Special features of the child’s visual system
![Children’s eyesight]( "Children’s eyesight")
Normal functioning of a child’s vision is an essential prerequisite not only for the visual process development but also for all the organs and systems of a child. The eye is not only an organ of sight but also a “consumer” of light energy. The stimulating effect of light helps to produce hormones of pituitary, adrenal glands, thyroid gland, sexual glands, etc. Rapid adaptation of a new-born child to the environment, its correct development and growth depend to a great extent on the correct functioning of the visual system. For this reason children develop their visual analyzer rapidly.The growth and development of the child’s eyesight is finalized by the age of 2-3 years. In the following 15-20 years there are fewer changes than in the initial period.
Particularly important for the normal functioning of the child’s visual system is the correct foundation and development of the visual organ prior to birth. There are several critical periods in the development of the fetus when the creation of some organs are particularly sensitive to various damaging factors. The results of clinical observations testify to the fact that abnormalities in the development of the eye may be caused by:
- avitaminosis of A (blindness)
- influence of lithium chloride (cyclopea, anophthalmos) and sodium thiocyanate (hydrophthalmos)
- hypoxia (cataract, maldevelopment)
- diagnostic X-ray investigation of a pregnant women (microphthalmos, cataract, blindness)
- contagious diseases, excessive or prolonged use of medications for diabetes (aplasia of the optic nerve, blindness, cataract), etc.
However, all these changes may be due to congenital factors. By the time of birth, in the event of normal prenatal development, a child’s eye has all the membranes but significantly differs in terms of size, mass, histological structure, physiology, and functions from those of an adult.
The eye of a new-born child
The eye of a new-born child has a much shorter frontal-rear axis (about 16-18 mm) and accordingly a much greater refracting power (80.0 – 90.9 D). Towards the end of the first year of life the frontal-rear size of the eyeball grows to 19.2 mm, by the 3rd year – to 20.5 mm, by 7 years – to 21.1 mm, by 10 years - 22 mm, by 15 years it is about 23 mm and reaches 24 mm by the age of 20-25. But the size and shape of the eyeball depend on the type and value of the refraction pattern (refraction disorder – myopia, hypermetropy, normal refraction – emmetropia). The size of a child’s eyeball is very important in evaluating the type and stage of ocular pathology (congenital glaucoma, myopia, etc.)
The eye of a new-born child has a much shorter frontal-rear axis (about 16-18 mm) and accordingly a much greater refracting power (80.0 – 90.9 D). Towards the end of the first year of life the frontal-rear size of the eyeball grows to 19.2 mm, by the 3rd year – to 20.5 mm, by 7 years – to 21.1 mm, by 10 years - 22 mm, by 15 years it is about 23 mm and reaches 24 mm by the age of 20-25. But the size and shape of the eyeball depend on the type and value of the refraction pattern (refraction disorder – myopia, hypermetropy, normal refraction – emmetropia). The size of a child’s eyeball is very important in evaluating the type and stage of ocular pathology (congenital glaucoma, myopia, etc.)
![The eye of a new-born child]( "The eye of a new-born child")
As a rule, new-born babies and infants display a hypermetropic refraction – farsightedness. According to medical studies, hypermetropia has been found in 92.8% of children under 3 years while 3.7 and 2% of children of the same age have shown normal refraction and myopia (shortsightedness) accordingly. The grade of hypermetropia ranges from 2.0 to 4.0 D. With the growing eye, its refracting power shifts towards normal value. The first three years of a child’s life are associated with intense growth of eyes as well as flattening of cornea and the crystalline lens in particular.
The cornea
is a basic refracting structure of the eye. The width (or horizontal diameter) of the cornea in a new-born child is on the average 8-9 mm. By the end of the first year, it is 10 mm, at the age of 11–11.5 mm which almost corresponds to the diameter of the cornea of an adult. The growth of the cornea comes as a result of the expansion and thinning of the tissue. The thickness of the central section of the cornea is reduced from 1.5 to 0.6 mm on the average, while in the peripheral section – from 2 to 1 mm. The radius of the corneal front curvature in a new-born child is 7 mm on the average. With time, the curvature flattens out and by the age of 7 it is 7.5 mm on the average becoming same as in adults (the curvature of the cornea may vary from 6.2 to 8.2 mm depending on the type and power of refraction of the eye). The refracting power of the cornea changes with age in inverse ratio to the curvature radius: for children under one year it is 46.0–48.0 D on the average; by the age of 7 it is about 42.0–44.0 D (the same to an adult eyes). The refracting power of the cornea in the vertical meridian is almost always by 0.5 D greater than in the horizontal meridian which accounts for what is called “physiological” astigmatism.
In the first months of the child’s life, the cornea is not very sensitive due to the continuing functional development of the cranial nerves. During this period, it is especially dangerous to have a foreign body penetrating the conjuctival sac and remaining there without causing irritation, pain, and other discomfort to the child. This can seriously damage the cornea (keratitis) to the extent of its total destruction. With further development of the child, the sensitivity of the cornea increases and almost reaches that of an adult in a 12-month-old child.
The iris
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The iris is the frontal part of the choroid of the eye. It forms a vertical diaphragm with an aperture in the center – the pupil, which governs the inflow of light into the eye depending on the external conditions. The iris may be coloured from blue to black and it depends on the content of melanin pigment – the more pigment there is in the iris, the darker it is. With low pigment content or its absence, the iris is bluish or light grey. Children have a low pigment in their iris and for this reason new-born babies and children under one have a bluish-grey iris. The colour of the iris gets its final colour by the age of 10–12. Infants have poorly developed muscular fibrils responsible for pupil expansion, and for this reason, their pupil is narrow (2–2.5 mm). During the period from 1 to 3 years, it grows further to 3–3.5 mm, i.e. the size of the adult’s pupil.
The retina
— is the major component of the optic analyzer, being its peripheral element. The sophisticated structure of the retina allows to perceive light, to process and transform the light energy into nerve impulses which are then transported through the chain of neurons into the visual centers of the cerebral cortex which perceives and processes the visual information. The retina is an internal membrane of the eyeball and lines the fundus of the eye. The most important region of the retina is the so-called yellow spot (macula lutea) with its central zone (fovea centralis) of 0.075 mm. This zone ensures maximum perception of visual images.
In a new-born child, the retina comprises ten layers:
- pigment epithelium;
- columnar layer;
- external limiting memrane;
- outer nuclear layer;
- outer plexiform (reticular) layer;
- inner nuclear layer;
- inner plexiform layer;
- ganglion and multipolar cell laye
- nerve fiber layer;
- inner limiting membrane.
The first four layers belong to the light-sensitive apparatus of the retina, whereas the others constitute the cerebral section. After the first six months of the child’s development, the eyes expand with thinning of the external and internal layers of the retina. This leads to considerable changes in the macular and particularly foveal (central) regions: here remain only the 1st, 2nd, 3rd, and 10th layers ensuring a high refraction power of the zone.
Anterior chamber of the eye
The anterior chamber of the eye is limited by the cornea’s innermost surface at its frontal region; along the peripheral zone (in the corner) it borders on the iris root, ciliary body; in the rear it borders on the frontal surface of the iris, while in the zone of the pupil it borders on the frontal capsule of the crystalline lens. By the time of birth, the child has a developed anterior chamber of the eye, but it differs considerably from that of an adult eye in terms of shape and size. This is explained by the existence of a short frontal-rear axis of the eye, specific shape of the iris, and spherical shape of the front surface of the crystalline lens. It is important to know that the back surface of the iris is in close contact with the interpupillary area of the anterior capsule of the crystalline lens.
The depth of the anterior chamber of a new-born infant in the center (from the cornea to the front surface of the crystalline lens) is up to 2 mm with an acute and narrow angle of the chamber. By the end of the first year of the child, the chamber grows to 2.5 mm; and by the age of 3, it is almost identical to an adult eye, i.e. about 3.5 mm. with a much more open angle of the chamber. In the prenatal period, the angle of the frontal chamber is covered by a mesoderm tissue which is dissipated by the moment of birth. Retarded inverse development of the mesoderm may lead to high intraocular pressure even during the prenatal period and the development of hydrophthalmos.
About 5 % of children are born with a blocked opening of the lacrimal-nasal channel. However, under the impact of the lacrimal fluid, the tissue (“blockage”) is almost always dissipated during the first few days to launch normal lacrimal outflow. Otherwise, the stoppage of lacrimal outflow takes place leading to stagnation and dacryocystitis of new-borns.
The orbit
The orbit is a protective bony socket, a cavity for the eye and its main appendages. A specific feature of a new-born child’s orbit is that its horizontal dimensions are greater than the vertical ones; orbital depth is small and in terms of shape looks like a three-sided pyramid with axis converging forward; sometimes it makes an impression of convergent strabismus. Only the upper section of the orbit is well developed. In the process of growth, the orbit deepens mainly through the increasing size of the greater wings of the main bone and the development of the frontal and maxillary sinuses. The orbit deepens and develops the shape of a four-sided pyramid. The axis direction is aligned, thus increasing interpupillary distance. By the age of 8-10, the shape and size of the orbit is almost the same as in adult eyes.
Following birth, the visual analyzer of the child goes through several stages of development, including the five main stages:
- formation of the yellow spot zone and the central zone of the retina during the first six months of life; out of the ten layers of the retina, there remain only four – these are the optic cells, their nuclei, and structureless limiting membranes;
- development of the functional mobility of the optic channels and their formation during the first six months of life;
- enhancement of the optic cellular elements of the cortex and cortical optic centers during the first two years of life;
- formation and consolidation of links between the optic analyzer with other analyzers in the course of the first few years of life;
- morphological and functional development of the cranial nerves during the first 2-4 years of life.
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Normal functioning of a child’s vision is an essential prerequisite not only for the visual process development but also for all the organs and systems of a child. The eye is not only an organ of sight but also a “consumer” of light energy. The stimulating effect of light helps to produce hormones of pituitary, adrenal glands, thyroid gland, sexual glands, etc. Rapid adaptation of a new-born child to the environment, its correct development and growth depend to a great extent on the correct functioning of the visual system. For this reason children develop their visual analyzer rapidly.The growth and development of the child’s eyesight is finalized by the age of 2-3 years. In the following 15-20 years there are fewer changes than in the initial period.
The iris is the frontal part of the choroid of the eye. It forms a vertical diaphragm with an aperture in the center – the pupil, which governs the inflow of light into the eye depending on the external conditions. The iris may be coloured from blue to black and it depends on the content of melanin pigment – the more pigment there is in the iris, the darker it is. With low pigment content or its absence, the iris is bluish or light grey. Children have a low pigment in their iris and for this reason new-born babies and children under one have a bluish-grey iris. The colour of the iris gets its final colour by the age of 10–12. Infants have poorly developed muscular fibrils responsible for pupil expansion, and for this reason, their pupil is narrow (2–2.5 mm). During the period from 1 to 3 years, it grows further to 3–3.5 mm, i.e. the size of the adult’s pupil.
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