Myopia
In myopia (short-sightedness), distant objects are blurred, while near objects can still be seen clearly. The eyeball in a myopic individual is longer than a normal eyeball, and we can measure this ‘axial length’. The longer the axial length, higher is the myopia. For every 1mm increase in the axial length, myopia will increase by roughly -3 diopters. The average axial elongation rate in children is 0.2 mm per year.
High myopia is defined as myopia more than -6 diopters. Because the myopic eyeball is too long, the retina tissue becomes thinner, and this increases the risk of retinal detachment and macular degeneration. In addition, the risk of developing glaucoma and cataract is also higher in high myopic individuals.
Although the need of wearing spectacles can be eliminated by doing laser vision correction after the age of 18, the underlying eyeball is still longer than normal. Laser vision correction only changes the curvature of the cornea (the most-front part of the eye), but does not change the overall eyeball length. Thus, the risks of high myopia-related complications stay the same even after laser vision correction. In developed countries, high myopia is becoming one of the leading causes of blindness. Therefore, we now emphasize the importance of myopia control - to slow down the myopia progression during childhood, so as to prevent them from developing high myopia and the related complications.
In Hong Kong, the prevalence of myopia was about 30% at age 6 to 7 years, 50% at age 10, and 70% at age 16 to 17. Among the 330,000 children aged 12 to 17, about 22,600 children have more than -6 diopters of myopia. Therefore, it is important to slow down the rate of myopia progression during ocular development.