To the best of our knowledge, this is the largest population study examining the ophthalmic and demographic associations of CVD. We found that ametropia, strabismus and amblyopia, are more common in individuals with color vision impairment. CVD prevalence is not uniform, varying not only by country of birth but also by socioeconomic background and cognitive function.
The reported prevalence of CVD varies in different populations, ranging from a low 1.4% in African-American males to a high 21% in Irish men to.2,3 The prevalence of CVD among white Caucasian males was previously described to be in the range of 5.6% − 8%,2,3,22 which is mildly higher than the 4% that we had found in our study consisting mostly of Jewish men. Our study population came from all over Israel, whereas in a study examining Israelis from the North of Israel the prevalence of CVD was 5.78%.14 Possible explanations for the population variance in CVD prevalence include natural selection processes, founder events or genetic drift. Considering the relative mild impact CVD has on survivability, previous researchers have favored the theory of founder events in some populations.22,23
In this study we found that CVD was more common in individuals from a higher socioeconomic background. A similar trend was also noted by Machluf et al. who reported on CVD rates of 5.10% and 6.73% in lower and higher socioeconomic groups, respectively.14 However, in their study this difference did not reach statistical significance, possibly due to lack of statistical power. This association is difficult to explain and may be multi-factorial but, interestingly, we also found an association between CVD and higher cognitive function tests as well as lower rates of attention deficit disorder, two factors which may potentially influence income capabilities.24 Cumberland et al. found that among academic performance in reading and mathematics were better in 16 years old individuals with CVD compared with normal controls.4 Previous studies reported that over a third of CVD individuals were influenced by their disorder when deciding upon their career and felt that it affected most aspects of their daily living.25,26
The association between CVD and ametropia is not well established with conflicting reports in the literature on this issue. Some authors found that CVD is associated with less refractive errors, while others reported the opposite. Ostadimoghaddam et al. found that both myopia and hyperopia were less common in primary school children with CVD compared with normal controls.11 Gan et al. have also shown a lower incidence of myopia and myopia progression in Chinese primary school children with CVD.13 On the other hand, Rajavi et al. found a higher incidence of refractive errors among primary school CVD Iranian children,27 and Machluf et al. showed that individuals with either myopia or hyperopia had higher incidence of CVD, compared with emmetropes.14 Our study, reporting on over 37,000 individuals with CVD, supports the latter studies, finding that CVD is associated with increased prevalence of both moderate and severe ametropia. It is possible that increased ametropia in CVD becomes more apparent with increased age. Additionally, we found increased prevalence of amblyopia in these subjects. This is not surprising as we found that the two most common risk factors for amblyopia, namely ametropia and strabismus were more prevalent in these individuals.28 Screening young children for CVD is an established practice in many countries, mostly for occupational and educational reasons.4,5,7 However, our study indicates that there is an added significance for identifying CVD at an early age, since these children are at risk for having amblyopia, that may benefit from early detection and prevention.
This study has several limitations. First, its design relies on the accuracy and completeness of recruits’ medical records including color vision testing. Second, we considered all recruits failing Ishihara color vision test as CVD, not differentiating degrees of severity (number of missed plates) and types of dyschromatopsia that were not recorded. Therefore, it is impossible for us to determine if our findings exist in all types of CVD. Third, our refractive errors data did not differentiate between hyperopia, myopia and astigmatism, not allowing us to better classify the association between CVD and type of refractive error. Fourth, our study includes mostly Jewish Caucasian Men, therefore additional studies are required in order to establish whether its results apply to other populations as well.
In conclusion, this large population study demonstrated that ametropia, strabismus and amblyopia are more common in individuals with CVD, suggesting that early screening for CVD is important, not only for educational and occupational purposes, but also for avoiding preventable vision loss.