Six Year Refractive Change among White Children and Young Adults: Evidence for Significant Increase in Myopia among White UK Children

OBJECTIVE:To determine six-year spherical refractive error change among white children and young adults in the UK and evaluate differences in refractive profiles between contemporary Australian children and historical UK data. DESIGN:Population-based prospective study. PARTICIPANTS:The Northern Ireland Childhood Errors of Refraction (NICER) study Phase 1 examined 1068 children in two cohorts aged 6-7 years and 12-13 years. Prospective data for six-year follow-up (Phase 3) are available for 212 12-13 year olds and 226 18-20 year olds in each cohort respectively. METHODS:Cycloplegic refractive error was determined using binocular open-field autorefraction (Shin-Nippon NVision-K 5001, cyclopentolate 1%). Participants were defined by spherical equivalent refraction (SER) as myopic SER ≤-0.50D, emmetropic -0.50D<SER<+2.00 or hyperopic SER≥+2.00D. MAIN OUTCOME MEASURES:Proportion and incidence of myopia. RESULTS:The proportion of myopes significantly increased between 6-7 years (1.9%) and 12-13 years (14.6%) (p<0.001) but not between 12-13 and 18-20 years (16.4% to 18.6%, p = 0.51). The estimated annual incidence of myopia was 2.2% and 0.7% for the younger and older cohorts respectively. There were significantly more myopic children in the UK at age 12-13 years in the NICER study (16.4%) than reported in Australia (4.4%) (p<0.001). However by 17 years the proportion of myopia neared equivalence in the two populations (NICER 18.6%, Australia 17.7%, p = 0.75). The proportion of myopic children aged 12-13 years in the present study (2006-2008) was 16.4%, significantly greater than that reported for children aged 10-16 years in the 1960's (7.2%, p = 0.01). The proportion of hyperopes in the younger NICER cohort decreased significantly over the six year period (from 21.7% to 14.2%, p = 0.04). Hyperopes with SER ≥+3.50D in both NICER age cohorts demonstrated persistent hyperopia. CONCLUSIONS:The incidence and proportion of myopia are relatively low in this contemporary white UK population in comparison to other worldwide studies. The proportion of myopes in the UK has more than doubled over the last 50 years in children aged between 10-16 years and children are becoming myopic at a younger age. Differences between the proportion of myopes in the UK and in Australia apparent at 12-13 years were eliminated by 17 years of age

Understanding myopia: Pathogenesis and mechanisms

Myopia is a common refractive error, characterized by an excessive increase in axial length relative to the refractive power of the eye. Despite much research, the mechanisms underlying the development of myopia are unknown. A large body of work on animal models (such as chicks, guinea pigs, and monkeys) has been instrumental to our understanding of visually guided ocular growth, and potential mechanisms leading to myopia. These studies have shown that experimentally degrading the quality of the image formed on the retina by introducing translucent diffusers (i.e., form-deprivation), or altering the focal point of the image with respect to the retinal plane by imposing plus or minus lenses to the eyes (i.e., lens induced defocus) results in abnormal eye growth and development of reflective errors. Ocular changes in response to form-deprivation and lens induced defocus are primarily associated with changes in axial length (mainly due to changes in vitreous chamber depth) and choroidal thickness. These experimentally induced ocular changes quickly revert to normal upon removal of the imposed optical treatment. Physiological changes in retinal cells and neurotransmitters (such as dopamine), presence of ocular aberrations, altered accommodative response to visual stimuli, and even subtle variations in natural circadian rhythms of axial length may all influence ocular growth, and hence susceptibility to myopia. In fact, several optical interventions alter ocular aberrations, peripheral refraction, and the accommodative response of the eye in an attempt to arrest myopia development. Epidemiological studies have also linked excessive near work, better socioeconomic status, and urbanization to myopia, although the exact cause for these associations remain elusive. Based on decades of work on the effects of ambient lighting on refractive development in laboratory animals, recent clinical studies have revealed protective effects of greater outdoor exposures on development and progression of myopia in children. Experimental models continue to provide valuable information on the cellular and biochemical mechanisms of myopia.</p