Myopia can be classified in various ways: cause (axial or refractive) (Emsley, 1953), age of onset (congenital, youth onset, early adult-onset, or late adult-onset) or degree (low, intermediate or high) (Grosvener, 1987). When classifying myopia by degree, high myopia is indicated where myopia is greater than 6.00 dioptres, intermediate between 3 and 6D and low 0.50 to 3.00D (Fitzgerald, Chung, and Krumholtz, 2005).
Axial myopia is due to the axial elongation of the vitreous chamber meaning that the eye becomes too long for its refractive power. (Benjamin, 1998) Refractive myopia is caused by an error in the dioptric power of the eye either due to variation in the refractive index of lens (index myopia), increase in curvature of the refractive surface of cornea (curvature myopia) or abnormality in anterior chamber depth (anterior chamber myopia) (Borish, 1970). Further research (Strang, Winn and Bradley, 1998) showed that most cases of significant myopia are mainly due to axial myopia.
Theories about the cause of myopia are currently incomplete. It is thought there is some genetic aspect but it can also be induced by an excessive amount of near work stimulating accommodation. (ref) Systemic conditions may also have an influence in myopia and its progression. (ref)These include: Marfans syndrome, foetal alcohol syndrome, Crouzoan syndrome and cerebral palsy. Another influence proven to be associated with myopia and its progression is birth weight and ...
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