Snellen test | Diseases and conditions: Medical tests.

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What is the Snellen test

The Snellen test is a test that is very commonly used by opticians and ophthalmologists. Thanks to him, the visual acuity of a person, i.e. the ability that has the human eye to distinguish objects at far distances can be studied. Visual acuity reached several tens of meters in people with a healthy view, but this capacity may be weakened by alterations eye like myopia or farsightedness that the eye has damaged its mechanism of refraction and focuses on evil.
The test was designed by Hermann Snellen, a Dutch physician, born in Utrecht in the 19th century. He was mathematically calculated the distance to which a healthy person correctly distinguishes two nearby objects in a separate way, so I can create a table with letters to different sizes that had to be interpreted within a specific distance each. Otherwise it is interpreted as a visual acuity deficit and had to investigate the cause.
It is a simple and very economical test (only required black letters of different sizes, drawn on a white surface), the Snellen test is used on a daily basis in consultations of Ophthalmology and optics around the world. In addition, it is the first step to study the optical diopters and its correction with lenses. Only does it take a few minutes to perform and is not no discomfort to the person who is subject to it.


When a Snellen test is done

The Snellen test must be performed to all persons who believe to have a lack of visual acuity. A person can realize it when you notice changes in the ability to see from far away. Some examples could be: watch TV at the same distance from always blurred, not read posters in stores and difficulty interpreting traffic signs, etc.
But the changes in visual acuity tend to be progressive, so that people don't often realize losing vision of coup and become accustomed to see worse. So it recommended, unofficially, be a Snellen test with some frequency or in the following cases:
Children: a Snellen test should be performed at least two times in children separated by 5-6 years. Also when suspected of having difficulty to read slate.
Teenagers: at the beginning of high school and at the end of these studies must be a Snellen test to check that the growth has not altered the optics of the eye.
Young: once the growth phase of adolescence, only has to perform a Snellen test kids long phases of study (e.g., college or opposition).
Adults: single Snellen test need to be repeated approximately every 10 years.
Nursing: the test should be performed when a visual deficit is suspected, but is recommended to go to the eye doctor because many times it can be blamed to the formation of cataracts in the lens.

The Snellen test prep

If you will do a Snellen test , these are issues that you should keep in mind about your preparation:
Duration: the Snellen test can take several minutes, approximately 5-7 minutes. However, all the consultation with an ophthalmologist or eye doctor can last up to 15-20 minutes because you will be asked for your vision.
Income: under no circumstances must be entering no one to perform the test. You can go to the clinic and back home in a few minutes.
Is necessary to be accompanied?: do not need to be accompanied.
Drugs: don't need to take any prior medication. You should tell your doctor all the medicines taken regularly, especially if you take anticholinergic drugs or you use eye drops eye.
Food: don't why stay in fasting prior to testing or after. Although it is recommended not to drink alcohol before the test.
Clothes: you can go dressed as you want, you won't have to take off any item at any time.
Documents: take the national document of identity and medical history about eye diseases, if you have them. After the test you can to give you a document explaining you the visual acuity deficit.
Pregnancy and breast-feeding: the Snellen test can be performed on any of these physiological situations without any problems. It is true that pregnancy can alter the visual acuity in some women temporary or permanently. He is recommended that expect to pass the stage of lactation to have final results.
Contraindications: do not have any specific contraindication.

How Snellen test

The ophthalmologist or optometrist is responsible for rating if it can be useful to make the Snellen test. That same query will perform the test, since no previous preparation is necessary. If you have dilated pupils to you you will not make you the test and will have to come another day.
Ask yourself that you feel in front of a white wall, about six meters from it. When you're sitting comfortably, they placed you glasses without lenses, but with one eye covered. Thus they will evaluate you an eye on each shift and the deficit of visual acuity in each eye must be measured separately. In front you will see the table with the letters that you have to identify. Sometimes these letters are projected on the wall, if so turn off the light.
The eye doctor will ask that you sign each letter one by one. You may say what or what you think they are. Sometimes you will not see them clearly and you can not say what, say what an eye doctor. Usually it will ask that you try to guess what, so you can see what is the degree of deterioration of visual acuity that you have.
When you take a certain number of right or wrong lyrics it will be over the test. If you've failed several can in sunglasses who have gotten you put lenses that alter the diopters of your eye, and you have to say if you see better or worse. Thus you graduate the vision to get glasses. After the test will give you a document (sometimes is not bigger than a card) with the visual deficit identified.
The Snellen test is a test devoid of risks and complications.

The Snellen test results

For the results of the Snellen test uses a very simple mechanism: each letter you've identified or failed under has a small number that helps the eye to know your degree of vision loss. This number indicates the meters to which letter should be identified. For example, a 12 will mean that that letter should be seen just 12 metres away.
To relate it to your loss of vision will be used a fraction whose denominator is the number of the table and the numerator the meters that you see it. For example, 6/12 means that a letter that should be 12 metres you can only see it at 6.
During the test you don't you walk away from the table, always remain approximately six metres of it. In order to study it without moving the letters change size from some very small to very large. Previous studies know which letter corresponds to far, so can be the test comfortably without leaving the site.
A common confusion related to the Snellen test results is to confound diopters as a measure of visual acuity. Diopter refer to the loss of the ability of the eye to focus for failures in its refraction. It really is the measure of correction of visual acuity.
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