What is a refractive error
We see the world around us because of the way our eyes bend (refract) light. Refractive errors prevent the eye from properly focusing light, causing blurred vision. The main refractive errors are nearsightedness, farsightedness and astigmatism.
Refractive errors can usually be “corrected” with glasses or contact lenses, or they can be permanently treated with vision correction surgery like LASIK. To find out more about refractive errors, let’s look a little closer at how the eye works.
How the Eye Sees
The process of vision begins when light rays that reflect off objects and travel through the eye’s optical system are refracted and focused into a point of sharp focus. For good vision, this focus point needs to be on the retina. The retina is the tissue that lines the inside of the back of the eye, where light-sensitive cells (photoreceptors) capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eye’s optic nerve to the brain for interpretation.
Causes of Refractive Errors
The eye’s ability to focus light sharply on the retina depends on three main features of the eye: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye. Both genetics and environmental factors (e.g. quantity of close work like reading, screen time etc) can affect these physical features of the eye.
- Eye length. If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.
- Curvature of the cornea. If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.
- Curvature of the lens. If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.
How do glasses help us see?
What is myopia or short-sightedness?
Someone with myopia has blurry distance vision, but generally has good near vision. In high myopia (i.e. someone who is very myopic), even near vision is affected. In this case, objects must be extremely close to the eyes to be seen clearly, so glasses would likely be needed even for close work like reading. Either one or both eyes can be myopic, and the eyes can be different strengths. This means a person may not know they have a vision problem, as one eye may have been compensating for blurred vision in the other eye without them even realising.
Source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=1817263
What is happening in the eye?
In cases of myopia, light is focused before it reaches the cornea, as shown in the diagram below. A lens (i.e. glasses or contact lenses) placed in front of the eye can help focus light on the cornea instead, giving a clear image.
What is hyperopia or far-sightedness?
Far-sightedness, also known as hyperopia, is a condition of the eye where light focuses behind, instead of on, the retina. This causes close objects to be blurry, while far objects may appear normal. In stronger cases, even objects at all distances may be blurry. Other symptoms may include headaches and eye strain.
Because far objects are generally clear, a child may not realise they have far-sightedness. In some cases, the additional work the eyes need to do to generate a clear image of closer objects such as words on a page can lead to letters becoming jumbled up, or frequent loss of concentration such as losing their place on the page.
What is happening in the eye?
The cause is an imperfection in the eye (often when the eyeball is too short or the lens cannot become round enough), causing the eye to not have enough power to see close or nearby objects. It is a type of refractive error.
Far-sightedness is usually corrected with convex corrective lenses, which help to focus light on the cornea.
What is astigmatism?
In a person with astigmatism, vision may be blurred both close-up and at a distance. Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football. In an eye with astigmatism, light fails to come to a single focus on the retina to produce clear vision. Instead, multiple focus points occur, either in front of the retina or behind it (or both). What this might look like for a person with astigmatism varies depending on the the type of astigmatism but some examples are shown below.
What is amblyopia or lazy eye?
Amblyopia, also known as lazy eye, is a vision development disorder in which an eye fails to see clearly, even with prescription eyeglasses or contact lenses. The cause of amblyopia can be any condition that interferes with focusing during early childhood. This can occur from poor alignment of the eyes, an eye being irregularly shaped so that focusing is difficult, one eye being more nearsighted or farsighted than the other, or clouding of the lens of an eye. After the underlying cause is fixed, vision is not fully restored as the mechanism also involves the brain.
Early detection improves treatment success. Glasses may be all the treatment needed for some children. In other cases, treatments which force the child to use the weaker eye are used. This is done by either using a patch or putting atropine in the stronger eye. Without treatment, amblyopia typically continues into adulthood.
Source: National Eye Institute, National Institutes of Health
What is strabismus or turned eye?
In a person with strabismus, one eye looks directly at the object they are viewing, while the other eye is misaligned inward (esotropia, “crossed eyes” or “cross-eyed”), outward (exotropia or “wall-eyed”), upward (hypertropia) or downward (hypotropia).
This interferes with binocular vision because it prevents a person from directing both eyes simultaneously towards the same fixation point. It also typically affects depth perception.
Strabismus typically involves a lack of coordination between the extraocular muscles. Treatment should be started as early as possible to ensure the development of the best possible visual acuity and stereopsis. To prevent double vision from congenital and early childhood strabismus, the brain ignores the visual input from the misaligned eye, which typically leads to amblyopia or “lazy eye” in that eye.
Newborns often have intermittent crossed eyes due to incomplete vision development, but this frequently disappears as the infant grows and the visual system continues to mature. Most types of strabismus, however, do not disappear as a child grows.
Routine children’s eye exams are the best way to detect strabismus. Generally, the earlier strabismus is detected and treated following a child’s eye exam, the more successful the outcome. Without treatment, your child may develop double vision, amblyopia or visual symptoms that could interfere with reading and classroom learning.
What is colour-blindness?
Colour blindness is the decreased ability to see colour or differences in colour. Colour blindness can make some educational activities difficult. Buying fruit, picking clothing, and reading traffic lights can also be more challenging. Problems, however, are generally minor and most people adapt.
There is no cure for colour blindness. Diagnosis may allow a person’s teacher to change their method of teaching to accommodate the decreased ability to recognize colour. Special lenses may help people with red-green colour blindness when under bright conditions. There are also mobile apps that can help people identify colours.
Males are more likely to be colour-blind than females. Boys are screened for colour-blindness as part of routine vision screening at age 11 or 12.
What do colour-blind people see?
It depends on the type of colour-blindness, but most common are difficulties in differentiating reds and greens. In the first picture below, depending on the computer displays, people with normal vision should see the number “74”. Many people who are color blind see it as “21”, and those with total colour blindness may not see any numbers.
Source: Wikipedia Public Domain https://commons.wikimedia.org/w/index.php?curid=1692985
Simulation of the normal (above) and dichromatic (below) perception of red and green apples.
What happens during an eye test?
Children’s eye exams include many of the same things as an adult one, with some changes to suit the individual child (e.g. differences for younger children who aren’t reading yet). Your child’s visit to the optometrist may include some or all of the following:
- Visual acuity: The visual acuity test helps find out how well a person can see without glasses or contact lenses. Your child will be shown an eye chart and asked what they can see. Older children are shown a letter chart and young children are shown pictures or shapes.
- Eye muscles: It is important to check for any problems in binocular vision (how both eyes work together) – especially for children. A simple assessment known as the “cover test” lets the optometrist identify how well the eye muscles work together.
- Pupil function: The optometrist will assess whether your child’s pupils function similarly using a bright torch. The pupils should expand or contract at the same rate and to the same size. A difference in pupillary reaction could signal to the optometrist to investigate further.
- Slit lamp assessment: The slit lamp consists of a microscope and powerful light source, which lets the optometrist look at different parts of the eye to make sure they are all healthy.
- Refraction: Refraction is the vision assessment required to find your exact prescription. The optometrist will work with your child to identify which lens power provides the clearest vision for them.