Evaluating Asthenopia and Binocular Vision Parameters in Diamond Polishing Workers, The Mumbai Eye Disease Study (MEDiS) 3

Abstract

Purpose:

Investigate the prevalence of asthenopia and binocular vision abnormalities (BVA) among diamond polishers in Mumbai, India.

Methods:

The data was collected from eye screening in people working in one of the diamond industries in Mumbai, India. This retrospective cross-sectional study included only diamond polishers aged 18–35 with uncorrected 20/20 distance vision and N6 near vision. Others working in the diamond industry were excluded. Comprehensive eye screening included measuring Snellen acuity, refraction, and binocular vision tests. A CISS (Convergence Insufficiency Symptoms Survey) questionnaire was administered to assess binocular vision parameters and asthenopia.

Results:

In the screening cohort of 1,093 diamond industry screened workers, 66 people engaged in polishing were included in the study. The prevalence of BVA was 77.3%; stereo acuity abnormalities were the most common (63.6%), followed by accommodative insufficiency (43.9%). No significant association was found between CISS scores with age, work hours, or experience, although 10.6% of participants had CISS scores above 21. Age was significantly correlated with the near point of accommodation (P < 0.001), and older participants (26–35 years) exhibited higher fusional vergence abnormalities though not statistically significant.

Conclusion:

The high prevalence of binocular vision abnormalities among diamond polishers underscores the need for targeted interventions to reduce visual fatigue and improve working conditions in this high-demand occupation. Further studies are required to suggest corrective measures to enhance eye health and work productivity.

INTRODUCTION

There is a strong link between near work-related asthenopia and accommodative disorders.[1] Subjective and objective studies on accommodative adaptation have shown that periods of intense near work can cause blurred vision due to the slowing of accommodative dynamics at near and far distances.[2] Asthenopia is a common eye complaint that can interfere with day-to-day work. Asthenopia occurs due to several causes, including uncorrected refractive errors, accommodative dysfunctions, vergence anomalies, inappropriate lighting conditions, extraocular muscle imbalance, and compromised image quality due to poor contrast.[3] These are more evident in predominantly near-vision occupations. Diamond polishing is one such occupation.

India is the world’s largest center for cutting and polishing diamonds, accounting for over 90 to 95% of the world’s polished diamond consumption (The Economic Times, May 2024). In addition to long hours of near work, the diamond polishing workers in India work in poor conditions (suboptimal lighting and ventilation) and improper work habits (prolonged work exposure, sitting in non-optimal posture). High concentration and continuous looking at a fixed object can contribute to binocular vision abnormalities (BVA), impacting productivity and work outcomes. The visual demands of jeweler manufacturing workers are like those of people engaged in computer-related work.[3] Both groups of professionals develop symptoms of computer vision syndrome (CVS). However, the earlier report from India did not analyze the accommodative parameters in the jeweler workers.[3]

The present study investigated the relationship between the extent of near work, dynamic accommodation, and asthenopia in the diamond polishers in one of the industrial hubs in Mumbai, India.

METHODOLOGY

It was a retrospective analysis of a prospectively collected cross-sectional study conducted at a diamond polishing industry in Mumbai, India. While we examined a larger section of people working in the factory as part of general eye screening, we specifically selected people engaged in actual diamond polishing work. We followed the tenets of the Declaration of Helsinki and the study was approved by our Institutional Ethics Committee. We excluded workers who performed tasks other than diamond polishing such as managers and designers.

The inclusion criteria were: (1) Age younger than 35 years; (2) uncorrected visual acuity (UCVA) 20/20 for distance and N6 for near vision in each eye; (3) People with low refractive error (myopia/hypermetropia <0.5 DS), but not wearing spectacles. The exclusion criteria were: (1) Refractive error - uncorrected ametropes, amblyopes, (2) history of any ocular surgery, (3) nystagmus, (4) people with manifest strabismus, (5) monocular subjects, (6) other ocular pathology (congenital anomalies, retinal pathologies, active ocular inflammation), and (7) people who were unable to understand the tests [Figure 1].

Figure 1.

Methodology of examination of the patients at the diamond workers factory. Abbreviations: IOP: Intraocular pressure, NPA: Near point of accommodation, NPC: Near point of convergence, NFV: Negative fusional vergence, PFV: Positive fusional vergence, MEM: Monoocular estimation method

Each participant received a comprehensive eye screening. This included measuring distance vision (on a Snellen chart placed at 6 m under good lighting conditions − 1000 lux), near vision (handheld N notation reading chart), automated refraction (InstaRef 2.0, Remidio, Bangalore, India), intraocular pressure (IOP) using the Pulsair IntelliPuff Tonometer (Keeler, Windsor, UK), and undilated fundus imaging using the handheld Fundus on Phone device (Remidio, Bangalore, India). One trained and qualified optometrist performed specific binocular vision tests. These included Stereoacuity, near point of accommodation (NPA), near point of convergence (NPC), negative fusional vergence (NFV), positive fusional vergence (PFV), monocular estimation method, cover and uncover test, ocular motility, accommodative flippers (AF), and vergence flippers (VF).

The procedures for each test were explained to the participants and demonstrated as necessary to ensure proper understanding and performance of the tests. For the near point of accommodation, Hofstetter’s formula was used to calculate the normative value for everyone.[4] All other binocular vision parameters were compared with the Schieman and Wick normative data.[5]

Definitions of parameters and testing

• Stereo Acuity was assessed with Randot stereo plates (Stereo Optical Company, Inc., Chicago, USA). Participants wore stereo goggles over the corrective lenses, if any. The stereo book was held at 40 cm with light shining (1000 lux). Participants kept their heads straight, ensured proper polarization alignment, and reported what the visualized in the vectogram. If no suppression was present, they identified the Wirt circles that appeared to pop out. Streoacuity was recorded in arc seconds. If there was a mistake on the first plate, but the next one was correct, the missed one was rechecked. Expected value: 40 sec of arc or better.

• Near Point Accommodation (NPA)[4] was measured with an RAF ruler (ASF Universal, Delhi, India) held against the participant’s cheeks and tilted slightly downward from eye level. Participants focused on a small reading print (N8) at the ruler’s far end, slowly moving closer until they reported the first blur. They were then asked to blink and try to clear the blur. The endpoint was when sustained blur was reported. NPA was recorded in cm and converted to diopters (Hofstetter formula). Normal value: 18–(1/3 age) ± 2.00 D.

• Near Point of Convergence (NPC)[4] was tested with an accommodative target on an RAF ruler. Participants reported sustained double vision, which was recorded in cm.

• Fusional Vergence[4] was tested using a horizontal prism bar (Luneau Technology Operations, France). Negative fusional vergence (base in) and positive fusional vergence (base out) were measured. Blur, break, and recovery points for distance and near were recorded in prism diopters (PD). Expected values were based on specific ranges for distance and near measurements.

• Accommodative Insufficiency[4] was defined as accommodation consistently lower than expected for age by 2.00 D or more, with the main symptom being general asthenopia related to near work.

• Monocular Estimate Method (MEM)[4] was measured by placing the MEM cards (Pioneer International, USA) 40 cm on the head of a retinoscope with an N8 target. Participants read the letters aloud, while quick retinoscopy (Heine Beta 200 LED retinoscope, Heine, Germany) was performed. The expected value was + 0.25 to + 0.75 DS.

• Cover Test[5]: Participants focused on a distant target (20/200) while an alternate cover test was performed. Expected value: 3 PD exophoria for distance and 6 PD exophoria for nearby.

• Ocular Motility[6]: The participant followed a moving target in all nine gazes with the head erect. Expected value: Full and free movement.

• Vergence Facility[4] was measured with vergence flippers (Bernell Corporation. 4016 North Home Street, Mishawaka, USA) (12 PD Base Out/3 PD Base In). Participants focused on a linear target at 40 cm, flipping the flippers upon reporting single vision. The number of times “single” was reported in 60 seconds was recorded as cycles per minute (cpm). The expected value: 15 cpm ± 3.

• Negative Fusional Vergence (NFV)[4] was assessed with a prism bar in the base-in direction. Participants fixated on a linear target and reported blur, double, or single vision. Expected value: Distance: Break: 7 ± 3PD/Recovery: 4 ± 2PD, near: Blur: 13 ± 4PD/Break: 21 ± 4PD/Recovery: 13 ± 5PD

• Positive Fusional Vergence (PFV)[4] was measured with a prism bar in the base-out direction. Participants fixated on a linear target and reported blur, double, or single vision. Expected value: Expected value: Distance: Blur: 9 ± 4PD/Break: 19 ± 8PD/Recovery: 10 ± 4PD, near: Blur: 17 ± 5PD/Break: 21 ± 6PD/Recovery: 11 ± 7PD.

• Accommodative Facility[4] was measured with accommodative flippers (Bernell Corporation, USA) (+2.00 D). Participants focused on an N8 target at 40 cm, flipping the flippers upon reporting clear vision. This was performed binocularly first, then monocularly. Expected values: Monocularly, minimum 12 CPM, average 17 CPM. Binocularly, minimum 10 CPM, average 13 CPM.

Questionnaire

Asthenopia was assessed using the CISS (Convergence Insufficiency Symptoms Survey)[7] questionnaire [Supplementary Table 1]. Before the survey, a single technician explained the questions to the participants in their regional language and instructed them to answer as best they could. The CISS questionnaire is scored on a five-point scale, with 0 being “never” and 4 being “always.” The recommended adult cut-off is ≥21.

Statistical analysis

All the variables for this study were analyzed using Microsoft Office Excel and STATA 14.0. (StataCorp LLC, TX, USA) Descriptive statistics for data (mean, median) were computed. The representation of continuous variables was expressed by mean, standard deviation (SD), and confidence intervals (CI); the categorical variables were shown with frequency and percentage. For intergroup analysis of metric parameters, the Chi-square test was used to determine the significance of the study parameters on a categorical scale between the groups. Correlation analysis was performed and tested using the Pearson correlation coefficient test, with 95% CI calculated for the correlation coefficients. Logistic regression was used to assess the independent association between the factors and the outcomes, with odds ratios (OR) reported along with their corresponding 95% CI. Significance was assessed at a 5% level of significance where P values were based on a two-sided test, and a P < 0.05 was considered significant.

RESULTS

A total of 1,093 participants were screened, and 66 participants aged 18–35 years were included in the study. All these participants were specifically engaged in diamond polishing, not in any other diamond industry-related work (for example, computer users, security, managers, distributors, and office staff). All of them were male. The mean age of examined participants was 28.7 ± 6.8. We divided the participants into two groups: 18–25 and 26–35 years. The prevalence of BVA in this study was 77.3% (n = 51/66). Abnormal stereoacuity was the most prevalent (n = 63.6%, CI: 50.8–75.1) BVA; others were accommodative insufficiency and convergence insufficiency [Table 1].

Table 1.

Prevalence of binocular vision abnormalities (BVA)

Binocular Vision Abnormalities	No. affected workers (n/total)	Percentage (95% CI)
Abnormal stereo acuity	42/66	63.6 (50.8–75.1)
Convergence Insufficiency	26/66	39.3 (27.5–52.1)
Accommodative insufficiency	29/66	43.9 (31.7–56.7)

We did not find an association between the CISS scores and age, hours worked per day, years of experience, or the number of diamonds polished daily. Only 10.6% of participants scored above 21 on the CISS questionnaire [Table 2]. However, there was a statistically significant association between the age and near point of accommodation in both eyes (NPABE), P value <0.001, where more than 57.4% of the polishers were in the age group of 26–35 years [Supplementary Table 2].

Table 2.

Association of factors relating to convergence insufficiency symptoms survey (CISS) SCORE

Factors	CISS score <21 n (%)	CISS score >21 n (%)	Significance (P)
Age (years)
18–25 (n=19)	17 (89.5)	02 (10.5)	0.992
26–35 (n=47)	42 (89.4)	05 (10.6)
Hours worked per day
<10 (n=48)	43 (89.6)	05 (10.4)	0.935
>10 (n=18)	16 (88.9)	02 (01.1)
Years of experience
<7 (n=26)	23 (88.5)	03 (11.5)	0.843
>7 (n=40)	36 (90.0)	04 (10.0)
No. of diamonds polished in a day
<100 (n=45)	39 (86.7)	06 (13.3)	0.292
>100 (n=21)	20 (95.2)	01 (04.8)

Supplementary Data 2.

Association of eye screening parameters with age

Eye Screening Parameters	Age (18-25 Yrs.) (n=19) n (%)	Age (26-35 Yrs.) (n=47) n (%)	P
Stereoacuity			0.167
Normal	05 (26.3%)	21 (44.7%)
Abnormal	14 (73.7%)	26 (55.3%)
NPABE			*<0.001
Normal	18 (94.7%)	20 (42.6%)
Abnormal	01 (05.3%)	27 (57.4%)
NPCObj			0.860
Normal	15 (78.9)	38 (80.9)
Abnormal	04 (21.1)	09 (19.1)
NPCsub			0.860
Normal	15 (78.9)	38 (80.9)
Abnormal	04 (21.1)	09 (19.1)
NFVbl (40cm)			0.658
Normal	05 (26.3)	10 (21.3)
Abnormal	14 (73.7)	37 (78.7)
NFVbr (40cm)			0.658
Normal	05 (26.3)	10 (21.3)
Abnormal	14 (73.7)	37 (78.7)
NFVre (40cm)			0.502
Normal	13 (68.4)	28 (59.6)
Abnormal	06 (31.6)	19 (40.4)
NFVbl (3m)			0.492
Normal	13 (68.4)	36 (76.6)
Abnormal	06 (31.6)	11 (23.4)
NFVbr (3m)			0.113
Normal	18 (94.7)	47 (100.0)
Abnormal	01 (05.3)	00 (00.0)
NFVre (3m)			0.113
Normal	18 (94.7)	47 (100.0)
Abnormal	01 (05.3)	00 (00.0)
PFVbl (40cm)			0.327
Normal	13 (68.4)	26 (55.3)
Abnormal	06 (31.6)	21 (44.7)
PFVbr (40cm)			0.150
Normal	13 (68.4)	23 (48.9)
Abnormal	06 (31.6)	24 (51.1)
PFVre (40cm)			0.501
Normal	18 (94.7)	46 (97.9)
Abnormal	01 (05.3)	01 (02.1)
PFVbl (3m)			0.384
Normal	16 (84.2)	43 (91.5)
Abnormal	03 (15.8)	04 (08.5)
PFVbr (3m)			0.860
Normal	15 (78.9)	38 (80.9)
Abnormal	04 (21.1)	09 (19.1)
PFVre (3m)			0.333
Normal	17 (89.5)	45 (95.7)
Abnormal	02 (10.5)	02 (04.3)
AFBE			0.385
Normal	17 (89.5)	41 (87.2)
Abnormal	02 (10.5)	02 (04.3)
VF			0.492
Normal	06 (31.6)	11 (23.4)
Abnormal	13 (68.4)	36 (76.6)

RE: Right eye, LE: Left eye, NPA: Near point of accommodation, NPC: Near point of convergence, NFV: Negative fusional vergence, PFV: Positive fusional vergence, AF: Accommodative facility, VF: Vergence facility, Obj: Objective, Sub: Subjective, bl: blur, br: break, re: recovery

We measured the fusional reserves for distance and near vision, documenting the break, blur, and recovery points. In the 26–35 age group, near vision NFV (break, blur, and recovery points) at 78.7%, 78.7%, and 40.4%, respectively, was higher than for distance vision NFV at 23%, 0%, and 0%, respectively, but not statistically significant [Supplementary Table 2].

DISCUSSION

Asthenopia is characterized by discomfort, lacrimation, and eyebrow pain.[7] It is mainly caused by extended periods of near work, inappropriate lighting (too dim or bright light), eye muscle imbalance, and uncorrected refractive errors. Diamond polishing requires precise work with small objects and demands high visual and cognitive attention. Due to improper workplace conditions such as glare, poor lighting, and improper sitting postures, the workers experience asthenopic symptoms.[7] An earlier study in India has shown that 65.2% of workers in the jeweler manufacturing business suffered from symptoms similar to computer vision syndrome; it was significantly higher than people engaged in computer work.[3] Given that the diamond polishers are also engaged in near work for longer hours, CVS-like symptoms are expected. Earlier studies on jeweler workers had assessed asthenopia.[3] Our study specifically corroborated the objectively collected and subjectively evaluated findings of asthenopia. We chose actual diamond polishers only and eliminated other near-work biases in other workers in the diamond industry (such as computer users and managers). Our study showed a high prevalence of binocular vision anomalies (72.3%), abnormal stereoacuity (63.6%), and accommodative insufficiency (43.9%) in the diamond polishers.

Stereoacuity is the smallest amount of horizontal retinal image disparity (measured in arc seconds), which gives rise to the perception of relative depth or stereopsis.[9] There are three unnatural causes for the disturbance of stereo acuity: excessive screen disparities, objects appearing farther than the fusional area, and vergence accommodation mechanisms. Other factors influencing stereo acuity include occlusion, perspective, texture gradients, relative size and height in the visual field, shadow, luminance, and aerial perspective. The abnormal stereoacuity seen in over 60% of participants in this study was probably related to suboptimal and improper lighting conditions, which can disrupt binocular summation.[10] When mean illumination differs substantially between the two eyes, sensory dominance is disrupted, reducing binocular summation. Also, reducing mean luminance in one eye increases binocular suppression and decreases stereoacuity.[11] The human visual system is not biologically adapted for such sustained near work.[9] Diamond polishing requires prolonged near focusing, putting a high demand on stereopsis, leading to abnormal stereoacuity. Accommodative insufficiency reduces the ability to perform near work.[12] It is a non-strabismic binocular vision anomaly characterized by an inability to focus or sustain focus for near vision.[13] It was seen in 44% of the participants in this study. It is probably related to awkward sitting postures, repetitive movement, long duration of work, and high visual demand. This condition leads to ill-sustained accommodation, finally manifesting in accommodative insufficiency.

The Convergence Insufficiency Symptom Survey (CISS) tool measures treatment effects, with different scores distinguishing symptomatic convergence insufficiency from normal binocular vision in children aged 16 and adults (≥21).[14,15] However, it is cautioned against using the CISS questionnaire as a screening tool in “asymptomatic” populations due to poor sensitivity and a high false positive rate.[8] Although the CISS was never designed as a screening tool, it is readily available online and has been used in several studies.[15,16] In our study, only 10% of participants scored above 21 on the CISS questionnaire; thus, it suggests that the CISS may not be entirely reliable in detecting non-strabismic binocular vision disorders in this population.[7] The young polishers naturally adapt to vision stress, which could explain the low scores in this study population.

Our cohort primarily consisted of individuals under 35 years of age, without refractive errors. We analyzed their accommodative and vergence parameters. We excluded participants with refractive errors to minimize bias from the influence of corrective lenses or uncorrected visual impairments on the study outcomes. We found that more than 50% [Supplementary Table 2] of the polishers in the 26–35 age group had an abnormal Near Point of Accommodation, which agrees with decreasing fusional reserves with age. This decline in fusional reserves typically affects people’s ability to maintain clear and comfortable vision during prolonged near tasks, resulting in symptoms such as eye strain, headaches, and difficulty focusing.

This study had several limitations. The sample size was small, which may not have captured all possible anomalies. The cross-sectional nature of the study also limits the ability to infer causality. Additionally, the reliance on self-reported data for the CISS questionnaire could introduce response bias.

Future research should include larger groups to validate these findings and suggest implementation of preventive measures. Comprehensive ophthalmic and optometric examinations are recommended to detect and treat binocular vision anomalies in the core people in jeweler making, including the diamond polishers. Furthermore, longitudinal studies would be beneficial in understanding the long-term effects of prolonged and intense near work on visual health in these people. Educating them about binocular vision anomalies and periodic eye examinations could help mitigate these symptoms.

CONCLUSION

This study reports a high prevalence of binocular vision anomalies among diamond polishers in an Indian metropolitan city. This is largely due to poor workplace conditions, inadequate lighting and longer working hours. The significant number of abnormal near-point accommodations in the older age group highlights the impact of prolonged near-work conditions on visual health. While the working conditions of these workers need significant improvement, mandatory comprehensive eye examinations would also help detect visual problems early for timely correction. An assessment with larger cohorts and a longitudinal evaluation would help frame the policy for them.

Conflicts of interest

There are no conflicts of interest.

SUPPLEMENTARY DATA

Supplementary data 1: CISS Questionnaire

Supplementary data 1

CISS Questionnaire

Funding Statement

Shantilal Shanghvi Foundation (SSF).