November 2012
Volume 53, Issue 12
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Letters to the Editor  |   November 2012
Author Response: Axial Length Changes with Shifts of Gaze in Myopes and Emmetropes
Author Notes
  • Contact Lens and Visual Optics Laboratory, School of Optometry and Vision Science, Queensland University of Technology, Brisbane, Australia. 
Investigative Ophthalmology & Visual Science November 2012, Vol.53, 7636. doi:10.1167/iovs.12-11190
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      Atanu Ghosh, Michael J. Collins, Scott A. Read, Brett A. Davis; Author Response: Axial Length Changes with Shifts of Gaze in Myopes and Emmetropes. Invest. Ophthalmol. Vis. Sci. 2012;53(12):7636. doi: 10.1167/iovs.12-11190.

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We thank Schachar and Levy 1 for their interest in our recent study “Axial Length Changes with Shifts of Gaze in Myopes and Emmetropes.” 2 They suggest that the differences we observed in axial length as a function of angle of gaze could be explained by small changes in fixation disparity within Panum's fusional area while “the subjects maintain bilateral fusion of a Maltese cross.” In this scenario described by Schachar and Levy, a slight misalignment of the measurement axis of the instrument with respect to the foveal axis could potentially account for some of the differences we observed. 
However, they appear to have misunderstood our methodology, which relies on a dichoptic view of the instrument fixation spot with the measured eye and the accommodation stimulus (Maltese cross) with the fellow eye, and not “bilateral fusion of a Maltese cross.” In our measurement technique, both eyes were viewing completely different stimuli and there was no binocular fusion of the Maltese cross. This dichoptic view was explained in the paper and was illustrated in Figure 1. 2 In this dichoptic view, the subject's primary task was to view the instrument fixation target in the optical biometer (measured eye), while the Maltese cross (fellow eye) position was adjusted until it was overlaid on the instrument fixation target. Since there was no fusion, and the primary fixation of the measured eye was on the instrument's fixation target, (which coincides with the instrument's measurement axis), these measurements must be along the foveal axis. 
Schachar and Levy cite Sullivan et al., 3 who reported that torsional disparity between two eyes can be about 2°, resulting from dichoptic stimuli. However, the dichoptic stimuli in this study by Sullivan et al. 3 involved identical targets presented to each eye, which were rotated by unequal amounts. The fusional response to these stimuli consisted of a compensating ocular rotation and a central fusion component. In our study, there was no fusion of the dichoptic stimuli. It is possible that some small amount of cyclotorsion may take place under dichoptic conditions with different gaze directions (particularly in oblique directions). But in our study, as the subject was always instructed to fixate on the biometer's fixation target during axial length measurements, the fovea of the measured eye was always aligned with the measurement axis of the instrument. The stability of monocular fixation is potentially less than a minute of arc. 4  
Schachar and Levy also question how the extraocular muscles could cause an increase in axial length in superonasal gaze and a decrease in superior and superotemporal gaze. The primary muscle involved in superonasal gaze is the inferior oblique; while in superior and superotemporal gaze, it is the superior rectus muscle. 5 Greene 6 modeled the potential mechanical influence of extraocular muscles upon the globe and concluded that tension from the extraocular muscles (particularly the obliques) could be a significant factor involved in myopic axial elongation associated with downward gaze. Based on Greene's theory, 6 we have hypothesized that because of its insertion location, the inferior oblique muscle may have the capacity to produce local stress on the posterior sclera, whereas the forces applied by the recti may be more anterior. The effects we have observed could also potentially be associated with changes in the choroid or intraocular pressure associated with shifts in gaze direction. Further studies could examine the changes in topographical retinal shape during shifts in gaze, to determine the effects of the extraocular muscle forces on the peripheral contour of the globe and better understand the mechanism underlying the axial length changes we have observed, using methods such as optical coherence tomography or magnetic resonance imaging. 
References
Schachar R Levy N. Axial length probably does not change with shift in gaze. Invest Ophthalmol Vis Sci . 2012;53:7425. [CrossRef] [PubMed]
Ghosh A Collins MJ Read SA Davis BA. Axial length changes with shifts of gaze direction in myopes and emmetropes. Invest Ophthalmol Vis Sci . 2012;53:6465–6471. [CrossRef] [PubMed]
Sullivan MJ Kertesz AE. Binocular coordination of torsional eye movements in cyclofusional response. Vision Res . 1978;18:943–949. [CrossRef] [PubMed]
Arathorn DW Yang Q Vogel CR Zhang Y Tiruveedhula P Roorda A. Retinally stabilized cone-targeted stimulus delivery. Opt Express . 2007;15:13731–13744. [CrossRef] [PubMed]
Von Noorden GK. Summary of the gross anatomy of the extraocular muscles. In: Lampert R Cox K Burke D eds. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St. Louis, MO: C.V. Mosby; 2002;38–51.
Greene PR. Mechanical considerations in myopia: relative effects of accommodation, convergence, intraocular pressure, and the extraocular muscles. Am J Optom Physiol Opt . 1980;57:902–914. [CrossRef] [PubMed]
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