By: Elizabeth Obigwe. Datelinehealth Africa Volunteer Freelance Writer, with medical review and editorial support by the DLHA Team
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Highlights
To see an object, light must travel into your eyes through the cornea, which is the transparent front part of the eye.
The cornea bends or refracts the light and passes it through the lens.
The lens then focuses the refracted light onto the retina at the back of the eye.
The retina converts the light into electrical signals that are sent to the brain through a nerve fiber pathway called the visual pathway, allowing you to see the image.
In a normal eye the cornea and the lens work together to focus the light directly onto the retina, resulting in a clear image.
However, in some cases, the shape of the cornea, lens, or eye globe is not ideal. This causes problems in focusing the light accurately.
These problems are called refractive errors and it is the subject of this report.
Refractive errors are common eye disorders that develop due to the eyes’ inability to focus images of objects clearly. As a result, the images formed are blurred. This leads to impaired vision.
Refractive errors of the eye are collectively referred to as ametropia.
On the other hand, the normal formation of an image that allows light rays to focus clearly on the retina is referred to as emmetropia.
According to the World Health Organization, about 153 million people worldwide live with visual impairment due to uncorrected refractive errors. However, this figure does not include the people living with uncorrected presbyopia, which is likely to be quite significant, according to some early evidence.
A study published in 2012 showed that in sub-Saharan Africa (SSA), uncorrected refractive error (URE) may cause moderate to severe visual impairment, but is not a significant cause of blindness.
There is not sufficient research to show the accurate burden of refractive errors among African adults. However, some of the reports available show that refractive errors of the eye have a significant impact on the vision of African adults.
Two studies carried out in Durban and Cape Town, two communities in South Africa showed that 57.3% and 30.8% respectively of the study population were affected by refractive errors.
In Nigeria, uncorrected refractive errors were responsible for 77.9% of mild visual impairment, 57.1% of moderate visual impairment, 11.3% of severe visual impairment, and 1.4% of blindness within a study population.
These data do not capture the exact impact of refractive errors among African adults. More research needs to be done in this regard.
There are four common types of refractive errors (ametropia). See figure 1. However, some authors argue that there are three. All of these errors of the eye are discussed below.
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Figure 1: Types of refractive errors. Click on image to enlarge.
If a person has nearsightedness, distant objects will look blurry.
This happens when the eyeball is longer than normal from front to back or when the shape of the lens or cornea is not normal (too steeply curved).
As a result, light rays passing through the eyes focus in front of the retina instead of on it. So, distant objects become blurred. However, near objects are clear.
Myopia is usually common among children especially those between age 6 to 14. It is an inheritable refractive error that progresses throughout the teenage years when the body changes rapidly.
Severe nearsightedness (hyper-myopia) can increase the chances of other eye conditions like retinal detachment (i.e., the retina is pulled from its normal position).
This is sometimes called hypermetropia and it is the opposite of myopia.
Here, either the eyeball is shorter than normal from front to back or the lens or cornea is abnormal (too flatly curved).
This causes light rays to focus after the retina. Hence, near objects appear blurred but far objects appear clear.
In cases of severe hyperopia, both far and near objects are blurred. Hyperopia can be inherited and it is also common among children.
In a normal, healthy eye, the cornea is smooth and curved evenly in all directions. So, when light enters the cornea, it is focused equally at one point.
However, in astigmatism, the front surface of the cornea is curved more in one direction than the other. This causes light rays to focus at different points, creating a distorted image. The vision is usually similar to looking into a distorted, wavy mirror.
Astigmatism causes blurred images at all distances and it can be present alone or in combination with hyperopia (hyperopic astigmatism) or myopia (myopic astigmatism).
The lens of a young person is flexible and is usually controlled by a set of muscles known as ciliary muscles. The ciliary muscles adjust the lens as needed so that objects at any distance are focused on the retina. This process is known as “accommodation.”
As we get older, the lens loses this elasticity, making it rigid and unable to focus near objects. This condition is called presbyopia and it is common among adults 40 years and older.
Because presbyopia is mainly associated with the inability of the lens to change shape, and not really the eye’s inability to refract (bend) light rays, some authors argue that it is not an error of refraction. Instead, it is “a condition of physiological insufficiency of accommodation.” In other words, it is caused by changes in the eyes, particularly the lens which affects its ability to accommodate and focus light.
Click and watch the video below to learn more about refractive errors of the eye.
Click on play icon to watch video.
Refractive error symptoms generally make your vision poor. Some of these symptoms are;
It is important to check your eyes regularly as some people may not have the above symptoms at the onset of their eye problem.
You can ask your doctor how often you need to come around for the checkup.
Also, regular checkup is important if you are at high risk (see below) of developing any of the refractive errors.
If you notice any of the symptoms above, do not hesitate to visit your eye care specialist.
Refractive errors occur due to one or more of the following factors:
1. Shape of the eye: The size and shape of the eye can affect how light is focused. Eyes that are too long or short, or have an irregular shape, can lead to refractive errors.
2. Cornea abnormalities: Any abnormalities in the curvature or thickness of the cornea can impact the way light is bent as it enters the eye.
3. Lens abnormalities: Changes in the shape or flexibility of the lens inside the eye can affect its focusing ability.
4. Hereditary factors: Refractive errors can run in families.
5. Environmental factors: Environmental causes of refractive error include; prolonged use of digital devices, reading in poor lighting conditions, or excessive strain on the eyes. These factors can also worsen existing problems.
The following may put you at risk for refractive errors of the eyes:
1. Family history: If your parents or siblings have refractive errors, you have a higher risk of developing them as well.
2. Age: Refractive errors, particularly presbyopia, tend to become more common as you get older.
3. Environmental factors: Prolonged close-up work, such as reading or using digital devices for extended periods, can contribute to the development of refractive errors.
4. Ethnicity: Certain ethnic groups have a higher prevalence of specific refractive errors. For example, myopia is more common in Asian populations.
5. Eye trauma or surgery: Previous eye injuries or surgeries can alter the shape and structure of the eye, potentially leading to refractive errors.
6. Medical conditions: Certain medical conditions, such as diabetes or Down syndrome, can be associated with an increased risk of refractive errors.
There is no known way to prevent refractive errors. However, you can avoid the environmental factors mentioned above that increase your risk of developing these conditions.
Regular eye checkups can also help with early detection and treatment.
Some of the tests that your eye doctor (ophthalmologist) or optometrist may carry out to check whether or not you have any refractive errors of the eye include:
This is a basic eye chart test where you read letters or symbols from a distance to determine how well you can see at various distances.
Figure 2: Visual acuity chart and test. Click on image to enlarge.
This test determines the exact prescription needed to correct your vision. The doctor uses a phoropter (or a series of lenses) and asks you to provide feedback on which lenses provide clearer vision. See figure 3.
Figure 3: Refraction test. Click on image to enlarge.
The doctor may use a retinoscope to shine a light into your eyes and observe how it reflects off your retina, helping to determine the kind of refractive error that you may have.
Your doctor may also perform a thorough examination of your external and internal eyes using different kinds of tests. The doctor will use eye drops to dilate or widen your pupils, allowing a better view of the internal structures of the eye. This examination is not limited to refractive errors only. It is used to test for other types of eye problems too.
There are different treatment options available for refractive errors. Your doctor will decide on the most appropriate treatment method for you after a proper examination.
Eyeglasses with lenses that have the appropriate prescription are the simplest and most common option for refractive error correction.
They help focus light properly onto the retina, providing clear vision.
The glasses for different refractive errors have lenses peculiar to them as seen in figure 1 above.
a. Myopia: Glasses for myopia have concave lenses, which are thinner at the centre and thicker at the edges. These lenses help focus light directly onto the retina, improving distance vision.
b. Hyperopia: Glasses for hyperopia have convex lenses, which are thicker at the centre and thinner at the edges. These lenses help focus light onto the retina, enhancing near vision.
c. Astigmatism: Glasses for astigmatism may have cylindrical lenses. These lenses have different curvatures at different points to compensate for the irregular shape of the cornea or lens. They correct the distorted or blurry vision caused by astigmatism.
d. Presbyopia: Glasses for presbyopia usually have multifocal lenses, such as bifocals or progressive lenses. Bifocals have two distinct sections for near and distance vision, while progressive lenses provide a smooth transition between different lens powers, allowing clear vision at various distances.
These are thin, curved lenses that are placed directly on the surface of the eye. Contact lenses also correct refractive errors and can provide an alternative to glasses. They require proper cleaning and maintenance.
There are different types of surgical procedures available to permanently correct refractive errors. The most common ones are LASIK (Laser-Assisted in Situ Keratomileusis) and PRK (Photorefractive Keratectomy). These surgeries use lasers to reshape the cornea, improving its focusing ability.
Uncorrected refractive error along with unoperated cataracts are the leading cause of vision impairment globally according to the WHO. Yet, there is a huge gap in healthcare interventions for this condition. Refractive errors affect people of varying ages and as a result, can greatly impact the educational and economic sectors of a country. Hence, there is a need for better planning and improvement of health services in Africa to cater to the unmet needs.
1. Blindness and vision impairment: Refractive Errors. World Health Organization; Oct 6 2013. Accessed June 19, 2023. Available from: https://www.who.int/news-room/questions-and-answers/item/blindness-and-vision-impairment-refractive-errors.
2. Dhaliwal DK. Overview of refractive error - eye disorders. MSD Manuals; Last updated Sept. 2022. Accessed June 19, 2023. Available from: https://www.msdmanuals.com/professional/eye-disorders/refractive-error/overview-of-refractive-error.
3. Ezelum C, Razavi H, Sivasubramaniam S, Gilbert CE, Murthy GV, Entekume G, et al. Refractive error in Nigerian adults: Prevalence, type, and spectacle coverage. Investigative Opthalmology & Visual Science. 2011;52(8):5449. doi:10.1167/iovs.10-6770
4. Mashige KP, Jaggernath J, Ramson P, Martin C, Chinanayi FS, Naidoo KS. Prevalence of refractive errors in the ink area, Durban, South Africa. Optometry and Vision Science. 2016;93(3):243–50. doi:10.1097/opx.0000000000000771
5. Otutu M, Nachega J, Harvey J, Meyer D. The prevalence of refractive error in three communities of Cape Town, South Africa. African Vision and Eye Health. 2012 Dec 9;71(1):32-8.
6. Refractive errors. University of Michigan Kellogg Eye Center. Accessed June 19, 2023. Available from: https://www.umkelloggeye.org/conditions-treatments/refractive-errors.
7. National Eye Institute. Types of refractive errors. U.S. Department of Health and Human Services. Last updated August 28, 2020. Accessed June 20, 2023. Available from: https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/refractive-errors/types-refractive-errors.
8. Sherwin JC, Lewallen S, Courtright P. Blindness and visual impairment due to uncorrected refractive error in sub-Saharan Africa: Review of recent population-based studies. British Journal of Ophthalmology. 2012;96(7):927–30. doi:10.1136/bjophthalmol-2011-300426
Related: Priority eye diseases in Africa.
Published: June 30, 2023
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