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ORIGINAL ARTICLE
Year : 2015  |  Volume : 3  |  Issue : 3  |  Page : 133-138

Does outside environmental humidity influence the outcome of laser refractive surgery? Results from the Hamburg Weather Study


1 Department of Ophthalmology, University Medical Center Hamburg-Eppendorf, Martinistraße, Hamburg, Germany
2 Department of Ophthalmology; Care Vision Refractive Centers, University Medical Center Hamburg-Eppendorf, Martinistraße, Hamburg, Germany

Date of Submission20-Jul-2014
Date of Acceptance25-Feb-2015
Date of Web Publication20-Aug-2015

Correspondence Address:
Andreas Frings
Department of Ophthalmology, University Medical Centre Hamburg-Eppendorf, Martinistraße 52, 20246 Hamburg
Germany
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2320-3897.163261

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  Abstract 

Purpose: This study was initiated to assess the impact of outside environmental humidity on the refractive and visual outcome of laser-assisted in situ keratomileusis (LASIK) in myopic eyes. Materials and Methods: One thousand and fifty-two eyes of 1,052 consecutive myopic patients (419 males and 633 females; mean age at surgery 35.0 ± 9.0 years) with mean preoperative refractive spherical equivalent (SE) of −3.88 ± 1.85 diopters (D) were studied. Two subgroups were defined, which had undergone surgery either during meteorological winter or summer. Result: By 1-month (33.0 ± 5.0 days) follow-up, a mean postoperative SE of −0.18 ± 0.44 D was obtained. Bivariate comparison showed that statistically significant better safety index (SI) was obtained for high outside environmental humidity. Robust regression methods indicated high humidity to be associated with significant better SI and postoperative SE. No change of more than one line on logMar scale was obtained. Conclusion: Although being statistically significant, there is no clinically relevant difference in outcome of LASIK, which demonstrates its highly standardized quality. However, we observed a tendency that high outside environmental humidity at the day of LASIK and during early perioperative time, tends to produce better refractive outcome. Prospective, longitudinal studies are warranted to address meteorotropic reactions through evaluating individual risk profiles.

Keywords: Ambient humidity, laser refractive surgery, outside environmental humidity, weather


How to cite this article:
Neuhaus-Richard I, Frings A, Görsch IC, Druchkiv V, Katz T, Linke SJ, Steinberg J, Richard G. Does outside environmental humidity influence the outcome of laser refractive surgery? Results from the Hamburg Weather Study. J Clin Ophthalmol Res 2015;3:133-8

How to cite this URL:
Neuhaus-Richard I, Frings A, Görsch IC, Druchkiv V, Katz T, Linke SJ, Steinberg J, Richard G. Does outside environmental humidity influence the outcome of laser refractive surgery? Results from the Hamburg Weather Study. J Clin Ophthalmol Res [serial online] 2015 [cited 2022 May 23];3:133-8. Available from: https://www.jcor.in/text.asp?2015/3/3/133/163261

Over the last 2 decades, the field of laser refractive surgery has seen dramatic advances introduced by new technologies. [1] Laser-assisted in situ keratomileusis (LASIK) has become the most popular excimer technique among refractive surgeons as it provides numerous major clinical and refractive benefits. [2] However, before undergoing LASIK, numerous factors have been established as having potential adverse effects on LASIK including ocular allergy, contact lens (CL) wear, herpetic keratitis, epithelial basement membrane dystrophy, eyelid disease, and lid abnormalities. [3],[4] Further factors, among them objective parameters like age and sex, have previously been examined as reliable predictor of medical treatment results. [3],[4]

Biological influence of weather, also called biotropy, can be 'health-endangering'. [5],[6],[7],[8],[9],[10] It has also been described in the field of ophthalmology. The eye can be damaged by visible light energy, ultraviolet (UV), and infrared wavelengths. [11] Moreover, epidemiologic evidence has revealed age-related macular degeneration (AMD) as being associated with severity of sun light exposure. [12] The role of season in developing endophthalmitis and retinal detachment has been described. [13],[14] Further data has indicated changes of intraocular pressure (IOP) in patients with open-angle glaucoma in relation to the passage of atmospheric fronts. [15]

Recently, Seider et al., [16] in a study looking at 1-month outcomes of over 200 thousand eyes undergoing LASIK in the UK and Ireland between 2008 and 2011, reported no clinically significant relationship between operating room humidity and postoperative refraction. In 2004, Walter and Stevenson [17] analyzed multiple environmental variables and their effect on post-myopic LASIK refraction. Interestingly, their conclusions were that 2-week-preoperative mean outdoor relative humidity and laser room relative humidity were statistically significant factors in predicting the need for enhancement. de Souza et al., [18] also published results on operating room humidity effecting LASIK outcomes, even after being checked as far as 60 days postoperatively.

This study was thus initiated to assess the impact of outside environmental humidity on the refractive and visual outcome of LASIK in myopic eyes.


  Materials and Methods Top


It was a retrospective study of myopic patients undergoing LASIK surgery. We examined one randomly selected eye from each patient. Exclusion criteria were ocular pathology and medication likely to influence the stability of refractive error. Patients suffering from hyperthyroidism, hypothyroidism, and malignant hyperthermia; and taking psychotropic drugs, beta blockers, and drugs interfering with temperature balance were excluded from the analysis. [19] Diabetics with ocular manifestation were not subjected to LASIK (patients with DM type I were not treated anyway). Patients with suspected dry eye were subsequently tested (Schirmer test) and did not receive LASIK surgery. For all patients, preservative-free hyaluronic acid artificial tears were applied post LASIK. This should address the issue of impaired visual function in low dry eye patients after LASIK. CLs, if any, were not to be applied before LASIK (10 days in case of hard CL and 5 days when soft CL was used). Eyes that could not be refracted to 20/20, but had no obvious pathology were included (i.e., mild amblyopia).

All patients were treated between 2010 and 2012 at Care Vision Refractive Centers in Germany. All patient data was de-identified prior to analysis. Our study adhered to the tenets of the Declaration of Helsinki and approval of local ethics committee (no. 2882) was achieved. Manifest spherical and cylindrical refractions, and visual acuity with and without correction were assessed pre- and postoperatively after 1 day, 1 and 6 months, and recorded electronically. All refractions were acquired by subjective refraction by expert optometrists.

The refractive outcome was analyzed according to standard graphs for reporting results of refractive surgery, as suggested by Dupps et al. [20]

Our results are based on first treatments only, enhancements were not included.

Geography and climate as well as definition of seasonality and treatment subgroups have been described elsewhere. [21]

The meteorological data included hourly readings and daily summaries of humidity (in %), recorded by meteorological stations of the German Weather Service. These stations were located in the cities of Berlin, Frankfurt a. M., Hamburg, Hanover, Cologne, Munich, Nuremberg, and Stuttgart. Each station was located close to the Care Vision Center, respectively.

After the data were compiled, they were entered into a spreadsheet program (Microsoft Office Excel; Hamburg Refractive Database) and were statistically analyzed using the Stata-PC program version 11.0 for Windows PC (Stata Corporation, Texas, USA).

We applied regression analysis based on robust regression methods. The idea of robust regression is to weigh observations based on their leverage or deviation from prediction obtained by ordinary least squares (OLS) analysis. It is a form of weighted and reweighted least squares regression.

The dependent variables were efficacy index (EI) and safety index (SI), measured on the log scale, and postoperative spherical equivalent (SE; diopter (D)). To describe EI and SI, the following formulas were applied:

  • EI = −log 10 (UCVA POSTOP) + log 10 (CDVA PREOP)
  • SI = −log 10 (CDVA POSTOP) + log 10 (CDVA PREOP)


Efficacy was defined as mean of ratios of postoperative uncorrected distance visual acuity (UDVA) to preoperative corrected distance visual acuity (CDVA). Safety was defined as mean of ratios of postoperative CDVA to preoperative CDVA; and thus, EI or SI could be interpreted as lines gained or lost after refractive surgery. By applying the abovementioned formulas, positive results indicated a less effective and negative results a more effective treatment. Therefore, negative coefficients in robust regression indicated a positive impact of the independent variable on the outcome, while this was negatively influenced by positive coefficients. Although approximately normal distribution was confirmed by analysis of residuals, we decided to apply robust regression methods instead of simple OLS-estimation to control for some minor violation of distributional assumption.

The independent variables were season (winter and summer) and humidity (%). In addition, we controlled for the possible confounding effect of preoperative CDVA and preoperative SE.

To assess a linear relationship between these independent variables and EI and SI and postoperative SE; humidity was subdivided by applying the 25 th and 75 th percentiles, thereby, defining three subgroups. With this in view, our analysis compared EI, SI, and postoperative SE between winter and summer in general, and between humidity as independent meteorological parameter. A value of P < 0.05 was considered statistically significant.

The LASIK procedure included mechanical flap preparation using an automated microkeratome (SKB, Moria, France) with a single-use 90-μm head. For all eyes, the excimer ablation was performed with an Allegretto excimer laser platform (Eye-Q 200 Hertz (Hz) or 400 Hz, WaveLight GmbH, Erlangen, Germany) under constant eye tracking (250 Hz). To minimize the induced spherical high-order aberration, an aspherical "wavefront-optimized" profile was used with an optical zone of 6.0, 6.5, or 7.0 mm depending on the mesopic pupil diameter and expected residual stromal bed. [22],[23] The manufacturer-recommended "WaveLight myopic astigmatic nomogram" was implemented to compensate for very short or long ablation time and for a cylinder-sphere coupling effect.

Cyclotorsion was minimized using a "NeuroTrack" system (WaveLight GmbH) in which four built-in blinking light sources eliminate cyclotorsion at its source by controlling optokinesis, a neural response of the visual system to use spatial cues to stabilize retinal images.

The laser treatments were performed in nine Care Vision Refractive Centers located in Berlin, Cologne, Frankfurt/ Main, Hamburg, Hanover, Munich, Nuremberg, Stuttgart, and Vienna by 23 experienced refractive surgeons. All surgeons followed a standard protocol of indications and preoperative, intraoperative, and postoperative management written by the first author (TK), and were trained by him in situ. Postoperative medication for LASIK included ofloxacin four times a day for 1 week, and dexamethasone four times a day for the 1 st week, and two times a day for the 2 nd and 3 rd weeks. Preservative-free hyaluronic acid artificial tears (Hylolasop, Ursapharm GmbH, Germany) were applied to all eyes for 1-3 months.


  Results Top


This study comprised 1,052 eyes of 1,052 consecutive myopic patients (419 males and 633 females; mean age at surgery 35.0 ± 9.0 years) with a mean preoperative refractive SE of −3.88 ± 1.85 D. All results reported herein are based on the 1-month (33.0 ± 5.0 days) follow-up data [see [Table 1]. To rule out systematic differences between 200 and 400 Hz lasers; and thus between our centers, a Kruskal-Wallis test was applied and no systematic differences were found. [23] [Table 2] summarizes EI, SI, and postoperative SE (in D) for humidity as independent meteorological parameter displayed by subgroups of quartiles. The cutoffs for grouping are shown in [Table 3]. Our data indicate that there was a significant difference in SI; thereby, suggesting that low humidity was a relative disadvantage. However, neither EI nor postoperative SE was significantly influenced by differing humidity levels.
Table 1: Descriptive statistics

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Table 2: EI, SI, and postoperative SE for independent meteorological parameter (humidity)

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Table 3: Cutoffs for the grouping of independent meteorological parameters

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To provide a thorough analysis of the influence of independent variables within a multivariate framework, robust regression methods were applied; thereby, defining EI, SI, and postoperative SE as dependent variable. The results are summarized in [Table 4], [Table 5] and [Table 6] and [Figure 1] and [Figure 2].
Figure 1: Subgroups of humidity, efficiency index (EI), and safety index (SI) are depicted as box plot with standard deviation

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Figure 2: Attempted versus achieved spherical equivalent (SE) correction at 1 month, displayed by subgroups of humidity

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Table 4: Robust regression with EI as dependent variable

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Table 5: Robust regression with SI as dependent variable

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Table 6: Robust regression with postoperative SE as dependent variable

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The EI of eyes with refractive surgery during summer was 0.023 higher (P = 0.032) compared to those treated during winter; thereby, indicating less efficiency [see [Table 4]. This meant less appropriate postoperative UDVA relative to preoperative CDVA in eyes being treated during summer. Humidity had no significant influence on EI based on robust regression analysis.

The SI of eyes with refractive surgery during summer was slightly lower compared to that of those treated during winter; thereby, resulting in no statistically significant difference [see [Table 5]. Higher humidity resulted in significantly better SI.

Although insignificant (P = 0.054), postoperative SE tended to be better with higher humidity. Independent of humidity, in approximately 81.00% the results were within ± 0.50 D of the attempted correction [see [Figure 2].

However, none of the coefficients on meteorological parameters or their combinations exceed the weight of (+/−) 0.1. No change by more than one line on log Mar scale was thus obtained as one decimal unit (0.1) represented one line on log Mar scale.


  Discussion Top


LASIK is one of the most widely used procedures for surgical correction of refractive errors. [24] The current study explores the outcome of LASIK in myopic eyes and the impact of humidity on the refractive and visual outcome. Although our data show that higher humidity statistically results in higher SI and better postoperative SE, the results also indicate that there is no clinically relevant impact on the actual outcome of LASIK because none of the coefficients on meteorological parameters or their combinations exceed the weight of (+/−) 0.1. Therefore, no change by more than one line on logMar scale is obtained as one decimal unit (0.1) represents one line on logMar scale. This is also confirmed by mean postoperative SE of −0.18 D ± 0.44 D obtained by 1-month follow-up, and the lack of significant differences of EI.

We concluded that our refractive surgical intervention and the procedure in general are highly standardized and independent of 'predictable' external factors, such as the influence of outside humidity. Of course, this conclusion is only valid applying the outside humidity data that were used in the current study. The literature, however, still gives a controversy. Recently, Seider et al., [16] in a study looking at 1-month outcomes of over 200 thousand eyes undergoing LASIK in the UK and Ireland between 2008 and 2011, reported no clinically significant relationship between operating room humidity or temperature and postoperative refraction. In 2004, Walter and Stevenson [17] actually looked at multiple environmental variables and their effect on post myopic LASIK refraction. Interestingly, their conclusions were that 2-week-preoperative mean outdoor relative humidity, procedure room relative humidity, outdoor temperature, and procedure room temperature all were statistically significant factors in predicting the need for enhancement. de Souza et al., [18] also published results on operating room humidity affecting LASIK outcomes, even after being checked as far out as 60 days postoperatively. All these studies emphasize why the question being asked in our article is important.

The operation room conditions are highly standardized and thus, need to be thoroughly controlled daily. That is due to the fact that the excimer laser is daily calibrated based on current room conditions (which, of course, are maintained).

The same LASIK procedure was applied to all eyes of our study. Limited refractive results, if any, due to technical purposes; therefore, should be similar. We applied the Wavelight nomogram recommended by the laser platform manufacturer. All surgeons followed a standard protocol and were senior consultants that had at least 500 LASIK surgeries performed.

The EI of eyes with refractive surgery during summer is 0.023 higher (P = 0.032) compared to that of those treated during winter, thereby indicating less efficiency. This means less appropriate postoperative UDVA relative to preoperative CDVA in eyes being treated during summer. High humidity results in higher SI. This condition is usually related to unfavorably outside conditions, thereby contributing to our assumption that uncomfortable outside conditions may be connected to better visual outcome. This might be due to the fact that less amenable weather may force patient's stay indoors. Specific meteorotropic conditions seem to be connected to minimally less appropriate results. This was suggested by robust regression analysis with SI and postoperative SE.

Of course, we cannot 100% control for the effect of the individual examiner/the set-up; but each patient was treated by the same optometrist pre- and post-operatively (which means individual examination techniques should equally influence the pre- and postop results). However, this is a limitation of almost all refractive studies (moreover, it is even more difficult when the study design is retrospective and a large patient pool is analyzed). Another limitation of our study is the fact that we only account for meteorological conditions on the LASIKday. This has to be interpreted with caution as we can only assume that pre- and postoperative days or weeks were of similar meteorological conditions. Therefore, an important implication in interpreting our data is that we actually account for meteorotropic reaction during the early postsurgical follow-up, which is 1month. Some might argue, we would had better addressed each patient's "risk" at being outside by a prospective study addressing more factors like the influence of smoking or go walkies. However, we see our methodological framework as sound approach, as differences between individuals are hardly to assess when a large patient population is analyzed. Only controlled, prospective studies can circumvent this selection bias.


  Conclusion Top


In the present study, we emphasized on the possible role of outside humidity on the refractive and visual outcome of LASIK in myopic eyes. To conclude, our results indicate that there is no clinically relevant correlation between outside humidity and the actual outcome of LASIK.

To assess meteorotropic reactions following LASIK, further investigations should be initiated evaluating individual risk profiles with the aim of providing the best possible visual acuity and subjective visual quality postoperatively in a stable, precise, and safe manner.

 
  References Top

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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