The tear film is an important contributing factor to the complex optical system of the eye as the tear film–air interface is the first refractive surface through which the light travels to reach the retina. The outermost lipid layer of the tears reduces the breakdown and evaporation of the tear film between each blink thus stabilizing it.[1 2
Dry eye disease (DED) is a condition in which there is a compromise of the quality or production of tears and inflammation of the ocular surface resulting in variable symptoms including ocular fatigue, irritation, burning sensation in the eyes, visual disturbance or fluctuating vision .[3 4 5 6 vision quality. It works on the double pass principle and allows the objective quantification of intraocular scattered light.[7 3 8
The aim of this study was to evaluate the impact of the tear film on quality of vision in normal patients versus those having dry eye disease. The tear film can be affected by various ocular surgical procedures like refractive surgery, hence, evaluating the tear film adequately is essential to provide good postoperative visual outcomes to our patients.[9
Methods
This is a prospective study of 100 eyes to compare the optical quality of the tear film in normal individuals versus patients diagnosed with dry eye disease (evaporative or aqueous deficient). The study was carried out as per the guidelines by the Declaration of Helsinki and was approved by the institutional ethics committee. 50 eyes of 25 patients diagnosed with dry eye disease (group 1) and 50 eyes of 25 normal subjects (group 2) were evaluated for their optical vision quality. Participants in both groups were age and gender matched with best spectacle-corrected visual acuity (BCVA) 6/12 or better with no history of ocular trauma, ocular surgery or systemic illness.
Examinations
The participants underwent visual acuity testing and refraction followed by a detailed slit lamp examination to rule out any other ocular conditions which could confound results. The dry eye evaluation included Schirmer's test (with and without topical anesthesia), tear film break-up time (TBUT) and corneal staining with fluorescein dye. The analysis of visual function was done on the Optical Quality Analysis System (OQAS). The visual quality evaluation was done first and followed by the dry eye evaluation. Patients who were on lubricant eye drops were advised not to instill the medications for at least 2 hours before the tests.
Schirmer's test was performed using the Schirmer's test strip and the results were noted at 5 minutes after placing the strip in the eye. This was followed by the TBUT test by instilling sodium fluorescein dye into the lower fornix, asking the patient to blink and then watch for the first area of break in the tear film which is taken as the tear break up time. 3 readings of TBUT were done to improve the reliability of results. Patients were divided into dry eye and normal subjects based on these tests. Patients with either Schirmer's (without anesthesia) value <10 mm at 5 minutes or TBUT test < 8 seconds were diagnosed as DED.[3
Optical quality analysis
The optical quality of the tear film and visual quality were measured using the Optical Quality Analysis System (OQAS, Visiometrics SL, Spain). Objective refraction of the patient was entered in the system prior to the examination to achieve the best focus. The double- pass technique is based on recording images from a point source after reflection in the retina and a double pass through ocular media.[7 7 Fig. 1 ]. Parameters used to measure the visual quality include Objective Scatter Index (OSI), modulation transfer function (MTF) which were calculated at an artificial pupil size of 4 mm. The tear film analysis program of OQAS records dynamic changes of the objective scatter index (OSI) values and calculates the mean value of the OSI over 20 s and it's standard deviation (Mean OSI). Mean OSI is only due to tear film alterations. This test therefore is useful for analyzing patients who have fluctuating vision and correlating this to their tear film and dry eye related changes.[10 11
Figure 1: Representative image of tear film analysis with OQAS. Objective scatter index value change over a time period and blinks noted. The graph that shows the OSI change over 20 seconds. Upper image is a good tear film, and lower a poor tear film
The test has been shown to have good repeatability and reproducibility for objective measurements of overall optical quality in normal and dry eye patients.[12 13
Results
This study compared the visual quality using the optical quality analysis system (OQAS) in 50 eyes of patients with DED (Group 1) to 50 eyes in patients who are normal (Group 2). The patients were age and gender matched in both groups. The age range for both groups was 25-40 years. There were 35 females and 15 males in both group 1 and 2. There were no contact lens users among the subjects and none of them had undergone any ocular surgery. Detailed slit lamp examination of all 50 subjects was done at screening to look for any ocular infections, lid abnormalities, ocular surface scarring, corneal opacities, non-dry eye-related inflammation or signs of ocular surgery which were exclusion criteria for the study.
In group 1 (DED), all 50 eyes had low TBUT or Schirmer's value or both as defined previously thus classifying them as aqueous deficient or evaporative dry eye. They were also graded as per the severity of disease and any staining or obvious irregularity of the corneal surface was noted. All the patients in group 1 had mild to moderate dry eye disease. 16 of 25 patients in group 1 had vision -related symptoms as well. Other causes for visual complaints were ruled out by a detailed refraction, corneal topography, lenticular and retinal examination. None of the eyes in group 2 control eyes had low TBUT or low Schirmer's values. The average TBUT among the patients in group 1 was 5.5 seconds with a range of 2 to 7 seconds and the average Schirmer's values was 8 mm as compared to an average TBUT 9 of and an average Schirmer's value of 25 in group 2. The ocular surface disease index (OSDI) score was also significantly higher in the group 1 (p < 0.05) compared to the control group. None of the patients had corneal staining within the visual axis or other severe corneal changes like filaments, scarring or thinning which could confound results of the visual quality analysis. The mean OSI of patients in both groups was calculated and was 9.50 in group 1 and 0.69 in group 2. This shows a significantly higher optical scatter in patients with dry eye as compared to the normal eyes (p < 0.05). The fluctuation in vision quality between blinks was also noted and found to be much higher giving a saw tooth like pattern in patients with poor tear film quality. An inverse association was noted between both TBUT and schirmer's values and the OSI [Fig. 2 ]. This implies that DED (aqueous deficiency or evaporative) in a patient is associated with relatively poorer quality of vision and there is also a significant deterioration of quality of vision with worsening DED test values.
Figure 2: Graphs showing the correlation between dry eye test values and the objective scatter index (OSI) demonstrating the negative relation between dry eye test values and OSI. The bar diagram shows a significant association between the dry eye and OSI
Statistical analysis
The analysis of the data was done using MedCalc Version 19.4.1. All parameters were assessed for normality of distribution. Continuous variables were tested using difference in their mean/median. Correlation between the dry eye parameters and those of optical quality were analyzed. Spearman's correlation was done to evaluate the correlation between the dry eye test values (Schirmer's and TBUT) in patients and the optical quality as defined by their OSI values from the OQAS.
Discussion
An adequate assessment of a patient's vision should include a measurement of the quality of vision and not just the visual acuity assessment on high contrast charts like the Snellen's visual acuity chart. A patient who has good visual acuity may still have other vision -related problems including fluctuating vision or loss of contrast. DED patients may have these complaints in addition to the more common symptoms like ocular discomfort, pain or ocular fatigue.[11 14 15
The reason for these visual symptoms is that an unstable tear film results in increased higher order aberrations after the blink and a consequent fluctuation of vision . The person can also complain of glare due to the scattering of light. As they try to accommodate to improve visualization in spite of this poor tear film, they can develop excessive ocular fatigue as well.[16
Poor visual quality can also have an impact on the quality of life of these patients.[17 vision therefore plays an important role in our approach to manage and treat these patients.[18 vision .
In our study we found a good correlation between the values of the dry eye tests and the optical scatter index (OSI). An inverse relation between the 2 implies a worsening of the quality of vision with severity of DED. This is similar to findings shown in other studies which also measured optical quality in patients with fluctuating vision due to dry eye and found that dry eye and a poor ocular surface can significantly contribute to a poorer optical quality in a person.[11 18 19 20 12 13 21 22 23 vision break up time (VBUT) are also useful in following up response to therapy in these patients. As dry eye can occur postoperatively in refractive surgery and cataract surgery as well and is a cause of postoperative dissatisfaction, it is important to adequately assess these patients preoperatively to obtain predictable outcomes.[24 25 26
Conclusion
The quality of vision is an important determinant of a person's quality of life as it has an impact on all our activities. Conditions like DED can affect the quality of vision , hence it is important to measure this when assessing a patient of dry eye if they have significant vision -related symptoms. This could also help in quantifying the severity of the condition and assessing the effect of therapy in ameliorating symptoms and signs. Our study assesses the effect of dry eye disease on the optical quality. The known effect of ocular surgeries especially refractive surgery on the ocular surface further emphasize the need to do a detailed preoperative evaluation of visual quality in these patients for better outcomes.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
1. Lemp MA. Advances in understanding and managing dry eye disease Am J Ophthalmol. 2008;146:350–6e1
2. Montés-Micó R, Alió JL, Muñoz G, Pérez-Santonja JJ, Charman WN. Postblink changes in total and corneal ocular aberrations Ophthalmology. 2004;111:758–67
3. . The definition and classification of dry eye disease: Report of the Definition and Classification Subcommittee of the International Dry Eye Workshop (2007) Ocul Surf. 2007;5:75–92
4. Diaz-Valle D, Arriola-Villalobos P, García-Vidal SE, Sánchez-Pulgarín M, Borrego Sanz L, Gegúndez-Fernández JA, et al Effect of lubricating eyedrops on ocular light scattering as a measure of
vision quality in patients with dry eye J Cataract Refract Surg. 2012;38:1192–7
5. Smith J, Nichols KK, Baldwin EK. Current patterns in the use of diagnostic tests in dry eye evaluation Cornea. 2008;27:656–62
6. Kanellopoulos AJ, Asimellis G. In pursuit of objective dry eye screening clinical techniques Eye Vis (Lond). 2016;3:1
7. Güell JL, Pujol J, Arjona M, Diaz-Douton F, Artal P. Optical quality analysis system; Instrument for objective clinical evaluation of ocular optical quality J Cataract Refract Surg. 2004;30:1598–9
8. Pflugfelder SC. Prevalence, burden, and pharmacoeconomics of dry eye disease Am J Manag Care. 2008;14(Suppl 3):S102–6
9. Li M, Gong L, Chapin WJ, Zhu M. Assessment of
vision -related quality of life in dry eye patients Invest Ophthalmol Vis Sci. 2012;53:5722–7
10. Xu CC, Xue T, Wang QM, Zhou YN, Huang JH, Yu AY. Repeatability and reproducibility of a double-pass optical quality analysis device PLoS One. 2015;10:e0117587
11. Herbaut A, Liang H, Rabut G, Trinh L, Kessal K, Baudouin C, et al Impact of dry eye disease on
vision quality: An optical quality analysis system study Transl Vis Sci Technol. 2018;7:5
12. Hu AL, Qiao LY, Zhang Y, Cai XG, Li L, Wan XH. Reproducibility of optical quality parameters measured at objective and subjective best focuses in a double-pass system Int J Ophthalmol. 2015;8:1043–50
13. Saad A, Saab M, Gatinel D. Repeatability of measurements with a double-pass system J Cataract Refract Surg. 2010;36:28–33
14. Mathews PM, Ramulu PY, Swenor BS, Utine CA, Rubin GS, Akpek EK. Functional impairment of reading in patients with dry eye Br J Ophthalmol. 2017;101:481–6
15. Deschamps N, Ricaud X, Rabut G, Labbé A, Baudouin C, Denoyer A. The impact of dry eye disease on visual performance while driving Am J Ophthalmol. 2013;156:184–9e3
16. Koh S. Mechanisms of visual disturbance in dry eye Cornea. 2016;35(Suppl 1):S83–8
17. Mertzanis P, Abetz L, Rajagopalan K, Espindle D, Chalmers R, Snyder C, et al The relative burden of dry eye in patients' lives: Comparisons to a US normative sample Invest Ophthalmol Vis Sci. 2005;46:46–50
18. Friedman NJ. Impact of dry eye disease and treatment on quality of life Curr Opin Ophthalmol. 2010;21:310–6
19. Uchino M, Schaumberg DA. Dry eye disease: Impact on quality of life and
vision Curr Ophthalmol Rep. 2013;1:51–7
20. Tan CH, Labbé A, Liang Q, Qiao L, Baudouin C, Wan X, et al Dynamic change of optical quality in patients with dry eye disease Invest Ophthalmol Vis Sci. 2015;56:2848–54
21. . Management and therapy of dry eye disease: Report of the Management and Therapy Subcommittee of the International Dry Eye WorkShop (2007) Ocul Surf. 2007;5:163–78
22. Shetty R, Tejal SJ, Shetty A. Optical quality and tear film analysis after various lubricating eye-drops J Vis Sci. 2015;1:1–4
23. Benito A, Pérez GM, Mirabet S, Vilaseca M, Pujol J, Marín JM, et al Objective optical assessment of tear-film quality dynamics in normal and mildly symptomatic dry eyes J Cataract Refract Surg. 2011;37:1481–7
24. Toda I. Dry eye after LASIK Invest Ophthalmol Vis Sci. 2018;59:DES109–15
25. Dohlman TH, Lai EC, Ciralsky JB. Dry eye disease after refractive surgery Int Ophthalmol Clin. 2016;56:101–10
26. De Paiva CS, Chen Z, Koch DD, Hamill MB, Manuel FK, Hassan SS, et al The incidence and risk factors for developing dry eye after myopic LASIK Am J Ophthalmol. 2006;141:438–45
