By contrast, Achison et al found that the average axial length of myope is about 350 microns (0.35 mm) longer per diopter than that of emmetropes.
...So someone with mild myopia of -3D would have an eye that is about a full millimeter (1000 microns) longer than someone with normal vision. Thats an increase of about 4% relative to a normal eye length of 25 mm. That's enough to produce real refractive effects, which I would argue constitute true axial myopia.
The Incremental Retinal Defocus Theory of Hung and Ciuffreda contents that true and permanent axial myopia results from the cumulative effect of thousands of cycles of defocus-induced axial lengthening, consolidated by sustained and repeated stimulus to scleral tissue growth.
I'm glad you referred to this study, because I think it needs some debunking -- one can frequently get misled by researchers' conclusions. I'd like to direct your attention to the raw data of the exact same study you quoted:
http://www.iovs.org/content/45/10/3380/T1.expansion.html
Please take a look at the eye dimensions reported for myopic groups from -1.60D through -5.5D, which represent the bigger part of the sample size. There is basically ZERO difference between the different groups' average axial length and little difference even in the extremes of the sample! If axial length was the primary determining factor of myopia and the prediction of this study is correct, then there should have been a 0.35 mm difference on average for each group or at least some measurable difference overall. There was none. This is quite disturbing given the conclusions drawn from the study.
I plotted out the data in Achison's Figure 1 (see attached figure), showing the axial length (by 3 different methods) for each group (labelled by the upper end of each diopter range). There does seem to be a trend to me. Sure, it's not monotonically linear, and flattens somewhat in the middle range, but keep in mind that these measurements had large variances, of between about
+0.5 to
+1.5 for each mean reported. So you have to view the data as having large error bars. Still, the mean axial length of -5.6 to -6.5D myopes is more than 2 mm longer than that of emmetropes, and that of -6.6 to -12 D myopes is more than 3 mm longer!
My point is not and never was that axial length perfectly correlates with or determines myopia. In fact, in the early stages, I suspect that most moderate myopia is pseudo myopia due to ciliary spasm or lens refractive errors, probably with little or no axial lengthening. Axial lengthening just makes things worse. While genetics and diet contribute, my contention is that the overprescription of minus lens, combined with excessive near work, drive a viscous cycle of further axial lengthening and worsening myopia.
But conceptually, you are right that axial length is not strictly a necessary or sufficient condition for myopia. (It may be a necessary condition for extreme myopia). That doesn't mean it is not a strong causal contributor in practice to most cases of myopia.
Todd, have you tried a refractometer to check whether your subjective VA has resulted in an objective refractive change?
I have not tried any such measurement. I have not visited an OD for more than 15 years since I stopped wearing glasses. The last prescription for which I have any record was made on December 18, 1995:
OD -1.00 -0.50 x 95
OS -1.75 -1.00 x 93
So my left eye was the more myopic (-1.75 D) at the time. You can see that I also had a bit of astigmatism. When I now test myself with the "astigmatic mirror" all the lines look about the same to me today:
http://www.i-see.org/astigmirror_bw.gifI have no need or reason to get my axial length measured or to do refractometry -- why spend the money? What's there to prove? Just to be clear, my view is that axial lengthening is not the sole determinant of myopia, and probably comes into play mainly for advanced myopia. Since my myopia was relatively mild, it is quite likely that I may have had mainly pseudo myopia and my eye may have never elongated for all I know. On the other hand, the fact is that my myopia in high school was barely noticeable (to me) until an OD told me I should wear glasses, and then got worse after he kept increasing my prescription. So maybe there was an axial component?
In any case, I suspect that those with myopia above -6D, and especially above -12D very likely have axial lengthening.
All I need to know is that my Snellen reading is 20/20 in the right eye and 20/40 (sometimes 20/30 on good days) in the left eye. Since my eyes are close enough and my less myopic right eye dominates, I see clearly in the distance. For close work, my left eye gets me a bit closer than the right - I see print in clear focus close up. So I have no need for glasses.
This is an interesting debate, but a bit academic. We may be right or wrong about the role that axial lengthening plays, vs. changes to the lens --- or perhaps even cognitive factors. All very interesting. But the real pragmatic question is whether the degree of myopia can be reversed by active focusing at the edge of blur, print pushing, reduced prescriptions, or by other techniques. My myopia resolved to the point that I can see distant objects that used to be very blurry. I can drive without glasses. Many people posting on this site or others have had similar experiences, many more dramatic than mine. Somehow it seems unlikely that we are all laboring under some massive delusion and our vision never actually improved.
Todd
