1. Nepal Medical College and Teaching Hospital (NMC), Nepal: mariesharma@gmail.com
2. Nepal Medical College and Teaching Hospital (NMC), Nepal: hinaaker67@gmail.com
3. Nepal Medical College and Teaching Hospital (NMC), Nepal: achayaakter@outlook.com=

*             Correspondence: mariesharma@gmail.com

Keywords: contrast sensitivity, color vision, diabetics, retinopathy

1. INTRODUCTION

A collection of physiological dysfunctions known as diabetes mellitus are typified by elevated blood glucose levels brought on by insulin resistance and insufficient insulin production. It is a long-term illness that impacts how well your body utilises glucose [1]. It poses a serious threat to health in both industrialised and underdeveloped nations. Nearly 4.6% (151 million) of the population had diabetes in 2000; this number nearly doubled to 8.8% (415 million) in 2015, and by 2040, it is predicted to reach 10.4% (642 million) [2-4]. Diabetes mellitus is becoming a major health problem due to its expanding epidemic and varied effects on different places. The Western Pacific area encompasses countries with the largest incidence of diabetes (37.5%) and has the highest number of adults diagnosed with the disease, although the Middle East and North Africa region has the highest prevalence of diabetes in adults (10.9%). (5). Diabetes affects several systemic systems, including neuropathy, retinopathy, nephropathy, and cardiovascular problems. Diabetes is a set of metabolic disorders. Retinopathy is the most well-understood symptom of ophthalmologic problems, which are the primary cause of blindness in affluent nations. Without resulting in diabetic retinopathy or maculopathy, it may have an impact on colour vision, contrast sensitivity, and visual acuity in the eyes [6, 7]. Diabetes lengthens the risk of diabetic retinopathy, which raises the possibility of permanent, sight-threatening consequences [8]. People with diabetes frequently experience altered visual acuity, and most of these cases are not related to diabetic retinopathy. Blurred vision, or the inability to focus on objects, is one of the most prevalent kinds of vision issues. Diabetes has the potential to produce this kind of eyesight issue. Horner (1873) was the first to report diabetes-related transient changes in refractive index. Patients with recently diagnosed diabetes may have transitory refractive alterations, such as hyperopia, myopia, or both. The resolution of this hyperopia and myopia is demonstrated by treating hyperglycemia [9]. In daily tasks like reading small print and the ability to resolve minute details, which is often measured by visual acuity, pattern recognition is essential [10]. The (ETDRS) graphic describes the visual activity using a linear trend across rows, an equal set of optotypes for each sized level, and letter size spacing in each row. As you move down the rows, letter size becomes the only variable that matters. The restricted test-retest variability of the ETDRS chart can be explained by these features: up to 87% of retest results can be within 0.1 logs MAR (or one line of letters on an eye chart) of the initial test result, and 98% can be within 0.2 log MAR [11]. A physical feature known as spatial contrast is the luminous transition of a border or boundary in an illustration that denotes the existence of an object or pattern. The amount of contrast needed to view a target is measured as a person’s contrast sensitivity. Measures of acuity are not the same as assessments of contrast-sensitivity. The target contrast threshold is being measured [12, 13]. Being able to identify objects in low contrast is essential for doing daily tasks including reading, driving, and navigating. Studies have also demonstrated that contrast sensitivity can help detect and track visual diseases such optic neuritis, cataracts, and glaucoma. One can assess someone’s contrast sensitivity to have a better understanding of their visual quality [14]. Despite good visual acuity and fundus examinations, diabetic patients may have reduced contrast sensitivity [15]. The most common methods for assessing contrast sensitivity these days are chart-based systems that may be seen on the wall. In these charts, the test targets are either letters or sine-wave gratings [12].  The capacity to distinguish between various colour hues is known as colour vision, and the lack of it is linked to colour vision impairment. Three different types of cone cells make up the majority of the human fovea. Short-wavelength (S) cones are most sensitive in the spectral blue region. Medium wavelength (M) cones are most sensitive in the vicinity of the spectral green. Cones with long wavelengths (L) are more sensitive in the spectral red region. When combined, they provide the basis of trichromatic colour vision. Thus, defects in colour vision may arise from the degeneration of these cells and the bipolar, ganglion, and horizontal cells that support them [15]. Studies have shown that people with diabetes have acquired impaired colour vision (ICV), which is the inability to distinguish between different colours. One of the earliest signs of neurodegenerative alterations in the retina is acquired decreased colour vision. One kind of acquired impaired colour vision is responsible for the incapacity to distinguish between colour tones that are identical (ICV). Deuteranopia or deuteranomaly (least severe kind) is the term used to describe red-green colour deficiency. A person with protanopia, also known as protanomaly (less severe variation), has difficulty telling different red, yellow, and green hues apart. Tritanomas are caused by deficiencies in the blue-yellow colour spectrum. Diabetes patients without retinopathy may have impaired colour vision, which could have a non-vascular cause [16, 17]. The blue-yellow fault on ETDRS patterns with increasing severities were the most frequently discovered patterns [18]. Only visual acuity evaluation is considered the most important visual function in most conventional eye exams, and visual acuity charts based on maximum contrast do not provide information concerning contrast sensitivity function or colour vision. A visual acuity assessment should be conducted in conjunction with an investigation of both visual functions, as contrast sensitivity and colour vision can fluctuate separately from visual acuity. Reduced colour vision, diminished contrast sensitivity, and decreased visual acuity are tests that indicate various types of visual information lost along the long optical and neural processing pathway.20, 19 Colour vision assessment and contrast sensitivity function are important even in diabetic subjects without retinopathy, suggesting that this is a useful way to identify and follow diabetic subjects without retinopathy because diabetic subjects need colour vision to adequately self-monitor their blood sugar and urine levels [19]. Tests for colour and contrast acuity provide clinically meaningful insights into functional vision. The earliest sign of diabetic retinopathy appears to be a loss in contrast sensitivity or colour vision in the absence of any other ocular abnormalities. This review concentrated on changes in contrast sensitivity and colour vision in diabetics without retinopathy because of the significance of contrast sensitivity in daily activities and how it can affect their quality of life.

2. LITERATURE REVIEW

According to studies, the prevalence of diabetes mellitus has increased fourfold worldwide over the past thirty years, making it the tenth most common cause of death. Nowadays, one in eleven people worldwide suffers from diabetes, with type 2 diabetes accounting for 90% of cases (T2DM). China and India are the top two major hubs for the rapidly increasing global T2DM epidemic [20–23]. Asia is a major contributor to this epidemic. The eye has long piqued the interest of scientists as a means of identifying systemic illnesses. diabetes retinopathy screening has emerged as a novel, cost-effective approach to both prevent sight-threatening conditions and identify individuals at higher risk of developing further diabetes comorbidities. Moreover, it has recently been determined that the severity and prevalence of diabetic retinopathy represent a distinct and substantial predictor of subclinical cardiovascular disease [24, 25]. Diabetes patients without clinical signs of diabetic retinopathy were shown to have thinner retinas in the foveal and temporal areas, and a higher HbA1c level was linked to thinner retinas in the temporal perifoveal area. These findings were reported by Jiang j et al. Additionally, it was discovered that retinal neuronal damage may start prior to vasculopathy in the very early stages of diabetes [26, 27]. A study found that in the early stages of diabetes, there were changes in the central macular thickness and retinal vascular diameter in patients without diabetic retinopathy. The arterioles and venules of diabetics were wider than those of healthy persons. The thickness of the central fovea decreases in diabetes mellitus. To study the human retina, OCT was employed. Before there are outward manifestations of diabetic retinopathy, changes in retinal vascular diameter in combination with structural retinal neurodegeneration may be an early indicator of retinal impairment in diabetes [28]. Additionally, it is hypothesised that neurodegeneration occurred in type 2 diabetics without diabetic retinopathy and with appropriate metabolic management, specifically in the macular region and the RNFL in the inferior and temporal sectors of the optic disc. Subclinical ischemia was found to be a major contributing factor to neuro-degeneration in those patients, along with other metabolic factors. Research revealed that retinal thickness was decreased in prediabetes and even more so in type 2 diabetes before diabetic retinopathy became clinically evident. In prediabetes, half of the thinning observed in DM2 without DR was already present. Generalised thinning of the macula may be associated with thinning of the temporal side of the optic nerve head via the papillo-macular bundle [29, 30]. Research has shown that those with diabetes are more likely than those without the disease to develop myopia, and although the relationship was not linear, poorer glycemic control has been linked to an increased risk of the condition. Individuals with diabetes have more noticeable age-related changes to their lenses, which could eventually cause myopia. Patients with Type 1 diabetes exhibit these physiological alterations more prominently than those with Type 2 diabetes, which may account for the former group’s higher prevalence of myopia. One of the chronic refractive shifts associated with diabetes is an increase in myopia [9]. According to a study, the biological underpinnings of the correlation between plasma glucose levels and visual impairment remain unclear. Whether the lens swelling is due to changes in its curvature or size, or whether the refractive alterations are caused by changes in the refractive index or its gradient, cannot be definitively answered. Nonetheless, it is possible for myopia or hyperopia to arise from a change in both the refractive index and the surface curvature. According to clinical observations, some diabetic patients appear to develop more hyperopic as their blood glucose level rises, while others appear to get more myopic [31]. A recent study revealed that astigmatism is more common in the diabetic community than hyperopia and myopia. A noteworthy finding that was made was emmetropia. A familial history of myopia has been connected to myopia. Another study found no correlation between poor glycemic management and development-related factors and a higher prevalence of myopia [32, 33]. It was also found that age-related changes in the optics of diabetic eyes do not need to be hastened if the disease is well controlled. DR lessens sensitivity to contrast. An examination of multiple systemic and ocular risk variables linked to CS in diabetic subjects demonstrates the connection between changes in contrast sensitivity and systemic and ocular characteristics, not just the severity of DR. This was noteworthy since lower contrast was linked to worse performance in tasks requiring a lot of vision, daily activities, and mobility [34]. A study discovered that subjects 50 years of age and older, both male and female, had reduced contrast sensitivity compared to younger subjects. It was also demonstrated that men over 50 had greater contrast sensitivity than women. Diabetes and alterations in contrast sensitivity were not related [35]. Some studies found that persons with diabetes had significantly lower CS. It was questionable, nevertheless, whether diabetes generally lowers CS or if it only affects specific components of the visual image. It was important to identify any anomalies in spatial contrast vision in diabetes since these could indicate altered photoreceptor/visual pathways caused by the illness process. Additionally, it was discovered that diabetes mostly decreased CS at low spatial frequencies, despite the fact that earlier studies revealed that the impairments were primarily noticeable in mid-range or high frequencies [14, 36].  A study found a strong correlation between CS measures and the thickness of the outer retina. However, there does not appear to be a strong correlation between outer-retinal thickness and visual acuity. These findings underscored the usefulness of CS as a functional evaluation, implying that in early-stage diabetic retinopathy, neurological impairment may manifest even in the absence of appreciable retinal thinning [37]. Reduced macular vascular density and permeability were found to have an impact on retinal function as measured by BCVA in one study [38]. However, more targeted changes in the foveal avascular zone seemed to have an impact on contrast sensitivity before clinical signs of DR manifested. Disruption of the inner retinal microvasculature can result in irregularities of fluid and alterations in metabolism, which can lead to the death of ganglion cells and cells in the inner nuclear layer of the retina. This is a vascular disorder. Before vasculopathy develops, changes in early type 2 diabetes have been identified by functional retinal monitoring. The inner retinal perfusion is the subject of another theory. It appears that colour vision tests may be used to identify individuals with glucose dysregulation as early as prediabetes. Higher HbA1c levels in those with prediabetes and diabetes are linked to these colour vision deficits [39]. A type 3 (Tritan) colour loss in acquired diabetes has been reported by certain researchers. Reduced sensitivity in the shorter-wavelength (S cone) circuits follows this. Later stages of the disease are associated with the red-green pathways [40].

3. CONCLUSION

When comparing diabetics without retinopathy to healthy individuals, there is a significant difference in contrast sensitivity using Pelli-Robson charts and colour vision deficiencies using Farnsworth D-15. Since diabetes is one of the main causes of blindness, changes in the retina brought on by the disease can be identified in an optometric setting by closely observing visual functions like colour and contrast sensitivity before the first clinical signs of diabetic retinopathy manifest in the presence of normal or near-normal visual acuity. In this situation, increasing public awareness necessitates thorough patient education. To fully understand, further study with a larger sample size is necessary because the general population is small.

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