Risk Factors and Prevalence of Contrast Sensitivity Impairment among Commercial Motor Vehicle Drivers in Benin City, Nigeria

1. INTRODUCTION

Globally, over 1.2 million individuals pass away in traffic-related incidents each year [1]. Millions of other individuals suffer injuries of varying degrees, some of which result in lifelong disability [1-3]. There are serious social and economic repercussions when a large amount of human potential is wasted in road traffic accidents [4, 5].

Poor vision among drivers is a significant risk factor influencing motor vehicle crashes. Hence, road safety is a significant global public health concern [1, 6]. Having good eyesight is imperative for safe driving. It accomplishes approximately 95% of all sensory needs for vision processes[7, 8]. Safe and high-quality driving requires commercial drivers to have the best vision [9, 10], particularly in crowded cities [11].

Contrast sensitivity (CS) is known as the innate ability of human eyes to discern any variation in luminance, or brightness, between neighbouring items in a certain area or locations within a given space [12]. CS also measures the capacity to identify variations in object features at a distance and in low contrast environments, such as at night or in foggy weather [13]. The ability to see contrast is important in a variety of situations, including recognizing lane markers, road signs, and oncoming traffic.

Driving at night in conditions of mesopic or low photopic illumination, or when there is glare from oncoming headlights, can cause visual difficulties for drivers [14], increase the frequency of night-time motor vehicle crashes [15], increase the number of drivers who report experiencing glare from oncoming headlights [16], cause reduced adaptation times after glare experience [15], and impair drivers’ ability to recognize lane markers and road signs [17]. When it comes to predicting real-world visual danger, CS may be more sensitive and accurate than visual acuity [18]. The different traditional vision tests, including visual acuity, visual fields, or patient self-reports, are often not very good indicators of one’s ability to undertake night-time driving [19]. This is why CS, which quantifies low contrast visual acuity (VA), remains a reliable indicator of one’s capacity for night-time driving [15].

An Iranian study revealed that while the mean CS for commercial drivers was 2.00 log units, 4.5% of drivers over the age of 58 reported CS values equal to or less than 1.25 log units [20]. Moreover, according to research by van Rijn et al. [21], 6.5% of European drivers over 75 years had less than 1.25 log units Pelli-Robson CS.

A number of ocular conditions including glaucoma, cataract, optic neuritis, and amblyopia decrease CS [12, 22, 23]. Existing research demonstrates that CS naturally declines at a proportion of approximately 0.45 log CS/decade in healthy eyes between the ages of 50 and 80. These natural changes in CS have been linked to a reduced ability to identify hazards when driving at night [15]. Research conducted in Iran on commercial and military drivers revealed that drivers over the age of 58 experienced a significant decrease in CS than drivers of younger age groups [20]. Moreover, various researches further revealed that older driver populations have higher rates of VA and CS impairments [24-26].

While, VA is a simple and valuable tool to assess an individual’s visual state, it is not a sufficient measure of good eyesight on its own [26]. Many researchers have highlighted the significance of CS assessment to gauge the competency to drive, since a subject with impaired CS may, for instance, find it difficult to identify a dark-coated pedestrian at night, even if his or her visual acuity is excellent during the day [18, 25, 27, 28].

Based on the information about the condition of the eyes and the prevalence of ocular diseases among drivers worldwide, scholarly investigations have emphasized the necessity of regular and comprehensive visual screenings and examinations prior to the issuing and renewal of driving licenses [20, 24, 29].

Despite the unequivocal significance of CS in driving safely, there is paucity of evidence-based literature in Nigeria, particularly regarding the link between CS impairment and road traffic collisions among drivers. Hence, this study was undertaken to point out the risk factors and prevalence of CS impairment among drivers of commercial motor vehicles in Benin City, Edo State, Nigeria.

2. MATERIALS AND METHODS

This study was an observational cross-sectional survey of contrast sensitivity or CS among commercial motor vehicle drivers aged 20-70 years (with the mean age of 48.09 years ± 11.21 years) in Benin City, Nigeria. Data was collected through the use of a modified self-administered National Eye Institute Visual Functioning Questionnaire-25, as well as comprehensive eye examination and CS assessment of 341 drivers of commercial vehicles in Benin City environs.

This survey was executed in the central area of Benin City, Edo State, Nigeria. Edo state is an agricultural and oil producing state of Nigeria. It is situated in the geopolitical zone called south-south and is home to 3,602,124 people [30]. The state is separated into three senatorial districts namely Northern Edo, Southern Edo, and Central Edo, with a total of eighteen administrative Local Government Areas. Benin City is the government hub of Edo territory as well as the economic nerve of the state [31].

The smallest sample required for this study was estimated using the formula for single proportion [32]

n = ,

where

n is the desired sample size (when population is >10,000),

Z is the normal curve’s abscissa, which divides a region α at the tails (1 - α equals the desired confidence level, e.g., 95%) which is 1.96,

e is the degree of accuracy which is 5% (0.05),

p is the estimation of the proportion of an attribute that is extant in the population, taken as 15% prevalence of commercial drivers with impaired CS in Rivers State, Nigeria [33],

q is 1-p, i.e (1-0.15)

n =

n =

n = 196       

10% attrition factor = 20

Minimum sample size = 216 Participants

The purposive random sampling technique was used to select a total of total of 341 commercial drivers. The survey was performed in Central Motor Park, Benin City, where majority of drivers enrooting the Benin City Metropolis load and disembark passengers. Data on demographics were gathered. Each participant completed an adjusted National Eye Institute Visual Functioning Questionnaire-25 [34], which asked about their age, sex, educational background, number of years of driving, amount of hours driven daily, visual complaints, and self-assessed road traffic collisions.

Presenting visual acuity or VA was evaluated using the Snellen’s chart, placed six meters (20 feet) away in a daylight open area by a licensed optometrist [35]. Presenting VA ≥ 6/12 in the best seeing eye was considered normal [36-38].

CS was assessed using Pelli-Robson CS test. Pelli-Robson CS is a white chart measuring 59 x 84 cm and uses black Sloan letters arranged in 16 groups of three letters [39]. It was used binocularly one meter away from the respondents. The resultant value obtained was expressed in logarithmic CS [40]. The Pelli-Robson chart ranges in contrast, from 0.00 log units (100%) to 2.25 log units (0.56%), and every triplet reduced by 0.15 log units [41]. Each letter was scored as 0.05 log units [25]. The Pelli-Robson CS scores ≥ 1.95 log units were considered normal for individuals with ages between 20 and 50 years. While, scores ≥ 1.80 log units were considered normal for individuals more than 50 years of age [42].

Data were analysed using IBM SPSS (version 29.0). Chi square test was used to observe significant differences linking CS and associated risk factors together.

3. RESULTS

CS test was conducted on a total of 341 drivers of commercial motor vehicles routing within the Benin City metropolis. The results revealed that more than half of the respondents were of Edo ethnicity, followed by Deltans (13.8%). The vast majority (95.6%) of the study population were males. The age groups 41-50 years and 51-60 years, with a prevalence of 45.5% and 24.0% respectively, comprised the largest segments of the study population. Moreover, most of the respondents were educated with 39.9% and 37.5% having secondary and tertiary education, respectively. Over two-thirds of the respondents were self-employed and over half of them had a driving experience of more than 10 years. Furthermore, over two-thirds of the respondents reported driving above 5 hours per day and experienced no road traffic collisions during the last one year. Meanwhile, 21.7% reported 1-2 road traffic collisions during the last one year (Table 1).

Figure 1 shows that the single largest group (41.1%) of respondents had a CS score of 1.95 log units and 27% of the respondents had a CS score of 2.25 log units. The mean (50^th percentile) CS score was 1.9135 log units ± 0.40168 SD and the 75^th percentile was 2.25 log units.

The results of this survey showed that over two-thirds of the respondents had a normal CS score, while the CS score of 21.7% of respondents showed that they were impaired (Figure 2).

Table 1. Sociodemographic Characteristics of the Respondents

Figure 1. CS Scores of Commercial Drivers in Benin City

Figure 2. Categories of CS Scores among Commercial Drivers in Benin City.

Table 2 shows the results of eye health risk factors linked with CS. The findings show that Deltan (46.8%) and Igbo (27.3%) ethnicities had the highest prevalence of CS impairment. The link between CS and ethnicity was statistically significant (p < 0.01). Although, the connection between CS and gender was not statistically significant (p > 0.05), CS impairment was higher in males (22.1%) than in females (13.3%). CS impairment increased with the increase in age from 7.9% among age group 20-30 years to 48.0% among age group > 60 years. The connection between CS and age groups was found to be statistically significant (p < 0.01).

Similarly, CS impairment decreased with higher levels of education, from 41.7% among respondents with no formal education to 18.8% among respondents with tertiary education. CS and educational status were significantly connected (p < 0.01). Furthermore, this survey demonstrated that there was no signification relationship of CS with employment status, years of driving experience, and hours driven per day (p > 0.05). The respondents’s self-reports of 1-2 (41.9%) and 3-4 (37.5%) traffic accidents during the past one year showed that the link between CS and the number of road traffic collisions in the past one year was statistically significant (p < 0.01).

Table 2. Demographic and Driving-related Factors Associated with Contrast Sensitivity among Respondents

4. DISCUSSION

Contrast sensitivity or CS is the adeptness of the eyes to accurately differentiate a figure in space from its background in dim illumination, such as foggy weather or nightfall. It plays a significant role in swift discernment and execution of decisions during driving. Poor vision among drivers, being a significant risk factor influencing motor vehicle crashes, remains a global public health concern [1, 6].

The study found that a total of 78.3% of respondents had a normal CS score, while the CS score of 21.7% of the respondents was impaired. This percentage is higher than the 15% prevalence of CS among drivers surveyed from the Rivers State of Nigeria [33], 4.5% prevalence among drivers above 58 years of age in Iran [20], and 6.3% prevalence among European drivers who were 75 years of age and older [43].

This survey demonstrated a statistically significant link between CS and ethnicity, age groups, educational status, and the number of road traffic collisions in the past one year (p < 0.01). CS impairment increases with the increase in age, from 7.9% among age group 20-30 years to 48.0% among age group > 60 years. These results are in conformity with several previous studies which reported that older drivers had Pelli-Robson CS below 1.80 log units, and a reasonably higher risk of susceptibility to road crashes [44, 45]. CS, in the normal eye, gets worse after 50 years of age. Moreover, natural variations in CS are associated with poorer hazard detection while driving, especially at night [15].

Similarly, CS impairment decreased with a higher level of education, from 41.7% among respondents with no formal education to 18.8% among respondents with tertiary education. This is in agreement with the findings of other studies indicating that higher educational attainment is associated with better overall visual health outcomes. While, specific studies on the direct relationship between CS and educational status are limited, education is often considered a proxy for socioeconomic status (SES), which can influence access to healthcare and health-related behaviours [46, 47].

This study showed that the link between CS and the number of road traffic collisions in the past one year was statistically significant (p < 0.01). While, 41.9% of the respondents reported 1-2 road traffic accidents, 37.5% of them reported 3-4 road traffic collisions during the past one year. This was in consonance with the study conducted by Owsley et al. [44], which demonstrated that drivers with impaired CS were three times more likely to get into an accident. Similarly, other studies reported the proportion of road traffic collisions to range from 8.3% to 45.5% [3, 29, 48-50]. However, these studies did not find a connection between CS and road safety [51, 52].

In this study, despite no significant relationship found between CS and years of driving experience, as well as hours driven per day (p>0.05), the findings revealed that reduced CS is associated with increased crash risk and poorer driving performance, particularly in older drivers [53, 54]. CS is essential for detecting objects against their background, which is critical for safe driving, especially in low-light conditions, fog, or glare [55].

This survey demonstrated that CS impairment was a major eye health burden among commercial drivers in Benin City Metropolis. Moreover, it was distinctly linked with ethnicity, age groups, educational status, and the number of road traffic collisions in the past one year. CS impairment increased with the increase in age among the drivers. Meanwhile, the CS score of drivers was better among drivers with higher educational attainment. Regular comprehensive eye examinations and appropriate eye care services are recommended to assess the CS status of the drivers of commercial or public vehicles in order to reduce the burden of visual impairment.

Author Contribution

John Esimaje Moyegbone: conceptualization, formal analysis, investigation, project administration & writing- original draft. Eghonghon Ehianata Oronsaye: investigation, methodology, writing-review and editing. Aghafekokhian Bose Osaiyuwu: conceptualization, supervision, validation, writing-review and editing. Patricia Tonbra Osunu: investigation, methodology, writing-review and editing. Jennifer Aleye Ebeigbe: conceptualization, supervision, validation, writing-review and editing. Ezekiel Uba Nwose: methodology, resources, writing-review and editing.

Conflict of Interest

The author of the manuscript has no financial or non-financial conflict of interest regarding the subject matter or materials discussed in this manuscript.

Data Availability Statement

The data linked to this study will be provided by the corresponding author upon request.

Funding

The authors received no funding for this study.

Generative AI Disclosure Statement

The authors did not used any type of generative artificial intelligence software for this research.

Ethical Approval and Consents to Participate

Ethical approval for this study was obtained from Ethical Committee of the Faculty of Life Sciences, University of Benin, Benin City, Edo State, Nigeria with protocol Number FLS/REC/018. Informed consent was obtained from each participating driver. Participation in this study was entirely voluntary.