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I. Stalmans, B. Siesky, T. G. Zeyen, A. Harris; Variability of Color Doppler Imaging Parameters and Consequences for Sample Size Calculations. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4608.
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Literature concerning color Doppler imaging (CDI) of the retrobulbar blood vessels suggests variability is higher in pathological conditions than in healthy controls. This concept, however, has not been proven. This study reveals the differences in variability between glaucoma patients and healthy controls and examines the sensitivity of CDI parameters over time. Utilizing the differences in variability and sensitivity of each specific outcome, sample size calculations for future CDI studies are presented.
CDI measurements were performed twice with one month intervals in normal tension (n=28) and high tension (n=19) glaucoma patients and healthy controls (n=22). The peak systolic velocity (PSV), end diastolic velocity (EDV) and resistive index (RI) of the central retinal, temporal and nasal short posterior ciliary and ophthalmic arteries were determined. Variability for log-transformed CDI measurements was quantified using coefficients of variation (CV). Sample sizes have been calculated to compare the change in CDI between two groups.
The variability of all CDI measurements was higher in glaucoma patients than in healthy controls. Moreover, EDV variability was consistently higher than PSV measurements in all retrobulbar vessels. Consequently, calculated sample sizes are indeed larger for glaucoma patients than for healthy controls and larger for EDV than for PSV measurements. The smallest sample sizes (6-10 subjects) would be required for CDI studies using healthy controls measuring changes in PSV. Conversely, to detect changes in EDV in glaucoma patients, sample sizes of up to 210 patients would be suggested to detect a change of ≤10% between measurements.
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