Cataract After Laser Iridotomy: Author Reply

Abstract

We thank Yip et al for their interest in our article and acknowledge their recognition of the importance of potential adverse sequelae of laser peripheral iridotomy (LPI). We hope to clarify some of the points raised in their letter.We agree with Yip et al that a number of our subjects were lost to follow-up and that this may affect the robustness of our results. Although it is a valid point that subjects lost to follow-up may have had fewer symptoms than those who remained in the study, logistic regression analysis of risk factors for cataract progression did not show an association between symptoms and progression of lens opacity. Yip et al also assumed that the existence of 5 subjects who underwent cataract extraction during follow-up implied a high rate of preexisting disease. The need for early cataract surgery might be attributed equally to even more rapid cataract progression than what was seen in the rest of the study population, and that these individuals might in fact reflect the more severe spectrum of LPI-induced lens opacity.We were interested particularly in the previous experience of the correspondents relating to Mongolian patients.1Nolan W.P. Foster P.J. Devereux J.G. et al.YAG laser iridotomy treatment for primary angle closure in East Asian eyes.Br J Ophthalmol. 2000; 84: 1255-1259Crossref PubMed Scopus (213) Google Scholar The Mongolian study, which showed equal decrease in visual acuity (VA) after 3 years in both the control and study groups, however did not examine lens opacity; cataract progression was only inferred by their results of VA.1Nolan W.P. Foster P.J. Devereux J.G. et al.YAG laser iridotomy treatment for primary angle closure in East Asian eyes.Br J Ophthalmol. 2000; 84: 1255-1259Crossref PubMed Scopus (213) Google Scholar The Mongolian subjects also underwent LPI with a neodymium:yttrium–aluminum–garnet laser alone, whereas subjects in our study underwent sequential LPI, with argon laser pretreatment, which may have different side effects.Differentiating between incidence and progression of cataract is a valid issue. The Lens Opacities Classification System III (LOCS III) is designed to provide semiobjective assessment of lens opacity, and comparisons between studies using this system often are performed. We employed a definition of progression and incidence very similar to that used in the Barbados Eye Study.2Leske M.C. Wu S.Y. Nemesure B. et al.Incidence and progression of lens opacities in the Barbados Eye Studies.Ophthalmology. 2000; 107: 1267-1273Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar In our article, incidence rates from our study were in fact calculated separately and presented distinct from progression rates, and this was mentioned in “Discussion.” Using a definition of cataract incidence as an increase in LOCS score by ≥2 units in eyes with baseline scores < 2 units, incidences of cataract in nuclear, cortical, and posterior subcapsular regions over 1 year calculated using this criterion in our study population were 0%, 8.2%, and 18.9%, respectively, demonstrating a marked preponderance of incidence of posterior subcapsular opacities. The corresponding rates in the Barbados Eye Study over 4 years were 9.2%, 22.2%, and 3.3% for nuclear, cortical, and posterior subcapsular opacities,2Leske M.C. Wu S.Y. Nemesure B. et al.Incidence and progression of lens opacities in the Barbados Eye Studies.Ophthalmology. 2000; 107: 1267-1273Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar whereas the figures from the Longitudinal Study of Cataract after 5 years of follow-up were 8%, 7.7%, and 4.3%, respectively.3Leske M.C. Chylack Jr, L.T. Wu S.Y. et al.Incidence and progression of nuclear opacities in the Longitudinal Study of Cataract.Ophthalmology. 1996; 103: 705-712Abstract Full Text PDF PubMed Scopus (48) Google Scholar, 4Leske M.C. Chylack Jr, L.T. He Q. et al.Incidence and progression of cortical and posterior subcapsular opacities the Longitudinal Study of Cataract.Ophthalmology. 1997; 104: 1987-1993Abstract Full Text PDF PubMed Scopus (49) Google Scholar We found that an increased LOCS score for any or all regions at baseline was not a significant risk factor for incidence of lens opacity.As we pointed out in the original article, our observational study was indeed limited by the absence of a control group that did not undergo laser iridotomy. Although we are able confidently to give a progression rate for the treated eyes, we cannot compare this with the background rate. However, we believe that an increase of 2 LOCS III units in the posterior subcapsular region in 16.7% of our subjects within 1 year of laser iridotomy is a significant finding, despite the lack of a control group.Finally, we clarify that it was not our intention to discourage ophthalmologists from performing laser iridotomies. Rather, it was to report a potentially important complication of a procedure that warrants further study simply by virtue of the great frequency with which it is performed. If large-scale population-based screening and preventive measures for angle closure are to be introduced, it is essential that even an infrequent adverse event needs to be investigated. We hope that other groups may improve on our work by conducting a better controlled study. We thank Yip et al for their interest in our article and acknowledge their recognition of the importance of potential adverse sequelae of laser peripheral iridotomy (LPI). We hope to clarify some of the points raised in their letter. We agree with Yip et al that a number of our subjects were lost to follow-up and that this may affect the robustness of our results. Although it is a valid point that subjects lost to follow-up may have had fewer symptoms than those who remained in the study, logistic regression analysis of risk factors for cataract progression did not show an association between symptoms and progression of lens opacity. Yip et al also assumed that the existence of 5 subjects who underwent cataract extraction during follow-up implied a high rate of preexisting disease. The need for early cataract surgery might be attributed equally to even more rapid cataract progression than what was seen in the rest of the study population, and that these individuals might in fact reflect the more severe spectrum of LPI-induced lens opacity. We were interested particularly in the previous experience of the correspondents relating to Mongolian patients.1Nolan W.P. Foster P.J. Devereux J.G. et al.YAG laser iridotomy treatment for primary angle closure in East Asian eyes.Br J Ophthalmol. 2000; 84: 1255-1259Crossref PubMed Scopus (213) Google Scholar The Mongolian study, which showed equal decrease in visual acuity (VA) after 3 years in both the control and study groups, however did not examine lens opacity; cataract progression was only inferred by their results of VA.1Nolan W.P. Foster P.J. Devereux J.G. et al.YAG laser iridotomy treatment for primary angle closure in East Asian eyes.Br J Ophthalmol. 2000; 84: 1255-1259Crossref PubMed Scopus (213) Google Scholar The Mongolian subjects also underwent LPI with a neodymium:yttrium–aluminum–garnet laser alone, whereas subjects in our study underwent sequential LPI, with argon laser pretreatment, which may have different side effects. Differentiating between incidence and progression of cataract is a valid issue. The Lens Opacities Classification System III (LOCS III) is designed to provide semiobjective assessment of lens opacity, and comparisons between studies using this system often are performed. We employed a definition of progression and incidence very similar to that used in the Barbados Eye Study.2Leske M.C. Wu S.Y. Nemesure B. et al.Incidence and progression of lens opacities in the Barbados Eye Studies.Ophthalmology. 2000; 107: 1267-1273Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar In our article, incidence rates from our study were in fact calculated separately and presented distinct from progression rates, and this was mentioned in “Discussion.” Using a definition of cataract incidence as an increase in LOCS score by ≥2 units in eyes with baseline scores < 2 units, incidences of cataract in nuclear, cortical, and posterior subcapsular regions over 1 year calculated using this criterion in our study population were 0%, 8.2%, and 18.9%, respectively, demonstrating a marked preponderance of incidence of posterior subcapsular opacities. The corresponding rates in the Barbados Eye Study over 4 years were 9.2%, 22.2%, and 3.3% for nuclear, cortical, and posterior subcapsular opacities,2Leske M.C. Wu S.Y. Nemesure B. et al.Incidence and progression of lens opacities in the Barbados Eye Studies.Ophthalmology. 2000; 107: 1267-1273Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar whereas the figures from the Longitudinal Study of Cataract after 5 years of follow-up were 8%, 7.7%, and 4.3%, respectively.3Leske M.C. Chylack Jr, L.T. Wu S.Y. et al.Incidence and progression of nuclear opacities in the Longitudinal Study of Cataract.Ophthalmology. 1996; 103: 705-712Abstract Full Text PDF PubMed Scopus (48) Google Scholar, 4Leske M.C. Chylack Jr, L.T. He Q. et al.Incidence and progression of cortical and posterior subcapsular opacities the Longitudinal Study of Cataract.Ophthalmology. 1997; 104: 1987-1993Abstract Full Text PDF PubMed Scopus (49) Google Scholar We found that an increased LOCS score for any or all regions at baseline was not a significant risk factor for incidence of lens opacity. As we pointed out in the original article, our observational study was indeed limited by the absence of a control group that did not undergo laser iridotomy. Although we are able confidently to give a progression rate for the treated eyes, we cannot compare this with the background rate. However, we believe that an increase of 2 LOCS III units in the posterior subcapsular region in 16.7% of our subjects within 1 year of laser iridotomy is a significant finding, despite the lack of a control group. Finally, we clarify that it was not our intention to discourage ophthalmologists from performing laser iridotomies. Rather, it was to report a potentially important complication of a procedure that warrants further study simply by virtue of the great frequency with which it is performed. If large-scale population-based screening and preventive measures for angle closure are to be introduced, it is essential that even an infrequent adverse event needs to be investigated. We hope that other groups may improve on our work by conducting a better controlled study. Cataract after Laser IridotomyOphthalmologyVol. 113Issue 8PreviewWe read Lim et al’s article suggesting a possible association between prophylactic laser iridotomy and progression of lens opacity with considerable interest.1 The World Health Organization rates glaucoma as the second greatest cause of blindness worldwide2 and the leading cause of blindness that is not potentially reversible. Angle closure accounts for around half of all cases of primary glaucoma worldwide.3 Laser iridotomy increases angle width significantly in the majority of people with narrow angles and is effective in preventing angle closure in those who have suffered a symptomatic attack. Full-Text PDF