Pseudophakic retinal detachments

LETTERS

that contrast sensitivity evaluation is necessary to assess visual performance in refractive surgery patients. As is well known, an acuity measurement does not provide all the information needed to describe spatial vision.2 Acuity is a 1-dimensional answer to a 2-dimensional problem (size ⫻ contrast)—3-fold if we include luminance as a variable.3 Based on their observations, Chan and coauthors argue that LASIK most affects the threshold of spatial frequencies 1.5 cycles per degree (cpd) and 3.4 cpd (relatively low-contrast region), while the effects to the thresholds of higher and lower spatial frequencies (highcontrast regions) are less obvious. We do not understand what the authors mean: What is the relationship between 1.5 cpd and 3.4 cpd frequencies and the lowcontrast region? And between the rest of the frequencies and the high-contrast regions? Contrast and spatial frequencies are 2 variables that are related by the contrast sensitivity function from which, for normal vision, we can derive that less contrast is required to detect medium-resolution targets than to detect low- or high-resolution targets (contrast sensitivity [1/contrast]). Because of this, a patient with a visual acuity of 20/20 but impaired broad-bar contrast acuity may be unable to see a truck in the fog and a patient with a refractive error only would have contrast decrements with the thinner bars. This is the reason for the larger scale in contrast sensitivity (y-axis, Figure 1) for 1.5 cpd and 3.4 cpd graphs than for the other frequency graphs. We think the authors agree with us, although this is not clearly stated in the paper and could cause some confusion for readers. Another point is that Chan and coauthors relate the contrast sensitivity reduction after LASIK to some optical factors and discuss the neural origin of the reductions. Refractive surgery has no effect on the neural components of vision, and any postoperative changes in visual performance should be due to optical factors only.2 Chan and coauthors speculate that the change in corneal endothelia, corneal birefringent properties, and retinal nerve fiber damage after refractive surgery could be the cause of contrast sensitivity reduction, although the authors conclude that these causes are unlikely true. Studies by Monte´s-Mico´ and Charman2,3 explain the source of reduction in contrast sensitivity after refractive surgery. The authors point out that decreases in contrast sensitivity are due to optical factors (defocus and optical

aberrations), which affect medium- to high-spatial frequencies, and haze, which attenuates uniformly across the spatial-frequency spectrum. It is not necessary to invoke new mechanisms to explain the reductions after refractive surgery. ROBERT MONTE´ S-MICO´ , OD, MPHIL JORGE L. ALIO´ , MD, PHD GONZALO MUN˜ OZ, MD, PHD Alicante, Spain

References 1. Chan JWW, Edwards MH, Woo GC, Woo VCP. Contrast sensitivity after laser in situ keratomileusis; one-year follow-up. J Cataract Refract Surg 2002; 28:1774 –1779 2. Monte´s-Mico´ R, Charman WN. Choice of spatial frequency for contrast sensitivity evaluation after corneal refractive surgery. J Refract Surg 2001; 17:646 –651 3. Monte´s-Mico´ R, Charman WN. Mesopic contrast sensitivity function after excimer laser photorefractive keratectomy. J Refract Surg 2002; 18:9 –13

Pseudophakic Retinal Detachments

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he study of pseudophakic retinal detachments (RDs) by Pokroy and coauthors1 reports that in eyes having cataract extraction with vitreous loss, implantation of anterior (AC) and posterior chamber (PC) intraocular lenses (IOLs) has the same incidence of RD. Although we share the authors’ opinion, we think the Patients and Methods section should be clarified in certain ways to allow more definitive results and conclusions. The lack of difference between the groups may be related to the relatively small study. The mean follow-up is also an important factor in concluding that there is no difference between AC and PC IOLs. A 6-month follow-up may not be long enough to see the effects of AC or PC IOLs on the RD incidence. The follow-up should be several years or longer for such a comparison to be made. To determine a risk factor for a condition such as RD, larger and longer studies are needed.2– 4 The incidence of RD development is dependent on other variables such as vitreous syneresis and posterior vitreous detachment, and it should be confirmed that the groups were matched for these variables. This would clarify whether the lack of differences in the study arose from the AC or PC IOLs or from the unmatched vitreous characteristics of the groups.

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LETTERS

The development of RD also depends on race.5 In the article, there is no mention of this and we do not know whether the groups were matched for race. If they were not, it might be a factor in the study results. The cataract extraction technique was not clearly stated. Was it extracapsular cataract extraction or phacoemulsification? Were the groups matched for technique? Because the development of RD depends on the surgical technique, occurring more with the extracapsular technique, this should be clarified. What technique was used for the anterior vitrectomy: vitrector or the scissors–sponge method? These 2 methods may affect the RD incidence. In the introduction, the authors state that the aim of the study was to evaluate the relationship of the AC IOL to RD. However, they compared the incidence of RD in eyes with a PC IOL to that in eyes with an AC IOL. In our opinion, the authors should have compared the eyes with AC IOLs to aphakic eyes to determine the relationship. We think that clarifying the points we have raised would make the study results more valid. MURAT OZDEMIR, MD GOKHAN OZDEMIR, MD ARZU CEVIK DURMUS, MD Kahramanmaras, Turkey

4. Smith PW, Start WJ, Maumenee AE, et al. Retinal detachment after extracapsular cataract extraction with posterior chamber intraocular lens. Ophthalmology 1989; 94:495– 504 5. Rosman M, Wong TY, Ong SG, Ang CL. Retinal detachment in Chinese, Malay and Indian residents in Singapore: a comparative study on risk factors, clinical presentation and surgical outcomes. Int Ophthalmol 2001; 24:101–106

Anterior Capsule Contraction

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was interested in the reports of anterior capsule contraction in eyes with piggyback silicone intraocular lenses (IOLs) in the paper by Yeh and coauthors1 and the letter by Eleftheriadis.2 This problem has not occurred in over 350 of my patients with nanophthalmic eyes who received piggyback IOLs—probably because I use poly(methyl methacrylate) (PMMA) lenses primarily rather than silicone or acrylic. I would like to suggest that this complication could be largely avoided by using PMMA IOLs for piggybacking in primary cataract surgery. Fortunately, the recent availability of PMMA IOLs in high powers has decreased the need for piggyback lenses in nanophthalmic eyes. JAMES P. GILLS, MD Tarpon Springs, Florida, USA

References 1. Pokroy R, Pollack A, Bukelmen A. Retinal detachment in eyes with vitreous loss and an anterior chamber or a posterior chamber intraocular lens; comparison of the incidence. J Cataract Refract Surg 2002; 28:1997–2001 2. Wilkinson CP. Pseudophakic retinal detachments. Retina 1985; 5:1–4 3. Cousins S, Bvoniuk I, Okum E, et al. Pseudophakic retinal detachments in the presence of various IOL types. Ophthalmology 1986; 93:1198 –1208

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References 1. Yeh PC, Goins KM, Lai WW. Managing anterior capsule contraction by mechanical widening with vitrector-cut capsulotomy. J Cataract Refract Surg 2002; 28:217– 220 2. Eleftheriadis H. Capsule contraction in polypseudophakia [letter]. J Cataract Refract Surg 2003; 29:8 –9

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