Refractive disorders: Astigmatism. (3) Different features

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Development

Astigmatism relatively changes a little throughout all life.

Astigmatism is not very frequent during school age and it has got few changes of frequency and degree. One study made in Orinda, California, showed that the frequency of increase in an astigmatism of 1 diopter or more, at 6 years old, rises gradually from 2% by 3%, at 14 years old.

Higher levels of astigmatism are associated with moderate to high hyperopia during infancy, but both tend to decrease by the age of 5 years.

If a child is going to have a high astigmatism, it should already exist before beginning school stage.

In adult age, astigmatism does not usually change; if it do it, it usually indicates tension-related one, as myopia (in both cases vision therapy is very useful in order to structure a correct vision).

The little astigmatism that appears during infancy can be due to the strength that the upper eyelid exerts on the cornea causing that the vertical meridian to be more curve than the horizontal one.
In a more mature age, this astigmatism may change its shape, turning the vertical axis flatter, because of the laxity of palpebral muscle that rests on the eyeball. That is the reason why the axis or degrees of our astigmatism change through the years.


Prevalence

According to an American study published in Archives of Ophthalmology, nearly 30% of children between the ages of 5 and 17 have astigmatism.
On the other hand, a recent Brazilian study found that 34% of the students in one city of the country were astigmatic.
The National Autonomous University of Mexico revealed that astigmatism is the visual problem with most prevalence among people younger than 23 years old, and even 23% of population younger than 14 years old, put up with it.
Regarding the prevalence in adults, a study in Bangladesh found that nearly 32.4% of those over the age of 30 had astigmatism.
Also, several studies have found that the prevalence of astigmatism increases with age.


Factors and Causes

FACTORS:
If a child is going to have a high astigmatism or hyperopia, these will appear from birth or in early age. This means the factors are hereditary.

CAUSES:

• High weight of the upper eyelid.
• Slightly fallen upper eyelid (Ptosis).
• Ocular contusions.
• Corneal scars or lacerations, due to hits, injuries and infections in the eye.
• Changes in corneal shape following eye surgery (refractive one, of catarata,…)
• KERATOCONUS (this disorder will have its own post later, but let me briefly explain that the cornea acquires a conical shape as time goes by, and each time it gets thinner).
• Metabolic changes, as for instance high sugar levels in the blood that changes the shape of the lens of the eye, and this causes astigmatism. When this sugar level is normalized, the lens usually gets back to its shape and said astigmatism disappears.

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