Good Hope, Heartlands, and Solihull Eye Clinics

Myopia

David Kinshuck

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

in a myopic eye ight focuses in front of the retina

 

 

 

myopia: light focuses in front of the retina

 

 

 

a minus lens focuses the light further back onto the retina

 

a minus lens focuses light further back onto the retina

 

Short-sighted

myopia... -3.00 dioptres, no reading glasses are needed

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.

 

 

3 optical causes of myopia

different causes of myopia

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. This is common, discussed below.
  2. 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
  3. 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.

 

enlarge

 

 

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, 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 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

Staphyloma 

Genes

There are many known genetic causes of myopia.

Close work/reading when young contributing to myopia

the feed back loop controlling eye 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.  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 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

 

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 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 with prosperity

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

 

lack of exercise and refined carbohydrate leads to higher insulin levels, and then high IG1 levels...the IGF1 stimulates eyeball growth

enlarge

 

 

 

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

 

 

Some syndromes etc related to high myopia

 

Systemic

  • Alport
  • Ehlers-Danlos
  • Homocysteinuria
  • Marfan
  • Noonon
  • Stickler

Ocular

  • Choroideraemia
  • Cone-Rod dystrophy.....retinitis pigmentosa
  • FEVR familial exudative vitreoretinopathy

Atropine

Eye 16

Diabetic retinopathy

  • High myopia reduces disabetic retinopathy BJO17