Sir,

We read, with great interest, the recent article by Zeitz, Vilchez, Matthiessen, Richard, and Klemm titled ‘Volumetric colour Doppler Imaging: a useful tool for the determination of ocular blood flow in glaucoma patients?' Two issues within the manuscript require attention. The authors state that the only technology providing quantification of volumetric flow as volume per unit time in a specific vessel is fluorescein angiography. They have ignored the Canon laser blood flowmeter. The technology is FDA approved, has been reported in the literature in multiple publications,1 and its very existence contradicts the authors' statement.

More importantly, the authors conclude that since they were unable to detect significant dorzolamide-induced increases in retinal blood flow with a volumetric CDI measurement, the technology is inapplicable in ophthalmology. This is a sweeping statement that is unsupported by research findings. In the present study, the authors' findings are consistent with those in the literature. In multiple studies, we also failed to detect significant changes in CDI parameters.2, 3, 4, 5 The present findings support the existing literature suggesting that arterio-venous passage times are more sensitive to changes in retinal haemodynamics than CDI.