Surgical Technique 3 (Ahmed Glaucoma Valve Drainage Implant)

Introduction

Glaucoma drainage implants can be characterized as either resistance (valved) or non-resistance (non-valved) devices. The Molteno and Baerveldt implants are non-valved devices that drain aqueous from the anterior chamber through an open, unobstructed tube.

The Ahmed Glaucoma Valve (New World Medical, Rancho Cucamonga, CA), introduced in 1993, was developed to address the problem of uncontrolled flow in non-resistance devices. It is the most frequently implanted resistance glaucoma drainage device. After placement of the Ahmed Glaucoma Valve, aqueous humor is directed out of the anterior chamber through a tube and across a flow-restricting valve before exiting onto a plate, which is implanted under the conjunctiva and Tenon's capsule, posterior to the limbus.

Device

The Ahmed Glaucoma Valve consists of a silicone tube attached to a valve mechanism on an end plate ( Fig. 113-1 ). The valve is comprised of two thin silicone elastomer membranes (8 mm long × 7 mm wide) positioned in a Venturi-shaped chamber ( Fig. 113-2 ). The elastic membranes of the valve are closed at pressures below 8–12 mmHg and open to allow flow when pressures are higher. In this way, the device is designed to reduce the problem of hypotony due to over-filtration. In vitro and in vivo studies have confirmed the ability of the Ahmed Glaucoma Valve to restrict flow by this valved mechanism.

The end plate of the Ahmed Glaucoma Valve is available in several sizes and materials ( Box 113-1 ). A firm polypropylene plate is used in the single-plate (S2 model) and the double-plate (B1 model). A more flexible silicone plate is used in the flexible single-plate (FP7 model) and the flexible double-plate (FX1 model). The double-plate Ahmed Glaucoma Valves allow for greater surface area for aqueous drainage, and may be implanted on either the right or left side of the eye. Both single-plated models (S2 and FP7) exist in smaller sizes (S3 and FP8) intended for pediatric patients, although many surgeons prefer to use adult-size implants in their pediatric patients.

Indications

The indications for implantation of the Ahmed Glaucoma Valve are the same as other glaucoma drainage implants ( Box 113-2 ). The indications include eyes which have previously failed or are at high risk of failing a trabeculectomy. Use of the implant for primary surgery can also be considered, though this is considered controversial by some.

Contraindications

There are no absolute contraindications for implantation of the Ahmed Glaucoma Valve. Glaucoma drainage implants are relatively contraindicated in patients who are noncompliant with self-care in the postoperative period. The American Academy of Ophthalmology's report on glaucoma drainage implants warns that the primary long-term complication of anterior chamber tubes is decompensation of the corneal endothelium, so care should be taken before implanting drainage implants in eyes with impaired endothelial function.

Surgical Technique

Local anesthetic is administered, which may be retrobulbar, parabulbar, or peribulbar. A 6/0 silk or polyglactin traction suture on a spatulated needle is placed through the corneal stroma adjacent to the quadrant chosen for implantation and the eye is rotated to achieve maximal exposure ( Video 113-1 ).

The Ahmed Glaucoma Valve is most often placed in the superotemporal quadrant and the plate positioned approximately 8 mm posterior to the limbus. Care should be taken when implanting in the superonasal quadrant because of proximity of the plate to the optic nerve and risk of inducing a Brown's syndrome.

In the chosen quadrant, a fornix-based incision is made through the conjunctiva and Tenon's capsule. Radial relaxing incisions on one or both sides of the conjunctival flap are often added to improve surgical exposure. Blunt scissors are used to dissect between the episclera and Tenon's capsule. Gentle blunt dissection continues posteriorly under Tenon's capsule to create a pocket between the rectus muscles. Wet-field cautery is often used near the limbus to achieve hemostasis.

Prior to implantation, the implant should be examined and primed ( Fig. 113-3 ). The device's sterilization process may cause the valve membranes to adhere to one another. Intraoperative priming of the valve with balanced salt solution though a 27- or 30-gauge cannula ensures the patency of the valve mechanism.

The anterior edge of the plate is grasped manually or with non-toothed forceps and inserted into the pocket between the rectus muscles ( Fig. 113-4 ). Care should be taken not to touch the valve mechanism with instruments during insertion, as this may damage the device. The valve is positioned 8–9 mm posterior to the limbus, and the plate is anchored to the sclera with 8/0 nylon, 9/0 nylon, or a similar permanent suture on a spatulated needle though the openings on the anterior edge of the plate.

The drainage tube is extended over the cornea and then cut to a length which will allow the tube to extend 2–4 mm into the anterior chamber ( Fig. 113-5 ). A 23-gauge needle is then used to make a track beginning approximately 0.5 mm posterior to the limbus and extending into the anterior chamber parallel to or angling slightly forward of the iris plane ( Fig. 113-6 ). Some surgeons inject viscoelastic as the needle leaves the eye in order to lubricate this needle track. The 23-gauge track is an appropriate size to allow tube entry but minimize leakage around the tube. Entry into the anterior chamber posterior to Schwalbe's line and anterior to the iris plane will minimize the risk of contact with the cornea or iris. The drainage tube is inserted into the anterior chamber through the needle track using non-toothed forceps or a specially designed tube inserter (New World Medical, Inc., Rancho Cucamonga, CA) ( Fig. 113-7 ). The tube is then loosely secured to the sclera using a single 9/0 or 10/0 nylon suture with care taken to avoid compression.

To prevent erosion of the tube through the conjunctiva near the limbus, a patch graft is sutured over the tube ( Fig. 113-8 ). Processed pericardium (Tutoplast; New World Medical, Inc., Rancho Cucamonga, CA; or IOP, Inc., Costa Mesa, CA) is the most commonly used patch graft, but other materials, including preserved donor sclera or cornea, fascia lata, or dura may be used.

Both corners of the conjunctival flap are then reapproximated to the limbus. The anterior edge of the conjunctiva may also need to be sutured to the limbus anterior to the patch graft. For conjunctival closure, monofilament 9/0 polyglactin suture is preferred over braided 8/0 polyglactin because it has a smaller diameter but a higher tensile strength. Relaxing incisions are also closed using the same suture in a continuous or interrupted manner.

A temporal clear corneal paracentesis should be considered to allow for postoperative reformation of the anterior chamber, if needed. This can also be used at the end of the procedure to reform the anterior chamber with balanced salt solution, if necessary.

Subconjunctival steroids and antibiotics are injected in a quadrant, preferably 180° away from the plate. Depending on the type of anesthesia used, an ointment containing antibiotic and steroid is applied to the eye, and it is lightly patched and shielded until postoperative day one.

Outcomes

Since the introduction of the device, multiple studies have reported outcomes with the Ahmed Glaucoma Valve implant in refractory glaucoma. In these studies, success was typically characterized as an intraocular pressure less than 21 or 22 mmHg and greater than 4 or 5 mmHg with or without medicines and without further glaucoma surgery or loss of light perception. These studies tend to report outcomes as probability of success. The majority of the studies are not randomized, prospective trials but retrospective, noncomparative case series.