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Editor,—We were interested in the article by Myung-Kyoo Ko et al 1 regarding the visualisation of the circle of Zinn by fluorescein angiography. However, we do not agree that it is possible to truly visualise the circle of Zinn by this method. As such, we feel that the point of view taken by the authors to reach their conclusions is flawed based on the following observations.
We have been investigating the location of the circle of Zinn by measuring the depth of the circle from the retinal side of the scleral surface, along with the distance between the circle of Zinn and the optic nerve sheath. We have chosen the classic longitudinal serial sectioning technique of the optic nerve and surrounding tissues. Observation at both sides of the sectioned optic nerve, aided by serial sections, has enabled us to conclude that the circle of Zinn lies at a relatively constant depth around the optic nerve in any individual eye.
Moreover, the mean depth of the circle of Zinn was 345 μm from the scleral surface in our study, which is larger than that in the report by Olver et al which is cited in Myung-Kyoo Ko and colleagues’ work. This is equivalent to 70% of the thickness of the retina at the posterior pole, roughly 1.5 times the thickness of the choroid. It is very unlikely that the small blood vessels at this depth in the sclera can be visualised by fluorescein angiography. There is no evidence that in myopic eyes with tilted discs, peripapillary sclera is “thinner” than normal.
A third point is the size of the blood vessels that were observed in Myung-Kyoo Ko et al. In our study, the mean radius of the lumen of the arteriole that consists the circle of the Zinn was 45.6 μm in 29 eye bank eyes. However, the vessels indicated in the photographs are much larger than this. These vessels appear to be as large as the proximal retinal arteries in all the photographs, and peripapillary arteries are about 200 μm wide, with a lumen of 100 μm in diameter.
Clinically, we have also noted these vessels and considered them choroidal vessels, readily visualised because of the pigment epithelial degeneration. Furthermore, the report of their vessels only on the temporal side in myopes needs additional explanation.
Editor,—We thank Dr Gauntt for comments on our article. He raises some questions about the depth of the CZH within the sclera, about visualisation of this vessel by fundus fluorescein angiography (FFA), and about the diameter of the vessel appeared on FFA. Though we have not investigated the mean depth of the CZH within the sclera, it is questionable that the CZH lies at a relatively constant depth within the sclera. If the vascular circle lies in such a constant position within the sclera, why have previous study using serial transverse or longitudinal histological sections to evaluate circulation of the anterior optic nerve failed to identify this intriguing vessel? This is partly because the CZH does not run in the same plane within the sclera, and we think that the serial histological section may give a little information about the CZH. As our previous study suggests, the CZH has tremendous morphometric and morphological variations between subjects and even in the same subject through its course around the optic nerve. These interindividual variations have also been reported by Olveret al. Additionally, the branching pattern of the vascular circle is so complexthat in simple histological section the main body of the circle may not be distinguished from its branches to peripapillary choroid or to the optic nerve.
Gauntt expresses doubt as to the visualisation of the CZH by FFA, because he suggested that it runs within sclera at such a depth (345 μm from the retinal side of the scleral surface) as to be invisible. However, we would like to point out again that the CZH has variable morphology in its course and does not run in the same plane anteroposteriorly and mediolaterally. Furthermore, chorioretinal atrophy and scleral thinning in the temporal crescent area, especially in pathological myopia, may offer a better chance to appreciate the temporal part of the CZH by imaging methods such as FFA or indocyanine green angiography (ICG). A recent report demonstrates the visualisation of the CZH by ICG in severely myopic eyes which supports our idea. However, the CZH could not be visualised even in severe myopic conditions when it is deeply seated within the sclera.
Regarding the diameter of the CZH, our previous study using the flat section method showed marked variation. It ranged from 20 μm to 270 μm with a mean of 123 (SD 75) μm. Even in the same circle it showed fluctuation during its course. In some cases the diameter of the main body of the CZH was less than that of its branch. Moreover, the abrupt decrease of the vessel calibre, the narrow portion of the CZH, was noted in many of our cases. Sometimes the peripapillary choroidal arterial arcade might be confused with the CZH in FFA. However we believe that it is distinguished from the CZH by several points. It is located along, the outer margin of the temporal crescent while the CZH appeared as concentric vascular filling within the temporal crescent or in the vicinity of the optic disc margin. It is obscured in the late phase of FFA by dye leakage from the intact choriocapillaris along the temporal crescent margin. Also, some branches from the arterial circle directed to the optic disc or to the peripapillary choroid can be observed on FFA.