What is myopia ..spectacles
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
Another term for myopia is short sighted...that
is everything close is clear, everything far away is out of focus and
If an eye is -3.00 diopters myopic, as opposite, distance vision is
blurred, reading is clear and focused. Enlarge.
3 optical causes of myopia
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.
Glasses or contact lenses etc
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.
The causes of (axial length) myopia...abnormal eye growth
Axial length myopia...that is a large eye,
- 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
- 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 education levels in Singapore Eye 12
- cataracts or anything preventing light reaching retina
- 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.
Close work/reading when young contributing to myopia
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.
Retinal cells and control of myopia
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
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
Diet and lack of exercise contributing to myopia
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
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
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
Some syndromes etc related to high myopia
- Cone-Rod dystrophy.....retinitis pigmentosa
- FEVR familial exudative vitreoretinopathy
- High myopia reduces disabetic retinopathy BJO17