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
http://www.ncbi.nlm.nih.gov/pubmed/22227914
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