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Editors Selection IGR 24-3

Clinical examination methods: Retrobulbar blood flow

Comment by Kaweh Mansouri on:

66622 Inter-device reproducibility of retrobulbar blood flow velocity measurements in healthy subjects using color Doppler imaging, Vercellin Alice C V; Cutolo CA; Dellafiore C et al., Journal of ultrasound, 2016; 19: 125-130

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The 'vascular' glaucoma theory implicates IOP-unrelated mechanisms such as reduced blood flow, suggesting that impaired regulation of ocular blood flow results in periods of relative ischemia and repeated reperfusion damage to the optic nerve. Color Doppler Imaging (CDI) can measure blood velocity but not actual blood flow. Using this device, velocity measurements have been reported for the ophthalmic, central retinal, and posterior ciliary arteries in different cohorts of glaucoma patients and healthy subjects. Most studies show impaired orbital blood flow in POAG. There are, however, important disparities including contradictory findings in between studies. The main reason, however, is the operator-dependent nature of CDI, leading to low reproducibility of measurements.¹ Furthermore, CDI is an expensive device (> $100.000) and, therefore, not readily available.

Data from CDI studies seem to support the hypothesis whereby glaucoma is related to compromised ocular blood flow

In this small but well-designed study Vercellin et al. evaluate the inter-device reproducibility of two widely-used CDI devices. Two ophthalmologists and two radiologists perform CDI measurements on ten eyes of young, healthy subjects. They found unacceptably low intra-device reproducibility (0.15 < CCC < 0.37, Lin's concordance correlation coefficient), with low precision (0.18 < Pearson's r < 0.47). Their results show that inter-device agreement is adversely and significantly affected by low precision of measurements, which in return is likely the result of high intra-subject variability.

The issue of intra-subject and intra-observer variability can be addressed through strict acquisition protocols and training. The lack of agreement between devices, however, is harder to address. As a consequence, data from different centers using different CDI devices are not comparable. In addition, creation of normative databases become more challenging.

Despite inconsistencies, data from CDI studies seem to support the hypothesis whereby glaucoma is related to compromised ocular blood flow. However, their numerous shortcomings have discouraged a generation of researchers from entering the field of ocular blood flow and prompted them to seek safer and less uncontroversial research activities.

Enter OCT-angiography (OCT-A) with its ease of use and higher degree of repeatability and reproducibility. Although it does not measure blood flow directly (for now) and may not replace CDI (for now), OCT-A will likely increase research efforts into this field. Thanks to this technology, advances in understanding vascular factors in glaucoma are expected for the near future.

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