Cortical Visual Processing In Axial Myopia

2792 words - 11 pages

Myopia is a refractive defect in which parallel rays of light are brought to focus in front of the retina when no accommodation is used (Grosvenor and Goss, 1999; Saw, et al., 1996). This causes distant objects to appear blurred while near objects can still be seen clearly.

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 prematurity. It has been found that 42% premature babies went on to develop more than 12D myopia, indicating this as a major influence for high myopia (Fitzgerald, Chung and Krumholtz, 2005).

Myopia is the most common refractive error and affects about 1 in 4 adults aged over 40 in the United States and Western Europe (Kempen et al, 2004) with around 1 in 24 of the general population exhibiting high myopia (Jackson and Wolffsohn, 2007). The prevalence of myopia within children varies highly with ethnicity. In Asian populations children of Chinese ethnicity are predisposed to myopia more than those of western populations whereas with adults the results are comparable (Pan, Ramamurthy and Saw, 2012).

With around 1 in 4 aged over 40 people suffering from myopia this indicates that it is a serious health issue due to its high prevalence but also due to the visual complications associated with pathological myopia. (Pan, Ramamurthy and Saw, 2012) Pathological myopia can be defined as myopia where there is excessive axial elongation that causes degenerative changes such as retinal detachments, myopic macular degeneration, cataract, glaucoma and peripheral retinal changes such as lattice degeneration (Duke-Elder, 1970)....

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