Control and optimisation of fluid flow in glaucoma drainage device surgery.

Avoidance of hypotony in the early post-operative phase following glaucoma drainage device surgery is paramount, if serious complications are to be avoided. Current strategies of preventing early hypotony are either a valve mechanism, as those found in Ahmed Glaucoma Valve or flow restrictor as used in Baerveldt or Molteno implants. The Ahmed glaucoma valve has a pre-tensioned valve which opens and closes at the pre-determined level of intra-ocular pressure. Although the opening and closing pressure can be slightly inconsistent, coinciding with the change of material used in the plate construction, the reliability appears to have improved in recent years. Flow restrictor method, such as vicryl tie and Supramid suture stenting, used in non-valved implants utilised the Poissuille's equation in predicting the pressure gradient across the tube and thereby avoiding the risk of hypotony. Newer generations of glaucoma drainage devices such as Xen, Microshunt, and eyeWatch all use the same principle. However, most of these do not address the issue of unnecessary residual flow resistance once the bleb resistance has formed around the drainage tube or plate except for the EyeWatch device.