Refractive Error: Definition, Causes, and Types

Refractive Errors

What is the SimpLe Definition of refractive error?

The first thing that people should remember is that “refractive error” is just a term or name given to a specific condition already existing in the eyes. So, when trying to understand  what refractive error means, consider that you shouldn’t get stuck with the term “refractive error” when trying to understand the condition.  So, think like the first scientists who discovered this condition to understand it.

When scientists wanted to discover why some people don’t see things as clear as others, they found that every image is created on the eye in this way:  a light rays strike an object and then reflect to the eye to form an  image of that object. These light rays of the object must be focused to form a definite image. So, the eyes have a mechanism for focusing all entering light rays. This focusing mechanism bends the entering parallel light rays and makes them intersect in one plane inside the eye to form a focused image of the object, and this is called “refraction”. For now the image is just focused, and it is not necessarily seen clear by the eye. For the eye to see this image clearly, it must be focused exactly on the retina. If the image is focused some place before or behind the retina, it is seen as non-clear image. The further this image is focused from the retina, the less clearer it seen by the eye.

So,  scientists realized  that the images don’t focus directly in the retina like they should do. This happens when the light rays entering the retina bend more or less than the required degree.

This bending of light rays is called “refraction.” The right refraction in people without eye problems bends light rays and focuses them directly in the retina.

Therefore, any condition in which the  light rays are not focused in the retina is a  “refractive error”.

is “refractive error” a disease?

The tissues of the eye with  “refractive errors” are totally intact. They have no bacteria, no infection, no inflammation and no pathologic imperfection. Thus, there is no disease in the tissues of your eyes since they are medically intact.

The only problem is that light rays don’t focus in the place they are supposed to be focused on – the retina.

correct refractive errorsThere are more than 150 million people worldwide  who are suffering from visual problems due to refractive errors.  However, the good thing is that these eye conditions can be addressed, by prescription eyeglasses or  contact lenses that accurately correct refractive errors.

what ARE THE COMMON causeS of refractive errors?

Refractive errors happen because the eye’s axial length and refraction or  “power of  bending light rays” do not match. Normally, the eye refracts light rays and makes them focus in a distance which is the same as the eye length.

In refractive errors, there are 3 possibilities:

1 – There is no problem with the eye’s axial length but the  light rays are focused in a distance that is not the same as the eye’s axial length.

This can happen in 4 ways which are:

Visual Impairment due to Refractive Errors

a – In myopia, refraction focuses light rays in a distance shorter than the eye length making them focused in front of  the retina.

b – In hypermetropia,  light rays are focuses in a distance longer than the axial length making them focused theoretically behind the retina.

c – In astigmatism, refraction is not equal in all meridians making the image focused in two planes one of them, at least, is in front or behind the retina.

d – In presbyopia, the eye has lost its ability and power of  bending light rays of near objects making them focused in a distance longer than the eye length and thus falling behind the retina.

2 – The eye is able to bend light rays and focus them but it has longer or shorter axial length than normal.

a – In myopia, the eye length is longer than what’s normal.

b – In hypermetropia, the eye’s axial length is too short.

3 – A combination of the above two causes. The eye has different power “refraction” and axial length is either shorter or longer than what’s normal.

There main types of refractive errors are:

1 – Myopia: which is also known as “nearsightedness”. This condition is optically corrected with contact lenses, and there are even single-use, daily contact lenses specifically for nearsightedness.

2 – Hepermetropia: also known as  “hyperopia” and “farsightedness”.

3 – Astigmatism

4 – Presbyopia: an age-related condition common in people 40 and above.

To know more about the causes and types of refractive errors, please watch the video below.