September 2016
Volume 57, Issue 11
Open Access
Letters to the Editor  |   September 2016
Author Response: Gravity Affects Lens Position During Accommodation
Author Affiliations & Notes
  • David A. Atchison
    Institute of Health and Biomedical Innovation & School of Optometry and Vision Science Queensland University of Technology Australia
  • Lucas J. Lister
    Institute of Health and Biomedical Innovation & School of Optometry and Vision Science Queensland University of Technology Australia
  • Marwan Suheimat
    Institute of Health and Biomedical Innovation & School of Optometry and Vision Science Queensland University of Technology Australia
  • Pavan K. Verkicharla
    Institute of Health and Biomedical Innovation & School of Optometry and Vision Science Queensland University of Technology Australia
  • Edward A. H. Mallen
    School of Optometry and Vision Science, University of Bradford, Bradford, United Kingdom.
Investigative Ophthalmology & Visual Science September 2016, Vol.57, 4568-4569. doi:10.1167/iovs.16-20068
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      David A. Atchison, Lucas J. Lister, Marwan Suheimat, Pavan K. Verkicharla, Edward A. H. Mallen; Author Response: Gravity Affects Lens Position During Accommodation. Invest. Ophthalmol. Vis. Sci. 2016;57(11):4568-4569. doi: 10.1167/iovs.16-20068.

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      © ARVO (1962-2015); The Authors (2016-present)

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In referring to our study,1 Schachar2 writes in the first paragraph of his letter “They concluded that the reduction in anterior chamber depth in the prone position during high levels of accommodation slackens the zonules sufficiently to allow the lens to move under the effect of gravity.” We made no such conclusion. Our conclusion was that slackening of zonules during high levels of accommodation in the prone position allows the lens to move under the effect of gravity. We assume that this is what Schachar meant. 
Schachar decides that a no-drug/minimum accommodation difference in anterior chamber depth between upright and downward head positions should be the baseline condition in our study to which other results should be compared. In his opinion, combining this comparison with measurement variability indicates that the effects we found are not real. 
Schachar's analysis in his third paragraph has a basic flaw. If there is a baseline condition, it would be the anterior chamber depth in the upright position for the no-drug/minimum accommodation combination, not the difference in anterior chamber depths between downwards and upright head positions for this combination. The other combination differences between downwards and upright head positions should be compared to zero, not by how much they vary from the no-drug/minimum accommodation combination difference. This gives significant mean effects of gravity for the young participants of 0.04 to 0.07 mm and for older participants of 0.08 to 0.12 mm (our Fig. 3C). Schachar writes that effects for drug conditions probably reflect drug-induced smaller pupil size; leaving aside that this claim is based on a study that dilated rather than constricted pupils,3 this is irrelevant as comparisons should be made within, not between, drug conditions. 
A second issue is Schachar's assertion that the no-drug/minimum accommodation combination difference in anterior chamber depths between downwards and upright head positions should have been greater for the older than for the young participants, rather than the opposite as we found. His basis for this is that older lenses have greater diameters in the unaccommodated state (giving reduced tension of the zonules) and greater weights than young lenses, making older lenses more susceptible to the effects of gravity. Some,46 although not all,7 studies of lens diameter indicates that it does not change with age in the unaccommodated state; Schachar's increase in diameter from 8.8 (young) to 9.5 (older) mm lenses comes from his own study of excised lenses8 and is irrelevant for his argument. The increasing weight of the lens with ageing may have been a contributing factor to a greater effect of gravity on anterior chamber depth for the older than for the young participants when pilocarpine was used. 
A third issue is that of measurement variability, which Schachar claims to explain some of our differences. He cites two papers concerning repeatability of the Lenstar biometer used in our study. One of these studies is concerned with intrasession repeatability,9 which is irrelevant as the appropriate comparison for a study involving combinations of conditions investigated at different times is intersession repeatability. Shammas and Hoffer10 found repeatability as given by the standard deviations of intersession differences of 0.10 mm. While this is of the order of our largest effects, the sessions were conducted 1 month apart rather than within a few days as in our study. It is interesting that the mean difference between sessions of 0.024 mm was nearly significant. A study which is a better comparison is that of Buckhurst et al.11 with an intersession repeatability of 0.013 mm, much less than the effects we reported. 
Relevant to the measurement variability issue is that our results were always in the direction supporting effect of gravity on anterior chamber depth, rather than sometimes in the opposite direction which might be expected if effects could be explained by measurement variability. 
In summary, our study demonstrated that gravity has small, but real, effects on lens position during accommodation. It supported the Helmholtz theory that accommodation is produced by ciliary muscle contraction relaxing zonular tension and enabling the lens to reduce in diameter and take up a more rounded form. It provided evidence against Schachar's alternative theory of accommodation that ciliary muscle contraction increases zonular tension and increases lens diameter. 
References
Lister LJ, Suheimat M, Verkicharla PK, Mallen EAH, Atchison DA. Influence of gravity on ocular lens position. Invest Ophthalmol Vis Sci. 2016; 57: 1885–1891.
Schachar RA. Gravity does not affect lens position during accommodation. Invest Ophthalmol Vis Sci. 2016; 57: 4566–4567.
Bakbak B, Koktekir BE, Gedik S, Guzel H. The effect of pupil dilation on biometric parameters of the Lenstar 900. Cornea. 2013; 32: 21–24.
Strenk SA, Semmlow JL, Strenk LM, Munoz P, Gronlund-Jacob J, DeMarco KJ. Age related changes in human ciliary muscle and lens: a magnetic resonance imaging study. Invest Ophthalmol Vis Sci. 1999; 40: 1162–1169.
Wendt M, Croft MA, McDonald J, Kaufman PL, Glasser A. Lens diameter and thickness as a function of age and pharmacologically stimulated accommodation in rhesus monkeys. Exp Eye Res. 2008; 86: 746–752.
Adnan Pope JM, Sepehrband F, et al. Lens shape and refractive index distribution in type 1 diabetes. Invest Ophthalmol Vis Sci. 2015; 56: 4759–4766.
Kasthurirangan S, Markwell EL, Atchison DA, Pope JM. MRI study of the changes in crystalline lens shape with accommodation and aging in humans. J Vis. 2011; 11 (3): 19.
Schachar RA. Growth patterns of fresh human crystalline lenses measured by in vitro photographic biometry. J Anat. 2005; 206: 575–580.
Zhao J, Chen Z, Zhou Z, Ding L, Zhou X. Evaluation of the repeatability of the Lenstar and comparison with two other non-contact biometric devices in myopes. Clin Exp Optom. 2013; 96: 92–99.
Shammas HJ, Hoffer KJ. Repeatability and reproducibility of biometry and keratometry measurements using a noncontact optical low-coherence reflectometer and keratometer. Am J Ophthalmol. 2012; 153: 56–61.
Buckhurst PJ, Wolffsohn JS, Shah S, Naroo SA, Davies LN, Berrow EJ. A new optical low coherence reflectometry device for ocular biometry in cataract patients. Br J Ophthalmol. 2009; 93: 949–953.
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