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Ahmed Glaucoma Valve


·      Developed by Mateen Ahmed

·      Glaucoma drainage devices are designed to divert aqueous humor from the anterior chamber to an external reservoir, where a fibrous capsule forms about 4-6 weeks after surgery and regulates flow.

·      AGV implantation is an effective and relatively safe surgical procedure, which allows to manage certain types of glaucoma (ie, secondary glaucoma) and glaucoma refractory to previous filtration surgeries (ie, second choice surgery)

 

Valved vs Non valved devices:

 

·      The amount of conjunctival scarring may determine the size of the implant and available area for a single-plate versus double-plate device.

·      The most important factor determining the type of implant selected is the target IOP.

·      Early IOP control is determined by the presence or absence of a valve in an implant, as the tube offers no resistance to aqueous flow.

·      The valved devices provide more immediate IOP control and a lower rate of hypotony.

·      Because nonvalved devices are often occluded with a stent or ligature suture, the postoperative IOP is unchanged and requires continuation of all preoperative medications until the fibrous capsule forms. With all devices, long-term IOP control depends on the surface area of the implant, which determines bleb size, tissue response to the implant, and thickness of the fibrous capsule controlling percolation of aqueous humor through the bleb wall.

 

 

Advantages:

 

·      For a beginning surgeon, valved devices may be preferred as the surgical technique is easier with localization to one quadrant without manipulation of the adjacent rectus muscles.

 

·      IOP control in the early postoperative period is more predictable

 

·      Require less postoperative follow-up and care.

 

 

 

Indications:

·      Severe uncontrolled glaucoma who have failed previous glaucoma surgery

·      High likelihood of trabeculectomy failure, including neovascular and uveitic glaucomas

·      Traumatic glaucoma

·      Aphakic and pseudophakic glaucoma

·      Post-keratoplasty glaucoma

·      Other secondary glaucomas

 

Parts

·      3 parts

·      Plate: in medical grade silicone, polypropylene, or porous polyethylene, depending on the model

·      Drainage tube: in medical grade silicone

·      Valve mechanism: in medical grade silicone.

·      Adult plate area: 180mm2

·      Pediatric plate area: 96 mm2

 

AGV with double plate: (total filtration area: 360 mm2) and one with a clip for pars plana tube insertion

The M4 AGV model: modified AGV S2 containing identical valve mechanism, with porous high-density polyethylene 

Total plate area is 160 mm2, not including the surface area of pores. The pores may facilitate fibrotic and vascular ingrowth and increase resistance to infection, exposure, extrusion, and mechanical deformation.

 

Physiology of valve:

·      Valve mechanism of AGV consists of thin silicone elastomer membranes, which are 8 mm long and 7 mm wide and create a Venturi-shaped chamber.

·      The membranes are pretensioned to open and close in response to IOP variations, in the range of 8–12 mmHg

·      After implantation, AH flows slowly and continuously into the trapezoidal chamber of the valve.

·      As the pressure reaches the preset threshold value, the valve opens, decreasing the IOP.

·      As the inlet cross-section of the chamber is wider than the outlet, a pressure differential is created across the chamber.

·      This pressure differential enables the valve to remain open even with a small pressure differential between the AC and the subconjunctival spaces surrounding the device.

·      This increased velocity and the nonobtrusive flow accounts for better evacuation and smaller valve friction.

·      The tension in the silicone membranes helps to reduce hypotony by closing after the pressure has decreased to normal level again.

 

Post operative course:

·      Topical antibiotic and steroid are started four times daily and continued for 4 to 6 weeks.

·      Initial follow-up is at 1 week, and the frequency of visits depends on the status of the eye.

·      For valved implants, preoperative glaucoma medications are discontinued to prevent hypotony.

·      For nonvalved implants, the glaucoma medications are usually continued until a fibrous capsule form around the plate, at which point the ligature suture may spontaneously open

 

Complications:

·      Hypotony

·      IOP increase

·      Excessive capsule fibrosis

·      Tube exposure due to conjunctival erosion

·      Tube migration or expulsion

·      Corneal complications – edema/ decompensation

·      Endophthalmitis or infections associated with grafting material covering the tube

·      Diplopia/ Strabismus


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