer than 1% bilateral emmetropes, if you use the > strict definition with no tolerances. But then most of my patients > come in because they have a problem with vision. General population, > probably 2-5%. People don't come to you just to have their eye health checked at least? Like they go to the dentist? I wish I could known how important eyesight is, I would always have my eyes checked. > Read their complete requirements, and how they define emmetropia. > You should find some tolerances... Wish I could > w.stacy, o.d. Dear John, The Austrailian study defined "emmetropia" to be refractive states between -0.5 diopters to +0.5 diopters. I do not know if there is any consistent or totally "accepted" definition of "emmetropia". Best, Otis |
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