The Ahmed Glaucoma Valve is a glaucoma drainage device commonly used in the treatment of recalcitrant glaucoma.
Manufactured by New World Medical, the device is made of a silicone tube attached to a polypropylene end plate. The anterior one-third of the end plate makes up the valve, which consists of a silicone sheet folded like a taco. The silicone tube opens into the middle of the closed end of the taco. The valve itself is enclosed in a rigid cage made of polypropylene and encased with four rivets. It is designed to open when IOP rises above 8 mm Hg.
The advantages of the Ahmed Glaucoma Valve (AGV) include one-quadrant dissection, easy insertion into the sub-Tenon’s pocket, no need to isolate the recti muscles and a low incidence of hypotony (9%). The AGV does have some problems, especially the incidence of a hypertensive phase (IOP greater than 21 mm Hg) in the first 6 postoperative months, which is greater than 40%.
AGV implantation has been performed using a modified topical anesthesia technique at Tulane University for the past 3 years with great patient satisfaction. Pain scale studies here have revealed that the degree of satisfaction is comparable to retrobulbar anesthesia. Using this technique, surgeons can avoid retrobulbar anesthesia with its complications in most patients.
We describe our surgical technique of AGV implantation under modified topical anesthesia in seven easy steps, as well as the management of the hypertensive phase with needling at the slit lamp.
Sub-Tenon’s dissection is performed using a 27-gauge cannula.
End plate is inserted using the eyelet. Note the plate itself is not touched.
The bleb is needled in a patient experiencing hypertensive phase.
Enlarged bleb is seen following needling in the same patient.
We routinely administer diazepam 10 mg 1 hour prior to surgery followed by 2 grams of midazolam and 100 mcg of intravenous fentanyl at the time of surgery. Before preparing the eye with betadine, topical tetracaine drops are inserted into the lower cul-de-sac. These drops are inserted again before surgery.
2. Corneal traction suture
A 7-0 vicryl suture on a spatulated needle is inserted through clear cornea (75% depth) at the 12-o’clock position, 0.5 mm anterior to the limbus. This step, which can be easily done without using a second instrument such as 0.12 -mm forceps, enables adequate exposure of the operation site. The eye is rotated inferiorly, and the traction suture is secured to the drape with a hemostat.
3. Sub-Tenon’s pocket
A fornix-based peritomy is performed at 12 o’clock with Westcott scissors, and the incision is extended by 1 mm into the chosen quadrant (supero-temporal or nasal). Preservative-free lidocaine 1% (2 cc to 3 cc) is injected into the sub-Tenon’s pocket using a 27-gauge cannula. The cannula is inserted into the sub-Tenon’s pocket as far posteriorly as possible, as the injection dissects the sub-Tenon’s tissue in a nontraumatic fashion.
This step is followed by completion of the fornix-based peritomy extending over three to four clock hours. Then closed Westcott scissors are inserted into the sub-Tenon’s pocket posteriorly and adhesions are dissected. A dry Weck-cel sponge is inserted into the pocket as far posteriorly as possible, providing further dissection and hemostasis.
Light cautery is applied to control bleeding points using preservative lidocaine drops instead of saline for underwater cautery. This decreases the pain during cautery.
During this part of the operation, patients may complain of pain. It is important both to warn the patient to expect some degree of discomfort during this step and to use light cautery. Patients with neovascular glaucoma may need more cautery than other patients.
4. Valve insertion
The AGV is brought to the operative site at this stage and primed with saline using a 30-gauge cannula. Then the conjunctiva/Tenon’s edge is lifted with 0.12-mm forceps to facilitate placement of the AGV end plate. The end plate is gently tucked into the pocket with the tips of nontoothed forceps held perpendicular to the plate or by holding the eyelet of the end plate.
No-touch zone: At no stage is the plate held or touched with the forceps, as this can break the rivets supporting the valve, which in turn can lead to early or late failure from fibrotic tissue entering the valve and causing obstruction. Not touching the plate will prevent denting its smooth surface. These dents might attract fibroblasts and scar tissue to the plate. It is critical to realize that the entire end plate of the AGV is a no-touch zone.
The end plate is secured to the limbus 7 mm from the limbus (even though the manufacturer recommends 8 to 10 mm) with the help of interrupted nonabsorbable sutures such as 10-0 nylon or polypropylene. Any blood vessels in the path of the needle should be lightly cauterized prior to needle insertion. The direction of the needle should be such that it should exit through the eyelets in the end plate. The 7-mm distance reduces the chance of optic nerve touch or degeneration from the AGV end plate, especially on the nasal side and in smaller globes.
Also, needling of the bleb is easier with an anteriorly located bleb. The nonabsorbable sutures prevent the end plate from sliding anteriorly or posteriorly during the postop period, especially on ocular movement.
5. Silicone tube insertion
The eye is repositioned into its natural position by releasing the hemostat. The silicone tube is cut with the bevel facing anteriorly using Westcott scissors, approximately 1 mm to 1.25 mm anterior to the limbus, taking care not to cross the pupillary margin. A 23-gauge butterfly needle is used to enter the anterior chamber while the eyeball/sclera is secured with the 0.12-mm forceps.
The needle is placed 0.5 mm posterior to the limbus to avoid postop dellen formation. The needle is directed parallel to and just anterior to the iris plane. The entry into the anterior chamber is done in a controlled fashion to avoid damage to the lens and the iris.
As the needle is being withdrawn, the 0.12-mm forceps are used to grasp the anterior lip, and the silicone tube is held with the angled smooth forceps close to the tip, allowing easy insertion of the tube without much struggle. The mistakes made by the novice surgeon are not to locate the needle entry point with the toothed forceps and to hold the tube too far away from the tip.
In patients with florid rubeosis and neovascular glaucoma, profuse bleeding is almost always seen following decompression with the needle. In these cases we routinely inject Healon (sodium hyaluronate, Pharmacia) into the anterior chamber, both to tamponade against further bleeding and to deepen the anterior chamber by pushing the iris away at the site of the tube insertion. This maneuver is especially helpful in a phakic eye in which the lens-iris diaphragm moves forward soon after decompression.
We also routinely use Healon in patients with corneal grafts, to decrease the risk of graft/tube touch.
6. Patch graft
We like to secure the silicone tube to the underlying sclera between the limbus and the end plate with a figure-eight 10-0 nylon suture; this is done so the suture is not too tight and just makes the tube flatten against the underlying sclera. The scleral patch graft is placed with one end along the limbus and secured to the underlying sclera with the help of two interrupted 10-0 nylon sutures. The graft is then trimmed to size with Stevens scissors.
This technique is much easier for shaping the scleral patch graft than trying to trim it freehand, and it will save several minutes during the surgery. The scleral patch graft is further secured with two additional sutures posteriorly, preventing the bleb from migrating anteriorly. If pericardium is preferred, the dry pericardium should be double-folded and secured to the sclera as described. If the pericardium is wet before suturing, it will become slippery.
7. Conjunctival closure
Preservative-free lidocaine 1% is sprinkled on the conjunctival edge before replacing it to the limbus. The tying forceps can be used to tease the conjunctiva down to the limbus. A 10-0 vicryl suture on a spatulated needle is used to secure the conjunctiva to the limbus. Two interrupted sutures (one on either end of the peritomy) are used to secure the conjunctiva to the underlying sclera. It is important to secure the conjunctiva always to the sclera and to the adjacent conjunctiva to prevent leaks. The middle portion is secured to the cornea, just anterior to the limbus, with a horizontal mattress suture.
Managing hypertensive phase
The hypertensive phase begins 3 to 6 weeks after the operation and lasts for 4 to 6 months. The bleb becomes visibly inflamed and dome-shaped, associated with an increase in IOP to higher than 30 mm Hg in some cases.
During this phase, antiglaucoma medications and digital massage are indicated when the IOP is considered to be too high, usually higher than 21 mm Hg. In cases that do not respond, needling of the bleb is indicated.
Needling of the bleb is performed in the outpatient clinic as follows. After instillation of topical anesthetic and antibiotic drops, a cotton-tipped applicator soaked in topical anesthetic is applied for 2 minutes to the conjunctival site where the needle will enter. A lid speculum is placed on the eye.
At the slit lamp, the patient is instructed to look down to expose all of the bleb. The slit lamp is set at the lowest magnification. This allows visualization of the bleb even if the patient moves the eye during the procedure.
Needling is performed with a 27-gauge needle on a tuberculin syringe. Because it is easier to approach the bleb from the midline for the right eye, needling is performed from the right for a superotemporal bleb and from the left for a superomedial bleb. The needle is introduced into the subconjunctival space 1 mm from the edge of the bleb and advanced for 3 to 5 mm into the bleb in the direction of the end plate. The surface of the end plate is scraped with a firm to-and-fro motion.
The needle is then partially withdrawn from the bleb and the direction changed to parallel the edge of the bleb. With sweeping motions, several tears are made in the capsular edge. The needle is then withdrawn.
Frequently, a change in the appearance of the bleb is seen during or soon after needling, with the size increasing and the bleb becoming less tense. We routinely give 5-fluorouracil injection (5 mg in 0.1 cc) into the subconjunctival space in the inferior fornix, away from the bleb. Slit-lamp examination with Seidel test is performed 30 to 60 minutes later.
Patients are instructed to use topical antibiotics with steroid for 5 days and to start gentle digital massage on the third day. They are usually seen again in 3 to 5 days. Some eyes need repeat needling for recurrence of fibrosis.
For Your Information:
- Ramesh S. Ayyala, MD, FRCS, FRCOphth, is director of glaucoma and anterior segment services in the department of ophthalmology of Tulane University Medical Center, 1430 Tulane Avenue, SL-69, New Orleans, LA 70112; (504) 584-2466; fax: (504) 584-2684; e-mail: email@example.com.
- New World Medical, makers of the Ahmed Glaucoma Valve, can be reached at 10763 Edison Court, Rancho Cucamonga, CA 91730; (909) 466-4304; fax: (909) 466-4305; e-mail: firstname.lastname@example.org.
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- Ayyala RS, Harman LE, et al. Comparison of different biomaterials for the glaucoma drainage devices. Arch Ophthal. 1999;117:233-236.