Perspective: How Might Emmetropization and Genetic Factors Produce Myopia in Normal Eyes?

Perspective: How Might Emmetropization and Genetic Factors Produce Myopia in Normal Eyes?
Review 
Siegwart, John T. Jr; Norton, Thomas T.
Optometry & Vision Science:
March 2011 - Volume 88 - Issue 3 - pp E365-E372


This article discusses the process the eye goes through to become emmetropic, that is, with good vision at near and far.   This process is very active during the first year of life, and then it slows down.  There's a strong genetic component, and children who have one or two myopic parents, usually have longer eyes.  Animal studies have shown that visual stimuli play an important role in the development of myopia and hyperopia, and that the impact is greater the younger the animal is.  Studies have shown than hyperopic defocus may cause the eye to grow longer and become more myopic.  Minus lenses at near cause hyperopic defocus; but if it they are removed for 2 hours a day, at least in animals, the elongation might be prevented. 

From the text:

"The animal data on which this speculative article is based suggest that there may be a normal decrease in the ability of the emmetropization mechanism to use myopia to slow axial elongation and that this may impart a natural bias toward developing myopia. Throughout evolution, it presumably was rare for juvenile or adult vertebrate eyes to encounter significant periods of hyperopia, so that once eyes had achieved emmetropia it was not an issue whether myopia became less effective in controlling eye growth. However, when humans developed environments with extensive and long-duration nearwork demands, if the accommodative mechanism did not sufficiently remove the hyperopia associated with nearwork, and if myopia, except in infants, is unable to slow the axial elongation rate, it would be natural that at least some eyes develop myopia for distant "

Link to full text:

http://journals.lww.com/optvissci/Fulltext/2011/03000/Perspective__How_Might_Emmetropization_and_Genetic.4.aspx 

Axial length changes during accommodation in myopes and emmetropes.

Axial length changes during accommodation in myopes and emmetropes.
Read SA, Collins MJ, Woodman EC, Cheong SH.
Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. 
Optom Vis Sci.  Sep;87(9):656-62.
2010

and also:


Axial elongation following prolonged near work in myopes and emmetropes.
Woodman EC, Read SA, Collins MJ, Hegarty KJ, Priddle SB, Smith JM, Perro JV.
Br J Ophthalmol. 2011 May;95(5):652-6.

These studies found that the eye elongates during periods of accommodation (when the eye changes to achieve clear focus), and it does so equally in people who are myopic as well as those who are not.  The lens thickness also changes.   In the second study, the myopic eyes elongated more than the non-myopic eyes, during a prolonged period (30 minutes) of near work.

From what I've learned so far, the myopic eye (with axial myopia) is elongated, and its crystalline lens does not function properly.   In induced myopia, these structural changes are induced and can be reversed.  But the studies of induced myopia are on animals.  If lens thickening and axial elongation are part of the process of focusing, I wonder why the myopic eye continues to elongate and loses its capacity to change.

Link to abstracts:

http://www.ncbi.nlm.nih.gov/pubmed/20562668

http://www.ncbi.nlm.nih.gov/pubmed/20829316 

Hereditary and environmental contributions to emmetropization and myopia.

Mutti DO.

Source

The Ohio State University College of Optometry, Optom Vis Sci., Apr;87(4):255-9.

2010

This abstract also discusses the role of the lens in myopic eyes.  The lens of myopic eyes does not vary its thickness in relationship to focal distance, as it's supposed to.  Time outdoors is also highlighted.  I haven't read the whole article, but time outdoors has recently been found to be a  deterrent to myopia development and progression, that is, the more time outdoors children spend, the less likely they are to be myopic.

From the text:

"The ciliary muscle may play a greater role in emmetropization and myopia than previously thought. Time spent outdoors, not near work, may be the more important environmental variable in myopia. The effect of time outdoors shows an important interaction with a substantial genetic contribution to the risk of myopia."

Link to the abstract:

http://www.ncbi.nlm.nih.gov/pubmed/20019643

Corneal and Crystalline Lens Dimensions Before and After Myopia Onset

Corneal and Crystalline Lens Dimensions Before and After Myopia Onset

Mutti, Donald O.*; Mitchell, G. Lynn†; Sinnott, Loraine T.‡; Jones-Jordan, Lisa A.§; Moeschberger, Melvin L.‡; Cotter, Susan A.‖; Kleinstein, Robert N.*; Manny, Ruth E.*; Twelker, J. Daniel*; Zadnik, Karla*; The CLEERE Study Group

Optometry & Vision Science:

March 2012 - Volume 89 - Issue 3 - p 251–262

This article found that the lens of children who become myopic stops functioning properly about a year before the onset of myopia; the lens doesn't thin and thicken in response to visual input at different focal lengths.  The lens impacts the axial length of the eye, so if it doesn't thin and thicken as it's supposed to, that has an impact on the elongation of the eye.   The ciliary muscle might have something to do with this.

I've read before that the lens will adopt the optimal thickness for near work and stay like that if, for example, a person spends most of her time doing near work (like reading or using a computer).  So the observations in this article were interesting in this respect to me because they add to the ongoing debate about whether the eye elongates primarily because it is genetically predisposed to, or if it elongates primarily because of visual experience (near work, minus lenses, diminished exposure to sunlight, etc.)  There are a lot of pieces in understanding how myopia develops, and I'm very interested in this dynamic view of the body and of the development of the eye and vision that finds that the eye changes as it responds to visual experience.  It seems that the mechanism that allows the lens to adjust to different focal lengths breaks down in myopes, and I've also read that the ciliary muscle, that pushes and pulls the lens into a thicker or thinner shape, is thicker  (contracted?) in the myopic eye.   If the lens doesn't adjust, the image doesn't fall on the retina at far, for myopes, and the eye elongates.  The question for me is here, what could be done to get the ciliary lens and the lens to gain back the ability to contract and relax?  I think this is what the Accommotrac biofeedback therapy was trying to do.   But it didn't work, apparently...

From the text: 

"The behavior of the crystalline lens before and after myopia onset indicates that the previously correlated, compensating response of the crystalline lens is interrupted at myopia onset. Optically this result is axiomatic; myopia would be impossible if the crystalline lens and axial length always changed in tandem. Yet myopia is almost always thought of first as excessive length. These results suggest that while excessive growth is important, growth only becomes myopic when an independence develops between the anterior segment (crystalline lens) and posterior segment (axial growth). For the sample as a whole, for each ethnic group (with the possible exception of African Americans), and for each gender (with girls showing a greater effect for lens thickness), this independence appeared at onset or within a year of onset. The crystalline lens ceased to thin, flatten, and lose power even as the eye continued to grow. It is noteworthy that despite differences in myopia prevalence between ethnicities, there seems to be some consistency in this process for most ethnic groups."

"In summary, the process of becoming myopic appears to be more than just one of excess axial elongation. The myopic eye is certainly elongated relative to the emmetropic eye, but the elongation is accompanied at myopia onset by an abrupt independence between axial growth and longstanding, compensatory optical changes in the crystalline lens likely in place from infancy up to the time of myopia onset. At onset and for at least the following 5 years, VCD-adjusted lens parameters stopped thinning, flattening, and losing power in children who became myopic compared with children who remained emmetropic. The stability of corneal power suggests that it is unrelated to this process. Future studies should seek to determine the source of this departure from correlated growth that characterizes myopia onset."
http://www.ncbi.nlm.nih.gov/pubmed/22227914

Link: http://journals.lww.com/optvissci/Fulltext/2012/03000/Corneal_and_Crystalline_Lens_Dimensions_Before_and.3.aspx