So...exactly what causes myopia?
Now you understand better what myopia is and the types of myopia, let's look at some of the causes of myopia. Many studies have been conducted to uncover what causes myopia and the current thinking suggests perhaps unsurprisingly that there are many factors that lead to myopia developing. The process itself can be part of the normal development of the eye, but this process seems to continue beyond the point it should stop.
Most modern theories of myopia revolve around the peripheral hyperopic defocus blur model, but there are also other causes and increased risk factors, such as genetic predisposition, lifestyle and more.
We'll take a look at them here:
A growing problem: development in children
Most new-borns and very young children tend to have something called hyperopia. This is where the focusing power of the eye is too weak and/or the length of the eyeball is too short. Therefore, a plus or convex lens is needed to add focusing power to bring light into focus on the retina, as shown in the diagram to the right. Somewhat incorrectly, this is sometimes called longsight or far-sight.
Up to the early teens, a process of elongation of the eyeball (and sometimes steepening of the optical surfaces) normally takes place in order to bring light into focus on the retina naturally without the eye needing to accommodate (add extra focussing power by steepening the lens internally).
This process of elongation of the eye is called Emmetropisation. How this takes place exactly is not yet fully understood by science, but it does appear that the peripheral retina has a part to play in it, caused by what we call peripheral hyperopic defocus blur. This may sound complicated, but we have described it in more simple terms below.
Myopia typically develops in childhood as a result of the eye growing too long from front to back which is referred to as the axial length of the eye
Peripheral hyperopic defocus blur - Axial length growth
In myopia, the process of elongation of the eyeball continues beyond the point of emmetropisation. In essence, the eyeball gets too long and the child becomes short-sighted. The measurement of the length of the eyeball is called the axial length.
There are several theories as to why this happens. Ultimately, where there is a lot of close work taking place, a mechanism in the eye detects that the image in the periphery is out of focus and is 'hyperopic', essentially trying to focus behind the retina (in other words the peripheral eye is too short). This triggers another mechanism that leads to the eyeball becoming longer to help bring these peripheral images into focus on the retina.
The problem is, continued close work leads to the same problem happening and the eye ‘tries’ to elongate once again. This cycle of progressive eyeball elongation is what leads to the axial length of the eye to continue increasing, resulting in higher and higher levels of myopia.
TL;DR Key Points
Myopia develops due to multiple factors and is not fully understood.
The peripheral hyperopic defocus blur model is a leading theory in myopia's explanation.
Hyperopia (longsightedness) is common in young children, but the eye elongates to achieve clear focus (Emmetropisation).
In myopia, the elongation of the eyeball continues excessively, causing nearsightedness.
Excessive close work, little outdoor time, genetics, and digital device use increase myopia risk.
Research on myopia is rapidly advancing, providing more knowledge for prevention.
The main causes of myopia
Too much close work - homework, reading, and writing
Too little time spent outdoors and exposure to sunlight
Genetics - parents having myopia and/or ethnic pre-disposition (oriental/Asian descent
Prolonged use of tablets and smartphones
There are many other potential causes and risk factors, but their effects are currently considered less significant than those above.
Research is taking place at a phenomenal rate and more knowledge and information around myopia and its causes are developing rapidly.