Refractive development of the eye

Refractive development of the eye
Photo: http://temeculacreekoptometrist.com/

Every single one of us has started his or her life as small as 2 tiny cells joined together. Those two cells soon became 4, and then 8, and then 16 and so on and so forth. Some 9 months later we were born, weighting around 3.5 kilos (7.5 pounds), possibly some much less, some a bit more. And then in the next 20-odd years every one of us has grown or is growing to their final height and to a certain degree weight. Pretty much all organs grow to their final shapes and sizes by then. Eye, of course, is no exception. But as you are about to see the growth of the eye is one of the most precisely regulated growths of all human organs.

Humans are born hyperopic

There are two entities that define the refractive properties of the human eye. (a) The axial length – the length of the eyeball and (b) the refractive power of cornea and lens. The axial length and the refractive power thus define one’s refractive state. People can either be (a)hyperopic – plus diopter, (b) emmetropic – no diopter and (c) myopic – minus diopter. For a more detailed description click here.

Humans are born hyperopic with strong refractive power of the cornea and lens and short axial length. Then, in the first few years the eyeball grows and the axial length increases, which leads to a shift towards emmetropization (i.e. 0.0 diopter – perfect refraction) that is usually completed around the age of 6. If, however, for whatever reason the eyeball continues to grow after the emmetropization has reached the eye will then become myopic (i.e. minus diopter – nearsightedness). It means that the eyeball has grown too much (the axial length is too long) for the refractive power of the cornea and lens. Indeed, myopia is the most prevalent refractive error in the industrialized world, reaching almost epidemic proportions. By some predictions, half(!) of the world’s population will be myopic by 2050. Why does the eyeball grow too much in so many people then?

Eyeball growth is under visual control

The growth of the eyeball is one of the most precisely regulated processes in humans. A difference in length as small as one-tenth of a millimeter (0.004 inches) represents a refractive error equivalent to 0.25 diopter. In other words, a person whose eyeball is 2-3/10 of a millimeter (0.01 inch) too long, or too short, will need corrective spectacles in order to see clearly. How is the growth so precisely controlled then?

There is evidence from several studies that the growth of the eye is under visual control. More precisely, the visual signals from the retina regulate the growth of the eyeball. In the early years of life, humans are, as mentioned, hyperopic. This means that the image the retina gets is not correctly focused on the retina which is the stimulus for the eyeball to grow. When, however, the eyeball grows enough so that emmetropization is reached, the image becomes focused correctly on the retina. In ideal circumstances, this should signalize the end of eyeball growth. In many people, though, the eyeball continues to grow, resulting in myopia. The factors that regulate myopia development were thoroughly scrutinized in many studies. Whilst some genetic factors that modulate predisposition to become myopic were discovered, it is generally believed that myopia development in the industrialized world is mostly due to environmental factors.

Strategies to halt eyeball growth

There were several reports from animal studies that encouraged the use of atropine to pharmacological terminate eyeball growth. Whereas animal studies were promising, the use of atropine is not conventionally recommended due to poorly understood side effects of prolonged application.

Another useful strategy to tackle myopia uses the evolutionary aspects of vision development to its advantage. Speaking strictly evolutionarily, humans are creatures that are created to spent most of our lives outdoors. This means that our eyes focus only on the objects that are close to us, the distance being blurry. The sharply focused image on the central retina combined with a blurry image on the peripheral retina then brings the eyeball growth to a halt. In the modern industrialized world, however, we spent more and more of our lives indoors, focused on the near objects with the whole visual field. The signals that would normally bring the eyeball growth to a halt are therefore attenuated and the eyeball continues to grow, resulting in the development of myopia. To make matters worse, we frequently focus on the objects that are so close that our lens has to accommodate [link blog: Following the Light – Part III: Iris, pupil, and lens] for us to see them clearly (e.g. reading or writing). The accommodation is perceived by our eyes as a strain and if this strain is too frequent or too prolonged our eyes confront it by signalizing further growth of the eyeball, which contributes to myopia development even further. Appropriate working distance is therefore crucial, as is the 20-20-20 rule (every 20 minutes focus on an object at least 20 feet (6 meters) away for at least 20 seconds). The special glasses that blur the periphery of the visual field were also shown to reduce the development of myopia in children.

Myopia development preventive measures

Myopia is slowly reaching epidemic proportions in the modern industrialized world. Whereas genetic factors do play some role in its development, the more important are the environmental factors. It is crucial that children in their most vulnerable period for myopia development spend as much time as possible outdoors, not focused on the electronic devices’ screens or being too close to paper when reading or writing.

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