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Additional Controls Required for Assessing In Vivo Accommodation
Author Response: Additional Controls Required for Assessing In Vivo Accommodation
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Ronald A. Schachar
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ron{at}2ras.com Ronald A. Schachar
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Using goniovideography in rhesus monkeys, Croft and co-authors1 measured the change in location of the lens equator and the ciliary processes in response to electrical stimulation of the Edinger-Westphal nucleus (EW). The authors placed 9-0 nylon sutures, of approximately 300 micra diameter, in the monkey cornea in the form of a loop to serve as a reference from which measurements were made. Critical to the accuracy of their measurements is that the image of the suture itself does not change dimensions, focus and location during EW stimulation. We measured the horizontal and vertical internal diameters of the corneal suture loops shown in the authors' Fig. 1A and 1B. We find that these dimensions changed by approximately 10%. To assess whether the authors' results are compromised by this dimensional change, additional experimental controls are needed. Although the authors attempted to assess the effect of induced extraocular muscle (EOM) movements on their measurements in two rhesus eyes without EW stimulation, they do not provide the results of these tests. Moreover, artificially induced eye movements are insufficient controls because they are different from the dynamic nonrandom eye movements that occur during EW stimulation. We suggest that the authors perform the following additional control experiments to really determine if eye movements affected their measurements: 1. In unrestrained eyes, which have undergone cycloplegia and have not received botulinum toxin, EW stimulation should be applied. This will induce EOM movements in eyes with paralyzed ciliary muscles but fully active extraocular muscles. It will demonstrate the effect of such EOM movements alone on the position of the ciliary processes and lens equator. We look forward to seeing studies that employ these controls. Ronald A. Schachar References 1. Croft MA, Glasser A, Heatley G, et al. Accommodative ciliary body and lens function in rhesus monkeys, I: normal lens, zonule and ciliary process configuration in the
iridectomized eye. Invest Ophthalmol Vis Sci. 2006;47:1076-1086. |
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Mary Ann Croft
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macroft{at}wisc.edu Mary Ann Croft
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Positive Controls Do Not Aid in Assessing In Vivo Accommodation We are writing to respond to the letter from Drs. Schachar and Kamangar regarding our recent study on "Accommodative ciliary body and lens function in rhesus monkeys, I: Normal lens, zonule and ciliary process configuration in the iridectomized eye."1 Drs. Schachar and Kamangar allege that the internal dimensions of the corneal reference suture changed by 10% in the low-resolution published, printed images of Figure 1. Drs. Schachar and Kamangar do not say in which direction these changes occurred, nor if the change they measured was statistically significant. Drs. Schachar and Kamangar do not report how the measurements were made. We undertook five repeated measures of both the horizontal and vertical internal dimensions of the reference suture in Figure 1 using the techniques described previously on the original, high-resolution images.1 Contrary to their claims, the dimensions do not differ significantly between the images in either direction. Appropriate control experiments to determine the effect of the residual eye movements are included in the study. We induced artificial eye movements that mimicked the direction and amplitude of the dampened convergence eye movements that were typically seen during EW stimulation.1 These movements are the same as those that occur with EW stimulated accommodation. As we state in the paper, "A small amount of variability was introduced by convergence eye movement, but the values were close to or less than the SD of the experimental measurements (±0.05 mm) and cannot account for the accommodative movements observed. The error bars (SEM) were small compared with the amplitude of movement (Figs. 3, 4). If there were an effect from eye movement, one would expect consistent, significant differences in CP and lens equator movement between nasal and temporal quadrants, irrespective of age."1 Drs. Schachar and Kamangar propose irrelevant positive control experiments, in which the amplitude of the convergence eye movement would be far greater than actually occurs during EW stimulation. We know from previous research2,3 that unrestricted, large amplitude, convergence eye movements interfere with assessing in vivo accommodative responses due to rotation of the eye. During our experiments,1 great care was taken to dampen convergence eye movement by botulinum toxin injection into the extraocular muscles precisely to avoid this issue. Further, we placed extraocular muscle sutures to dampen any residual eye movement.1 In addition, a frequency of electrical stimulation was used during EW stimulation, which further minimized convergence eye movement without diminishing accommodation. We very carefully controlled convergence eye movement without dampening the accommodative response for each monkey eye throughout the age range. In multiple monkeys of the same age, CP and lens movements were similar in magnitude regardless of the presence or absence of residual eye movements. Similarly, we carefully controlled eye movement during ultrasound biomicroscopy imaging and oriented the anterior/posterior ends of the ciliary body in a horizontal direction within all images for each monkey eye throughout the age range reported. Comparisons were made between the images in the unaccommodated and accommodated state. The dimensions of the suture shown in our original Figure 1 do not change to the extent that Drs. Schachar and Kamangar suggest, the controls they suggest are irrelevant to the study performed, and the results we report are not influenced by the extent of eye movements that do occur. Mary Ann Croft,1 Adrian Glasser,2 Gregg Heatley,1 Jared McDonald,1 Paul L. Kaufman1 1Department of Ophthalmology and Visual Sciences, Wisconsin Regional
Primate Research Center, University of Wisconsin, Madison, Wisconsin References 1. Croft MA, Glasser A, Heatley G, et al. Accommodative ciliary body and lens function in rhesus monkeys, I: Normal lens, zonule and ciliary process configuration in the iridectomized eye. Invest Ophthalmol Vis Sci. 2006;47:1076-1086. |
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