10 What Is the Suprachoroidal Space and Can It Scar? Uveoscleral outflow (uveoscleral bulk flow) has been long recognized in rabbits, cats, and by Anders Bill in some monkeys as early as 1962.1 This “unconventional” flow, perhaps accounting for 25 to 57% of total aqueous outflow in healthy subjects, is pressure independent at normal intraocular pressure (IOP) and is thought to include seepage into the ciliary muscle, through the sclera, and into the choroidal space vessels. Current researchers, however, doubt the preeminence of percolation through the sclera.2 Aqueous flow into and absorption by choroidal vessels is of current interest due to ongoing experimentation with suprachoroidal shunt devices such as the CyPass Micro-Stent® (Transcend Medical, Menlo Park, CA), a 6.35-mm-long polyimide tube with an internal diameter of 300 µm and an outer diameter of 510 µm with built-in stabilizing rings. In a preliminary 6-month clinical study, it proved to be a remarkably uncomplicated minimally invasive filtering option, with IOP lowering from a baseline of 30.5 to 19.9 (N = 52).3 A larger 12-month clinical study of CyPass as an adjunct to cataract surgery found a 14% IOP reduction from baseline and a 49 to 75% reduction in medications without any problematic hypotony.4 The compartment into which such devices can be placed is anatomically limited to the suprachoroidal space between the most external layers of the choroidal veins and the lamina fusca anterior to vortex vein ampullae and their extensions through the sclera to the superior and inferior orbital veins. Aqueous drainage, in theory, could occur throughout the entire suprachoroid, ideally as a micrometer-thin low-pressure fluid layer. Prolonged high-pressure flow would seem likely to precipitate at least transient choroidal effusion, undesirable inflammation, and vision compromise, none of which were encountered in the CyPass studies.3,4 Preplacement evaluation of the space available for safe device insertion requires careful indirect ophthalmoscopy and ideally high-resolution ultrasound and care to avoid vortex vein ampullae in the suprachoroid, which rarely enlarge as varices.5 Vortex veins, five to eight in number, and typically more numerous nasally, lie 15.5 to 16.5 mm posterior and nasal from the lateral rectus insertion in the superior temporal quadrant and ~ 3 mm posterior to the globe equator.6 As with orbital drainage devices, the superior temporal quadrant would seem the most favorable quadrant for suprachoroidal device placement. The microanatomy of the suprachoroidal space into which shunting devices might be safely placed and aqueous drained has been detailed by Torczynski7
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