AHMED GLAUCOMA VALVE

Guest author

TANZEELA KHAN

Ajmal Khan Tibbiya College

Aligarh, India

INTRODUCTION

• Intraocular pressure (IOP) is the main risk factor for the development and progression of glaucoma.
• IOP reduction can be achieved by medical, laser or surgical therapy.
• Glaucoma filtration surgeries have high success rates during the early years (70-90%) but the effects tend to decrease with time.
• The success of trabeculectomy is largely influenced by the healing process in the anterior subconjunctival space.
• Aqueous shunts (also called Glaucoma Drainage Devices [GDDs]) are a reliable alternative to trabeculectomy.
• By shunting aqueous to the posterior subconjunctival space these devices may avoid healing issues, especially in patients who have already undergone previous glaucoma surgeries or conjunctival manipulation.
• Ahmed Glaucoma Valve (AGV) is a highly successful GDD which has been implanted in thousands of individuals world-wide.

AGV: DEVICE DESCRIPTION AND TECHNICAL DATA

• AGV provides a complex mechanism to control aqueous humor outflow.
• The device was developed by Dr Abdul Mateen Ahmed, a biomedical engineer of Indian descent.

• The AGV consists of 3 parts:

1. Plate (made of medical grade silicone, polypropylene or porous polyethylene, depending upon the model).
2. Drainage tube (made of silicone).
3. Valve mechanism (made of silicone).

• The adult model (S2) of AGV provides 180 mm² of total plate area.
• The pediatric model (S3) has a total plate area of 96 mm².
• A variant with 2 plates (total filtration area= 360 mm²) and one with a clip for pars plana tube insertion have also been designed.
• The M4 model is made of porous high-density polyethylene (Medpor).
• The 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 open and close in response to IOP variations over a range of 8-12 mmHg.
• After implantation aqueous humor flows slowly and continuously into the trapezoidal chamber of the valve.
• As the pressure reaches the reset threshold value, the valve opens, thus decreasing IOP.
• Since 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 anterior chamber and subconjunctival spaces surrounding the device.
• AGV complies to Bernoulli’s equation (fluid flowing into section A= fluid flowing out of section B) by increase in fluid velocity as it leaves the chamber through the drainage tube.
• The increased velocity and non-obtrusive flow account 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 again.

Mechanism of action of the AGV

SURGICAL TECHNIQUE

• The AGV is primed prior to implantation by injecting 1 cc of balanced salt solution.
• A fornix or limbus based conjunctival flap is fashioned.
• Usually the supero-temporal quadrant between the superior and lateral recti muscles is used.
• The body of the implant is positioned 8-10 mm from the limbus.
• The plate is sutured to the sclera by 9-0 or 10-0 nylon sutures.
• The drainage tube is trimmed so that 2-3 mm enters the AC. The tip is cut at a bevel of 30 degrees to facilitate insertion.
• The AC is entered 2-3 mm behind the corneo-scleral limbus.
• In patients with previous vitrectomy, the tube is inserted into the vitreous and fixed to the sclera by a pars plana clip.
• The drainage tube is covered with preserved donor sclera, pericardium, cornea or any other suitable patch graft material.
• Alternatively a 2/3rd thickness scleral flap is created, a needle tract is made and the tube inserted through this passage.
• The flap should be securely sutured to prevent aqueous egress from around the tube.
• Recently fibrin glue has been used to paste the scleral or bovine pericardial graft over the tube.
• The use of antimetabolites in AGV implantation is debatable. There are studies which do not show any significant difference when compared with no antimetabolite use. Others have reported good success rates when intra-operative MMC and post-operative 5-FU were used.

COMPLICATIONS:

1. HYPOTONY:

In the Ahmed versus Baerveldt (AVB) Study 6.1% patients in the Baerveldt group experienced vision threatening complications related to hypotony at 3 year analysis while none in the Ahmed valve group had similar complications.

In the Ahmed-Baerveldt Comparison (ABC) study, 1 eye (2%) in AGV group had persistent hypotony, compared to 6 eyes (13%) in the Baerveldt group at 5 years of follow-up.

2. IOP INCREASE AND EXCESSIVE CAPSULAR FIBROSIS:

The hypertensive phase after AGV implantation is quite common, peaking at 1-3 months post-operatively and resolving within 6 months. It is probably less common in those with silicone AGV implantation compared to polypropylene. Primary reason for IOP increase is from capsular fibrosis.

Digital massage is an option for the management of the hypertensive phase. If the IOP rises later than 6 months, capsular fibrosis and encapsulation of the plate is evident, then needling may resolve the situation.

3. TUBAL EXPOSURE:

Tubal exposure is reported in 5-14% patients. It can lead to inflammation, hypotony, poor vision and late endophthalmitis. In a study of AGV and Baerveldt implants, no significant differences in tubal exposure rates were found between the two devices. Younger age, prior inflammation and inferior quadrant implantation were significant risk factors for development of tube exposure.

4. CORNEAL COMPLICATIONS:

Silicone tubes in the AC can disturb the corneal endothelium and induce corneal decompensation. Long term frequency of this complication is reportedly 9-27%.

In the AVB study, corneal edema was present in 7% patients of the AGV group and 14% patients in the Baerveldt group at 3 years of follow-up.

Risk factors such as high IOP, long duration of high IOP, toxicity of preservative in the eyedrops and duration of surgery play a part in corneal decompensation.

5. INFECTION AND ENDOPHTHALMITIS:

It is a rare complication following GDD implantation. In a study conducted in Saudi Arabia (1994-2003), out of 542 implantations, 9 developed endophthalmitis (5 cases being in pediatric age group).

Younger age and conjunctival erosion over the tube are significant risk factors for the development of endophthalmitis. The exposed tube may act as a direct channel for intraocular passage of conjunctival flora from the ocular surface.

6. DIPLOPIA AND STRABISMUS:

Diplopia and strabismus are well known post-surgical complications of GDD implantation. Patients with Baerveldt device implantation have the highest rates of these complications. The cause of diplopia is likely restrictive strabismus, either from the plate itself or from plate impinging on muscle insertion.