Abstract

The present study investigated the effect of refractive error or ametropia on the spatial array of vision among young adults in Owerri Municipal, Imo State, Nigeria. The examined ametropias were myopia and hyperopia. A total of 100 persons were used for this study, 40 % had normal binocular central visual field, while 60 % of the total number had abnormal central visual field. The result revealed that ametropes whose degree of error > ±7.00 D had no normal binocular central visual field, while those with < ±3.00 D had a higher percentage normal visual field in decreasing diopters. A non-significant difference (p<0.05) was observed on the effect of myopes and hyperopes on the central visual fields of the examined subjects. The result obtained from the effect of gender on ametropic condition indicated no association between central visual field involvements with sex in ametropic condition of the eye. However, the ametropic condition of the higher degree affects the central isopter size drastically. There were significant relationship (p< 0.05) observed in myopia, hyperopia and the blind spot size. The present study has established that ametropia affects the central visual field efficiency in young adults.

1. Introduction

Worldwide, the scourges of visual impairment owed to uncorrected refractive error affect over 200 million people ¹. The global statistics of World Health organization (WHO) has revealed population strength of over 160 million with visual impairment, while 37 million out of the population are blind people, and 1.4 million of the score are children under the age of 15, while 125 million people have severely impaired vision ².

These problems often progress by inability of the eyes to focus on a central point. The sensational spatial array of vision available to observation which includes the total area covered by the eye when focused on a central point is referred to as visual field. Visual systems provide relevant information and offer molecular regulatory network to guide various cellular and feedback eye relay to the brain such as locomotion, reading and accommodation ^{3, 4}. Again, visual field can be examined to measure the extent and distribution of the field of vision. The test may be performed by a number of methods including what are termed tangent screen exam, confrontation, and automated perimetry. Furthermore, many deviations from the normal health status can adversely affect the visual field. Hence, visual field assessment could serve as a monitoring parameter for the diagnosis of recurrent diseases and acts as guide for the treatment of some idiopathic intracranial hypertension (IIH), multiple sclerosis, glaucoma and some other optic neuropathy ⁵. In a similar role, some medications such as antimalarial drugs like chloroquine and hydroxychloroquine and lifestyle activities like excessive use of cigarettes, or chronic exposure to its compounds, affects visual discrimination, supporting the existence of overall deficits in visual processing of unhealthy lifestyles ⁶.

It has been established that peripheral vision is potentially important for this activity performed by the cell. However, there may be some observable conditions where the image fails to focus as a result of refractive errors. This is a clinical condition called ametropia. The central visual field is significantly impaired in ametropic conditions of the eye especially in myopia and hyperopia. Most of these conditions can cause a reduction in the visual performance of an individual which unfortunately leads to unpleasant ocular symptoms. The most commonly known symptoms are repeatedly complaints including: eye strain, headaches, blurred vision and intermittent double vision ⁷.

Recently, an increasing numbers of myopia cases have been globally felt. However, the leading cause and reliable ameliorative measures has not been fully established ⁸. Myopia and hyperopia are frequent types of refractive error. People with myopia remain at risk for conditions caused by excessive axial elongation. Similarly, retinal detachment, myopic maculopathy, myopic retinopathy and glaucoma are predisposing factors for high myopia. Again, restriction in the lower visual field during stair walking results in more cautious locomotor behaviour such as walking slower and using the handrails. In daily activities, tasks or conditions that restrict or alter the lower visual field information may elevate the risk for missteps and falls ⁹.

In all, the prevalence of refractive errors in children and young adults has long been documented and becomes more frequent in the population as age increases. Also, certain pathological conditions of the eye such as optic nerve defect, Retinitis pigmentosa, glaucoma, affect the size of the field. Therefore, the present study investigated the effect of major ametropia; myopia and hyperopia on the central visual field of young adults.

2. Materials and Methods

The present study was conducted in a large cross-sectional examination of young adult in Owerri Municipal, Imo State. The ametropic conditions investigated were myopes and hyperopes. A total of hundred (100) persons were used for this study, 40 % of them had normal binocular central visual field, while 60 % of the total number had abnormal central visual field.

The magnitude of error existing in these patients was determined through Retinoscopy and subjective refraction on these patients.

The refractive error readings were reported as a spherical equivalent (SE) (sphere power plus half-negative cylinder power). Hyperopia was defined to be spherical equivalent higher than +0.5 D and emmetropia to be higher than −0.5 and lower than +0.5 D. Myopia was defined to be with a SE lower than −0.5 D.

Descriptive statistical analyses were performed to characterize the participants’ clinical data using GraphPath Prism statistical software version 7.03. The data were tested at 95 % level of confidence using Chi-square.

3. Results

The effect of magnitude error (diopters) on percentage central visual field result is shown in (Table 1). The table indicates that ametropes whose degree of error is above ±7.00 D had no normal binocular central visual field.

Table 2 revealed the degree of ametropic conditions on central visual field. The effect was tested on myopes and hyperopes. There were no significant relationship (p<0.05) observed on the effect of myopes and hyperopes on the central visual fields. The abnormal central isopter for myopes were higher than those of the hyperopes.

Table 3 showed the effect of gender on central visual field in ametropic condition of the eye. From the result, there was no significant difference observed (p<0.05) between the central isopter and abnormal central isopter in both male and female gender.

The result of the normal blind spot at various distances 1 and 2 m showed an increase in 2 mm stimulus target as the distance changes in both directions (vertical and horizontal) as shown in (Table 4).

There were significant relationship (p< 0.05) observed in myopia, hyperopia and the blind spot size (Table 5).

4. Discussion

In the world today, visual impairment has been a challenging threat especially in technological less advantaged countries. Again, visual impairment secondary to uncorrected refractive errors in childhood is an important health problem, and can lead to social, educational and economical disability in adulthood ^{10, 11}. Olunye and Sargent ¹² has reported that visual impairment is generally correlated along with hearing impairment, as a sensory disorder. There is no scientific documentation that evaluated the effect of refractive error on central visual field in Owerri municipal, Imo State, Nigeria. In this study, the empirical correlation between ametropia and the central visual field of the eye especially on the central isopter and the blind spot size were evaluated.

The result revealed that ametropes whose degree of error > ±7.00 D had no normal binocular central visual field, while those with < ±3.00 D had a higher percentage normal visual field in decreasing diopters. Traquair indicated that slight increase in the central isopter size from fixation point is seen in myopic condition and slight decrease in central isopter size from fixation point occurs in hyperopia condition of the eye. Hrynchak et al. ¹³ describes the prevalence of refractive errors for myopia is <−0.50 D. The reports from National Health and Nutrition Examination Survey (NHANES) revealed a mean ocular refraction (MOR) for refractive errors between -0.5 D and + 0.5 for people between the ages of 20 and 59, while those above 60 years had a higher refractive peaks between + 0.5 and + 2.0 D ¹⁴. Wood and Guggenham ¹⁵ revealed that refractive error usually myopia and hyperopia have age-related effects.

The purpose of the eye’s optical system is to cast an image of the external world onto the photoreceptor layer of rods and cones in the retina to support vision ¹⁶. In this study, there were no significant relationship (p<0.05) observed between ametropes (myopes and hyperopes) on the central visual fields. Although, refractive errors are the major causes of visual impairment, they can be easily corrected with spectacles as a cost effective treatment modality ¹⁷. Myopia or short sightedness exists when parallel light rays enter the eye and focus in front of the retina. Concave or minus lenses are often used in the correction of myopia. Minus or concave lenses are used due to its divergence property of light rays entering the eye. It moves light to the focal point back to the plane of the retina ¹⁸. On other hand, hyperopia or long sightedness exists when parallel light rays enter the eye and focus behind the retina. Plus lenses are used in the correction of hyperopia. Plus or convex lenses are used due to its light ray converging ability which moves light to the focal point up to the plane of the retina ¹⁹. Irving et al. ²⁰ reported that mean ocular refraction and refractive error distribution vary with age. The highest magnitude of myopia is found in young adults

The result also revealed a non-linear significant relationship between genders (male and female) in ametropic condition of the eye. Gong et al. ²¹ had reported that gender is one of the risk factors accounting for the high prevalence of myopia in adolescent. They opined that myopia incidence of female is higher than that of male. The present study suggested that the central visual field loss in ametropic condition of the eye may not be sex-related

One of the most striking factors in visual field assessment is the evaluation of the size of the blind spot. The blind spot is the area on the retina without receptors that respond to light. However, an image that falls on this region will not be seen. It is in this region that the optic nerve exits the eye on its way to the brain. The result of the present study revealed significant relationship (p< 0.05) in ametropes (myopia and hyperopia) and the size of blind spot. Furthermore, blind spot enlargement can be caused by a range of medical conditions and monitoring the size of the blind spot scotoma can indicate progression of disease ²². The nerve and the retinal elements had been implicated to be involved in the ametropic condition of the eye. This nerve and retinal elements is often manifested in size and location of the blind spot especially in patients with high refractive error.

5. Conclusion

The present study has established that ametropia affects the central visual field efficiency in young adults. Again uncorrected refractive error, particularly myopia, is often a problem in young people. Poor vision and the inability to read material written on the blackboard can have a serious impact on the performance of an individual both at school and at work. Clearly, we suggest that a large proportion of vision loss can be corrected with the simple provision of an appropriate pair of spectacles. Also to assess and improve vision by providing corrective measures should be taken seriously so as to ameliorate the dangers of blindness

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