• What is myopia
• Short-sighted
• 3 optical causes of myopia
• Glasses or contact lenses
• Axial length myopia.....abnormal eye growth
• Close work/reading when young contributing to myopia
• Diet & lack of exercise contributing to myopia
  
  
• Myopic macular degeneration
• BMJ
• Field defects in myopic eyes AJO 2011; high myopia and glaucoma page
  
  
• Causes of myopia
• visual field defects in high myopia

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.

Spectacles or contact lenses help most people see really well. But if you have significant myopia, and cannot wear contact lenses, laser refractive surgery may help BBC.

This auother strongly recommends glasses and contact lenses. Children.

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

• in utero or at a very young age ...the eye may grow quickly or 'ballistically'.
  • High myopia may be due to genetic problems such as PAX6 polymorphism
  • congenital glaucoma
• due to lack of control of visually guided growth
  • genetic causes (including some of those causing high myopia above)
  • reading & close work when young does lengthen the eye and causes myopia BJO 2011
  • cataracts or anyhing preveintig light reaching retina
• due to diet /lack of exercise etc
• taller people are more myopic    (Archives 2011)

'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 the generic conditions below.

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.

There are many known genetic causes of myopia.

• 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 reaching a 'normal' size. Also
• In Ehlers Danlos myopia and Marfans, suspect genes have been identified, but it is not known how they cause myopia.
• Lumican (LUM) promotor polymorphism (Nature 2010)
• See review 2010.
• MYP1 gene Archives 2010
• Archives 2011

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 growth 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.

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 light hitting the retina,.such as congenital cataracts.

• some of the genetic conditions above involve this mechanism, such as the PAX6 gene

• Other genes are also involved.

• Pirenzapine can stop this growth.

• other genetic factors         ZNF644 mutations

World wide the number of people with myopia is increasing, and dietary factors probably contribute, although school work and near work probably contribute more as above.

See increase in US myopia. Epidemiology China Archives2011

Certainly, environmental factors are involved.  Less exercise leads to more myopia and Rose 2008 and outdoor activity reduces it.

Environmental influences does not cause very high myopia that is often associated with major ocular genetic problems, but lower degrees of myopia affecting 5-45% of populations.

One postulated dietary mechanism is that the

myopia increases in highly developed countries

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• 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).

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A possible mechanism of dietary influences contributing to the epidemiological increase in myopia across the world
(but close work is likely to contribute more)

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