Why are we able to see?
The primary organ of the visual system is the eye. Light rays coming from a source, after passing through the cornea, lens and vitreous body, are refracted and eventually focus on the retina, which acts as the light-sensitive layer.
The image formed on the retina is reduced, real and inverted. With the help of special photoreceptors the nerve cells of the retina change the incident light into electrical impulses, which are sent to the brain (to the occipital lobe of the cerebral cortex) via the optic nerve. There the image is reversed and interpreted. It is important to remember that each of the two eyes sees the observed object slightly differently. The brain combines the independent images produced by both eyes, providing three-dimensional vision. The lens of the eye can change its focal length by changing its shape, and this is its remarkable and amazing ability. Any healthy human eye can see sharply objects located at distances from about 20 cm to infinity, e.g. stars in the sky. However, the eye is not able to see several objects sharply at the same time if each of them is at a different distance from the observer. We therefore see sharply only those objects on which we can focus our gaze. The lens then adjusts its shape and distance so that the focal length coincides with the distance between the centre of the lens and the retina, ensuring that a sharp image of the object is produced. The ability of the lens to change its focal length is called accommodation of the eye.
When we have healthy eyes and want to watch or read something, we often unconsciously position the object we are looking at at a distance of about 25 cm from our eyes. This is called the distance of good vision for a healthy human eye.
Unfortunately, not everyone has healthy eyes. Eye diseases are genetically predetermined, sometimes they also occur as a result of a lack of proper care, e.g. we often read in poor light or look too close and too long at a computer or TV screen.
Moreover, the eyes age with age. Refractive errors are usually associated with changes in the structure of the eyeball, damage to the cornea or loss of the eye’s accommodative ability.
The most common visual impairment in the world is myopia. The development of civilization means that children start using various electronic devices at a very early age. They spend many hours a day staring blankly at the screens of phones, tablets and other devices. The smaller the device, the closer we hold it to the eyes. The longer we don’t take our eyes off it, the faster myopia can develop. According to the projections of WHO (World Health Organization) 66% of people in Asia will be short-sighted in 2050, and 55% in Poland. For this reason, many Asian countries have banned the use of phones and tablets in schools, and have also set a minimum sports time at around 1.5 hours per day.
Types of refractive errors
- Shortsightedness (myopia) – a refractive error associated with an excessively long eye, causing light rays to converge in front of the retina. As a result, a sharp image is formed where there are no photosensitive cells, so what reaches the retina is already an out-of-focus image. To compensate for this error, spectacle lenses with minus power are used, which are biconvex lenses that diffuse light rays.
The greater the myopia, the closer you need to bring your eyes to the observed object to see anything; for example, a myopic person with a -1 dioptre can see well from a distance of one meter, while his friend with a -10 dioptre can only see well from a ten times shorter distance, i.e. from ten centimetres.
- Farsightedness (hyperopia) – a refractive error in which the eyeball is too short, causing the retina to be too close to the lens and cornea, so that light rays converge outside the eyeball. To compensate for this error, spectacle lenses with plus power are used, which are convex lenses that focus light rays. In people with hyperopia both far and near vision is low. This error is corrected by eyeglasses or contact lenses, the basic element of which is a focusing lens.
- Astigmatism – a cylindrical refractive error which may accompany both myopia and hyperopia. In astigmatism, because of the disturbed symmetry of the eye, vertical and horizontal rays have different points of focus on the retina. As a result patients experience blurred vision in one direction, and visual distortions, such as straight line deformations.
Astigmatism can be of corneal, lenticular or mixed corneal-lenticular origin. Astigmatism it is corrected with special spectacle lenses or contact lenses, called toric lenses. There are also surgical methods: corneal astigmatism is corrected with a laser, while lenticular and mixed astigmatism is corrected by implanting an artificial lens. In difficult cases, a combination of both methods is needs to be used.
Astigmatism in children can be detected through eye examinations performed at 6 months of age, again at 3 years of age, and one more time just before the start of kindergarten or primary school. If the risk of developing this refractive error is high, an eye examination should be performed every year. It is very important to regularly check children’s eyesight, as astigmatism may sometimes not be detected for many years.
Astigmatism that is incorrectly treated or not treated at all has a negative impact on a child’s ability to read and concentrate, not only at school but also in everyday functioning.
Is it possible to cure shortsightedness (myopia) in children?
It is important to distinguish between simple non-morbid myopia, in which there is only a refractive error, and morbid myopia, in which the changes in the eye increase with age.
Typically, morbid myopia is characterized by a very rapid progression of the refractive error, associated with excessive elongation of the eye. In half of the cases, drops containing 0.01%-0.05% atropine instilled once a day, overnight, help to control the condition.
In children with high myopia, apart from the measurement of the refractive error, the doctor also performs biometry to measure the length of the eyeball. For every 0.3 mm elongation of the eye the refractive error increases by 1.0 dioptre. In addition to monitoring the development of the eyeball, which in most cases completes growth around the age of 14, it is necessary to compensate for the refractive error with contact lenses or glasses. Also, which is very important, time spent looking at the computer screen, tablet or mobile phone should be strongly limited.
If a child needs to use the computer for many hours, they need to take a break at least every hour. During these breaks, the child should shift their gaze for 2-3 minutes to objects located further away, preferably look out of the window.
Is it possible to cure farsightedness (hyperopia) in children?
Hyperopia is normal in newborn babies – we are usually born hyperopic (far-sighted). It can persist for as long as three years, in which case we speak of physiological hyperopia. It occurs because the eyeball in toddlers needs time to fully form. Physiological hyperopia, in the absence of other distressing symptoms, is a perfectly normal phenomenon and you should not be overly worried about it. However, when a child after the age of three complains of rapid eye fatigue, recurring headaches, double vision, is reluctant to read, constantly squints and rubs their eyes frequently, it is necessary to consult an ophthalmologist.
Untreated hyperopia in children causes problems with vision, problems with concentration, problems at school – it makes it difficult to learn and perform everyday activities.
In smaller children, we correct hyperopia with glasses. In older, more independent children, contact lenses can also be used. Hyperopia may be accompanied by accommodative strabismus. In such case, the doctor may additionally recommend orthoptic exercises, which directly affect the optical system of the eye and are very helpful in reducing the effects of farsightedness.