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Editor,—I read with interest the article by Koet al,1 in which the authors describe their findings on the morphology of the circle of Zinn–Haller (CZH) obtained with fluorescein angiography. It was not mentioned in the article that by using scanning laser Doppler flowmetry, a non-invasive clinical method, CZH can also be clearly identified.2 Like Ko et al, the authors we found CZH easily visible in healthy myopic eyes within the temporal peripapillary area, especially if peripapillary atrophy was present.2 Using Heidelberg retina flowmeter images focused on the retinal surface, the temporal part of CZH was seen even in moderate degree myopia (−5.0 D), as well as the centripetal arterial branches, which originate from the circle and lead to the optic disc.2 It is even more interesting that the arterial perfusion in the CZH was also clearly detectable using scanning laser Doppler flowmetry even if the circle was not visible on the screen because of the retinal capillary network.
In healthy myopic eyes CZH was detected in 83% on the temporal side and in 8% on the nasal side of the disc.2 This may suggests that CZH is frequently incomplete. In healthy non-myopic eyes the figure was 23% on both sides.2 In glaucoma the temporal part of CZH was detected in 71% of the myopic eyes and in 75% of the non-myopic eyes. This suggests that retinal thinning or atrophic changes for any reason within the temporal peripapillary area may enhance the detectability of CZH.
Since fluorescein angiography reflects the plasma circulation while scanning laser Doppler flowmetry reflects the moving red blood cells the two methods may provide complementary information for anatomical and functional evaluation of peripapillary perfusion.
Editor,—We greatly appreciate Dr Hollo’s interest in our study. As was mentioned by some authors, the scanning laser Doppler flowmetry or indocyanine green angiography are valuable imaging methods for detecting the peripapillary circle of Zinn–Haller (CZH), especially when myopic crescent or glaucomatous peripapillary atrophy are present. In this regard we agree that the chance to detect the CZH by any imaging methods can be enhanced by peripapillary chorioretinal thinning under any circumstances.
According to Olver and associates, although the completeness of the vascular circle was about 77%, this did not mean the functional completeness. However, we should keep in mind the tremendous morphological variations of the CZH in the intraluminal diameter and its distance from the optic disc margin. The CZH does not have uniform intraluminal diameter during its course and does not run in the same plane within the sclera. In our study using flat section of human eyes, the minimal diameter of the vascular circle was 20 μm.
This narrow portion of the CZH may be not detected by scanning Doppler flowmetry owing to resolution or functional incompetence. In addition, we think that the imaging methods have limitations in detecting the deep seated portion of the CZH with minimal peripapillary chorioretinal atrophy and in observing the detailed branching patterns. We agree that it will be interesting to study peripapillary perfusion in patients with normal or high tension glaucoma or anterior ischaemic optic neuropathy by scanning laser Doppler flowmetry.