Exploring the world of vision

Monday, August 10, 2009

Refractive disorders: Astigmatism. (4) Symptoms.

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Behavior of astigmatic people

A low degree of astigmatism USUALLY causes more symptomatology, because the visual system tries to compensate it, and does not show blurred vision symptoms; therefore, at the beginning people do not usually associate the symptoms to a vision problem.

The main symptom is the distorted and blurred vision in high astigmatism; and headache, visual strain, ocular itching and red eyes in low astigmatism.

As I explained in a previous post, a young hyperopic person with the low degree is able to “disguise” her disorder if her accommodative function works correctly. But in the case of an astigmatic person, although her accommodation is in correct condition, it “gets crazy”, because it has several points or images that fall in different planes regarding the retina; accommodation is constantly trying to focus all of them, without distinction. The closest points of the retina will cause less tension, but those that are farther will be more difficult to compensate. Visual system will be exhausted and the symptoms (red eyes, itching, burning, tearing, headache, visual strain,…) will appear.

Although a person with astigmatism is usually born with it, if this is low, she could compensate it during the infancy. But as the child grows up and the school request is higher (more number of homework or study hours, more reading, more understanding,…), and her accommodation naturally decreases, the child has got more problems to compensate it; and this moment is when the astigmatism is obvious, showing several symptoms. That is why, many people say their astigmatism appeared between 10 and 20 years old.

So, an astigmatism will cause more symptoms if it is associated with a hyperopia than with myopia, because hyperopic people have a more exhausted accommodative system due to the excessive effort that she makes to compensate her refractive error.

That is why astigmatism can also cause learning disabilities.

It can develop wrong postural habits, like tilting her head when looking far away or looking up close, since when the astigmatic person tilts her head, can find the position in which she sees better, or where the image is less distorted. This can cause cervical problems in the long run.

“Blurred” or double vision greater at near than at distance.

While she reads, she jumps the line or the letters seem to be moving.

Visual strain and feeling sleepy.

Headache mainly in the fronthead and in the eyes.

Photophobia (light sensitivity).

Frequent conjunctivitis or blepharitis.

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:

IN CONJUNCTIVA (5)

IN CORNEA (1)
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.

Painful superficial keratitis
Photokeratitis or snow blindness
Climatic droplet keratopathy or Spheroidal Degeneration

IN AQUEOUS HUMOR (4)
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.

IN CRYSTALLINE LENS (8)
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.

Cataratas

IN RETINA (10)
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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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.

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.

Tuesday, June 30, 2009

Refractive disorders: Astigmatism.(1) Vision

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This third refractive disorder is the most common of them all, although the most known is myopia. This can coexist with myopia as well as hyperopia (13 percent of population has only got astigmatism, and 20 percent has astigmatism associated with myopia or hyperopia).

Astigmatic person’s vision

The typical complaints in astigmatism are “`BLURRED VISION´ MAINLY WHEN TRYING TO VIEW DISTANT OBJECTS, AND FREQUENT FRONT HEADACHE”. Only when the astigmatism is high, the complaint is also, “`BLURRED VISION´ AT CLOSE TASKS”.

If you realize I write 'BLURRED VISION' in quotation marks, because this refractive error, usually congenital, causes the outlines of the letters or the objects to be seen distorted; as if there would be some shadows behind every letter or every object, or as if they were seen double or distorted. Actually it is not a BLURRED VISION.

For low and medium quantities of refraction:

An uncorrected myopic person has low Visual Acuity at distance but it is good at near.

An uncorrected hyperopic person can have a good Visual Acuity at any distance as long as she has enough accommodation.

But, in spite of that a little astigmatism of about 0.50 diopter can not appreciably interfere in Visual Acuity in general, a person with uncorrected astigmatism does not have any distance where the image shaped in her retina is perfectly clear. In the case of a hyperopic astigmatism, by using help of the accommodation, it will be easier for itself to achieve better sharpness than a myopic astigmatism.

I am going to show you two simple ways for checking if you or your child has got astigmatism or not:

Put this image in the computer screen and get yourself away about 1 meter or 1 meter and a half, and covering one eye (and without wearing your glasses, if you have it), check if you can see ALL lines with the same contrast, that is, all of them have the same degree of “blackness” or all of them are clear or all of them are blurred.

If you do not have any astigmatism, the answer will be “yes”; you see it as it is in the drawing, or maybe all drawing is blurred (if you have some myopia or hyperopia).

But if you have it, the answer will be that you see something like this:

(Unless myopia or hyperopia, when we give the value of the astigmatism in a prescription, we give the power of the lens and also the axis of said lens).

This is the most common answer. It would belong to pure astigmatism between 180º and 150º (the astigmatic axis is perpendicular to the lines that you see better). If you have some myopia or hyperopia, as well the rest of axises will be seen out of focus.
Another clear way to check it is looking at the full moon. If you have some astigmatism, it will be impossible to perceive its outlines and it will be seen elongated and out of focus.

Wednesday, June 17, 2009

Refractive disorders: Hyperopia. (4) Solutions

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Solutions

Low or even medium hyperopias are difficult to diagnose because, as I wrote in one previous post, they “consume” the refraction. Besides, they are difficult to be detected by parents or teachers because these hyperopic people have a more or less clear vision in any distance. That is why these people need a COMPREHENSIVE EYE EXAM from early age and a chekup once a year.
In that comprehensive visual examination the refraction that the visual system of the hyperopic person is constantly compensating is checked, along with other skills of the visual system, specifically her accommodation which is the one that is working all time, and if it (accommodation) may have disturbed other visual skills.

In the case that the refraction error can be diagnosed or that the error is causing a certain symptomatology, these are the possible solutions:

Eyeglasses, either if she needs them to see clear because she has a high hyperopia, or if she has a low one and needs them to avoid that constant effort for close-up tasks, and this way, avoiding certain symptomatology. In the last case, eyeglasses are not used for seeing clear.

In the case of a high hyperopia, the use of eyeglasses can make sense both for short and long distance seeing, if the visual acuity for long distance is reduced.

In the case of lower hyperopia, that is causing some symptoms, the option of eyeglasses is good because she does not require wearing them during all day.

Hyperopia is corrected with plus, positive or convex spherical lenses (thicker at the middle than in the edges of the lens), that optically falls in front the image, on the retina.

If hyperopia is higher than +1.50 diopter, another option for compensating it, is by wearing contact lenses for a long period of time. In this case, unlike myopia, each patient may use the best contact lens for her, that is, there is no suitable specific one. There is no one that stops the hyperopia, simply because as I have explained, the hyperopia does not increase.

There exists a research about Orthokeratology, carried out specifically in Australia, which tries to find a contact lens that model the cornea and reduce the hyperopia; but even, as I say, it is under research.

Hyperopic people are usually more uncomfortable wearing contact lenses than eyeglasses, because although her visual field is bigger wearing contacts, the size of images is more real; but wearing eyeglasses with plus lenses, they magnify the objects (I will explain this effect later); therefore, as they see everything bigger wearing the glasses, they are more comfortable wearing them than with contact lenses.
Vision Therapy is the best allied in this refraction disorder. A hyperopic person does not often respond to lens correction alone, but they are required to “remediate” accommodative dysfunction. With therapy we teach her to control her accommodation and her convergence and to perform her close-up tasks without effort. We will avoid that this to happen again.

Within vision therapy, besides some simple visual activities, the use of eyeglasses with a low positive refraction will be able to relieve her symptomatology in order to be able to perform the daily close-up tasks. This way, she relieves all accommodative effort that she constantly performs.
Besides, as it happens with myopic people, some simple VISUAL HYGIENE AND ERGONOMIC RULES will help prevent her reaching that visual stress. Modification of the patient's habits and visual environment is occasionally useful as an adjunct therapy.

Mainly:
- Improving lighting or glare reduction
- Using better quality printed material.
- Decreasing temporal demands, with frequent rests.
- Looking far away and focusing on something, frequently, when performing close-up tasks.
- Etc.

Lastly, surgery: hyperopia is a refractive defect that can be operated as well, but except for high hyperopias, it is not considered as the best option.

Monday, June 01, 2009

Refractive disorders: Hyperopia. (3) Symptoms

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Hyperopia degrees

Low: 0 - (+2.00) D
Medium: (+2.25)-(+5.00) D
High: More than +5.00 D

Behavior of hypermetropic people (symptoms)

Usually, if hyperopia is low, hyperopic people do not have any symptom, and the time can go by until they show any. Besides, the younger the person is, the lesser the symptoms she will have, since her accommdation works perfectly and therefore, she can compensate the problem without any effort.

Either medium hyperopic, or not so young people or in certain cases will show (without wearing the refraction) the following symptoms:

Constant or intermittent blurred near vision.

If hyperopia is medium/high, it also affects distant vision.

Visual inconvenience when she performs close-up tasks.

Headache, visual strain, ocular pain, burning, itching, tearing, red eyes… (due to incapacity of keeping the effort of accommodation that is demanded).

In the case of low hyperopia that people have always been able to compensate without any problem (both as from a distance and as close up), as time goes by or in a special period with too many close-up tasks (at work or at school), they show inconvenience and discomfort and they do not know the reasons why they are caused. They have always seen well and have not had any previous visual problems; and they do not think, that this can be the cause of their problems. Before seeing blurred at near (while they can still keep the accommodation with effort), they usually show the symptoms above (headache, visual strain, itching, red eyes,...).

In the case of children or young people, they usually have problems with the reading: line jumping, jumping letters, letters “seem to dance”, “they don’t stop!”… (presbyopic or old sight people also say these same words). These symptoms cause that they have aversion to reading.

They as well perform facial contortions or frequent blinking while reading, that shows the effort that they are doing in their close-up tasks.

Poor eye-hand coordination.

In children (younger than 3 years old), if hyperopia is high, it can cause accommodative convergent strabismus (crossed-eye). This is because as they are young, they can perform much effort in order to see the image clear; but since accommodation and convergence are related (I will explain this better later), when the eyes perform too much accommodation, they also converge a lot, and one of eyes gets crossed. Consequently, this can also cause lazy eye or amblyopia on that eye.

In the case of low hyperopic people who have always been able to compensate without any problem and they have always had a enviable sight in their youth, they inevitably undergo presbyopia. But unfortunately for the hyperopic, they suffer it before that the rest of the people, before they are 40 years old. These people will think that their arms are shorter each time, because they need put their reading text farther (but this refractive disorder deserves its own post later).

Exploring the world of vision

Monday, August 10, 2009

Refractive disorders: Astigmatism. (4) Symptoms.

Friday, July 24, 2009

Ocular injuries after solar exposure

Tuesday, July 21, 2009

Refractive disorders: Astigmatism. (3) Different features

Monday, July 06, 2009

Refractive disorders: Astigmatism.(2) Appearance

Tuesday, June 30, 2009

Refractive disorders: Astigmatism.(1) Vision

Wednesday, June 17, 2009

Refractive disorders: Hyperopia. (4) Solutions

Monday, June 01, 2009

Refractive disorders: Hyperopia. (3) Symptoms

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Monday, August 10, 2009

Friday, July 24, 2009

Tuesday, July 21, 2009

Monday, July 06, 2009

Tuesday, June 30, 2009

Wednesday, June 17, 2009

Monday, June 01, 2009

Blog en español

My web sites

About me

Followers

Blog Archive

IT IS FOR...

Purpose of this blog

Acknowledgements

Subscribe to this blog

My another blog about vision

Contribution to other blogs

My Blog List

Interesting links