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Editors Selection IGR 12-2

Basic Research - Outflow: Ferritin as ouflow tracer

Comment by Nils Loewen & Julius Oatts on:

47027 Morphologic study of the drainage pathway using a tracer after a bypass filtering procedure in rabbit eyes, Lei J; Sun N; Zhao X et al., Ophthalmic Surgery Lasers and Imaging, 2011; 42: 254-262

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Is histologic assessment of uveoscleral outflow feasible? Lei et al. (1256) use intracameral ferritin as a tracer to compare outflow in trabeculectomy and deep sclerectomy in rabbits. As trabecular sclerectomy theoretically provides access to the suprachoroidal space, one would expect an increase in uveoscleral flow, which the authors claim citing an increased number of intra- and episcleral drainage vessels. This is a daring conclusion that is not strongly supported by the findings in this very interesting paper. The authors chose a challenging animal model, the rabbit, that is prone to severe inflammation and failure within days. The choice not to use mitomycin C is appropriate as ciliary body toxicity may reduce aqueous production in rabbits. Various prior studies have examined uveoscleral outflow ranging from intracameral fluorescent microspheres.¹ to protein dye complexes and fluorescein.² Tracers to measure uveoscleral outflow is challenging. There is wide variability in uveoscleral outflow across species (humans 38%, rabbits 3-8% in rabbits of total outflow³). Second, tracer-techniques do not truly quantify uveoscleral outflow and may obstruct flow channels depending on tracer size.⁴ Intracameral ferritin as an outflow tracer technique requires histology and can therefore not be used in patients. A potential problem of iron based dyes and very small molecules may be that they are prone to stain surrounding tissues that did not originally contain the dye. As glaucoma surgeons move toward bleb-free surgeries, the surgical technique described here is highly relevant. It will compete with newer internal, suprachoroidal, drainage devices but will not incite a foreign body reaction and may potentially be less affected by fibrosis as a result. It will be interesting to see whether the space created in trabecular sclerectomy will be maintained or collapse and disappear.

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