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S. Goyal, E. Jones, P. Alaghband, L. Beltran-Agullo, K. S. Lim; Controlling Flow: Gravity Driven Flow Test for the Measurement of Ahmed Valve Opening and Closing Pressure and Its Effect on Early Post Operative Complications. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4448.
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© ARVO (1962-2015); The Authors (2016-present)
Ahmed glaucoma valve (AGV) was developed to provide better immediate intraocular pressure (IOP) control while minimising the risk of hypotony in the early post operative phase, prior to the development of bleb resistance. However, there have been some concerns about the AGV FP7 and an in vitro study showed that in 3 of the 6 AGVs tested (Moss, J of glaucoma 2008) the closing pressure was ≤5 mm Hg. In July 2008 we introduced a simple in vivo pre-implantation gravity driven flow test for the AGV and we report the outcomes in terms of early IOP control and complications.
Restrospective case series. We identified AGV implanted between July 2008 and November 2009 from our database. At the time of surgery the AGV was cannulated with a Rycroft cannula, primed by flushing with Balanced Salt Solution (BSS), and connected to bottle of BSS via an infusion set. The gravity driven flow test measures the opening and closing pressure by observing the onset and cessation of flow through the valve when altering the BSS bottle height. A head of 13.6 cm water is equal to 10mmHg. We rejected AGV where flow was still present at 10cm (7mmHg) and there was no flow at 17cm of water (12mmHg). Surgery was done with a fornix based conjunctival flap without mitomycin C, anterior chamber (AC) was enterered with a 25g orange needle and a corneal/scleral patch graft was used.
20 AGVs were implanted in 16 patients (Table 1) in this period and 4 AGVs were rejected (17%). Hypotony (IOP <6mmHg) was present in 2 eyes (10%) despite testing. One eye had a shallow AC with peripheral choroidal effusion at one week that resolved after viscoelastic injection in to the AC. The second eye had ‘kissing’ choroidal effusions associated with leakage around the tube entry site, which was successfully resited without further complication. However, there were 6 eyes (30%) with early post-op IOP >15 mmHg (range 18 to 52 mmHg).
Though in vivo testing is a useful safety check for the AGV, there are other factors that can lead to hypotony or high IOP e.g. leakage around the tube, secondary valve failure, or other unknown mechanisms.
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