What is the difference between myopia, hyperopia, presbyopia and astigmatism?
The eye can be thought of as a camera with light rays from an object being directed by the lens to form an image on the retina. Myopia and hyperopia can be viewed as a state where the eyeball is too long or too short respectively. In the former, the image of the object viewed is formed in front of the retina whereas in the latter, the image is projected posterior to the retina. This is why a concave lens to diverge the original rays of light from the object is used to correct myopia whilst a convex lens helps to converge the original rays of light in hyperopia. These lenses allow the image of the object to be formed precisely on the retina, allowing good clarity.
Presbyopia refers to the inability to autofocus for near objects when one reaches the age of forty years and above. This is related to weakening of the ciliary muscles of accommodation.
In astigmatism, the problem could reside in the cornea or less frequently, in the lens. In the case of corneal astigmatism, it can be said that the corneal surface is not perfectly spherical or round. The degrees of curvature are different at different axes. Cylindrical lenses are used to correct this.
Why is myopia control such a big concern in Singapore?
In Singapore, 10% of kindergarten children, 60% of primary 6 students, and 80% of 18-year-olds are myopic. These figures are amongst the highest in the world.
This has largely been attributed to both genetic predisposition as well as the tendency for children to prefer near-work activities including reading and game playing on handheld gaming devices, mobile phones or computers. Although children can still see clearly with the use of spectacles, the major concern is that, left unchecked, the child may develop high degrees of myopia which could lead to increased risk of more serious conditions such as retinal tear, retinal detachment, myopic macular degeneration and glaucoma.
What are the risk factors for myopia?
Asian ethnicity is a risk factor, as is a positive family history (twice the usual risk if one parent is myopic and 8 times the risk if both parents are myopic), congenital glaucoma, retinopathy of prematurity and connective tissue disorders. A modifiable risk factor is the amount of near work a child engages in. It is advisable for children to spend more time outdoors playing sports that require distance viewing in order to mitigate against the genetic predisposition. Natural sunlight also appears to confer some protection against the development of myopia.
How can I avoid myopia progression in my child?
Parents can consider atropine eye drop medication. Initially used in the late 1880s to relax the eye muscles of accommodation, it has been found that atropine can help slow down the progression of myopia. The exact mechanisms are unclear.
The first ATOM study done in Singapore showed that 1% atropine drops slowed myopia progression by 77 %. However, problems such as glare and poor near reading were encountered and children would need photochromatic glasses and a reading add component to their glasses.
The second ATOM study showed that much lower concentrations of 0.01% were still effective, albeit at 50-60 %, at slowing down myopia progression. Some clinicians have advocated a 0.05% concentration to improve success rates whilst keeping at bay the uncomfortable side effects.
Unfortunately, myopia progression can still be seen in up to 10% of children even when higher doses of atropine are prescribed.