• What is myopia
• Short-sighted
• 3 optical causes of myopia
• Axial length myopia.....abnormal eye growth
• Visually guided growth failure causing myopia
• Diet & near work contributing to myopia
• BMJ

Myopia occurs when light is focused in front of the retina. A minus lens will focus the light further back on the retina. enlarge

Another term for myopia is short sighted...that is everything close is clear, everything far away is out of focus and blurred.

If an eye is ' -3.00 diopters ' myopic, as opposite, distance vision is blurred, reading is clear and focused. Enlarge

Theoretically myopia can occur for 3 reasons...

1. the eye is longer than usual...this is termed axial myopia. This is the commonest form of myopia.
   
   
2. the cornea is more curved than usual and so is stronger. The light is focused too far forward, in front of the retina.
   
   
3. the lens of the eye becomes stronger, as in early cataract formation. The stronger lens focuses the light too far forward.

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In practice axial myopia is the major problem around the world, and is discussed below.

Axial length myopia...that is a large eye, may develop

1. in utero or at a very young age ...the eye may grow quickly or 'ballistically'. High myopia may be due to gen problems such as PAX6 polymorphism
2. due to lack of control of visually guided growth
3. due to diet etc

'Ballistic' is the term used by Gregson to describe early onset rapid growth of the eyeball (it is not a generally used term).

Rapid eye growth may occur in conditions such as those opposite.

In glaucoma, the eye reaches a high pressure, and the eye ball is 'stretched'. If the pressure is reduced to normal, then a normal growth pattern is restored.

In Sticklers, there is a collagen problem, and it may be the scleral collagen does not respond normally to the various growth factors involved. It keeps growing even after reachign a 'normal' size. Also

In Ehlers Danlos myopia and Marfans, suspect genes have been identified, but it is not known how they cause myopia.

In healthy eye development, the retina can 'switch off' eye growth. In one experiment chicks are reared in the dark, and resulting in extra eye growthr see. Effectively the light is controlling growth.

One current theory suggests that the amacrine cell in the retina produce a chemical that influences the serotonin transmitter in the retina, and this produces a chemical that influences scleral chondrocyte growth.
Several genes such as the PAX6 gene are involved. Other genes are also involved.
Pirenzapine can stop this growth.

Light hits the retina...amacrine cells in the retina release chemicals that slow down scleral growth. The growth factor acts on the chondrocytes in the sclera.

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Amacrine cells somehow control eye growth by means of the ZENK transcription factor

Neurons and their synaptic connections in the retina:

1. Photoreceptors, Rods and Cones
2. Horizontal cells
3. Bipolar cells
4. Amacrine cells
5. Ganglion cells
6. Optic Nerve fibres.

This 'visually guided' feedback pathway sees to be at fault in such conditions as

• CSND congenitally stationary night blindness and related conditions
• any disease that prevents ight hitting the retina,.such as congenital cataracts.

World wide the number of people with myopia is increasing. It is not known whether dietary factors or school work and near work or both are responsible.

Certainly, environmental factors are involved.  Less exercise leads to more myopia and Rose 2008. This is not the very high myopia that is often associated with major ocular genetic problems, but lower degrees of myopia affecting 5-45% of populations.

There remains a lot of discussion as to whether near work and school work causes myopia directly, or whether this is an indirect association. Those who read and study more execise less, and participate in less sport, playing less outside, etc. There is limited hard evidence that reading itself causes permanent myopia.

A postulated mechanism is that the

• lack of exercise causes muscles to be resistant to insulin, and this leads to higher blood insulin levels
• a diet rich in sugar and refined carbohydrate leads to higher insulin levels

The higher blood insulin level stimulates the liver to produce more IGF1, leading to higher blood IGF1 levels. The IGF reaches the eye where it stimulates growth of the eyeball ( diagram below).

myopia increases in highly developed countries

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A possible mechanism of the epidemiological increase in myopia across the world.

1-2
less exercise leads to insulin resistance; a diet with higher refined sugars (and more carbohydrate) leads to higher insulin levels.

2
Muscles become resistant to insulin, so higher blood insulin levels are needed for the same effect

3
refined carbohydrate leads to higher insulin levels also

4-5
higher insulin levels lead to more IGF1 production from the liver

6-7
Higher IGF1 levels lead to greater eyeball growth and more myopia