Friday, July 24, 2009

Ocular injuries after solar exposure

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A digression on the “Astigmatism series”.

Summer is here and many of you had already been on holiday or you are thinking about that, preparing everything to enjoy a well-deserved time for rest :-)

Do not forget to put in the suitcase sunglasses for all your family members (adults and children). Just as you do not forget the sun protection lotions, sunglasses are something indispensable to enjoy some safe holiday.

Think about the time that you are going to be performing outdoor activities, without your eyes being protected. Furthermore, there are more harmful radiations in the beach, because we have to add the ones that are reflected in the sea; but many surfaces reflect those harmful rays (sand, buildings, sidewalks,…). Even in cloudy days, with high clouds, the solar radiation that crosses them is nearly the same one that when there are no clouds at all. Only rain, fog and low clouds reduce UV radiation in a significant way.

Any tissue of the eye, as it happens with the skin, can be burnt. Solar UVA or UVB radiations on the tissues cause a cellular death process and a transformation of cell DNA, which can create irreversible disorders. Therefore, we must take care of our eyes by wearing lenses that comply with health regulations of visual quality. In the case of European sunglasses, the CE mark identifies glasses fulfilling quality regulations.

Atmospheric ozone is a barrier against very detrimental UVC radiation and other UV types; and this moderates the quantity of UVB that arrives to the earth (more harmful than UVA).
The constant slimming of ozone layer causes an increase of the UVB radiation that we receive daily. So, while this keeps being like that (unfortunately), we will have to do everything in our power to protect our eyes.

Some injuries that are caused in the eye due to solar radiation are the following ones:


Cornea and crystalline lens absorb most of UVA and UVB radiation that get into the eye, damaging these structures; therefore, if a person has got a keratoconus or she has undergone a refractive surgery, her cornea will be thinner and it will be able to absorb less radiation, so, this radiation might get into the eye, causing some injuries.

The aqueous humor of the anterior chamber has a lot of Vitamin C (ascorbic acid), which is responsible for filtering UV radiation, and this way, achieving that the least amount possible of radiation gets to the crystalline lens.
  • A prolonged exposure to sunlight decreases the quantity of vitamin C in the aqueous humor.

The same as solar exposure can cause that your skin aged sooner, the same happens with the lens; it might suffer from a premature aging, because of the damages in DNA.

Looking directly at sun (in a eclipse or not), without adequate eye protection, causes a photochemical damage of the photoreceptors, inducing a burn at macular level (area of maximum vision of the retina) and being the reason of an irreversible blindness.
  • Early development of Macular degeneration
  • Melanomas

Tuesday, July 21, 2009

Refractive disorders: Astigmatism. (3) Different features

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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.


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

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.

  • 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.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...

Monday, July 06, 2009

Refractive disorders: Astigmatism.(2) Appearance

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Appearance of astigmatic eye

As I wrote in a previous post, one characteristic of the astigmatic eye is that the external face of its cornea -1- is not spherical (as a pure myopic or hyperopic eye), but elliptical. It is similar an American football cut in half (CORNEAL ASTIGMATISM).

But astigmatism may be more complicated than all that, since because the cornea has a thickness, the curvature of the internal face of the cornea may also cause an astigmatism that is more complicated to diagnose and treat; the same happens with the one that is caused also because of an unusual slope of the crystalline lens -8 (INTERNAL ASTIGMATISMS). Those last cases are less frequent.

But both of them (corneal and internal ones), may exist at the same time; and the addition of both, is the result of the total astigmatism that one person has got.

This typical shape of the ocular surfaces causes that their different "meridians" (horizontal and vertical ones) do not have the same power; therefore, some of them are more curve than others, and this causes the light is focused on two or more planes regarding the retina, instead of only on the retina itself. The light is clearly focused along one plane but is blurred along the other. The result is blurred vision at all distances.

More ...

Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...