MNREAD Reading Vision in Adults With Glaucoma Under Mesopic and Photopic Conditions

Abstract

Purpose: Despite good photopic visual acuity, glaucoma patients report difficulty performing daily activities under dim light such as reading. Here we investigated the impact of mesopic lighting conditions on reading vision of glaucoma patients.

Methods: The study design included 39 patients with glaucoma and 40 healthy controls. Reading vision was assessed with MNREAD charts under mesopic (2 cd/m²) and photopic (220 cd/m²) conditions. Four reading indexes: maximum reading speed (MRS), critical print size (CPS), reading acuity (RA), and reading accessibility index (ACC) were obtained from the MNREAD test yielding a plot of reading speed versus print size.

Results: Compared to photopic conditions, reading vision of both healthy controls and glaucoma patients significantly decreased under mesopic conditions (P < 0.05). For glaucoma patients (85% with mild or moderate glaucoma), MRS and ACC decreased by six words per minute and 0.1, respectively under mesopic conditions; CPS and RA increased by 0.25 and 0.18 logMAR, respectively. Moreover, under both photopic and mesopic conditions, reading vision of glaucoma patients was significantly worse than that of healthy controls, but the difference was greater under mesopic conditions (P < 0.05) even after controlling for age and visual acuity.

Conclusions: Mesopic conditions make reading more challenging for both healthy controls and glaucoma patients. However, reading in dim light appears to be more burdensome for glaucoma patients. Mesopic reading tests mediated by both cone and rod photoreceptor systems likely provide a more sensitive and comprehensive assessment of a patient's reading impairment than testing under photopic conditions.

The human visual system deals with a wide range of ambient light levels, encompassing photopic (ranging from 10 to 10⁸ cd/m²), mesopic (0.01 to 3 cd/m²), and scotopic (10⁻⁶ to 10⁻³ cd/m²) light levels.¹ Although cone photoreceptors are responsible for photopic vision, mesopic vision is mediated by both cone and rod photoreceptors.² Visual function such as visual acuity or contrast sensitivity has been shown to be significantly compromised under dim light conditions,³ particularly in older adults and visually impaired individuals.^4,5 For this reason, individuals with visual impairment often report a great deal of difficulty with everyday tasks in low or dim lighting conditions.^6,7 This difficulty becomes more prominent in individuals with rod dysfunction, because mesopic vision is mediated by both cone and rod photoreceptors.^4,8–10 

Glaucoma, a leading cause of irreversible blindness worldwide, is characterized by progressive loss of retinal ganglion cells and associated visual field defects. Although glaucoma mainly involves ganglion cell damage, recent evidence suggests that mesopic vision may be compromised in glaucoma. More specifically, recent studies suggest that rod bipolar cell dysfunction is present in glaucoma and can occur before ganglion cell damage in animal models of glaucoma.^11,12 Additionally, rod loss or dysfunction is implicated in loss of peripheral vision, a common characteristic of glaucomatous damage, primarily because of the higher abundance of rod photoreceptors in the peripheral retina.¹³ 

Furthermore, despite normal photopic visual acuity, patients with early stages of glaucoma often report difficulty in everyday tasks under low or changing luminance conditions including driving, mobility, and reading.^14–23 In particular, reading difficulties have been a major complaint among glaucoma patients,^18,24 with subjective complaints of greater difficulty under low luminance conditions.^18,25,26 For typical text reading, humans rely mostly on foveal vision, where cone photoreceptors are densely packed because this central region of the visual field provides high-acuity vision.^13,27,28 Because the foveal region (i.e., 1.25° in diameter) is free of rod photoreceptors,¹³ the functional consequences of any rod damage are expected to be negligible for tasks involving central vision, even under mesopic conditions. However, the parafoveal region, which covers the central 4° to 5° of the visual field²⁷ and is composed of cones and rods with a sharp increase in rod photoceptor density, is also known to play a critical role in efficient reading behaviors. These include previewing upcoming words, planning optimal saccades, and facilitating subsequent foveal processing.^29–34 Previous research on reading performance has demonstrated that skilled readers of alphabetic writing systems extract letter information extending up to three or four letters to the left of fixation and 14 to 15 letters to the right of fixation,^35–38 supporting the crucial role of the parafoveal vision in the reading process. 

Based on the compilation of these findings, we hypothesize that significant functional deficits may manifest in the early stages of glaucoma under low luminance conditions, aligning with the reported reading difficulties. To the best of our knowledge, most studies examining the functional reading vision of glaucoma patients have focused on photopic conditions.^39–41 Thus the question of whether and to what extent (if any) reading vision indeed deteriorates under mesopic conditions in glaucoma remains to be answered. 

The current study aims to evaluate the effect of mesopic luminance level on the reading vision of patients with primary open-angle glaucoma. Reading vision was assessed using the MNREAD acuity chart (i.e., a continuous text reading acuity test⁴² that has been widely used to evaluate the effects of eye disorders, treatment, and visual rehabilitation on reading vision).^43–47 Reading vision was evaluated based on four MNREAD indices obtained from the MNREAD test: (i) Maximum oral reading speed (MRS)—a person's reading speed when reading is not limited by print size; (ii) Critical print size (CPS)—the smallest print that the person can read with maximum speed; (iii) Reading acuity (RA)—the smallest print that the person can read without making significant errors; (iv) Reading accessibility index (ACC)⁴⁸—the person's access to text across the range of print sizes found in everyday life. By comparing MNREAD reading vision under both mesopic and photopic conditions between glaucoma patients and age-matched healthy controls, here we investigate whether the adverse effects (if any) of mesopic conditions are more prominent in patients with glaucoma. 

Methods

Participants

Glaucoma patients were recruited from the Massachusetts Eye and Ear glaucoma service, and their diagnosis was confirmed through medical records based on both structural (i.e., glaucoma specific changes of the optic nerve and nerve fiber layer defects) and functional (i.e., glaucoma specific visual-field defects) changes. All participants were native or fluent English speakers without known cognitive or neurologic impairments, confirmed by the Mini Mental Status Exam (score ≥25). Exclusion criteria included the following: (i) Diagnosis of age-related macular degeneration, other retinal conditions, optic nerve conditions, or corneal disease; (ii) Diagnosis of diabetic eye disease; (iii) Diagnosis of Alzheimer's disease, Parkinson's disease, brain injury, or other neurological or psychiatric conditions as revealed by the medical record or self-report; (iv) Known dyslexia; (v) Non-native English speakers. Healthy control participants had normal retinal health (i.e., no age-related macular degeneration, glaucoma, or diabetic retinopathy in either eye) and normal binocular vision with no known history of neurologic disease or ocular disease other than cataracts or cataract surgery. 

The experimental protocols followed the tenets of the Declaration of Helsinki and were approved by the Institutional Review Board of Northeastern University. Written informed consent was obtained from all subjects before the experiment and after explanation of the nature and possible consequences of the study. 

Apparatus

The MNREAD test was administered with the MNREAD iPad app (1.2 version; © 2017 University of Minnesota; https://apps.apple.com/us/app/mnread/id1196638274) running on an iPad Air 2 tablet with Retina display (2048 × 1536 pixel resolution at 264 ppi). The iPad was mounted vertically on a stand in landscape mode. For photopic conditions, iPad screen brightness was set to a value of 75 (equivalent to 220 cd/m²). Testing under mesopic screen luminance (2 cd/m²) was carried out by setting the iPad screen brightness to a value of 0 (equivalent to 3.9 cd/m²) in combination with a neutral density filter (Kodak ND 0.30) applied on the screen. The filter had a factor of 2 luminance reduction, and the luminance attenuation of the filter was confirmed with photometric readings from a luminance meter (Minolta LS-110 Luminance Meter; Konica Minolta, Inc., Tokyo, Japan). All the text was black on a uniform white background with a luminance contrast of 99%. The luminance contrast of the text remained unchanged for both mesopic and photopic conditions. 

Study Design and Task Procedure

This cross-sectional study used a within-subject design to compare reading vision between mesopic and photopic conditions for each participant group (i.e., glaucoma and healthy controls). For each participant, the MNREAD test was administered in the following sequence (i.e., photopic-mesopic-mesopic-photopic). This reverse order was used to minimize any potential confounding effects such as fatigue or training effects. For each MNREAD test, a different MNREAD chart was used so that a participant never read the same text more than once. We reported the average value of the two measurements for each viewing condition (i.e., mesopic vs. photopic viewing). 

Participants read binocularly at a viewing distance of 40 cm up to 80 cm. The viewing distance was adjusted to vary the angular size of the font subtended at the eye if needed. We made sure to use an appropriate viewing distance to yield a proper plateau and drop-off point on the MNREAD curve as illustrated in Figure 1A. Each participant's near refractive error was corrected for their testing distance with either their habitual reading glasses or with a trial lens refraction. If they were wearing bifocal or progressive lenses, participants were instructed to use the correct portion of their lens to read the text on the screen. 

Figure 1.

 

(A) Examples of an MNREAD curve (i.e., reading speed as a function of print size) from which the four MNREAD parameters were extracted: MRS, ACC, CPS, and RA. Any impairment in MNREAD reading vision can be represented as a downward or rightward shift of the MNREAD curve (e.g., orange curve vs. blue curve). B to E plot the differences in MNREAD parameter values between mesopic (solid bar) and photopic (open bar) conditions for glaucoma patients (blue color) and healthy controls (orange color). The bar graph indicates the mean value of each MNREAD parameter collapsed across subjects. (B) Maximum reading speed (wpm). (C) Reading accessibility index (D). Critical print size (logMAR). (E) Reading acuity (logMAR). Error bars represent ±1 standard error of the mean (SEM). *** P < 0.001. n.s., no statistical significance. In D and E, the colored crosses with error bars indicate the mean binocular near visual acuities (in logMAR units) under photopic testing condition for either glaucoma patients (blue cross) or healthy controls (orange cross).

Participants performed the reading task in a dimly lit room (approximately 0.3 cd/m²) while seated in a comfortable position and were given on average eight minutes to adapt to this dim lighting condition before the main task. Sentences were presented one at a time, centered on the iPad screen while participants sat at the fixed viewing distance. The MNREAD chart uses sentences of 10 standard word length (60 characters) to determine reading speed across print sizes that decrease logarithmically in steps of 0.1 log units.⁴² 

Like the standard print chart, the iPad app uses the same short sentence and sentence layout (three lines per sentence) with a reduced range of print sizes (14 sentences compared to 19 in the printed version).⁴⁹ Sentences were displayed on three lines in Times Roman font, with print size decreasing in steps of 0.1 log unit. The physical print size ranged from 6.3 M to 0.32 M (in Sloan M notation) or 9.3 mm to 0.5 mm, corresponding angular print size ranges from 1.2 to −0.1 logMAR at a viewing distance of 40 cm.⁴⁹ Sentence presentation was initiated by the experimenter using an external keyboard, and each sentence was displayed instantly. Participants were instructed to read the test sentences aloud as quickly and accurately as possible, beginning with the largest print size and progressing toward the smallest print size that could be read. After each sentence reading, a score screen was displayed, where the experimenter entered the number of errors via the external keyboard (if any had occurred during testing). The testing ended when the print size was so small that the participant could no longer read any words. Once reading was completed, the experimenter ended the trial, and the app recorded the reading time. The same procedure was used for both mesopic and photopic conditions. Note that reading speed was estimated by excluding words that were missed or read incorrectly. If more than 10 errors were made, then reading speed was assumed to be zero. The number of errors was also considered to estimate reading acuity (acuity = 1.4 – (sentences × 0.1) + (errors × 0.01)). 

As shown in Figure 1A, the app displayed the corresponding MNREAD curve of log reading speed as a function of print size, along with the following four MNREAD parameters: MRS, CPS, RA, and ACC. The ACC (a scaled score of 0–1) is defined as the mean reading speed across the 10 largest physical print sizes (e.g., 0.4 to 1.3 logMAR for a viewing distance of 40 cm) on the MNREAD chart, normalized by 200 words per minute (wpm), which is the mean value for a group of normally-sighted young adults.⁴⁸ Thus a value of 1 indicates normal reading performance, whereas a value of 0 means that he/she could not read any of the sentences in the designated range. 

For each participant, contrast sensitivity and distance visual acuity were also obtained for both eyes. Distance visual acuity was measured at 2 meters with Early Treatment Diabetic Retinopathy Study (ETDRS) charts. Contrast sensitivity was measured at 1 meter using Pelli-Robson contrast sensitivity charts. Pelli-Robson contrast sensitivity was scored based on the letter-by-letter method,⁵⁰ whereby each letter correctly identified was scored as 0.05 log units. Binocular near visual acuity was also measured with either trial lenses or habitual near prescription at the set working distance with a near ETDRS chart. The chart luminance for ETDRS visual acuity and Pelli-Robson contrast sensitivity ranged from 120 cd/m² to 145 cd/m². 

Data Analysis

We used a within-group analysis to compare reading vision between mesopic and photopic viewing conditions for both glaucoma and healthy controls. We also performed the between-group analysis after controlling for age and binocular near visual acuity to compare reading vision between glaucoma and healthy controls. More specifically, to examine if there are any significant differences in reading vision (i.e., four MNREAD parameters) between (i) two viewing conditions and between (ii) glaucoma patients and healthy controls, we performed an Analysis of Covariance (ANCOVA) on reading vision – 2 (viewing condition: mesopic and photopic) × 2 (subject group: glaucoma and healthy controls) repeated measures ANCOVA with viewing condition as a within-subject factor, subject group as a between-subject factor, and with age and binocular near visual acuity as covariates. “Reading vision” refers to reading performance indicated by the four MNREAD parameters: MRS, ACC, CPS, and RA. We performed a separate ANCOVA analysis for each MNREAD parameter. To determine which specific level differs from each other while controlling for multiple comparisons, we performed post hoc tests with Bonferroni correction. Statistical analyses and data visualization were performed using IBM SPSS statistics (version 28.0.0.0) and MATLAB R2020b (The MathWorks Inc., Natick, MA, USA). 

Results

A total of 79 subjects participated in this study: 39 patients (17 males, mean age 67 ± 11 years) with primary open-angle glaucoma, and 40 older adults with normal or corrected-to-normal vision (22 males, mean age 62 ± 7 years). According to the Hodapp-Parrish-Anderson glaucoma grading system,⁵¹ the majority of patients (85%, 33 out of 39) had mild or moderate glaucoma: 28 patients (71.8%) had mild VF loss (<−6MD), 5 (12.8%) had moderate VF loss (<−12 MD and ≥−6 MD), and 6 (15.4%) had severe VF loss (≥−12 MD). The severity of glaucoma for each patient was classified by the better seeing eye. The mean deviation of glaucoma patients was −11.09 ± 9.20 dB for the worse eye and −5.99 ± 7.46 dB for the better eye. The monocular distance visual acuity was 0.25 ± 0.26 logMAR for the worse eye and 0.18 ± 0.21 for the better eye. The binocular near visual acuity was 0.11 ± 0.15 logMAR. The mean log contrast sensitivity was 1.32 ± 0.31 for the worse eye and 1.44 ± 0.18 for the better eye. The binocular contrast sensitivity was 1.53 ± 0.19. 

The monocular distance visual acuity of healthy controls was 0.11 ± 0.17 logMAR for the right eye and 0.08 ± 0.l1 logMAR for the left eye. The binocular near visual acuity of healthy controls was 0.06 ± 0.12 logMAR. The mean log contrast sensitivity of healthy controls was 1.51 ± 0.19 for the right eye and 1.57 ± 0.15 for the left eye. Their binocular contrast sensitivity was 1.81 ± 0.18 (Table 1). 

Table 1.

 

Characteristics of Study Participants

 

The Reading Vision of Glaucoma Patients Is Significantly Worse Compared to That of Healthy Controls for Both Photopic and Mesopic Conditions Even After Controlling for Age and Visual Acuity

Figures 1B through 1E plot the average MNREAD parameter values for mesopic and photopic viewing conditions for glaucoma patients and healthy controls. We first found that the reading vision of glaucoma patients noticeably differs from that of healthy controls regardless of the viewing condition. These results were substantiated by the statistically significant main effect of the subject group on reading vision for all MNREAD parameters (P < 0.05) even after adjusting for age and binocular near visual acuity, except for reading acuity (P = 0.763). More specifically, compared to healthy controls, glaucoma patients overall exhibited slower reading speed (by 14 wpm, F_{(1, 75)} = 6.78, P < 0.05), reduced reading accessibility (by 0.1, F_{(1, 75)} = 10.12, P < 0.005), and larger critical print size (by 0.1 logMAR, F_{(1, 75)} = 8.76, P < 0.005) regardless of the viewing condition. 

Reading Vision Is Significantly Worse Under Mesopic Condition for Both Glaucoma Patients and Healthy Controls

As shown in Figures 1B through 1E, we observed that mesopic reading vision was significantly worse than under photopic conditions for all four MNREAD parameters (MRS: F_{(1, 75)} = 9.12, P < 0.005; ACC: F_{(1, 75)} = 135.04, P < 0.001; CPS: F_{(1, 75)} = 236.82, P < 0.001; RA: F_{(1, 75)} = 173.86, P < 0.001). In other words, mesopic conditions led to slower reading speed, reduced reading accessibility, larger critical print size, and worsened reading acuity for both glaucoma and healthy controls. 

Glaucoma patients had slower reading speed (by 6 wpm, F_{(1, 75)} = 14.52, P < 0.001), reduced reading accessibility (by 0.08, F_{(1, 75)} = 107.14, P < 0.001), larger critical print size (by 0.25 logMAR, F_{(1, 75)} = 105.56, P < 0.001), and worse reading acuity (by 0.18 logMAR, F_{(1, 75)} = 82.12, P < 0.001) under mesopic conditions compared to photopic conditions (Table 2). Similarly, healthy controls also had reduced reading accessibility (by 0.05, F_{(1, 75)} = 33.56, P < 0.001), larger critical print size (by 0.27 logMAR, F_{(1, 75)} = 124.29, P < 0.001), and worse reading acuity (by 0.18 logMAR, F_{(1, 75)} = 86.33, P < 0.001) under mesopic conditions compared to photopic conditions. 

Table 2.

 

Mean Values for MNREAD Parameters for Two Viewing Conditions

 

It is also worth noting that photopic critical print size was markedly larger than photopic near visual acuity for both glaucoma patients (by 0.25 logMAR, t₍₃₈₎ = 9.54, P < 0.001) and healthy controls (by 0.16 logMAR, t₍₃₉₎ = 8.40, P < 0.001). These findings are consistent with previous research showing that the print size requirement for reading is much larger than what is expected from visual acuity (Table 2).^52–54 

The Adverse Effect of Mesopic Conditions Is Exacerbated in Glaucoma Patients Compared to Healthy Controls Even After Controlling for Age and Visual Acuity

As shown in Figures 1B through 1E, the difference in reading vision between the two viewing conditions was more prominent for glaucoma patients compared to the difference in healthy controls. This pattern of results was confirmed by the statistically significant interaction effect of subject group and viewing conditions on reading vision: maximum reading speed (F_{(1, 75)} = 5.83, P < 0.05) and reading accessibility (F_{(1, 75)} = 10.54, P < 0.005). It should be noted that these results held true even after controlling for age and binocular near visual acuity. 

More specifically, the reading speed of glaucoma patients decreased by 6 wpm (F_{(1, 75)} = 14.52, P < 0.001) under mesopic conditions while the reading speed of healthy controls remained unchanged (F_{(1, 75)} = 0.14, P = 0.71). Similarly, as shown in Figure 1C, although the reading accessibility index of healthy controls decreased by 0.05 (F_{(1, 75)} = 33.56, P < 0.001), the decrease in glaucoma patients was larger, i.e., 0.08 (F_{(1, 75)} = 107.14, P < 0.001). We, however, did not find any significant interaction effect for critical print size (P = 0.61) and reading acuity (P = 0.94). 

Next, we further examined whether reading vision under mesopic and photopic conditions significantly differs as a function of glaucoma severity (i.e., mild, moderate, and severe glaucoma). As shown in Figure 2, we found a significant main effect of viewing condition on reading vision: maximum reading speed (F_{(1, 34)} = 10.17, P = 0.003), reading accessibility (F_{(1, 34)} = 52.74, P < 0.001), critical print size (F_{(1, 34)} = 38.45, P < 0.001), and reading acuity (F_{(1, 34)} = 27.53, P < 0.001). We, however, did not find any significant main effect of glaucoma severity, nor an interaction effect between glaucoma severity and viewing condition. This may be attributed to the small sample size for moderate (N = 5) and severe (N = 6) glaucoma subgroups. However, our detailed analysis reveals that the adverse effect of mesopic conditions on reading vision is present as early as in the mild stage of glaucoma. 

Figure 2.

 

Reading vision as a function of glaucoma severity for mesopic and photopic conditions. A to D plot the differences in MNREAD parameter values between mesopic (solid bar) and photopic (open bar) conditions for patients with mild (light blue color, N = 28), moderate (dark blue color, N = 5), and severe glaucoma (gray color, N = 6). The bar graph indicates the mean value of each MNREAD parameter collapsed across subjects. (A) Maximum reading speed (wpm). (B) Reading accessibility index. (C) Critical print size (logMAR). (D) Reading acuity (logMAR). Error bars represent ±1 SEM. *** P < 0.001. n.s., no statistical significance.

Discussion

Although relatively little attention has been paid to understanding central vision deficits in glaucoma, converging evidence suggests that even early or moderate stages of glaucoma are associated with noticeable macular retinal damage,^22,55–58 including cell death of RGCs and shrinkage of dendritic structures translating to dysfunction of central vision tasks such as reading and face or object recognition.^{18,39,40,59–67} In line with these findings, our current study demonstrated a significant impairment in reading vision in glaucoma patients in comparison to that of healthy controls, observed under both photopic and mesopic reading conditions. Even after adjusting for age, glaucoma patients exhibited a significant reduction in reading speed — 12 wpm under photopic conditions and 17 wpm under mesopic conditions, surpassing the clinically significant threshold of 10 wpm difference. Our findings further confirm the presence of functional impairment in central vision, even in early and moderate stages of glaucoma. It is noteworthy that the majority (85%) of the patients in our study had mild or moderate glaucoma. 

Although visual function measured under optimal viewing conditions appears to be asymptomatic during early stages of glaucoma (e.g., a photopic luminance setting with an uncluttered background and fully focused), glaucoma patients report difficulties with reading under poor lighting conditions or adapting to different levels of lighting^14,18,22 despite relatively normal visual acuity.^18,24,68,69 Our functional measures of MNREAD reading vision corroborate these subjective complaints. While most functional measurements in the clinic are assessed under optimal photopic conditions, activities of daily living are performed under varying illumination conditions (i.e., mesopic light conditions), including at dusk and in overcast weather conditions. Indoor lighting is often below what is recommended, and studies have shown median home lighting is typically three to four times lower than lighting under clinical testing.⁷⁰ Here we showed that reading vision of both glaucoma patients and healthy controls deteriorates under mesopic conditions with a greater impact on reading vision in glaucoma patients. 

Importantly, the impairment of reading vision when transitioning from photopic to mesopic conditions was even more pronounced in glaucoma patients compared to healthy controls. As shown in Figure 2, our results further confirmed that the adverse effect of mesopic conditions on reading vision is present as early as the mild stages of glaucoma. Under low-luminance conditions, glaucoma patients required at least two times magnification of print size (0.61 vs. 0.36 logMAR) to achieve their maximum reading speed, which was 6 wpm slower than that achieved under photopic conditions. Additionally, their lower reading accessibility index indicates that under dim lighting glaucoma patients experience a further reduced access to everyday print sizes as compared to photopic conditions. Binocular near visual acuity was comparable between glaucoma patients and normal controls (0.11 vs. 0.06 logMAR, P > 0.05) indicating that the reading impairment observed under mesopic conditions is not likely due to any difference in the visual acuity between the two subject groups. 

Although speculative, decreased contrast sensitivity under low luminance conditions may contribute to the additional reading impairment observed under mesopic conditions. Luminance contrast, defined as the difference in intensity between light and dark regions of an image, serves as the fundamental building block of human pattern vision. It is known that contrast information is first encoded by the center and surround receptive field structure of a ganglion cell typically modeled as a difference-of-Gaussian (DOG)^71,72 and further processed in downstream cortical areas. For this reason, contrast sensitivity provides the basis of ophthalmic testing (i.e., standard perimetry) to identify and quantify the pattern of visual field defects resulting from RGC loss or damage in glaucoma. Importantly, it has been shown that the suppressive or inhibitory surround of a ganglion cell becomes noticeably weaker under mesopic conditions,^73–75 likely resulting in inadequate contrast coding under low luminance conditions. As glaucoma primarily involves ganglion cell loss/damage, inadequate contrast coding under low luminance conditions could be more pronounced in glaucomatous vision. Previous studies have shown contrast sensitivity is greatly reduced in glaucoma as compared to age-matched healthy controls,^64,69,76,77 and such impairment is found to be greater under mesopic conditions when compared to photopic conditions.^5,78 In addition, a study by Burton et al.⁶² found that when lowering the contrast of the text itself, glaucoma patients had a greater decrease in reading speed compared to healthy controls (i.e., a median of 20% vs. 11% reduction). Consistent with our prediction, after statistically controlling for binocular contrast sensitivity as a covariate in our model, we found no significant differences in reading vision between glaucoma and healthy controls (all P > 0.1). This underscores the significant role of contrast sensitivity in reading vision, consistent with previous findings.^79–82 Although our current study cannot determine the exact mechanism for the additional impairment experienced in glaucoma patients under mesopic conditions in glaucoma, it is the first to offer objective measures of functional reading deficits in such lighting conditions. 

For both glaucoma patients and healthy controls, the critical print size obtained under photopic conditions was much larger than photopic visual acuity. This finding was consistent with results from previous studies showing print size requirements for reading are often larger than the measured visual acuity.^52–54 The results from a study by Legge and Bigelow found that, even under ideal lighting conditions, there is an optimal range of print sizes (approximately 0.3 to 0.7 logMAR) for fluent reading performance for normally sighted individuals.⁵² Interestingly, as shown in Figure 1E, photopic reading acuities were better than photopic near visual acuity for both glaucoma and control groups, a finding that is consistent with studies of reading with low vision.^43,83,84 For example, a study by Xiong et al.⁸³ has found that visual acuity in healthy older adults was 0.09 logMAR worse than reading acuity, and for non-macular low-vision subjects, visual acuity was worse by 0.11 logMAR. Together, this evidence further underscores that visual acuity alone may not be the best predictor for functional reading vision. 

There are several limitations to our study. First, our methodology involved subjects reading out loud, which might have led to an underestimation of reading speed. We also did not assess sustained reading. Sustained reading has a greater impact on reading fatigue,³⁹ and the burden of mesopic conditions on reading for patients with glaucoma might be more exacerbated with long passage reading. Moreover, we could not rule out the possibility that some of our healthy controls might have had undisclosed or undetected ocular conditions that could have affected our results, such as decreased reading speed and increased critical print sizes. That said, any misclassification would likely have led to an underestimation of the differences between glaucoma patients and healthy controls. Also, the current study only measured photopic visual acuity and contrast sensitivity. It would have been more insightful if we had evaluated changes in both visual acuity and contrast sensitivity between photopic and mesopic conditions and then correlated these changes with the differences observed in reading vision. Furthermore, in our current study, the assessment of reading vision was made solely at a single mesopic luminance level, which approached the upper limit of mesopic conditions. To comprehensively characterize the influence of mesopic conditions on reading vision, a future study is warranted to investigate a range of mesopic light levels. Finally, a future study with a larger sample size should investigate potential variations in mesopic reading deficits among different stages of glaucoma, specifically through subgroup analysis comparing mild, moderate, and severe stages of the disease. 

In summary, our study reveals reduced reading vision under mesopic conditions for individuals with mild to moderate stages of glaucoma and healthy controls, requiring a larger print size for optimal reading for both groups. However, the detrimental effect of dim light is more pronounced for glaucoma patients compared to that in healthy controls, resulting in a larger decrease in maximum reading speed and reading accessibility. Our findings underscore the need for clinical assessments to include a broader range of luminance conditions, enabling a more comprehensive evaluation of the day-to-day reading performance of glaucoma patients. Our findings further advocate for increased education regarding lighting when counseling individuals with glaucoma. 

Acknowledgments

Supported by NIH/NEI Grant R01 EY027857 and Research to Prevent Blindness (RPB) / Lions’ Clubs International Foundation (LCIF) Low Vision Research Award. 

This study was presented as a poster presentation at the 2023 Vision Sciences Society Annual Meeting, May 20, 2023. 

Disclosure: T-L. Goddin, None; H. Yu, None; D.S. Friedman, None; C. Owsley, None; M. Kwon, None