- What is myopia
- 3 optical causes of myopia
- Glasses or contact lenses
- Axial length myopia.....abnormal eye growth
- Close work/reading when young contributing to myopia
- Retinal cells and control of myopia
- Diet & lack of exercise contributing to myopia
- Myopic macular degeneration
- high myopia and glaucoma
- Some syndromes related to high myopia
- Myopic retinoschisis
Myopia occurs when light is focused in front of the retina. A minus lens will focus the light further back on the retina. enlarge
myopia: light focuses in front of the retina
a minus lens focuses light further back onto the retina
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...
- the eye is longer than usual...this is termed axial myopia. This is the commonest form of myopia. This is common, discussed below.
- Less common: the cornea is more curved than usual and so is stronger. The light is focused too far forward, in front of the retina
- A low degree of myopic can be caused as we get older by lens changes: the lens of the eye becomes stronger, as in early cataract formation. The stronger lens focuses the light too far forward.
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. This another 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
- due to diet /lack of exercise etc
- taller people are more myopic (Archives 2011)
- febrile illness Eye 16
- Pathological myopia Retina 17, the mechanisms
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
- Causes of severe myopia include
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. enlarge
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.
There is 2% reduction in myopia for every additional hour per week spent outdoors BJO 16. Also, ethnicity is important.
One current theory suggests that when stimualted by light the amacrine cells in the retina produce a chemical that influences the serotonin transmitter, and this produces a chemical that influences scleral chondrocyte growth. Amacrine cells somehow control eye growth by means of the ZENK transcription factor.
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. Certainly, environmental factors are involved. Less exercise leads to less myopia and Rose 2008 and outdoor activity reduces it.
Environmental influences contribute to lower degrees of myopia affecting 5-45% of populations. Vvery high myopia (very high myopia is often associated with major ocular genetic problems as above. See increase in US myopia. Epidemiology Archives2011
One postulated dietary 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).
- could vitamin D pathways be involved
- outdoor playtime reduces myopia Wu 2013 Cui 2013 French 2013 Rose 2013
- Prediction of Juvenile-Onset Myopia JAMA15
Increase in myopia
myopia increases in highly developed countries enlarge
Myopia is increasing. his seems to be due to a combination of
- lack of exercise,
- more time spent reading,
- possibly less vitamin D as a result of less time exercise
- possibly also higher IGF as below again with less exercise
- and possibly a higher glycaemic index diet leading to higher IGF levels.
- OTE 16 Eye16
less exercise leads to insulin resistance; a diet with higher refined
sugars (and more carbohydrate) leads to higher insulin levels.
Muscles become resistant to insulin, so higher blood insulin levels are needed for the same effect
refined carbohydrate leads to higher insulin levels also
higher insulin levels lead to more IGF1 production from the liver
Higher IGF1 levels lead to greater eyeball growth and more myopia
- Excellent article 2015 p19 (p113 print)
- Cone-Rod dystrophy.....retinitis pigmentosa
- FEVR familial exudative vitreoretinopathy
- High myopia reduces disabetic retinopathy BJO17
Dome shaped macula
Macula hole with detachment
Myopic CNV (choroidal neovascularisation)
Deformation of optic disc