Until recently, myopia has been thought of as a simple refractive error, with its origin mainly genetic. This view has recently changed and in this article, we are going to have a look at the reason behind this shift in perception.
Consider the following definitions of disease:
• A disease is any harmful deviation from the normal structural or functional state of an organism, generally associated with certain signs and symptoms and differing in nature from physical injury.
• A disease is an abnormal condition affecting a living organism. Diseases are generally understood to be medical conditions that involve a pathological process associated with a specific set of symptoms.
Clinical trials and work with animal models of myopia have provided ample evidence that axial elongation is the primary factor driving myopic progression. Refractive myopia, in which the optical power of the cornea and/or lens is abnormally high in eyes with a normal axial length, also exists. However, axial myopia is far more common than refractive myopia.
When comparing myopia management treatments to reduce myopic progression, there is a clear relationship between the impact of an intervention on refraction and axial length.
Ocular morbidity increases per diopter and per millimetre of axial length, thus increasing the risk of serious, sightthreatening complications.
Normal structural and functional development in a healthy eye:
The human eye is programmed to achieve emmetropia in youth and to maintain emmetropia with advancing years. This is despite the changes in all eye dimensions during the period of growth and the continuing growth of the lens throughout life. The process of normal emmetropisation in children’s eyes is demonstrated by a shift from refractive errors around a hypermetropic mean value at birth, to around an emmetropic mean value in adulthood.
Axial Length
The Axial Length (AL) is measured as the distance from the anterior corneal surface to the retinal pigment epithelium. Full term babies have a mean axial length of 16.8 mm at birth,13 which typically ranges between 16-18mm,12 and increases to an average of 23.6 mm for near emmetropic adults. The most significant increase in axial length occurs during the first 3 to 6 months of age. Large scale studies on eye growth of the ocular components suggest that the eye reaches its adult emmetropic axial length by the age of 13 years. The anterior chamber normally reaches its maximum depth, and the crystalline lens its minimum thickness by about 15 years of age.
Refractive Error
The widest range of refractive errors in children is found at birth and during the first year of life.
Most infants are hyperopic, with a mean cycloplegic refractive error of approximately +2.00D. A few infants are myopic at birth, and most of those who are either myopic or hyperopic will emmetropise. There is also a higher prevalence of astigmatism at birth, with as many as 69% of full-term newborns having astigmatism of 1.00D or more. Significant with-the-rule (WTR), against-therule (ATR) and oblique astigmatism are all more prevalent in young children than adults. Of these three types, oblique astigmatism is the least common. In most populations there is a decrease in both the prevalence and degree of astigmatism in the first few years of life.
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