1. Introduction
Visual acuity is the measure of the spatial resolution of the eye (the clarity or sharpness of vision) and it determines the highest spatial frequency or smallest detail that the eye can perceive at high levels of contrast. However, visual acuity is measured on a fixed target (optotype) and it may be the basic assessment of vision but does not fully meet the daily requirements of the human visual functions, which comes into contact with a variety of stimuli of varying intensity. The purpose of a visual acuity test is to determine the patient’s optical performance or sharpness of vision (1). Contrast sensitivity, which is the ability of the eye to detect small changes in illumination at targets that do not have clearly defined limits, defines the threshold between visible and non-visible, which has both elementary and clinical significance in the science of vision (1). It is just as important and accepted as complementary to visual acuity as it reflects the quality of vision. It is a well-known fact that refractive errors decrease the contrast sensitivity, so in cases where visual acuity is 6/6, a person may not be happy with the quality of his vision and still may complain of decreased vision due to low contrast sensitivity. Therefore, this study was conducted to evaluate the changes in contrast sensitivity in relation to myopia using the Pelli–Robson chart.
2. Research elaborations
2.1. Methodology
A total of 500 patients with myopia who came to Ophthalmology OPD of a tertiary healthcare center were studied for a period of 1 year, i.e., from 1 June 2021 to 1 June 2022. Only patients who were using glasses with the best corrected visual acuity of 6/6, for the correction of myopia without any retinal pathology, were included in the study. Patients of both genders in the age group of 16–70 years, who were literate, were included after receiving informed written consent. The study was started after the review and approval of the protocol of study by the Institutional Ethical Committee and Research Committee. Contrast sensitivity was recorded by the Pelli–Robson chart (Figure 1) with respect to visual acuity by the Snellen chart. The refraction was recorded on the basis of automated refraction, subjective test, and post-mydriatic test (PMT). The data included degree, duration of myopia (which was recorded based on the history and duration of chief complaints), use of spectacles, profession, and the degree of astigmatism and were collected in the form of an excel sheet. The Pelli–Robson chart consists of letters of equal size, in sixteen triplets, with a decrease in brightness of 0.15 log units per triplet. The Pelli–Robson chart measures contrast sensitivity thresholds ranging from 100 to 0.56% and is a simple, easy-to-use, low-cost test with a strong test–retest repeatability. The chart is wall mounted at one meter from the person to be examined and the letter size is 4.9 × 4.9 cm and consists of eight rows of letters (Figure 2). Table 1 shows the contrast sensitivity grouping. The Pelli–Robson chart is easy to use as it resembles the visual acuity measurement that most patients are familiar with; it is fast with good repeatability, as shown in Figure 3. Contrast sensitivity is determined by the last triplet letter in which the patient should be able to read at least two. The degree of myopia is shown in Table 2. A value of less than 3D corresponds to low myopia, that of equal to or more than 3D and less than 6D is moderate myopia, and that of more than 6D corresponds to high myopia.
Figure 1. Pelli–Robson chart.
Figure 2. Contrast sensitivity testing by Pelli–Robson chart.
Table 1. Grouping of contrast sensitivity.
Figure 3. Showing combined parameters and their frequency percentage.
Table 2. Grading of myopia.
3. Results
Refractive error is a known cause of decline in contrast sensitivity in patients. We studied factors such as age, gender, profession, degree of myopia, duration of myopia, and astigmatism, affecting the decline in contrast sensitivity of myopic patients.
Patients were studied in three age groups, namely, under 30 years (233, 46.6% patients), 30–60 years (159, 31.8% patients), and over 60 years (108, 21.6% patients), as shown in Table 3 and Figure 4). It was found that a mild decrease in contrast sensitivity was present in all age groups (81.5, 80.5, and 86.1%, respectively).
Table 3. Showing the degree of contrast sensitivity in various parameters of the study, with the chi-square values and their significance in the study.
Figure 4. Age and severity of contrast sensitivity.
A male-to-female ratio was 1:1.76. Notably, 89.5% (162 patients) of males and 73.9% (249 patients) of females showed a mild decline in contrast sensitivity (Table 3 and Figure 5). A severe decline was seen only in 10 (3.1%) females.
Figure 5. Sex and severity of contrast sensitivity.
Patients were studied in 3 subgroups with durations of myopia of (Figure 6) <12 months (414 patients), <13–24 months (31 patients), and ≥25 months (55 patients). A mild decline in contrast sensitivity was noted in all subgroups (82.9, 64.5, and 87.3%, respectively). A total of 10 (2.4%) cases had a severe decline in contrast sensitivity but these patients had myopia with a duration of > 12 years.
Figure 6. Duration of myopia and severity of contrast sensitivity.
The females showing a severe decline in contrast sensitivity also had myopia of high grade with a duration of > 12 months. Therefore, a definitive relation of CS and gender and duration is difficult to establish unless we keep one of these variables constant.
We took into account various professions for comparison with contrast sensitivity such as students (269 cases, 53.8%), engineers (56 cases, 11.2%), managers (31 cases, 6.2%), doctors (29 cases, 5.8%), housewives (25 cases, 5%), athletes (31 cases, 6.2%), drivers (29 cases, 5.8%), and laborers (30 cases, 6%). We found a mild decline in contrast sensitivity in each of the above professions, where the maximum number of cases with a mild decrease was found in students (213 patients, 79.2%) (Table 3) (Figure 7).
Figure 7. Profession and severity of contrast sensitivity.
All three grades of myopia showed a mild decline in contrast sensitivity (Figure 8), i.e., in low myopia, 312 (88.4%) cases, in moderate myopia, 46 (73%) patients, and in high myopia, 53 (63.1%) cases. A severe decline in contrast sensitivity was seen only in patients with high myopia of more than 6D (10 patients, 11.9%). This shows that as the grade of myopia increases, we see a decline in contrast sensitivity.
Figure 8. Grade of myopia and severity of contrast sensitivity.
A mild decrease in contrast sensitivity was seen in all 3 categories of astigmatism, i.e., 62.3% in compound myopic astigmatism, followed by 28.2% in simple myopic astigmatism and 9.5% in simple myopia. Of note, 3.1% (256, 116, and 39, respectively) of patients with severe loss of contrast sensitivity were found to be having compound myopic astigmatism (Table 3 and Figure 9).
Figure 9. Astigmatism and severity of contrast sensitivity.
4. Discussion
A vision of the patient is not just the visual acuity. Many parameters affect the visual potential of a person. Visual acuity, color vision, and contrast sensitivity work hand-in-hand for providing good vision to a person (1).To improve the quality of life of a patient with refractive error, sharpness vision plays an important role (2–6). Myopia affecting an estimated 22.9% of the world’s population or 1.406 billion people has become a worldwide public health issue (7). Prescribing just glasses to a myopic patient does not always lead to patient satisfaction. We compared contrast sensitivity in myopic patients with demographic profile, i.e., age, sex, occupation, and degree and duration of myopia. We included patients in the age group of 16–70 years. This was similar to the study conducted by Zhouyue Li, Yin Hu et al. (3) which had patients in the age group of 20–70 years. A mild decline in contrast sensitivity was seen in all the age groups (81.5, 80.5, and 86.1%), respectively. A severe decline in CS was seen only in 10 (2.1%) patients. Therefore, a decline in contrast sensitivity was seen in all age groups. However, an incremental relation was not seen. However, many other studies (4, 6, 8) indicated that functions of contrast sensitivity were directly proportional to increasing age and are more severely affected in the old age group of >69 years. This contrast with other studies can be attributed to a smaller sample size (92 patients) in these studies (9) as compared to ours (500 patients). In addition, the age distribution was more toward younger patients (46.6%) in our data in contrast to older patients in data collected by other studies. We had a male-to-female ratio of 1:1.76, which was almost similar to other studies (8, 10) with a sex ratio of 1:1. Notably, 78.1% of females and 89.5% of males had a mild decrease in contrast sensitivity. We could not comment on gender dependence of contrast sensitivity because of the variable distribution of grade and duration of myopia in both genders. To comment on definitive correlation of gender and contrast sensitivity, we need studies with constant grade and duration of myopia. We studied the effect of profession on contrast sensitivity, which to date has not been mentioned in any other study. (10) We noted that professions included by us (students, engineers, athletes, managers, doctors, housewives, laborers, and drivers) had a mild degree of decline in contrast sensitivity. Students being majority (269 cases, 53.8%) had all 3 grades of decline in contrast sensitivity. It can be attributed to long hours dedicated to laptops and mobile screens during the lockdown phase due to online classes. An increased dependence on mobiles and laptops in every aspect of life such as work from home, buying supplies online, business transactions, and bill payments, compared to the pre-COVID era has contributed to contrast sensitivity being affected as well. Therefore, not only the people involved in the IT sector but also every professional, including housewives, showed a mild decline in contrast sensitivity. These unprecedented conditions of the COVID-19 pandemic have made digital media interface a necessary evil. Therefore, the relationship of contrast sensitivity with the profession of the patient should be explored in the future.
By comparing contrast sensitivity with the degree of myopia, (9, 11) it was interpreted that the majority of them were low myopia and had a mild decrease in contrast sensitivity (312 cases, 88.4%), and in the high myopic group, cases of severe decline in contrast sensitivity were very less (11.9%). We can infer that grade of myopia shows a positive correlation with contrast sensitivity. The duration of myopia was not a significant factor because the time of diagnosis and presence of myopia can be different in the Indian population because of social stigma related to the use of glasses. This was in conflict with the results of the study conducted by Bistra Stoimenova, (12) which suggested that contrast sensitivity is negatively related to the degree and duration of myopia. He studied 60 myopes and showed that 89% of subjects with myopia of more than 10 years had severe decline in contrast functions (9, 16) This contrast in our studies can be attributed to a larger sample size of our study (500 patients) as compared to the aforementioned study (12) (60 patients). Contrast sensitivity and astigmatism were found to have significant relation in our research.
A severe decline in contrast sensitivity was seen in compound myopic astigmatism in 10 (3.1%) cases, whereas maximum (78.5%) cases belonged to mild decline in contrast sensitivity, showing a direct relationship between astigmatism and contrast sensitivity. This was similar to the study conducted by Yumi Hasegawa et al. (13) on 12 emmetropic volunteers, which also suggested that astigmatism deteriorates contrast sensitivity depending on the amount of astigmatic power. The only difference between the studies was that we did not have any comparison group of emmetropic patients.
5. Conclusion
We concluded from our study that out of 500 patients, there were 82.2% of patients who had a mild decline in contrast sensitivity followed by 15.8% of patients having a moderate decline in contrast sensitivity, whereas only 2% accounted for severe decline in contrast sensitivity. We saw that age and contrast sensitivity are not linearly related. The relationship between gender and duration of myopia is non-definitive because of the variability of parameters. A need for more streamlined studies is reflected in our research. The degree of myopia and astigmatism are positive risk factors for decline in contrast sensitivity. Occupation of the patient had emerged as a new parameter post-COVID era affecting contrast sensitivity, which should be explored more thoroughly. Therefore, the crux of the study is that despite having fully corrected refractive error with a visual acuity of 6/6, myopic patients showed reduced contrast sensitivity, even without any retinal pathology, making it an essential part of a routine ophthalmic examination.
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