Influence of Corrected Astigmatism on Photopic and Mesopic Visual Acuities in Young Adults

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Influence of Corrected Astigmatism on Photopic and Mesopic Visual Acuities in Young Adults

Pengaruh Astigmatisme Yang Dibetulkan Terhadap Akuiti Penglihatan Fotopik Dan Mesopik Dalam Kalangan Dewasa Muda

PUI JUAN WOI, NAI ZHENG HUONG & WAN NUR AMIRAH IBRAHIM

ABSTRACT

Visual performance testing under mesopic condition is often neglected in optometric examination. The patients sometimes complain of visual disturbances even with the full optical correction under mesopic condition such as night driving. The influence of corrected astigmatism on visual acuity (VA) under mesopic condition still remain unclear. The aim of this study was to investigate the low and high contrast VA under photopic and mesopic conditions in young adults with different severities of corrected astigmatism. Thirty-nine subjects with the mean age 22.31 ± 2.74 years were equally divided into three group based on the severity of astigmatism.

Low and high contrast VA were measured under both photopic and mesopic conditions across three groups of subjects. Low contrast VA was significantly poorer when measured under mesopic condition (p < 0.01). There was no significant interaction between illumination conditions, contrast of acuity chart, and severities of astigmatism, that was, the low contrast VA was poorer under mesopic condition than photopic condition for all severities of astigmatism (p > 0.05). High contrast photopic VA was correlated with other types of VA in astigmatic subjects. In conclusion, the severity of astigmatism does not contribute to the reduction in low contrast VA tested under mesopic condition. This visual performance deterioration could be due to the neural factors that associated with decreasing retinal illuminance.

Keywords: Astigmatism, mesopic, visual acuity, contrast

ABSTRAK

Pengukuran prestasi penglihatan di bawah keadaan mesopik sentiasa diabaikan semasa pemeriksaan mata.

Pesakit akan mengadu bahawa kadang-kala terdapat gangguan penglihatan di bawah keadaan mesopik seperti semasa memandu waktu malam walaupun mereka telah memakai pembetulan refraksi yang optimum. Pengaruh astigmatisme yang telah dibetulkan terhadap akuiti penglihatan (VA) di bawah keadaan mesopik masih kurang jelas. Kajian ini bertujuan untuk mengkaji VA berkontras rendah dan tinggi di bawah keadaan fotopik dan mesopik dalam kalangan dewasa muda dengan tahap astigmatisme yang berbeza. Tiga puluh sembilan subjek dengan min umur 22.31 ± 2.74 tahun telah dibahagikan kepada tiga kumpulan berdasarkan tahap astigmatisme.

VA berkontras rendah dan tinggi telah diukur di bawah keadaan fotopik dan mesopik. VA berkontras rendah didapati lebih lemah apabila diukur di bawah keadaan mesopik (p < 0.01). Keadaan iluminasi, kontras carta VA dan tahap astigmatisme tidak mempunyai interaksi yang signifikan (p > 0.05). VA berkontras rendah lebih lemah di bawah keadaan mesopik untuk semua tahap astigmatisme. VA fotopik berkontras tinggi berkolerasi dengan jenis-jenis VA yang lain. Kesimpulannya, tahap astigmatisme tidak menyumbang kepada kemerosotan VA berkontras rendah di bawah keadaan mesopik. Kemerosotan prestasi penglihatan ini boleh disebabkan oleh faktor saraf yang berkaitan dengan iluminans retina yang berkurang.

Kata kunci: Astigmatisme, mesopik, akuiti penglihatan, kontras

INTRODUCTION

Astigmatism is one of the most common low order aberrations in the human eye and has an effect on vision when uncorrected, even when relatively low amounts of astigmatism are present (Miller et al.

1997; Wolffsohn et al. 2011). There are between 47%

and 51% of patients presenting for clinical care having at least 0.75 D astigmatism in one eye, while around 35% have at least 1.00 D of astigmatism in one eye (Luensmann et al. 2018;

Young et al. 2011). In clinical practice, astigmatism is usually corrected by using cylindrical optical lenses or toric contact lenses.

Astigmatic patients have been assumed that they are adapted to the distortion produced by their natural astigmatism (Vinas et al. 2013). However, the extent to which astigmatic patients are adapted to their own astigmatism and recalibrate upon correction of their astigmatism, and the extent to what these perceptual changes affect visual performance is still not well known (de Gracia et al.

2011; Vinas et al. 2012).

Visual acuity (VA) measurement serves as the quick yet effective procedure to allow eye care practitioners to know about the visual status of their patients. The standard clinical VA measurement for individual with or without correction is commonly performed under photopic condition using a high contrast visual acuity chart (Elliott 2013). However, the standard clinical VA test is unable to predict the visual status under mesopic condition (Haegerstrom-Portnoy et al.

2000). Visual-related activities under low light or mesopic condition is unavoidable in daily life such as conditions during night time and weather conditions like rain or fog. Under mesopic conditions, standard high contrast VA is less important to overall visual function than the ability to discriminate low contrasts (Owsley 1994).

Spectacles or contact lens correction is commonly being prescribed based on the optometric examination conducted under photopic condition. Visual performance testing under mesopic condition is not a common practice during optometric examination procedures and thus tend to be neglected. Previous study has shown that fully corrected myopes show a significant decline in mesopic acuity compared to emmetropes. A decreased retinal illumination can result a reduction in visual acuity of corrected myopic subjects (Coletta et al. 2007). To date, the mesopic VA of participants with corrected astigmatism is still remain unclear.

The purpose of this study was to compare and investigate the low and high contrast VA under photopic and mesopic conditions in young adults with different severities of corrected astigmatism.

MATERIALS AND METHODS

SUBJECTS

This cross-sectional study was conducted at the Optometry Centre of UCSI University in Kuala Lumpur. Thirty-nine young adults (mean age=

22.31 ± 2.74 years) were recruited from the general campus community and the surrounding general population through purposive sampling method.

Only subjects with refractive astigmatism equal to or greater than 0.50 cylindrical dioptre (DC) were included. The subjects were categorised into three groups based on the severity of their astigmatism.

Subjects with astigmatism less than 1.00 DC on both eyes were defined as low astigmatism group (n= 13, mean age= 22.54 ± 1.94 years); subjects with astigmatism between 1.00 to 2.00 DC on both eyes were defined as moderate astigmatism group (n= 13, mean age= 22.15 ± 0.69 years); subjects with astigmatism greater than 2.00 DC on both eyes were defined as high astigmatism group (n=

13, mean age= 22.23 ± 4.39 years). Table 1 details subject demographics and refractive characteristics.

All subjects underwent complete ocular health examinations to ensure that no ocular pathologies or binocular anomalies were present. None of them had any history of systemic diseases, history of ocular or refractive surgery, history of ocular injury or medication with known ocular involvement. All subjects wore their full refractive correction, with corrected distance VA of 0.10 logarithm of the minimum angle of resolution (logMAR) or better in each eye.

ETHICS STATEMENT

Written consent was obtained from all subjects before the start of any data collection. The procedures complied with the Declaration of Helsinki and were approved by the Institutional Ethics Committee of UCSI University (IEC-2019- FMHS-017).

PROCEDURES

Table 1 details subject demographics and refractive characteristics. All subjects were tested binocularly and were fully corrected during the tests. Both low and high contrast VA were measured with an illuminated ETDRS chart with adjustable luminance levels (ESC2000 ETDRS Illuminated Cabinet, Good-Lite, US) at a test distance of 3 m.

The luminance level of the chart was set at 160 cd/m² for high contrast VA while the luminance level of the chart was set at 3 cd/m² for low contrast VA.

Low and high contrast VA were measured under photopic and mesopic lighting conditions.

An ETDRS chart was randomly selected from three ETDRS charts for each VA measurement to avoid memorization. The ambient lighting was provided by a source of light adjusted to study conditions and was measured using a lux meter (LT10, Extech, US). For photopic condition, the lighting was adjusted to 500 lux (Tidbury et al. 2016). After the measurement of low and high contrast VA under photopic condition, the lighting was adjusted to 28 lux (Nor et al. 2013) for achieving mesopic condition. Low and high contrast VA were measured again under mesopic condition. Subjects were seated in the room for 10 minutes of adaptation period before each VA measurement

(Asgari et al. 2018; Hiraoka et al. 2015; Lin et al.

2015).

STATISTICAL ANALYSIS

Data were entered and analysed using SPSS Statistics version 26.0 (IBM, Armonk, New York, United States) at 95% confidence interval. The normality test was conducted using a Shapiro- Wilk test. Mixed analysis of variance was performed to compare the mean differences of VA between three groups of participants with low and high contrast charts under photopic and mesopic conditions. Pearson correlation was performed to determine the correlation between photopic high contrast VA with photopic low contrast VA, mesopic high contrast VA, and mesopic low contrast VA. A p-value less than 0.05 was considered statistically significant.

RESULTS AND DISCUSSION

Table 1 shows mean values of low and high contrast VA in three different severities of

astigmatism groups. All data were normally distributed based on the result of Shapiro-Wilk test.

In Figure 1 and Table 2, the VA under photopic condition was significantly better than VA under mesopic condition for both low and high contrast VA [F (1, 36) = 90.78, p < 0.01]. There was no significant interaction between illumination and severity of astigmatism on VA [F (2, 36) = 0.11, p

= 0.90]. The low contrast VA was significantly poorer than high contrast VA under both illuminations [F (1, 36) = 150.21, p < 0.01]

(Figure 1 and Table 2). There was no significant interaction between contrast of acuity chart and severity of astigmatism on VA [F (2, 36) = 0.61, p

= 0.55]. There was a significant interaction between illumination and contrast of acuity chart on VA, that was, the low contrast VA was significantly poorer when measured under mesopic condition [F (1, 36) = 113.31, p < 0.01]. However, there was no significant main effect between illumination, contrast of acuity chart, and severity of astigmatism, that was, the low contrast VA was poorer when measured under mesopic condition, regardless of severity of astigmatism [F (2, 36) = 1.86, p = 0.17] (Figure 1 and Table 2).

TABLE 1 Subject demographics, refractive characteristics, and mean values of VA in low, moderate and high astigmatism groups

Parameters Low astigmatism Moderate astigmatism High astigmatism

Subjects, n 13 13 13

Age, mean ± SD (years) 22.54 ± 1.94 22.15 ± 0.69 22.23 ± 4.39

Cylindrical dioptre on right eyes, mean ± SD (DC)

-0.62 ± 0.13 -1.21 ± 0.29 -2.92 ± 0.77

Cylindrical dioptre on left eyes, mean ± SD (DC)

-0.63 ± 0.17 -1.25 ± 0.25 -2.77 ± 0.74

Photopic high contrast VA, mean ± SD (logMAR)

-0.11 ± 0.06 -0.07 ± 0.04 0.02 ± 0.05

Photopic low contrast VA, mean ± SD (logMAR)

-0.11 ± 0.06 -0.06 ± 0.04 0.04 ± 0.06

Mesopic high contrast VA, mean ± SD (logMAR)

-0.09 ± 0.08 -0.05 ± 0.05 0.05 ± 0.05

Mesopic low contrast VA, mean ± SD (logMAR)

0.02 ± 0.10 0.07 ± 0.08 0.14 ± 0.05

TABLE 2 Result of mixed analysis of variance

Variables F-value p-value

Illumination 90.78 <0.01

Illumination  Astigmatism 0.11 0.90

Contrast 150.21 <0.01

Contrast  Astigmatism 0.61 0.55

Illumination  Contrast 113.31 <0.01 Illumination  Contrast  Astigmatism 1.86 0.17

FIGURE 1 Mean low and high contrast VA under photopic and mesopic conditions in three severities of astigmatism groups

The mean differences between low contrast VA under mesopic condition and standard high contrast VA under photopic condition were 0.13 logMAR in low astigmatism group, 0.14 logMAR in moderate astigmatism group, and 0.16 logMAR in high astigmatism group respectively. These mean differences of VA which approximately 6 to 8 letters worse on the VA chart were clinically

significance as well. All VA values in logMAR were converted into MAR for correlation analysis.

Strong positive correlations were found between photopic high contrast VA with photopic low contrast VA (r = 0.95, p < 0.01), mesopic high contrast VA (r = 0.83, p < 0.01) and mesopic low contrast VA (r = 0.72, p < 0.01) in all participants (Figure 2).

FIGURE 2 Correlations between photopic high contrast VA with photopic low contrast VA, mesopic high contrast VA, and mesopic low contrast VA

This study contributes to understanding the functional effect of astigmatism on low contrast VA under mesopic condition, and highlights the importance of assessing mesopic VA during optometric examination. To the best of our knowledge, this is the first study to investigate the photopic and mesopic VA using the subjects with corrected astigmatism. In this study, we demonstrated that low and high contrast VA significantly deteriorated under mesopic condition compared to photopic condition in all types of severity of astigmatic subjects. Our results are in agreement with the study done by Lin et al. (2015).

Monocular VA in normal subjects were assessed at one photopic and three mesopic condition levels with an E-ETDRS testing system in their study.

Several factors were investigated as confounding factors to the decrease in VA at mesopic light levels, including accommodative error, pupil size, and higher-order aberrations. They conclude that light level contributes the most to the reduction in VA tested under mesopic conditions. However, the refractive error status was not stated in their study.

It has been shown that a myopic shift can occur under mesopic conditions (Leibowitz and Owens 1978; McBrien and Millodot 1987). This myopic shift might contribute to the reduction in VA under mesopic condition in our study. Hiraoka and colleagues (2015) compared the VA between photopic and mesopic conditions using optotype system with myopic shift correction. VA was significantly decreased in mesopic condition compared with photopic condition even after myopic shift correction. Their result suggests that factors other than myopic shift influence VA under mesopic condition. It has been postulated that the reduction in VA under mesopic condition is due to the neural factors that associated with decreasing retinal illuminance, instead of optical blur from myopic shift (Arumi et al. 1997; Johnson 1976).

In this study, low contrast VA under mesopic condition in young adults was significantly poorer regardless of severity of corrected astigmatism. The differences between low and high contrast VA were more than one line acuity (> 0.1 logMAR) under mesopic condition in all severities of astigmatism. This indicates that the optimum optical correction is not sufficient for viewing a low contrast object under mesopic condition.

Mesopic vision relates to the light level of the environment is between photopic and scotopic vision regime. Both rod and cone cells are activated in mesopic vision (Kang et al. 2009). Under cone vision, gain control mechanisms maintain perceptual constancy for contrast over a wide range of illumination levels, following Weber’s law and avoiding response saturation (Shapley and Enroth- Cugell 1984). On the contrary, rod vision favours sensitivity, therefore abandons gain control

mechanisms and follows the deVries–Rose law:

increment threshold is proportional to the square root of the background illumination (Rose 1948).

Mesopic vision is a combination of cone and rod vision, therefore it follows a pattern in between deVries–Rose and Weber behaviour. Taken together, this can explain the result of deterioration in low contrast VA under mesopic condition.

Positive linear relationship was found between photopic high contrast VA with photopic low contrast VA, mesopic high contrast VA and mesopic low contrast VA in corrected astigmatic subjects. This study postulates that the photopic high contrast VA, which is independent of the severity of astigmatism, is strongly correlated with other types of VA in astigmatic patients. If the photopic high contrast VA is poor, the other types of VA will be negatively affected regardless of the severity of astigmatism. Therefore, it is vital to fully correct the photopic high contrast VA, which is the standard optometric examination procedure in order to optimize the visual performance which involves low contrast target or under mesopic condition, such as when driving at night.

The study findings need to be considered in light of its strengths and limitations. Strengths include the assessment of participants with true, rather than simulated, astigmatism and each subject was having similar dioptres of astigmatism in both eyes. Limitations of this study include a relatively small sample size and higher order aberrations which were not measured. Future studies need to explore these effects for a larger sample size and the higher order aberrations should be taken into consideration. The age range is worth to be extended as the visual performance might be more affected due to the decline of higher visual areas during ageing process (Woi et al. 2016).

CONCLUSION

In conclusion, this study demonstrated that low contrast VA under mesopic condition in young adults is significantly poorer regardless of severity of corrected astigmatism. The dioptric value of astigmatism does not contribute to the reduction in low contrast VA tested under mesopic condition. Neural factors that associated with decreasing retinal illuminance could be counted for this deterioration in low contrast VA under mesopic condition. The optimum optical correction is not able to secure a good VA under mesopic condition, especially with low contrast target. The quality of mesopic vision is often ignored or neglected. Our results warrant further investigation on mesopic functional vision for patient care.

ACKNOWLEDGEMENT

The authors received no financial support for this research. Special thanks are extended to all the participants who contributed to this research. No potential conflict of interest relevant to this article was reported.

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Pui Juan Woi

Optometry and Vision Sciences Programme Center for Community Health Studies Faculty of Health Sciences

Universiti Kebangsaan Malaysia, Kuala Lumpur Campus Jalan Raja Muda Abdul Aziz

50300 Kuala lumpur, Malaysia Nai Zheng Huong

School of Optometry

Faculty of Medicine and Health Sciences UCSI University, Kuala Lumpur Campus Jalan Puncak Menara Gading

56000 Cheras, Kuala Lumpur, Malaysia Wan Nur Amirah Ibrahim

Centre of Optometry Studies Faculty of Health Sciences Universiti Teknologi MARA

Selangor Branch, Puncak Alam Campus 42300 Selangor, Malaysia

Corresponding author: Pui Juan Woi E-mail: woipj@ukm.edu.my Tel: +60169334657

Fax: +60326910488 Received: 31 August 2021 Revised: 4 October 2021

Accepted for publication: 12 October 2021