Myopia is a condition where in there is relatively a higher plus power(convex lens) in the eye than required to focus on the retina than required. So it is corrected by minus spectacle number. Plus power in the eye ( around 60 D) is contributed by the corneal power ( around 40 D) and natural lens(around 20 D). Amount of power required to focus the image on the retina in an individual eye depends on the axial length of eyeball( front to back length of the eye).
Normally the eye ball is slightly shorter at birth and hence most of young children have plus power at birth which does not require to wear glasses. Eyeball grows in its axial length and reaches normal size at around 4–6 years and reaches zero power state in most of the people.
However certain eyes continue to grow especially more during adolescence( 12–18 years of age) resulting in longer eye. Longer eye needs less plus power to focus on the retina. As a result relatively higher plus power compared to the power required to focus the image on retina results in myopia which needs to be corrected by minus spectacle lens. This is called axial myopia which is the most common type.
Less common cause of myopia is curvature myopia where corneal power is more to increased curvature of the cornea. This may be occasionally due to corneal condition called keratoconus.
Whatever is the cause of myopia, it cannot be reversed by any exercise as longer eye ball does not become short or curvature of the cornea does not change. Only hypermetropia ( corrected by Plus spectacle number) tends to remain stable decrease till the age of 18 years due to increase in the axial length of the eye.
Vision can be restored in myopia by using glasses or contact lens.
Myopia can be treated by changing the curvature of the cornea by laser corneal procedure or introduction of minus lens inside the eye as in phakic lens(ICL)
Myopia in older people (above 50–60 years) may be induced by cataract which is reversed b cataract surgery.
