People who are short sighted (myopic) have blurred vision for distant viewing, but clear vision for near, and this occurs because their eyes grew too long during childhood and early adolescence. Myopia is becoming more common, and can lead to loss of vision in middle and from degenerative and other changes. Studies show that "nature and nuture" are both important in the development of myopia and the "epidemic" of myopia in Asian children is attributed to as yet little understand factors associated with urban living. As it is becoming increasingly realized that what happens to individuals early in life, indeed, even before they are born, can influence later disease risk, we plan to analyse data collected from a cohort of children when they were aged 15 years who have been followed up regularly since they were born. To our knowledge this will be the first study of myopia during children to use the "life course" approach to analysis although data from the 1958 birth cohort (who are now middle aged) is ongoing, providing a very useful comparative study.

Background: The World Health Organization (WHO) estimate that 153 million people worldwide are visually impaired from uncorrected refractive error, 8 million of whom are blind. The report suggests that 12 million school-age children (5-15 years) are visually impaired from uncorrected refractive errors, and population based surveys undertaken using standard methods and techniques suggest that 90-95% of visual impairment in this age group is due to myopia. Uncorrected myopia is, therefore, a major public health problem.

The following 4 components of the eye determine its refractive status: axial length (AL), corneal curvature, anterior chamber depth, and the thickness, curvatures and internal refractive indices of the lens. After birth the eyes grow in all dimensions, increasing almost threefold in volume by the time adult size is reached. Eye growth is exquisitely controlled so that in most people the AL of the eye increases in tandem with changes in corneal curvature and dimensions of the lens so that the image of distant objects remains focused on the retina. Evidence from laboratory, animal and clinical studies suggests that growth of the cornea and lens are largely controlled genetically whereas AL is influenced at least in part, by visual stimuli. How the visual system detects and adapts to blur, and how this is then translated into altered growth is not known. However it is known that emmetropisation is an active and dynamic process which persists throughout childhood and adolescence, involving an image-processing feedback mechanism in the retina which ultimately influences growth of the sclera. Failure or perturbation of emmetropization can give rise to myopia. The plasticity of this process has been demonstrated in animal experiments in which eye growth has been manipulated by lid closure, diffusers and plus or minus lenses but there is considerable species variation. It seems reasonable, therefore, that changes in the environment in children may also modify eye growth.

There is compelling evidence from twin, family and association studies, and epidemiological research that myopia is a complex disease. However, the rapidly increasing incidence in SE Asia can only be explained by exposure to environmental risk factors that are new or where exposure at the population level is greater and/or more intense. Changes in behaviour and life style consequent to urbanization have been implicated but there are controversies surrounding which elements in childrens' environments are the more important. To date all studies have been either cross sectional or relatively short-term longitudinal studies in which standard multivariate statistical methods have been used. The latter cannot take account of the sequence and timing of the different exposures which are likely to be critical in the development of myopia.

The ALSPAC birth cohort enrolled 14,000 children born in Avon in 1991/92. The cohort have had eye examinations at different time points (see below) and there is a wealth of other data on relevant exposures (e.g. socioeconomic status of parents prior to enrolment and throughout the study; birth weight; breast feeding; early growth; parental myopia; close work; outdoor activities; intelligence etc). We will use the life course approach to analyses which will be the first time these methods will have been used in myopia research in children although we understand that data from the 1958 birth cohort are being examined in this manner. The following data on myopia and ocular dimensions are available from the ALSPAC cohort:

* undilated (ie without cycloplegic drops) autorefraction at ages 7, 10, 11 and 15 years. The prevalence of myopia (defined as less than 1.5D to take account of non-dilation) increased from 1.5% at the 7-year clinic (n=8003 right eyes) to 3.6% at the 10-year (n=7467 right eyes) and 4.9% at the 11 -year (n=6730 right eyes) clinics; 5500 attended and were autorefracted at the 15-yr clinic (data in preparation for analysis).

* vision related questionnaire data at 7 years and clinical examination at 7 years which included eye examination and the following measurements: LogMAR visual acuity, contrast sensitivity, stereo acuity, motor fusion; ocular alignment, eye preference, subjective accommodation.

* 2900 participants also have data on axial length and corneal curvature from the 15-year clinic

* Up-to-date optometric records for approximately 3000 participants who came to the 15-year clinic, and their siblings and parents

* Opportunistic autorefraction of approximately 2500 parents of the study participants who came to the 15-year clinic.