PREVALENCE OF CONGENITAL COLOUR VISION DEFICIENCY IN NIGERIANS LIVING IN UGEP, CROSS RIVER STATE

Transcription

1 1 PREVALENCE OF CONGENITAL COLOUR VISION DEFICIENCY IN NIGERIANS LIVING IN UGEP, CROSS RIVER STATE A DISSERTATION SUBMITTED TO: THE UNIVERSITY OF NIGERIA, NSUKKA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF SCIENCE (M.Sc) IN PHYSIOLOGY BY UBOM, RUKU EDWIN (PG/M.Sc/06/45962) DEPARTMENT OF PHYSIOLOGY COLLEGE OF MEDICINE UNIVERSITY OF NIGERIA ENUGU CAMPUS March, 2011

2 2 CERTIFICATION This is to certify that the present study titled Prevalence of Congenital Colour Vision Deficiency in Nigerians Living in Ugep, Cross River State, was carried out by Ubom, Ruku Edwin (PG/M.Sc./06/45962) under the supervision of Professor J.C. Igweh. That, the report has not been submitted to this University or any other institution for the award of a degree Prof. J. C. Igweh Dr. U. S. Anyaehie Chief Supervisor Co-Supervisor... Dr. U. S. Anyaehie Head of Department

3 To Jesus Christ 3

4 4 ACKNOWLEDGEMENT In the course of the study, several individuals made useful contributions and it is my pleasure to acknowledge them. Professor J.C. Igweh, Chief Supervisor and Dr. U. S. Anyaehie, the Co-Supervisor who supervised the study and ensured an accurate presentation of the report. The Head of Department, Dr U.S. Anyaehie, who facilitated the study and the entire M.Sc. programme. Dr J.F. Ude who kindly provided me with the Ishihara s album used for the mass screening of the study subjects. Dr U.I. Nwagha, Dr E.E. Iyare, Mr M.I. Nweke and Mr D.C. Nwachukwu as well as the entire staff of faculty of Medicine in the University whose suggestions in various stages of the study are reflected in the content and style of presentation of the report. My colleagues in the M.Sc. class and in School of Nursing, Itigidi were also helpful to me in the course of the study. Mr N.U. Ikoi is acknowledged for the help he offered in the collection of data. My spouse, children, entire family and brethren deserve a happy return for their various contributions.

5 5 ABSTRACT Colour vision deficiency and colour blindness are synonymous terms describing poor colour discrimination by the visual senses. Congenital colour vision defects are common, x-linked inherited, non-progressive and untreatable disorders. Elsewhere, screening for these disorders are an established practice so that those affected can be advised about occupational preclusions. However, population-based study on the broader impact of colour vision defects is limited. A descriptive crosssectional survey was conducted using Plates 1-17 of the 2008 edition of the Ishihara s colour album. The study was undertaken in Ugep, a rural community in Cross River State, Nigeria. A convenient sample of 1500 male and female subjects ranging from years of age was used and the selection was based on cluster sampling. The study reveals that the prevalence of congenital colour vision deficiency in Nigerians living in Ugep is 1.87%(28 of 1500 subjects) and that of total colour blindness is barely 0.20%. The gender distribution of colour blindness in the sample 2.8% for males and 0.7% for females indicates a significantly greater frequency of defect among males than females (p<0.001,df=1). The distribution of colour blindness based on age brackets 10-20,21-30,31-40,41-50,51-60 years was 16,2,1,9,0 and this reveals no sequence between age and the defect (p<0.001,df=1). The findings which serve as base line data for the area under investigation are inconsistent with Nigerian samples reported for other regions in the country but the regional variations are not accounted for. Populationbased screening for colour vision deficiency helpful for prevocational counselling is recommended.

6 6 Certification Dedication Acknowledgement Abstract Table of Content List of tables List of figures TABLE OF CONTENT Page i ii iii iv v vii viii Glossary ix CHAPTER ONE 1.0 Introduction 1.1. Background of the Study Statement of Problem Significance of Study Purpose of Study Objectives of Study Research Hypotheses Research Questions Glossary CHAPTER TWO 2.0 Review of Related Literature 2.1. Basic Theories of Colour Vision Classification of Congenital Colour Vision Prevalence of Congenital Colour Vision Deficiency Importance of Investigating Colour Vision Deficiency Clinical Tests of Colour Vision

7 7 CHAPTER THREE 3.0 Methods and Methodology 3.1. The Research Design The Study Area The Sample Sampling Procedure Instrument for Data Collection Procedure for Data Collection Method of Data Analysis Validity of Ishihara Test Plates Ethical Consideration CHAPTER FOUR 4.0 Presentation of Results 4.1 Frequency of Colour Vision Deficiency Frequency of Different Types of Colour Vision Defect Frequency of Colour Vision Deficiency Based on Gender Frequency of Colour Vision Deficiency Based on Age -24 CHAPTER FIVE 5.0 Discussion of Findings 5.1 Discussion Summary and Conclusion Recommendation Suggestion References Appendix I Appendix II Appendix III Appendix IV

8 8 LIST OF TABLES Page Table 1a: Prevalence of Normal/Defective Colour Vision Table 1b: Chi square Analysis of Data on Prevalence of Normal/Defective Colour Vision Table 2a: Frequencies of the Various Types of Colour Vision Defects Detected in the Sample Table 2b: Chi square Tests Statistics on the Frequencies of Types Of Colour Vision Defects Table 3a: Gender specific Prevalence of Colour Vision Deficiency 22 Table 3b: Chi square Tests Statistics on Gender specific Prevalence of Colour Vision Deficiency Table 4: Age specific Prevalence of Colour Vision Deficiency - 24

9 9 LIST OF FIGURES Fig. 1: Pie Chart Showing the Prevalence of Normal/Defective Colour Vision 19 Fig. 2: Bar Chart Showing the Prevalence of Dichromats and Monochromats 21 Fig. 3: Bar Chart Showing Gender Specific Distribution of Colour Vision Deficiency Fig. 4: Bar Chart showing age specific Prevalence of Colour Vision Deficiency 24

10 10 GLOSSARY Colour Vision Deficiency Inability to identify one or more colours of the spectrum. Colour Blindness Misleading term for deficient or anomalous colour vision. It does not mean that objects are seen only in black and white but refers to types and degrees of colour confusion. Trichromat A person who is able to perceive the three primary colours red, green and blue. Dichromat A person who is able to perceive only two of the primary colours and colour matching is done with only the two colours. Monochromat A person who cannot perceive any colour but sees the whole spectrum in different shades of gray. Protan A person who has red colour blindness due to absence of red sensitive pigment in the cone Deutan A person who has green colour blindness. Tritan A person who has blue colour blindness. Ishihara s Colour Album Charts by Japanese Ophthalmologist, Shinobu Ishihara, used for detecting different types of colour vision defects.

11 11 CHAPTER ONE INTRODUCTION 1.1 BACKGROUND TO THE STUDY Colour identification is one of our most important visual abilities and nearly everyone including colour vision defective individuals can see colour and make discriminations based on colour. This general tendency seems to query the rationale for screening colour vision and minimizes the benefits derived from available reports on colour blindness. In the course of studying normal colour vision, investigators have observed a wide range of colour discrimination ability especially under such circumstances as the absence of cues, poor illumination, working at speed and viewing objects that subtend a narrow angle at the eye. It is further observed that colour vision defectives show colour vision deficits when compared with those with normal colour vision (Ishihara, 2008; Williams et al, 1998; and Balasundaram and Reddy, 2006). However, some claim that colour vision deficiency does not interfere with daily routine or lifestyle since a reduced visual acuity is not associated with it. Most people with colour vision defects develop effective adaptive strategies and behaviours, and they use other cues such as colour saturation, to deal with any potential limitations in their professional personal life. This makes it possible for most colour blind individuals not to be aware their deficiency (Holroyd and Hall, 1997). Others speculate that clinicians are reluctant in colour vision investigations because should a congenital deficiency be identified there is no treatment for those affected (Adams and Haegerstrom, 1987). Ishihara(2008) identified colour vision deficiency of congenital origin as the commonest form of colour vision disturbances and explained that

12 12 most cases of congenital colour vision deficiency are characterized by a red-green deficiency which may be red colour blindness(protan defect) or green colour blindness(deutan defect). The main peculiarity of redgreen deficiency is said to be the fact that red and green colours appear as grey or dark while blue and other colours appear remarkably clear. The application of this peculiarity to the test for colour vision deficiency is the distinguishing feature of Ishihara test which is used to survey the prevalence of congenital colour vision deficiency in Ugep. The use of Ishihara colour album has been practiced routinely for many years as screening tools for the assessment of congenital colour vision deficiency. Besides their role as simple diagnostic devices, they are of sufficient sensitivity to allow investigators use the results in a clinically meaningful way. 1.2 STATEMENT OF PROBLEM The screening of colour vision is not well appreciated and even clinicians often conduct only a cursory examination of this aspect of vision. Perhaps, this is because it is generally believed that colour blindness is a minor inconvenience without any functional disadvantage on an individual (Cumberland et al, 2004). More so, in a society where there is less social discrimination coupled with increasing emphasis on equal opportunity, people with impaired colour vision are allowed to undertake jobs that require critical colour judgment. Hence, many do not consider the need to assess their colour vision status. However, the problem situation is that most of the items in daily use, including colour computer monitors, colour pictures, symbols and printed matter are coloured materials that place a demand on us to interact with and distinguish numerous shades and tints of colour. Again, the rising technology poses basic challenges. Firstly, several careers now require

13 13 critical colour judgment and employees are thus expected to possess fine colour discrimination ability. Secondly, there is continuously lower cost of colour printing that makes available more coloured materials and further increases our chances of relying on our colour vision function. The situation tends to be critical in that what is largely available is the extrapolations of prevalence of colour blindness not based on specific data sources. Such estimates have very limited relevance to the actual prevalence of colour blindness in any region. 1.3 SIGNIFICANCE OF STUDY Counselling a subject concerning colour vision defect is an important component of colour vision testing. Therefore, the present survey may help to control the handicap of colour vision deficiency when used for counseling subjects concerning the effects of defective colour vision on daily routine of life, learning progress and effectiveness in occupations that require critical colour judgment. Also, those who are congenitally colour defective may be unaware of their own deficiencies and early diagnosis is valuable not only in this respect, but also in planning vocational choices (Taylor,1971). Again, an assessment of colour discrimination helps to determine the functional and structural intactness of the sense of vision (Tusa and Newman, 1995). Finally, the study might provide a basis for comparison of the prevalence of colour blindness in the area under investigation with what is observed in other areas as researchers attempt to formulate hypotheses on the significance of colour vision deficits in human populations.

14 PURPOSE OF STUDY The aim of the study is to assess the status of visual perception of colour among individuals in the study area. 1.5 OBJECTIVES OF STUDY To determine the prevalence of colour vision deficiency in the study area. To verify the claim that total colour blindness is a rare condition. To examine the occurrence of colour vision deficiency in the area on the basis of gender and age. 1.6 RESEARCH HYPOTHESIS There is no significant difference between the occurrence of normal colour vision and defective colour vision in the population. Total colour blindness is not a rare condition. The prevalence of colour vision deficiency is not associated with gender and age. 1.7 RESEARCH QUESTIONS What is the prevalence of defective colour vision in the population? Is total colour blindness a rare condition? Is the prevalence of colour vision deficiency associated with gender and age?

15 15 CHAPTER TWO REVIEW OF RELATED LITERATURE 2.1 BASIC THEORIES OF COLOUR VISION Several theories have been proposed to explain colour vision. However, a complete explanation of colour vision is still largely theoretical and most assertions are based on the few universally accepted theories. Young-Helmholtz Trichromatic Theory Palmer(1777) hypothesized the presence of a three-receptor system in the retina but Young(1802) is generally given credit for the trichromatic theory of colour vision because of his lecture in 1801 to the Royal Society of London formally describing the theory using physiological and physical terms(bounton,1979). Helmholtz later revived the theory, stating that there are three basic colour sensations, red, green and blue, across the entire visible spectrum. That, these three sensations relate to the activation of three distinct photo pigments found in the cone cells of the retina. That the three types of cone cells have maximum sensitivities within the visible spectrum in the red, green and blue portions, and the interaction of these cells gives rise to colour perception. White colour results from equal stimulation of the three types of cells, black results from an inactivity of the cells of the retina and other colours are compound sensations brought about by different combinations of stimulation of the three types of cells(sherman, 1981). Hering s Opponent Theory Hering(1964) developed the opponent colour theory when he noted that the trichromatic theory is inadequate to explain the perception of colour under all viewing conditions. To clarify the phenomenon that deal with the psychological aspects of colour perception, Hering

16 16 explained such colour phenomena as simultaneous colour contrast, dissimulative-assimilative processing and how certain colours cannot be seen in combination. He further proposed that opponent processing channels are present within the visual system. Granit s Modulator and Dominator Theory Granit(1974) observed that the cells of the retina are stimulated by the whole of the visual spectrum and that the cells are classified into dominators and modulators. According to Granit(1974), the dominators are responsible for brightness or intensity of light while the modulators are responsible for colour vision. That, if green light falls on retina, modulators of green are stimulated and other two are less affected(sembulingam and Sembulingam, 2006). Hartridge s Polychromatic Theory It is proposed that the retina has seven types of receptors divided into three units. A tricolour unit consisting of receptors for orange, green and blue; a dicolour unit with receptors for yellow and blue, and another dicolour unit with red and blue-green receptors (Sembulingam and Sembulingam, 2006). 2.2 CLASSIFICATION OF COLOUR VISION DEFICIENCY Deviations from the Trichromatic theory of the normal human colour vision is said to form the basis for which colour vision deficiencies are classified. If there is a reduction in the number of the cone types present it is either dichromacy or monochromacy. If there is an alteration in the specific wave length sensitivity of one of the cone types it is anomalous trichromacy(robert and Terry, 1990). Von-Kries first classified colour vision deficiencies in 1897, noting that there are congenital and acquired defects. The congenital defect

17 17 has been attributed to an abnormality of one of the three pigments within the cones transmitted as X-linked recessive trait due to an abnormal gene on the X-chromosome. The acquired defect is said to result from damage at any level along the visual pathway. Von-Kries (1897) also introduced the classification system for congenital defects using words from the Greek: protan = first, deutan = second and tritan = third, depending on the cone type affected. He explained that protons (protanopes) lack the long wavelength sensitive cones (protanic dichromacy) or the cones are present but exhibit an abnormal spectral sensitivity (protanomalous trichromacy). Protan defect is also called red colour blindness and is characterized by insensitivity in colour discrimination at the long wavelength end of the visible spectrum. The subjects use blue and green to match the colours. Thus, they confuse red with green. Deutan and tritan are terms used to specify similar defects in the middle wavelength sensitive or short wavelength sensitive cone types. Fletcher and Voke (1985) and Wald (1966) had consistent reports with Von-Kries (1897). They agreed that deutans(deutaranopes) lack the middle wavelength sensitive cones either as deuteranopic dichromats or deuteranomalous trichromats. Hence deutan defect is characterized by insensitivity to green colour and is called green colour blindness. The subjects use blue and red to match the rest of the colours. Only a few investigators have tested the colour vision of large number of people with congenital tritan defect because the condition is said to be rare (Piantanida, 1990 and Wright, 1952). Several other investigators simply refer to hereditary blue-yellow losses as tritan defects without specifying whether they are dichromatic or anomalous. Tritan defect is however observed to show autosomal dominant inheritance pattern.

18 18 Von-Kries (1924), Ruddock (1991) and Pokorny et al(1979) had consistent reports on the classification of protans and deutans as redgreen defects, having noted that there is similarity of colour confusions and inheritance patterns for protans and deutans. According to Boynton (1979), monochromacy is an extremely rare condition. He referred to this as true colour blindness because no colour discrimination is present. Researchers explained that monochromacy is present when only a single type of cone in the retina is functional and that visual perception of colour is based solely differences in brightness (Partnall et al, 1983). Verriest (1963) developed a classification system for acquired colour vision deficiencies based on the chromatic discrimination loss exhibited by the subject. That, a type 1 defect produces a red-green discrimination loss, Verriest type 2 defect shows a red-green loss with a milder blue-yellow loss and type 3 defect is characterized by a loss of discrimination along the blue-yellow axis. Kollner (1912) also studied acquired colour defects and developed a general guide to help identify the primary location of the causative lesion. Kollner s rule stated that damage within the outer retinal layers is characterized by a blue-yellow defect, whereas damage of the inner retina and optic nerve gives rise to a red-green defect. Although this general rule is a useful memorization tool, investigators have noted that numerous pathologic conditions give rise to colour deficiencies contrary to these guidelines. 2.3 PREVALENCE OF CONGENITAL COLOUR VISION DEFICIENCY The English chemist John Dalton published the first scientific paper on the subject in 1798 Extraordinary Facts Relating to the Vision of Colour after the realization of his own colour blindness (Tusa and

19 19 Newman, 1995). Thereafter, there has been increasing interest in colour vision status of individuals of different regions of the world. Available population data on the prevalence of colour vision deficiency reveal that the commonest form of deficient colour vision is the congenital red-green deficiency with a marked male predominance owing to its X-linked recessive inheritance. It is added that total colour vision deficiency is a rare trait (Harrison et al, 1964; Stern, 1973; Balasundaram and Reddy, 2006). Congenital colour vision deficiency is reported to be 8% in males of European descent, having had a number of studies of European males that consistently detected frequencies generally higher than 7.5% (Post, 1962; Mckusick, 1994). However, among the Basques of Western Europe, a frequency as low as 4.02% and 3.4% were reported for Americans of mixed African and Caucasian parentage and those of apparently consistent African parentage, respectively (Post, 1962). In a study of 1214 school children in petaling Jaya, Malaysia, the prevalence of colour vision deficiency was found to be 2.6 % (Reddy and Hassan, 2006). A survey of colour vision deficiency among 1427 students and health care personnel in Seremban Malaysia revealed a prevalence of 3.2% as another Asian data (Balasundaram and Reddy, 2006). In Africa, a high frequency of 3.9% was reported for a sample of Bantu-speaking Ugandans, but most frequencies for other parts of Africa were less than 3%(Pickford and Pickford, 1981). Zain (1990) however, reported a higher frequency, 4.2% among Ethiopians. In two studies of red-green colour blindness among males in Nigeria, Roberts (1967) observed frequencies of 1.81% (11 of 609) among the Hausa/Fulani of Katsina State in the Northwest of 3.33% (2 of 60) among the Yorubas in the Southwest in the first study and 2.11%(8 of 380) among the Abuo/Ogoni of Rivers State in the Niger

20 20 Delta. In spite of the differences in habitats between the Northern (savanna) and Southern (forest) populations, Roberts (1967) found that the observed frequencies of red-green colour blindness among the three groups were not significantly different from those in a number of subsaharan African populations. However, Odeigah and Okon (1986) reported a frequency of 7.01% (53 of 750) for red-green colour blindness in a sample of Nigerian males in primary and secondary schools and Universities in Lagos, a cosmopolitan city in the Southwest. The frequency, excluding children and grand-children from unions between Nigerians and Europeans, was 6.76% (38 of 562). Both frequencies were statistically not significantly higher than the frequencies detected in Nigeria by Roberts (1967) and also the frequencies reported for other African populations (Pickford and Pickford, 1981). 2.4 IMPORTANCE OF INVESTIGATING COLOUR VISION DEFICIENCY Hart (1987) asserted that colour vision performance is a sensitive measure indicating the relative integrity and function of the visual pathway. It was explained that the cones are relatively few in number within the retina and may be especially vulnerable to certain alterations. Therefore, a test of colour vision may be regarded as a test of the intactness of the visual pathway. Again, owing to its X-linked recessive inheritance, colour blindness prevalence is used as a marker in studies concerning genetically transmitted traits in a community and hence the importance of wide cross sectional repeat prevalence surveys (Natu, 1987). Many systems for teaching both reading (example Gattegno, 1964) and arithmetic (Cuisenaire and Gattegno, 1957) utilize colour. While colour cueing can facilitate the acquisition of vocabulary and visual sequencing, confusion over colour can impair learning at a critical age of educational development (Hallahan and Kauffman, 1975). Mistakes made over colours can destabilize associations between vision

21 21 and language for some children (Litton, 1979). It has been suggested that defective colour vision may be a factor in some cases of specific learning disability because a greater incidence of defective colour discrimination has been observed in the educationally handicapped and those having a poor primary school reading performance (Espinda, 1973). Deficiency in colour vision has been directly implicated in intelligence test performance (Mitchell and Pollack, 1974). Some have argued that dichromatism can be responsible for a non-achieving attitude in school (Synder, 1973; Heath, 1974). Others however, have failed to observe any association between defective colour vision and academic performance (Mandela, 1969; Lampe et al, 1973). Dwyer (1991) suggested that such children are incorrectly diagnosed as colour defective because they cannot respond appropriately to the test as a result of language or other difficulties. He concluded that the production of false positives in a colour blindness test thus represent a mismatch between the intellectual capacity of the subject and the cognitive demands of the test, and does not undermine the evidence of exhibiting a poor educational performance. The data of Verriest (1963) indicate that colour vision normals have variable colour discrimination ability and it is therefore necessary to screen the sensitivity of potential employees in jobs that require fine colour discrimination. According to Taylor (1971), a colour vision defect may prevent entry into a particular career mainly for reasons of safety (example, the armed forces, transport service and fire service) or because critical colour judgments have to be made (example, pharmacy, laboratory sciences, chemical engineering, textile and printing trades). In some careers, a colour vision defect does not debar entry but can be a handicap (example, cartography, decorating and design work) depending on the type and severity of the defect (Pater and

22 22 Buckingham, 1993). According to Balasundaram and Reddy (2006), detection of certain clinical signs by health care practitioners requires unimpaired colour vision. That, such conditions as cyanosis, jaundice, colour of body fluids (example, haematuria) and blood in vomitus require colour differentiation, and those who are colour vision deficient tend to perform poorer. The extent to which these difficulties translate into actual professional error is yet to be determined. 2.5 CLINICAL TESTS OF COLOUR VISION In the first instance, Pokorny et al (1976), Higgins et al (1978) and several others noted that proper quality of illumination that approximates true day light should be considered central in colour vision tests due to the precision in chromaticity required for the various colours in the tests. Birch et al (1979) listed several standard illuminants that are commercially available including 100-W tungsten lamp, Macbeth easel lamp, Richmond products, Boca Raton and FL COLOUR NAMING According to Birch et al (1979), requesting an individual to name the colour of readily available objects has been an unreliable test of chromatic discrimination traditionally practiced. They added that, although the results of the naming of coloured spots are not useful in diagnosing a defect, the spots can be useful to demonstrate a child s colour confusion to a parent or teacher. - PSUEDOISOCHROMATIC PLATES The Ishihara, Standard (Part 1 and 2), American Optical HRR, Dvorine and Farnsworth F 2 pseudoisochromatic plates are listed as tests commonly found in the clinical setting (Good and Terry, 1986). The tests are all said to show relatively good predictive efficiency in screening for

23 23 congenital red-green defects, with only the American Optical HHR plates having significant testing capacity along the blue-yellow axis (Taylor, 1975). Adam et al (1983) reported the F 2 to be comparable to the American Optical HHR plates as a screening device and that even though the F 2 is not commercially available, it can be constructed using Munsell papers. The Ishihara test is preferred because it allows rapid yet sensitive screening of congenital colour vision deficiency. Familiarization with all the colours is not necessary since the answer is given in terms of numbers and not in terms of colours (Ishihara, 2008). - NAGEL ANOMALOSCOPE Steward and Cole (1989) recommended the modern version of Nagel Anomaloscope as the sincle instrument that can reliably classify deutans and protans, and discriminate extreme anomalous trichromats from redgreen dichromats. It is not often found in smaller clinics or in private hospitals perhaps due to its cost effectiveness. - FARNSWORTH-MUNSELL 100-HUE TEST Farnsworth (1949) introduced FM 100-Hue test for evaluating overall colour discrimination ability, even in those subjects with normal trichromatic vision. Verriest (1963) presented FM 100-Hue test as a device used to indicate fine colour discrimination ability among colour vision normals and thus considered relevant in screening the sensitivity of potential employees in occupations requiring fine colour discrimination. The data of Verriest (1963) indicate that colour discrimination ability is widely variable and appears to worsen with age.

24 24 - FARNSWORTH PANEL D-15 TEST Bowman(1982) and Vingrys and King-Smith (1988) assert that Panel D-15 is similar in format to the FM 100-Hue test but was designed to identify those individuals with a moderate to severe colour vision deficit. They developed a screening method for the Panel D-15 test that provides a number that can be used to specify the depth of a congenital defect or to monitor the progression of an acquired defect (Steward and Cole, 1984; Bowman and Cameron, 1984; Collins, 1986).

25 25 CHAPTER THREE METHODS AND METHODOLOGY 3.1 THE RESEARCH DESIGN Descriptive cross-sectional survey was conducted to determine the prevalence of congenital colour vision deficiency in Nigerians living in Ugep, Cross River State. 3.2 THE STUDY AREA Ugep is an urban settlement in central Cross River State, Nigeria with a projected population of about 100,000. It has boundaries in the North with Ekori, in the South with Idomi and Adim, in the East with Mkpani and in the West with Ediba and Usumutong communities. There are diverse natural heritage in the area but majority of the people engage in subsistence agriculture. 3.3 THE SAMPLE 1500 subjects were selected from the population in the study area. Inclusion Criteria Male and female children above 9 years of age in selected schools within the area under investigation. Adults residing in the area. People who are willing to participate in the study. People who are present at the time of data collection. Exclusion Criteria Children under the age of 10 years. People who are not willing to participate in the study. People who are unable to read and write numerals. People with impaired visual acuity as well as the totally blind. People who are not present at the time of data collection.

26 SAMPLING PROCEDURE Cluster Sampling - the population was considered in clusters of homogenous units such as families, classes of schools, offices of employees, church congregations as well as social and business gatherings. The selection was done by chance occurrence. 3.5 INSTRUMENT FOR DATA COLLECTION Plates 1 17 of the 2008 Edition of the Ishihara s colour album were used. The individual plates or charts have symbols composed of numerous disks of varying colours and brightness. The background for the symbols are similarly configured but have colours that can be differentially discriminated by persons with normal versus defective colour vision. Only small differences are present between the symbol and background to make the charts sensitive enough to identify mild colour defects, but the differences are great enough to allow subjects with normal colour vision to pass. The diagnostic effectiveness of Ishihara s test is maximized when the directions for use are followed. 3.6 PROCEDURE FOR DATA COLLECTION Plates 1 17 of the 2008 Edition of the Ishihara s colour album were administered to the subjects in rooms brightly lit by daylight. The subjects were screened with Plates 1,3,5,13 and 15. Those judged according to the Ishihara criteria to be possible colour defective were retested with the full set of Plates If the defect was ascertained then Plates 16 and 17 were used to determine the type of red-green defect. Of the Plates 1 15, if 13 or more plates were read correctly, the colour vison was regarded as normal. If only 9 or less than 9 plates were read normally, the colour vision was regarded as red-green deficient and thus classified as colour blind. Among the colour blind individuals, those who read not more than two plates (Plate 1 inclusive) as either normal or red-green defective are classified as totally colour

27 27 blind (monochromats) while the red-green defectives (dichromats) were further classified as deutans or protans based on their ability to read Plates 16 and 17 correctly. Subjects were tested 2 or 3 at a time standing about 50cm apart and about 75cm from the Plates, which were held so that there was no glare. Subjects were given about 3 or 4 seconds to read and write the number seen in a plate. The writing was done in a proforma (Appendix 2) prepared to facilitate the collection of data. 3.7 METHOD OF DATA ANALYSIS The data obtained was arranged in tables, bars and charts on the basis of simple percentages. The level of significance in chi square tests determined whether the number of affected in two groups were different from each other at 1% (P<0.001, df=1). 3.8 VALIDITY OF ISHIHARA TEST PLATES Ishihara s test plates are standard tools proved to be effective for the diagnosis of congenital red-green colour vision deficiencies but unreliable when used alone to test for acquired colour vision defects. The test is not useful for detecting blue-yellow patterns of defect in differential diagnosis. Ishihara s test confirms that subjects have normal trichromatic vision but it cannot be used to grade overall colour discrimination ability (Ishihara, 2008; Hart, 1995; Good and Terry, 1990). Justin and Hart(1976) assert that, to a child of limited experience, the coloured dot numbers of Ishihara s plates lack clarity and have configurations which differ from those numbers commonly shown to children in that age group. Hence, its use is inappropriate for children less than 7 years of age. Adams (1974) found that about half of those who fail pseudoisochromatic plate tests, and are subsequently tested by

28 28 the D-15 test, do not have a colour defect of such severity to warrant consideration as an educational handicap. 3.9 ETHICAL CONSIDERATION A written informed consent was obtained from the study subjects (Appendix 1).

29 29 CHAPTER FOUR PRESENTATION OF RESULTS 4.1 Frequency of Colour Vision Deficiency Table 1a Prevalence of Normal/Defective Colour Vision No. Tested Normal Colour Vision Defective Colour Vision (98.13%) 28(1.87%) A total of 1500 subjects participated in the study subjects representing 98.13% were found to have normal colour vision while only 28 (1.87%) subjects were found with defective colour vision. Defective Colour Vision Normal Colour Vision Fig.1: A pie chart showing the prevalence of normal/defective colour vision Table 1b Chi-square Analysis of data on Prevalence of Normal/Defective Colour Vision Colour vision Freq. observed Freq. expected Normal Defective df X 2 cal X 2 table p

30 30 Chi-square test showed a highly significant difference between the subjects of normal colour vision and those of defective colour vision. Therefore, hypothesis 1 which states that there is no significant difference between the occurrence of normal colour vision and defective colour vision in the population was rejected. Rather, there is a very low prevalence of defective colour vision as against a significantly high occurrence of normal colour vision in the population. 4.2 Frequency of Different Types of Colour Vision Defect Table 2a Frequencies of the various types of Colour Vision Defects detected in the Sample No tested No. of defects Dichromats Monochromats Total defects Classified *Unclassified Total Deutans Protons 1, * Plates 16 and 17 could not classify this as deutan or protan. The distribution of the subjects in terms of the various types of colour vision defects detected in the sample revealed that only 0.2% (3 of 1500) of the entire sample was classified as monochromats and 1.67% (25 of 1500) as dichromats. Among the dichromats, there was a greater frequency of deutans than protans, 64% (16 of 25) as against 32% (8 of 25). Using Plates 16 and 17 of the study instrument, it was not possible to classify 4% (1 of 25) of the dichromats as either deutan or protan.

31 N o o f d e f e c t Dichromats Monochromats Type of defect Fig.2: Bar Chart Showing the Prevalence of Dichromats and Monochromats

32 32 Table 2b Chi-square test statistics on the Frequencies of types of Colour Vision Defects Defect Freq. observed Freq. expected Dichromats Monochromats 3 14 df X 2 cal X 2 table p The number of dichromats was significantly higher than monochromats. The monochromats on the other hand were scarcely observed in the entire sample. Therefore, hypothesis 2 which states that total colour blindness is not a rare condition was rejected. 4.3 Frequency of Colour Vision Deficiency Based on Gender Table 3a Gender-specific Prevalence of Colour Vision Deficiency Gender No. tested Defective colour vision Total Dichromats Monochromats Male of 830 (2.8%) Female of 670 (0.7%) The gender distribution of colour vision deficiency suggests a greater frequency of defect among males than females, 2.8% as against 0.7%. Within one sex, there was a marked discrepancy between dichromats and monochromats.

33 Dichromat Monochromat Male Female Fig.3 Bar Chart Showing Gender Distribution of Colour Vision Deficiency Table 3b Chi-square Test Statistics on Gender-specific Prevalence of Colour Vision Deficiency Gender Freq. observed Freq. expected Male Female 5 14 df X 2 cal X 2 table p

34 34 Chi-square test showed that the frequency of male defect is significantly higher than female defect. Therefore, hypothesis 3 which states that the prevalence of colour vision deficiency is not associated with gender was rejected. 4.4 Frequency of Colour Vision Deficiency Based on Age Table 4 Age-specific Prevalence of Colour Vision Deficiency Age Group No. Tested Defective colour vision (in years) Fig.4: Bar Chart Showing Age-specific Prevalence of Colour Vision Deficiency

35 35 The distribution of colour vision deficiency on the basis of age is shown in table 4. It revealed that the age of the subjects ranged from years with the mean age being 25.3 years and a prevalence of 0.9% (2 of 227) detected in the age bracket which bears the mean age. Using the Statistical Package for Social Sciences (SPSS) to analyse linear by linear association or interval by interval Pearson chi-square, a p-value of was obtained as against table value of (p>0.001,df=1). This showed that there is no sequence between age and colour vision deficiency. Therefore, hypothesis 3 which states that the prevalence of colour vision deficiency is not associated with age was accepted.

36 36 CHAPTER FIVE DISCUSSION OF FINDINGS 5.1 Discussion Ishihara s album was chosen as the instrument for data collection because it is an easier and quicker device for mass screening of congenital colour vision deficiency. Familiarization with all the colours is not necessary since the answer is given in terms of numbers and not in terms of colours. The prevalence of colour vision deficiency detected in the present study was 1.87% (28 of 1500 subjects). Roberts (1967) had observed in separate studies frequencies of 1.81% in Northern Nigeria, 3.33% in Southwest Nigeria and 2.11% in the Niger delta. Comparison of each of these Nigerian samples with a frequency of 1.87% in the present study showed that there was no significant difference in the prevalence of congenital colour blindness among Nigerian populations in spite of the differences in habitat between the Northern and Southern regions. Odeigah and Okon (1986) in a study of separate ethnic groups in Nigeria concluded that the frequency of colour blindness cannot be accounted for on the basis of ethnic composition of the samples. On the contrary, Choudhury (1994) found the people of North East India to be extremely variable with regard to this trait, the frequency ranging from 0 to 8.05%, and this generates questions on the factors leading to variations in colour blindness among different populations. Referring to the literature of frequencies of other African populations, Asian and American data as well as the 8% prevalence rate consistently detected for males of European descent, there is sufficient indication that the prevalence of colour blindness among black Africans and Negros generally tend to be low compared to the Caucasians.

37 37 A wide cross sectional repeat prevalence survey as suggested by Natu (1987) might be needed to test the influence of habitat and occupation on colour blindness, especially for studies carried out in rural areas where the uncertainties of ancestry can be minimized. The prevalence of total colour blindness detected in the present study is 0.20%. This is not significantly different from 0.08% detected by Odeigah and Okon (1986) and 0.19% by Williams et al (1998). That the prevalence of monochromats in the population is almost negligible verifies the claim that monochromacy is a rare autosomal trait (Harrison et al, 1964; Stern, 1973). The barely few monochromats detected has made it impossible to state whether there is a significant difference between male and female frequency of total colour blindness. On the other hand, there was a marked discrepancy between male and female dichromats in the data. This higher occurrence of the condition in males suggests that the prevalence of colour vision deficiency in the population is gender specific. Williams et al (1998) reported a similar disparity of 3.6% prevalence for males and 0.81% for females, and suggested that these values be regarded as the frequency of colour blindness among Nigerians, especially Yorubas in Lagos. That the trait occurs more in males than females is consistent with the assertion of Von-Kries (1897). He explained that the hereditary defect is an abnormal gene linked to the X-chromosome. That, since males have only one X-chromosome, the chances of manifesting the defect is higher than in females with double X-chromosomes should there be a transmission of the X-linked recessive trait on one of the chromosomes. The frequency of female colour vision deficiency is however similar to the frequency in a number of populations which have similar frequency of affected males (Post, 1962). Rebato and Calderon (1990) and Zein (1990) reported frequency of affected males as 0.46% for Basques and 0.20% for Ethiopians

38 38 respectively. The frequency of 0.71% female prevalence in the present study is not significantly different from the male value in these other places. An observation of the frequency of the various types of colour vision defects detected in the sample showed that the frequency of deutans was twice as much as protans. This agrees with the report of Post (1962) in his review of previous work as well as by Odeigah and Okon (1986) and by Zein (1990). However, it should be noted that equal frequency of deutans and protans were detected in some African samples by other investigators (Pickford and Pickford, 1981) and in India (Dronamraju, 1963). Williams et al (1998) had approximately equal frequency of deutans and protans. The present data showed that colour blindness is not age-specific since it did not indicate any relationship between age and colour vision defects. Odeigah and Okon (1986) could not account for this trait on the basis of age composition. Their reports did not reveal any significant difference among the age ranges in their samples, as it is true of the present study. Adam (1969) and Mckusick (1994) among others have noted shortcomings associated with the Ishihara s album. The present study also highlights some of them. Two people, of normal colour vision, looking at the same thing might come out with different results. It is therefore not surprising that even though the Ishihara charts are made up of colour dots, both normal and defective individuals, contrary to expectation, read some plates alike. Some of the defective subjects saw numbers which were different from those expected of either normal or defective individuals while others saw numbers in plates which should have appeared blank to them, but the numbers were different from those seen by normal individuals.

39 39 Pickford (1969) reported that the two plates which are supposed to differentiate between deutans and protans were unsuccessful while Dronamraju(1963) also reported some difficulty. These plates, Plates 16 and 17, were also not completely effective in the present study. There was a case that could not be clearly classified as either deutan or protan using Plates 16 and 17, and the same subject who read Plate 16 as normal and Plate 17 as blank was noted as unclassified. Williams et al (1998) observed similar difficulty and recorded 21 of 245 dichromats as a group that could not be clearly classified as deutans or protans using Plates 16and 17. It was noted that some of the unclassified dichromats saw numbers not present in the key, others either read one plate as normal and the other as blank or read both as normal. However, as Adam (1969) pointed out, in the absence of other readily available tools, close adherence to the guidelines for use of the Ishihara s album produce results that approximate anomaloscope results. As reports on colour vision abnormalities increase, Roberts(1967) and Adam(1969) noted that there might not be any simple explanation for the biological significance of the deficiency, especially as Roberts(1967) has pointed out that heavily pigmented peoples generally have the lowest prevalence of the trait, and so do Eskimos and north American Indians. On the other hand, Adam (1969) also referred to unpublished data showing some rather high prevalence among some Papua-New Guinean populations.

40 SUMMARY AND CONCLUSION *The 1.87% prevalence of congenital colour vision deficiency detected in Nigerians living in Ugep serve as base line data. *The findings are inconsistent with Nigerian samples reported for other regions in the country, suggesting regional variation of the trait in different populations in Nigeria in spite of the difference in habitat between the Northern and Southern regions. *Matched for age, congenital colour vision defect is not associated with age. *Matched for gender, the trait is gender specific, having a higher prevalence in males, 2.8%, than females, 0.7%. 5.3 RECOMMENDATION *Population-based screening of colour vision deficiency helpful for prevocational counseling is recommended. *Teachers should be trained to perform colour vision screening and to modify their teaching methods to accommodate the child with colour vision deficiency. *Individuals with colour blindness should be legally barred from occupations in which colour perception is an essential part of the job or in which colour perception is important for safety. *Instructors should avoid using colour coding or colour contrasts alone to express information. Rather, visual systems should operate in shades of grey. *Test for normal colour vision should be part of the medical certification that is a prerequisite for obtaining drivers licenses.

41 SUGGESTION It is suggested that the study on colour blindness be extended to other geopolitical regions in Nigeria in order to ascertain regional variations in colour blindness and determine the factors implicated for such variations.

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48 48 APPENDIX I INFORMATION LETTER Please, may I ask you to participate in a survey of congenital colour vision deficiency in Ugep, Cross River State, conducted to assess the colour vision status of individuals in the area. A series of test chats will be presented requesting you to read and write down the numerals seen on the charts. Your ability to identify the numerals indicates your colour vision status. There are no anticipated adverse effects to you as a result of your participation in this study. All information collected from participants in this study will be aggregated and all identifying information removed. Thank you. Ubom R. E Investigator Dept. of Human Physiology College of Medicine University of Nigeria Enugu Campus CONSENT FORM I, agree to participate in this study being conducted by Ubom, Ruku Edwin of Physiology Department, University of Nigeria, Enugu campus. I have made this decision based on the information I have read in the information letter. Address: Signature:... Date......

49 49 APPENDIX II FORM FOR THE COLLECTION OF DATA ON COLOUR VISION DEFICIENCY NAME: SEX: AGE: L.G.A: TOWN: OCCUPATION Write down the number seen on the chart shown to you. CHART NUMBER SEEN

50 50 APPENDIX III METHOD OF DATA ANALYSIS Chi, X = (0 E) 2 E = Group 1 Group 2 (0 E) 2 + (0 E) 2 E E Where 0 = observed frequency E = expected frequency Estimated Standard Error = p (1 p) n = sample size p = sample proportion = r n where r is observed frequency. n