Measurement of Foveal Avascular Zone Dimensions and its Reliability in Healthy Eyes Using Optical Coherence Tomography Angiography

We read with interest the article by Shahlaee and associates describing the reliability of measurements of foveal avascular zone (FAZ) dimensions using optical coherence tomography angiography (OCTA). In that study, the authors reported good interobserver agreement for the superficial FAZ measurements but a lower reliability for the deep FAZ measurements.

The authors suggested that differences in the morphology and anatomy of the vessels between the superficial and deep vascular networks, as well as current limitations on the spatial resolution of OCTA, may explain the observed discrepancies in the intergrader reliability. Another potential factor to consider, however, is the influence of shadow artifacts arising from the superficial vascular network. This becomes more relevant when one considers that in most studies, the superficial FAZ has been reported to be smaller than the deep FAZ (0.27 mm 2 vs 0.34 mm 2 in the current paper). Hence shadow artifacts arising from the vessels at the edge of the superficial FAZ would project onto what would otherwise be an avascular area in the deep layer. Manual grading of the FAZ requires the grader to determine whether a “vessel” seen on the deep vascular plexus is really part of that network, or is caused by a shadow artifact from the superficial vascular network. It is not surprising that this results in more intergrader variability for measurements of the deep FAZ.

We agree with the authors that the patient’s ability to fixate is an important factor that affects imaging quality, since current commercially available OCTA devices are susceptible to motion artifacts. Younger subjects, especially those with good visual acuity, generally tend to fixate better. We have observed in our clinical practice, however, that even some young adults have difficulty cooperating with the OCTA scans, resulting in significant motion artifacts that make accurate assessment of the FAZ difficult. These are important factors to consider in any study describing FAZ characteristics.

The authors mentioned that FAZ area may be independently affected by age, although the evidence for this remains uncertain, and varies between different studies. In addition, other factors such as sex, central retinal thickness and volumes, and refractive error have been shown to influence the size of the superficial and/or deep FAZ. It is important for ophthalmologists to be cognizant of the fact that both the superficial and deep FAZ size show considerable variability even in normal subjects. This would potentially confound assessment of whether the FAZ appears “normal” on an OCTA scan, and we agree with the authors that comparing the FAZ dimensions with that of the contralateral eye is useful.

In summary, we congratulate the authors on their study, which has measured FAZ dimensions in healthy eyes using OCTA.

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Jan 6, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on Measurement of Foveal Avascular Zone Dimensions and its Reliability in Healthy Eyes Using Optical Coherence Tomography Angiography

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