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Medical Forum / General / Vision / November 2009Would Prism from bad PDs cause damage?
Hi all. I wore a pair of glasses for a long time, circa 8 years actually, with the PDs set wider than my eyes. I'm a -5.5 myope with some cylinder (-0.5 in left, -0.75 in right - I think it should be -1 but whatever...). The PDs were probably about 63 and I'm 57. I know now, after some research, that a lot prism was being induced. How bad is this? Are my eye muscles damaged now? I have glasses now where (presumably) the PDs are correct and I get the opposite effect from the previous glasses - my eyes feel pulled together a bit and things are hard to focus on. Not always, but sometimes... If I turn my head slightly sideways it's better - I rarely sit in front of the TV or movie screen, mostly to one side now. I'm at the stage where I'll either fly to Wauwatosa, Wisconsin or get LASEK ! Seriously! Contact lenses are better for me but I can't wear them too long... Why 8 years with those bad glasses? Well, I did have trouble initially and they (Specsavers, Ireland) gave me new glasses with rounder lenses (PD still at 63) and that helped. They said "you'll have to get used to them". And I did I suppose. Then the industry went to tiny rectangular glasses that mash your face and l didn't get new ones after my usual 4 years. I did get optician check ups several times though. Only when I was at an optician in Canada for a checkup did the guy look at my glasses and say "whoa. The PDs are way off". First optician ever to check this. Great. I got new glasses when I came back and had to return to the dispenser to get the panto tilt done so I could read. I think I know more than they do now which is a poor reflection on the industry... Cheers, Conor. > Hi all. I wore a pair of glasses for a long time, circa 8 years > actually, with the PDs set wider than my eyes. I'm a -5.5 myope with > some cylinder (-0.5 in left, -0.75 in right - I think it should be -1 > but whatever...). The PDs were probably about 63 and I'm 57 The induced prism is (pd error in cm) x power, so 0.6 x 5.5 or about 3prism diopters Base In. Not a lot. Did you tend to see double with the old glasses? > I know now, after some research, that a lot prism was being induced. > How bad is this? Are my eye muscles damaged now? I have glasses now > where (presumably) the PDs are correct and I get the opposite effect > from the previous glasses - my eyes feel pulled together a bit and > things are hard to focus on. No permanent damage. For many people a little prism Base In makes reading more comfortable. Maybe the previous doc did it on purpose to create the Base In effect if you have a large exophoria at near. Judy (late reply after weekend, sorry) > > Hi all. I wore a pair of glasses for a long time, circa 8 years > > actually, with the PDs set wider than my eyes. I'm a -5.5 myope with [quoted text clipped - 4 lines] > 3prism diopters Base In. Not a lot. Did you tend to see double with > the old glasses? No I didn't. My eyes could feel strained though and I blinked heavily a lot. When I was doing a lot of sports it was sometimes a relief to put in contact lenses, which are simple -5.25 non-torics. Can't get that wrong. But I don't like wearing contacts a lot because my eyes start burning after a while; so I don't really consider contacts as a solution. Which is why I've started throwing around the idea of Lasek. Bit extreme though! I'm allergic to pain! > No permanent damage. For many people a little prism Base In makes > reading more comfortable. Maybe the previous doc did it on purpose to > create the Base In effect if you have a large exophoria at near. I'm certain it was by accident (well, incompetence really - the PDs were simply not supplied to the lab and they guessed) and it didn't make reading more comfortable. Plus I started to become very sensitive to minor alignment changes in my glasses fit. Still am with these glasses. I guess higher myopes are a bit sensitive to slight changes in cylinder and angle. Anyway, I was worried that a prism would be like wearing dental retainers - ie: over time my sight would be realigned. I guess that is not the case so thanks for the replies. Conor. > Anyway, I was worried that a prism would be like wearing dental > retainers - ie: over time my sight would be realigned. No, your eyes grow further apart over time. :) -MT >The induced prism is (pd error in cm) x power, so 0.6 x 5.5 or about >3prism diopters Base In. ANSI Z80.1-2005 fabrication standards for horizontal prism imbalance are ± .67^ total, or 2.5mm total if the power at 180 is over ±2.75 D. Assuming a mid 60's IPD, there will be an additional 2^ BI induced when the gaze is at the near point (40cm), for a total of 5^ BI. > Maybe the previous doc did it on purpose to create the Base In effect if you have > a large exophoria at near. >Judy Good point. There's no way to know if this is prescribed prism or an error in measuring/fabrication unless you call the doctor. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > ANSI Z80.1-2005 fabrication standards for horizontal prism imbalance are ± .67^ > total, or 2.5mm total if the power at 180 is over ±2.75 D. [quoted text clipped - 10 lines] > > Robert Martellaro Well given that Specsavers here is akin to Lenscrafters over in the US, the quality of the labs tends to vary a bit. I'm positive that it was an error given that there was space on the original prescription form (which I still have) for prism and it was left blank. That's the form that went to the lab. I do know that they specifically measure PDs now and they didn't back in 1997 so I'm guessing they must have had trouble with unsatisfied customers. I was just concerned that unwanted prism could cause me binocular fusion issues into the future, especially as I'm considering Lasek. I'm aware that people with incorrect glasses PDs who then get a new pair of glasses with correct PDs sometimes report dissatisfaction with the new pair! Indicating that they've become accustomed to a certain degree of induced prism. Thanks for the replies, Conor. > > ANSI Z80.1-2005 fabrication standards for horizontal prism imbalance are ± .67^ > > total, or 2.5mm total if the power at 180 is over ±2.75 D. [quoted text clipped - 29 lines] > > Thanks for the replies, Conor. From a fairly informed patient (not an eye doctor), I have to agree with all who said it's unlikely to cause you any harm ... whether or not it WAS intentional. My $0.02: if you were NOT diagnosed with an eye alignment issue as a young child, then you probably developed a fairly healthy and robust fusion system (in your brain). You may have ... even if you DID have an alignment issue, by the way. I've heard some AMAZING stories about how elastic these "fusion mechanisms" are ... such as ... a pro volleyball player who had a horrible car wreck, was "wall-eyed" as a result, and whose well- developed fusion mechanism "forced" his deviating eye to turn back to "straight ahead" in a matter of months. Whether or not you have THIS sort of plasticity ... I don't know ... but it doesn't sound like your binocular vision suffered THAT sort of insult, either. If you are genuinely concerned, though, you may want to locate a strabismus ophthalmologist (or an optometrist who has some specialty in binocular function disorders), and ask that they thoroughly check your binocular function. If they DID find something wrong, it would be verrry difficult to blame it on the specs. If they found NOTHING wrong, though, your mind might be put at ease. Best of luck... Neil > My $0.02: if you were NOT diagnosed with an eye alignment issue as a > young child, then you probably developed a fairly healthy and robust > fusion system (in your brain). You may have ... even if you DID have > an alignment issue, by the way. Um. Well, I did have a right-eye strabismus (inward squint), corrected surgically in 1978 when I was 6 years old or so (damn I'm old). Which is why I was thinking about it so much. > If you are genuinely concerned, though, you may want to locate a > strabismus ophthalmologist (or an optometrist who has some specialty > in binocular function disorders), and ask that they thoroughly check > your binocular function. Yup. Went and did that. They're pretty darn expensive aren't they!? 230 euro later he said I was fine and my eyes were healthy. If you aren't seeing double you are good to go pretty much. I went for another (standard) prescription today from a place I hadn't gone to before. She said I was slightly esophoric but nothing she'd put prism in for. She also said that the fact that my OD and OS cylinder axes were back to front in my current glasses really isn't helping matters much (90 and 75 degrees so the effect isn't terrible). I was pretty disappointed at that but given the competence of the labs over here, not overly surprised. I guess the labs just don't give a crap. It also seems common for the optician to ramp up cylinder without realising the patient is accomadating. One of my earlier prescriptions has -1.75 cylinder instead of -0.75. Those glasses got binned almost immediately. Idiots... > If they found NOTHING wrong, though, your mind might be put at ease. Well... My mind is at ease in one way - my eyes are pretty much fine apart from minor muscular disparaties that most myopes have to one degree or another. Next time I get glasses I'm going to get them to dot the centres and recheck the prescrip. I'll bring a baseball bat for leverage. Getting LASEK makes me wonder. If the process is the same - take a standard glasses prescription - but then burned into your eyes, I can't imagine I have much confidence. Ooops, we got the prescription wrong (again), too bad, this time it's permanent, sorry. Cheerfully, Conor. > It also seems common for the optician to ramp up cylinder without > realising the patient is accomadating. One of my earlier prescriptions > has -1.75 cylinder instead of -0.75. Those glasses got binned almost > immediately. Idiots... It's the test, called Jackson Crossed Cylinder. Sometimes the instructions are poor, sometimes misunderstood. For whatever reason, some people "eat cylinder" meaning the more you give them the more they want. Of course it doesn't happen to me because I'm the only refractionist who knows how to do it properly. :) -MT I agree with Judy. 3-4 prism diopters base-in would be appropriate if you're a little exophoric (which most people are.) Even if it were 20 or 30 prism diopters, the worst you get is headaches and double vision. The notion of tearing a muscle or doing permanent damage is pretty far-fetched. -MT > Hi all. I wore a pair of glasses for a long time, circa 8 years > actually, with the PDs set wider than my eyes. I'm a -5.5 myope with [quoted text clipped - 31 lines] > > Conor. |
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