April 2015
Volume 56, Issue 4
Free
Letters to the Editor  |   April 2015
Reliability of Field Chromatic Pupillometry for Assessing the Function of Melanopsin-Containing Retinal Ganglion Cells
Author Affiliations & Notes
  • Wuying Zhou
    Department of Ophthalmology Quhua Hospital, Quzhou, Zhejiang, China
  • Yitian Lou
    Department of Ophthalmology Quhua Hospital, Quzhou, Zhejiang, China
  • Bingxin Pan
    Department of Ophthalmology Quhua Hospital, Quzhou, Zhejiang, China
  • Jinhai Huang
    School of Ophthalmology and Optometry, Wenzhou Medical University, Wenzhou, Zhejiang, China.
Investigative Ophthalmology & Visual Science April 2015, Vol.56, 2519. doi:10.1167/iovs.15-16672
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      Wuying Zhou, Yitian Lou, Bingxin Pan, Jinhai Huang; Reliability of Field Chromatic Pupillometry for Assessing the Function of Melanopsin-Containing Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2015;56(4):2519. doi: 10.1167/iovs.15-16672.

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

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Research on test–retest reliability of a new method to measure melanopsin-driven postillumination pupil response (PIPR) induced by hemifield, central-field, and full-field light stimulation was performed by Lei et al.1 The new PIPR method is useful in clinical practice. We would like to share our opinion and expertise with the authors and other readers in order to broaden the discussion. 
Table 3 shows a large variation in the measurements. The 95% confidence interval of intraclass correlation coefficient (ICC) was 0.69 to 0.96 for the lower hemifield and 0.71 to 0.97 for the upper hemifield. These values undermine the clinical utility of this method for assessing the PIPR. Merely reporting the ICC and coefficient of variation is not adequate to demonstrate whether a method is precise enough for clinical diagnostics. The authors should perform more rigorous statistical analyses and thoroughly understand the statistical parameters suggested by the British and International Standards, such as test–retest repeatability (TRT).25 Test–retest repeatability is defined as the 2.77 within-subject standard deviation, which means an interval within which 95% of the differences between measurements are expected to lie. 
An important question is whether a sample size of 20 visually normal subjects is sufficient for such an interesting study. The authors need to provide statistical power calculations for ICC and coefficient of variation. It is more appropriate to use the sample calculation method suggested by Bland and Altman.3,6 For precision studies, the sample size calculation is independent of the instrument or technology. The only variable factors are the number of repeated measurements (n') and the confidence with which precision is estimated, typically 10%, which corresponds to a confidence of 0.10. So the formula is  where n is the sample size and m is the number of repeated measures. Therefore, if the authors assessed reproducibility with three repeated measurements and wanted to be within 10% confidence interval, they would require the following sample size:  Therefore, n = 96 subjects.  
Alternatively, a weaker confidence interval of 20% would require  Therefore, n = 24 subjects. 
We encourage the authors to recruit more subjects to reinforce the quality of their research and improve the clinical application of PIPR testing. 
The lower and upper hemifield stimulations were repeated in the second session scheduled within 1 month of the first session. Read and Collins7 demonstrated that significant physiological diurnal variation occurs in corneal thickness and shape, including the anterior and posterior corneal surfaces. The melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) also exhibit circadian rhythm variations.8 It is unclear whether the authors used the same unit time. To investigate the intersession reproducibility, we suggest that measurements be performed at the same time as the first session, by the same examiner, using the same protocol to minimize ipRGC circadian variations. It is also important to test the reproducibility of the new method in different diseases, such as age-related macular degeneration and glaucoma, where higher variability as compared to normal subjects may be expected. 
References
Lei S, Goltz HC, Chandrakumar M, Wong AMF. Test–retest reliability of hemifield central-field, and full-field chromatic pupillometry for assessing the function of melanopsin-containing retinal ganglion cells. Invest Ophthalmol Vis Sci. 2015; 56: 1267–1273.
Bland JM, Altman DG. Measurement error. BMJ. 1996; 313: 744.
McAlinden C, Khadka J, Pesudovs K. Statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology. Ophthalmic Physiol Opt. 2011; 31: 330–338.
International Organization for Standardization. Accuracy (Trueness and Precision) of Measurement Methods and Results – Part 2: Basic Methods for the Determination of Repeatability and Reproducibility of a Standard Measurement Method. London: HMO 1994; BS ISO 5725.
International Organization for Standardization. Accuracy (Trueness and Precision) of Measurement Methods and Results – Part 1: General Principles and Definitions. London: HMO 1994; BS ISO 5725.
Bland JM, Altman DG. How can I decide the sample size for a repeatability study? 2010. Available at: http://www-usersyorkacuk/∼mb55/meas/sizerep.htm. Accessed April 2, 2015.
Read SA, Collins MJ. Diurnal variation of corneal shape and thickness. Optom Vis Sci. 2009; 86: 170–180.
Hattar S, Liao HW, Takao M, et al. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science. 2002; 295: 1065–1070.
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