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Medical Forum / General / Vision / May 2005Irregular astigmatism vs HOAs
I've read that irregular astigmatism created by LASIK can be treated with INTACS. But not HOAs. What's the difference? Especially if both are the result of LASIK. I always thought HOAs was a newer term for irregular astigmatism but they were essentially the same. c...@wyoming.com wrote: > I've read that irregular astigmatism created by LASIK can be treated > with INTACS. But not HOAs. What's the difference? Especially if both > are the result of LASIK. I always thought HOAs was a newer term for > irregular astigmatism but they were essentially the same. HOA stands for higher order abberations. they are not the same as irregular astigmatism. HOA includes coma, spherical aberrations, etc. Using Intacs to treat any refractive aberration caused by LASIK is lunacy. it is difficult to control the precise refractive outcome of Intac implants. it is much easier to do so with LASIK. Wavefront LASIK using a laser-generated flap would likely give you the best refractive outcome. I'm going to respectfully disagree with most of what you have said. While an opinion is never "wrong", I believe the facts would substantiate a different opinion. Since irregular astigmatism is often used as a catch-all phrase for just about any irregular cornea, let us assume that this patient's irregular astigmatism is it's most simple form. Intacs change refractive by adding material to the cornea at the periphery. Adding this material causes a flattening of the center of the cornea, changing the arc by stretching the cornea ever so slightly. Depending upon the nature of the irregularity of the patient's cornea, this stretching effect may provide significant relief. Be clear that Intacs are not approved for the treatment of astigmatism (regular or otherwise), and so this would be an off-label use of the medical device. I also do not know of any studies that have been published on the use of Intacs for irregular astigmatism. There is no medical device approved for the treatment of HOAs. Even the excimer laser is approved only for sphere and cylinder, while considering HOAs as a part of the aberrometry evaluation. HOAs are not a "cause", but are a consequence of something else. You don't treat the symptom, you attempt to treat the cause. Irregular astigmatism often will cause an elevation of quatrefoil and other 5th - 6th level HOA as represented in Zernike polynomials. These higher levels are virtually impossible to resolve with refractive surgery of any kind. My suggestion for the patient would be to start with contacts, moving up to RGPs if needed. These are very likely to resolve irregular astigmatism before any surgery will. Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. > I'm going to respectfully disagree with most of what you have said. What part are you disagreeing with > Be clear that Intacs are not approved for the treatment of astigmatism > (regular or otherwise), and so this would be an off-label use of the > medical device. I also do not know of any studies that have been > published on the use of Intacs for irregular astigmatism. You are both in agreement. Intacs are inappropriate for such a use. > My suggestion for the patient would be to start with contacts, moving > up to RGPs if needed. These are very likely to resolve irregular > astigmatism before any surgery will. The patient was not asking about RGP's > I am not a doctor. Thanks Seldom do I require others to tell me what I said, but I do find that on occasion my statements are misinterpreted. I do not agree that Intacs would be inappropriate. I do not agree that wavefront-guided ablation would be appropriate. The process by which Intacs reduce myopia is a process that should with reasonable probability reduce some types of irregular astigmatism. This is not a technique that, to my knowledge, has extensive study. I have contacted the CEO of Intacs' manufacturer, Addition Technology, and asked their researchers to provide any case studies they may have available. I'll follow up here. Of course, much would depend upon the precise nature of the patient's irregular astigmatism. I've seen doctors call all manner of corneal problems "irregular astigmatism". There are several case studies that have been presented as free papers at several ophthalmic meetings over the past couple of years where Complex Wavefront Retreatment (CWR) was attempt to resolve irregular astigmatism induced by previous LASIK. Results were never excellent, and seldom good. Ask the medical directors of any of the excimer laser manufacturers and they will readily acknowledge this limitation of wavefront-guided ablations. I don't believe the patient was harmed by suggesting consideration of soft contact lenses and possibly RGPs, which have a very solid history of resolving irregular astigmatism in post LASIK patients. Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. >This is not a technique that, to my knowledge, has > extensive study. i think your reply and your arguments are academic. you arguments are not from the perspective of refractive surgery practitioners. what i was trying to state before-- to plan to use a second refractive surgery technique (intacs) to solve any problems created by using an initial refractive surgery technique is inappropriate. i meant to suggest that, in the initial surgery, a wavefront ablation would be preferred to reduce the likelihood of an outcome which includes HOA and/or irregular astigmatism. nevertheless i would "think" (no experience) that a successful outcome of a secondary procedure to improve irregular astigmatism would likely be more successful with wavefront lasik than would intacs. after all, you stated in your reply that secondary wavefront procedures have been reported in the literature and that intacs have not been used that way to the best of your knowledge. the practice that i work in has tried intacs. we are not impressed. other practioners we communicate with have had similar experiences. obviously RGPs would be a solution, but if a patient found that acceptable in the first place why would they have refractive surgery? Ah, I had missed your point before. Thanks for the clarification. Yes, I agree that planning in advance of any refractive surgery to use two different refractive surgery techniques (one to screw things up, and one to try to fix the screw-up) would be inappropriate. I was assuming that the patient was discussing an existing problem and seeking resolution. For an initial surgery on virgin eyes, wavefront-guided excimer laser ablation for LASIK, IntraLASIK, PRK, LASEK, or Epi-LASIK have consistently shown better outcomes than conventional ablation. Wavefront-guided surface ablation techniques PRK, LASEK, and Epi-LASIK have shown better outcomes than wavefront-guided LASIK and IntraLASIK. And although the procedures are very different, wavefront-guided PRK, LASEK, and Epi-LASIK have shown better outcomes than Intacs on virgin eyes. Complex Wavefront Retreatment (CWR) for patients with irregular astigmatism problems induced by previous refractive surgery do not have such good results. The current wavefront-guided laser systems are not designed for, nor do they have the capabilities for, correction of irregular astigmatism. It can happen, but not predictability. A lot of patients who have problems from refractive surgery are delighted to get back good vision with contact lenses. The reason they had refractive surgery in the first place is undoubtedly because they did not expect to be in that unfortunate 3% who have an unresolved complication after the normal six month healing period. Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. At least one peer-reviewed study affirms that Intacs can reduce irregular astigmatism. J Cataract Refract Surg. 2004 Aug;30(8):1685-95. The article can be viewed at http://www.PubMed.com with the search keywords "Intacs irregular astigmatism" Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. As a lark, I looked at that study. Here's what I found: ============================================================ In one of the 13 eyes, the intracorneal ring segments were removed because of progressive stromal melting. CONCLUSIONS: The use of corneal ring segments in selected eyes with residual myopic refractive errors after LASIK was safe and effective. ============================================================ In the first place, the sample size is pretty small to make any claims about safety and efficacy. In the second place, one of the thirteen eyes had progressive stromal melting. Was that from the Intacs? It seems to me that 12/13 isn't all that great. If I took a sample of thirteen widgets with one defective, the batch wouldn't be shipped. Well, maybe it would, but to some third world country. DrG The term "irregular astigmatism" was very much in use before any discussions of "higher order aberrations" and Zernicke polynomials. Basically, it meant a refraction that contained a cylindrical component that did not fully correct the patient's refractive error with the result that the patient's vision was not fully correctable. In the days when most optometrists used ophthalmoscopes, irregular astigmatism could be seen as an irregular light reflex. When the doctor attempted to dial-in the optimum combination of sphero-cylindrical lenses, the light reflex still remained irregular. In those days, there was little or no discussion of starbursts, and halos were primarily the result of corneal edema or a small optical zone in a contact lens. Now we are more sophisticated and analytical with digital "ophthalmoscopes" using laser beams and lens arrays and mathematical models to describe what we used to call "irregular astigmatism." Sorry about the typo. I meant to say "retinoscopes" instead of ophthalmoscopes. A retinoscope is a hand-held light source that projects either a streak of spot of light into the eye. The light is reflected back out of the eye from the retina. By moving the light back and forth, the optometrist can judge the refractive status of the eye, and can neutralize the eye's lower order aberrations using the lenses in the phoropter, or hand-held lenses. Prof. D.Z. Reinstein once referred to the retinoscope as an analogue aberrometer. I thought that was a very apt definition. DrG In the days before wavefront aberrometry and Zernicke polynomials, irregular astigmatism was used to describe a complex refraction that could not be completely neutralized with sphero-cylindrical lenses. The word "astigmatism" simply refers to the condition in which the image is not focused to a point or a single plane. The only means of resolving the condition involving multiple- or complex-refracting surfaces was with spheres and cylinders. If the spatial frequency was low and symmetrical, then those lenses were adequate. A retinoscope is used to "read" the wavefront as visible light is reflected out from the back of the eye. Combinations of lenses are then dialed in place to "neutralize" this wavefront. There are situations in which the wavefront is so irregular that no combination will make it regular, and the patient will report the quality of vision as never being satisfactory. This is termed "irregular astigmatism." Back in the day, irregular astigmatism was most frequently encountered in eyes with keratoconus or eyes that had undergone a keratoplasty. Halos were pathognomic of corneal edema, and starbursts meant cataracts or the edge scotoma of a small hard contact lens. DrG |
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