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Editors Selection IGR 16-4

Surgical treatment: Aqueous drainage devices

Comment by Tanuj Dada on:

59586 Five-Year Treatment Outcomes in the Ahmed Baerveldt Comparison Study, Budenz DL; Barton K; Gedde SJ et al., Ophthalmology, 2015; 122: 308-316

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Villamarin et al. report on the in vivo testing of a novel noninvasively adjustable glaucoma drainage device (AGDD). The AGDD is designed to drain aqueous with a resistance to outflow that can be externally regulated.

The implant is inserted surgically under a scleral flap and contains a mechanism that allows for a variable compression of the tube, altering its cross-sectional area and thus changing the fluidic resistance using an external magnet. In the present study AGDD was implanted on one eye of seven white normotensive rabbits for a duration of four months with the other eye serving as control. The IOP dropped significantly when the AGDD was opened from its fully closed (maximum outflow resistance) to fully opened (minimum outflow resistance) position.

Safety issues which need to be investigated include magnetic resonance imaging (MRI) compatibility and the influence of an external magnetic field on the device resistance

Although this is an exciting new technology to regulate postoperative IOP and prevent hypotony in glaucoma filtering surgery there are several inherent limitations on its use in penetrating glaucoma filtering surgery. The study evaluated the efficacy AGDD in IOP reduction in normotensive eyes and not in glaucomatous eyes, for which it is actually intended. The IOP was reported only with fully open or closed positions of the device and there was no control/gradient of outflow reported with change of device resistance. Despite being implanted in a closed position there was a transient hypotony in the early postoperative period. A very large (7 x 7 mm) sclera flap was used and the resistance to outflow may vary with the number and tension of sutures used to close the sclera flap. Safety issues which need to be investigated include magnetic resonance imaging (MRI) compatibility and the influence of an external magnetic field on the device resistance. Finally, a four-month postoperative follow-up is too short because episcleral and subconjunctival fibrosis will eventually limit the aqueous outflow and reduce the efficacy of the device making the adjustable outflow technology redundant in the long term.

Despite these limitations this is a commendable innovation which may also have potential in the prevention of early hypotony if coupled with non-valved glaucoma drainage devices.

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