- Email: [email protected]
Reply: Environmental factors and LASIK
LETTERS
suggest the development of a nomogram that considers indoor room humidity “since controlling indoor humidity is difficult.” While LASIK nomogram refinements are certainly 1 method to address the issue of humidity, the most obvious solution would be to control the indoor humidity. The Liebert Mini-Mate2威 by the Liebert Corp. and CeilAiR威 by Stultz Air Technology Systems provide excellent procedure humidity and temperature control with 2 degrees of variation of temperature and 5 units of variation of humidity through the year, even in the extreme temperature fluctuations of the Midwest. While these systems can cost between $20 000 and $60 000 to install, the expense would be justified considering the desire to further improve LASIK outcomes. LOUIS PROBST, MD Chicago, Illinois, USA Larry Stitt, MSc, Biostatistical Support Unit, Department of Epidemiology & Biostatistics, University of Western Ontario, London, Ontario, Canada, assisted with the statistical analysis.
tion software. Whether humidity has more effect through corneal adaptation before surgery or through direct effect of laser performance in the operating room may be irrelevant. Furthermore, we do not wish the importance of outdoor humidity to be underestimated or, even worse, ignored. In our univariate analysis, the 2-week outdoor humidity was the most significant variable ( P ⫽ .001). Indeed, referring to Figure 2 in the article, between August and September, the procedureroom humidity dropped from 43.5% to 39.8%, while the outdoor humidity increased from 71.4% to 76.7% and the enhancement rate increased from 23.3% to 50.0%. Anecdotally, since Wake Forest University Eye Center has begun compensating for outdoor humidity, our enhancement rates have dropped from 15.5% to 3.5% (unpublished data). As our article states, further investigation is warranted. Ideally, it would be best to control the humidity to less than 1% variability and then study what role outdoor humidity really plays. I am excited that this research has provoked so much controversy and rhetoric and am hopeful that it will serve our future LASIK patients well. We would like to acknowledge the statistical assistance provided by Greg Russell, MSc, of Wake Forest University.— Keith Walter, MD
References 1. Walter KA, Stevenson AW. Effect of environmental factors on myopic LASIK enhancement rates. J Cataract Refract Surg 2004; 30:798–803 2. de Souza IR, de Souza AP, de Queiroz AP, et al. Influence of temperature and humidity on laser in situ keratomileusis outcomes. J Refract Surg 2001; 17:S202–S204
Reply:
Dr. Probst’s statement that indoor humidity was the most important variable in a multivariate analysis and could be controlled with commercial equipment is not only valid but helpful. However, after reviewing the statistics with our statistician, it became apparent that we may have misled the reader by our method of relaying that information. We used a stepwise model, which eliminates the least-significant variable and then refits the data. This is repeated until only 1 variable remains, which in our study was procedure-room humidity. The mistake was in the way the sentence was worded originally; if it had said “only room humidity remained in a stepwise model,” the confusion might have been eliminated. Dr. Probst states that his procedure-room humidity is controlled within 5%. While this is superb control of humidity, our article states, “Each 1% rise in (procedure room) humidity increased the odds for enhancement by 9.3%.” So, a 5% change in room humidity would mean a 46% greater chance for an enhancement. The value in our study lies in the awareness that environmental factors play important roles in determining outcome and may be more significant than the latest customiza1818
Single-piece acrylic intraocular lens implantation in children
W
e read with interest the article by Trivedi and Wilson on single-piece acrylic intraocular lens (IOL) implantation in children.1 We agree with the authors that currently, acrylic IOLs have become the first choice, especially for pediatric cataract surgery, because of their superior design and better biocompatibility. We have been using a 3-piece acrylic IOL in pediatric cataract patients. We routinely perform primary posterior capsulorhexis and anterior vitrectomy followed by optic capture of the IOL. Posterior capsule opacification (PCO) is 1 of the most frequent postoperative complications of pediatric cataract surgery. Use of primary posterior capsulorhexis with and without anterior vitrectomy has decreased the rate of occurrence and the severity of this complication.2,3 Although combining anterior vitrectomy with primary posterior capsulorhexis decreases the incidence of PCO, it has been shown that optic capture in conjunction with primary posterior capsulorhexis and anterior vitrectomy further lowers the rate of PCO compared with primary posterior capsulorhexis and anterior vitrec-
J CATARACT REFRACT SURG—VOL 30, SEPTEMBER 2004
suggest the development of a nomogram that considers indoor room humidity “since controlling indoor humidity is difficult.” While LASIK nomogram refinements are certainly 1 method to address the issue of humidity, the most obvious solution would be to control the indoor humidity. The Liebert Mini-Mate2威 by the Liebert Corp. and CeilAiR威 by Stultz Air Technology Systems provide excellent procedure humidity and temperature control with 2 degrees of variation of temperature and 5 units of variation of humidity through the year, even in the extreme temperature fluctuations of the Midwest. While these systems can cost between $20 000 and $60 000 to install, the expense would be justified considering the desire to further improve LASIK outcomes. LOUIS PROBST, MD Chicago, Illinois, USA Larry Stitt, MSc, Biostatistical Support Unit, Department of Epidemiology & Biostatistics, University of Western Ontario, London, Ontario, Canada, assisted with the statistical analysis.
tion software. Whether humidity has more effect through corneal adaptation before surgery or through direct effect of laser performance in the operating room may be irrelevant. Furthermore, we do not wish the importance of outdoor humidity to be underestimated or, even worse, ignored. In our univariate analysis, the 2-week outdoor humidity was the most significant variable ( P ⫽ .001). Indeed, referring to Figure 2 in the article, between August and September, the procedureroom humidity dropped from 43.5% to 39.8%, while the outdoor humidity increased from 71.4% to 76.7% and the enhancement rate increased from 23.3% to 50.0%. Anecdotally, since Wake Forest University Eye Center has begun compensating for outdoor humidity, our enhancement rates have dropped from 15.5% to 3.5% (unpublished data). As our article states, further investigation is warranted. Ideally, it would be best to control the humidity to less than 1% variability and then study what role outdoor humidity really plays. I am excited that this research has provoked so much controversy and rhetoric and am hopeful that it will serve our future LASIK patients well. We would like to acknowledge the statistical assistance provided by Greg Russell, MSc, of Wake Forest University.— Keith Walter, MD
References 1. Walter KA, Stevenson AW. Effect of environmental factors on myopic LASIK enhancement rates. J Cataract Refract Surg 2004; 30:798–803 2. de Souza IR, de Souza AP, de Queiroz AP, et al. Influence of temperature and humidity on laser in situ keratomileusis outcomes. J Refract Surg 2001; 17:S202–S204
Reply:
Dr. Probst’s statement that indoor humidity was the most important variable in a multivariate analysis and could be controlled with commercial equipment is not only valid but helpful. However, after reviewing the statistics with our statistician, it became apparent that we may have misled the reader by our method of relaying that information. We used a stepwise model, which eliminates the least-significant variable and then refits the data. This is repeated until only 1 variable remains, which in our study was procedure-room humidity. The mistake was in the way the sentence was worded originally; if it had said “only room humidity remained in a stepwise model,” the confusion might have been eliminated. Dr. Probst states that his procedure-room humidity is controlled within 5%. While this is superb control of humidity, our article states, “Each 1% rise in (procedure room) humidity increased the odds for enhancement by 9.3%.” So, a 5% change in room humidity would mean a 46% greater chance for an enhancement. The value in our study lies in the awareness that environmental factors play important roles in determining outcome and may be more significant than the latest customiza1818
Single-piece acrylic intraocular lens implantation in children
W
e read with interest the article by Trivedi and Wilson on single-piece acrylic intraocular lens (IOL) implantation in children.1 We agree with the authors that currently, acrylic IOLs have become the first choice, especially for pediatric cataract surgery, because of their superior design and better biocompatibility. We have been using a 3-piece acrylic IOL in pediatric cataract patients. We routinely perform primary posterior capsulorhexis and anterior vitrectomy followed by optic capture of the IOL. Posterior capsule opacification (PCO) is 1 of the most frequent postoperative complications of pediatric cataract surgery. Use of primary posterior capsulorhexis with and without anterior vitrectomy has decreased the rate of occurrence and the severity of this complication.2,3 Although combining anterior vitrectomy with primary posterior capsulorhexis decreases the incidence of PCO, it has been shown that optic capture in conjunction with primary posterior capsulorhexis and anterior vitrectomy further lowers the rate of PCO compared with primary posterior capsulorhexis and anterior vitrec-
J CATARACT REFRACT SURG—VOL 30, SEPTEMBER 2004