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In Vivo Observation of IOP-dependent Reactions of the Episcleral Venous Plexus in the Rabbit

1Selbach J. M., 1Kremmer S., 2Steuhl K. P., 3Lütjen-Drecoll E.,
1Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde (Essen)
2Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde, Abt. für Erkrankungen des vorderen Augenabschnitts (Essen)
3Friedrich-Alexander-Universität Erlangen-Nürnberg, Anatomisches Institut LS II (Erlangen)

Purpose: To investigate the episcleral vascular network and its reactions to changes in intraocular pressure (IOP), which might play a role in aqueous humour outflow regulation.
Methods: For in vivo observation of the episcleral vasculature the microendoscopic technique as described by Funk and Rohen (1989; Ophthalmic Res. 21, 8-17) was applied. The endoscope together with a camera and a magnifier were attached to a micromanipulator. In 10 anesthetized New Zealand rabbits the episcleral vasculature was studied. IOP was monitored by a transducer connected to an anterior chamber needle. Mean arterial pressure was monitored, too. After an initial period of 10-15 min to reach steady state conditions IOP was raised or lowered by means of a hydrostatic reservoir and vascular reactions were observed for 15 min. The microendoscopic images were recorded continuously on a video recorder. Vessel diameters were analyzed at definite time intervals before (control) and after changing intraocular pressure with an image analysis software.
Results: At an IOP of 15 mmHg the plexus of the anterior episclera drains aqueous humor in aqueous veins which are either devoid of blood or show a typical lamellation of blood and aqueous. There is no predominant flow direction. After a stepwise experimental elevation of IOP from 20 to 60 mmHg, a 30-50% dilation of the veins and venules can be observed after 2-3 minutes. At high IOP levels the veins are mainly blood filled with a rapid flow from anterior to posterior. Reduction of IOP down to 5 mmHg is followed by a constriction of arteries and veins (up to 40%) after 3-6 minutes. No reflux of blood into the most anterior aqueous veins (loops) is observed. Topical application of sodium nitroprusside leads to a slight initial IOP increase (1.5 mmHg) followed by a marked decrease of IOP (4.3 mmHg) after 2 minutes which parallels a 30-40% dilation of the episcleral veins.
Conclusions: The results indicate that their might be an interaction between intraocular pressure and the densely innervated aqueous draining episcleral venous plexus which probably allows a subtle modulation of reflux of blood into the outflow pathways and presumably of aqueous humor outflow.

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