Glaucoma Drainage Devices
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Careful preoperative examination and planning are essential for successful surgical outcomes. Clinicians should assess mobility of the conjunctiva to determine the best quadrant for drainage implant insertion. The iris should be inspected under high magnification to detect neovascularization to consider preoperative use of anti-vascular endothelial growth factor (VEGF) agents to minimize intraoperative and postoperative bleeding. Anterior chamber depth should be assessed to determine if tube insertion in the anterior chamber would be safe without touching the iris or cornea. Gonioscopy should be performed preoperatively to determine the locations of peripheral anterior synechiae which may interfere with the tube insertion into the anterior chamber intraoperatively. The lenticular status of the eye should be noted. The tube may be placed in the sulcus in a pseudophakic eye or pars plana in an aphakic, vitrectomized eye. In an eye with a cataract, a combined surgery may be considered.
Glaucoma drainage devices are typically reserved for patients with severe uncontrolled glaucoma who have failed previous glaucoma surgery. In addition, the devices appear to be advantageous as a primary procedure in patients with a high likelihood of trabeculectomy failure, including neovascular and uveitic glaucomas. They are commonly used in the management of congenital and developmental glaucomas. Additional indications include traumatic glaucoma, aphakic and pseudophakic glaucoma, post-keratoplasty glaucoma, and other secondary glaucomas.
Implantation of a glaucoma drainage device requires careful attention to detail at every step of the procedure to improve results and minimize postoperative complications. Initially, a fornix-based or limbus-based conjunctival incision is created to allow adequate exposure for insertion of the plate. A corneal or scleral suture can be placed to improve exposure in the working quadrant. The implant is anchored between two rectus muscles with the anterior edge approximately 8 to 10 mm posterior to the limbus. Larger implants (Baerveldt) are inserted with the long axis directed toward the apex of the orbit and then rotated horizontally so that the tube points directly toward the anterior chamber and the wings of the implant are under the rectus muscles. If a two plate implant is used, one plate is positioned in each of two quadrants. The tube connecting the two plates may be passed under or over the intervening rectus muscle. With all valved implants, prior to the plate anchorage, the tube should be primed with balanced salt solution with a 30-gauge cannula to ensure that the valve leaflets are not fused after sterilization techniques. The tube of the nonvalved implant should be irrigated as well to ensure its patency.
Once the implant has been appropriately positioned, the plate is secured to the globe with two non-absorbable sutures (8-0 or 9-0 nylon sutures on a spatulated needle). The suture knots should be rotated into the fixation eyelets to prevent erosion through the conjunctiva. Secure attachment to the underlying sclera is essential to prevent anterior, posterior, or lateral migration of the implant during the postoperative period.
After the plate is attached to the globe, the tube is laid across the cornea and cut with a sharp scissor to create a beveled edge with the opening toward the cornea. The tube should extend approximately 2.5 to 3 mm into the anterior chamber to minimize the risk of tube-cornea touch or retraction out of the anterior chamber. A 23-gauge needle is used to create a track through which the tube is inserted into the anterior chamber just anterior and parallel to the iris. The tube may be secured to the sclera a few millimeters anterior to the plate with 7- 0 or 8-0 Vicryl suture. This suture helps to stabilize the tube and should not be tight; otherwise, it will restrict flow in valved devices.
The tube is covered to prevent its erosion through the conjunctiva. Patch graft materials include processed pericardium, sclera, fascia lata, dura, or cornea. The patch graft should be secured to the globe with interrupted sutures at the anterior corners by using either 8-0 Vicryl or nylon sutures. If the patch graft material is not available, a partial thickness scleral flap can be constructed. The needle track and tube entry are done under this flap. The flap is then sutured with 10-0 nylon sutures. After the patch graft has been placed, the conjunctiva and Tenon layers are pulled over the plate, tube, and patch graft and secured into place with 8-0 Vicryl suture. In some cases, the monofilament 9-0 Vicryl suture is preferred because of its higher tensile strength and finer vascular needle to prevent buttonholes when handling thin conjunctiva.
At the end of the operation, the eye should be inspected to ensure that the implant plate, patch graft, and intraocular portion of the tube are in good position. Fluorescein drops or strips can be used to inspect the conjunctiva for leaks. Any buttonholes found in the conjunctiva should be closed with 9-0 Vicryl suture.
Following glaucoma drainage implant surgery, the patient is seen on postoperative day 1, and attention is paid to the tube position and wound architecture. A topical antibiotic is started four times daily for one week. A topical steroid is started eight times daily for one week, four times daily for 3 more weeks, 2 times daily for 4 more weeks, and 1 time daily for 4 more weeks. Initial follow-up is at 1 week, and the frequency of visits depends on the clinical 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 forms around the plate, at which point the ligature suture may spontaneously open. If further IOP lowering is required, the stent can also be removed.
Complications and Management
Glaucoma drainage devices can be associated with various postoperative complications. The early postoperative complications are similar to other filtration procedures including flat chambers, hypotony, and suprachoroidal hemorrhage.
Hypotony: Hypotony and its related sequelae, choroidal effusions or suprachoroidal hemorrhage, are more commonly observed with the nonvalved drainage devices. Early postoperative hypotony usually results from wound leak, inflammation, incomplete occlusion of the tube, or larger venting slits with nonvalved implants. Valved implants usually reduce, but do not eliminate hypotony. Hypotonous eyes are conservatively managed as long as the anterior chamber depth is maintained. If there is lenticular-corneal touch, then a viscoelastic should be injected to reform the anterior chamber. Associated choroidal effusions are generally treated with corticosteroid and cycloplegic agents. If these measures fail, surgical revision may be required.
Valve Malfunction: This is a rare complication. All valved devices should be primed at the time of the surgery as sterilization techniques may lead to adhesion of the valve membranes that prevents flow to the plate. In addition, careful surgical handling during insertion is required to prevent damage to the plastic rivets holding the valve.
Hyphema: Hyphema may occur following procedures performed on eyes with neovascular glaucoma. It is seen less commonly now with the preoperative use of anti-VEGF agents.
Scleral Perforation: Scleral perforation is a rare complication during anchorage of the plate to the sclera. Care must be taken in eyes with collagen vascular diseases.
Tube-Related Problems: Care should be taken to place the tube in the anterior chamber correctly. If the tube is placed too anteriorly, it will cause decompensation of the corneal endothelium. If the tube is placed too posteriorly, it will cause inflammation by rubbing on the iris and may also result in cataract formation if it touches the anterior lens capsule. If the tube is inadvertently cut too short, either an angiocatheter or tube extender can be used to obtain the desired length.
Tube block from blood, vitreous, fibrin, or iris incarceration in the early postoperative period can occur. A Nd:YAG laser may be helpful in some cases, whereas in others, a return trip to the operating room to clear the tube end may be required, especially in eyes with retained vitreous that may necessitate vitrectomy. If the tube is clogged by a blood clot, tissue plasminogen activator (0.1-0.2 ml of 5-20 µg) may be beneficial to dissolve the clot. Tube obstruction because of kinking of the tube has been reported after pars plana AGV insertion. The obstruction was treated with a pars plana clip.
Tube retraction and anterior migration are more commonly seen in children. As the eye grows, the tube may retract or touch the corneal endothelium. Retracted tubes can be lengthened with tube extenders or, alternatively, can be placed in the pars plana. Anterior migration of the tube can be fixed by shortening its length and a more posterior reinsertion.
Tube Erosion and Endophthalmitis: Tube erosion usually results from conjunctival melting near the limbus overlying the tube and may be related to poor patch graft preparation or placement; however, even without these factors, melting of the conjunctiva and patch graft occur over time and result in tube exposure. Replacing the patch graft and mobilizing a conjunctival graft may be useful in repairing these defects. An exposed tube or plate is considered an ocular emergency requiring prompt surgical intervention to prevent endophthalmitis. The risk of endophthalmitis has been reported to be more common in children.
Migration or Expulsion of the Plate: These are seen less commonly than are tube migration and tube erosion. Migration and expulsion usually result from placing the plate too anteriorly.
Corneal Decompensation: Poor tube placement with lens-cornea touch or persistent flat chamber from hypotony may result in corneal edema. In eyes that have undergone penetrating keratoplasty, the risk of graft failure in eyes that develop a flat chamber postoperatively is high. The use of valved and stented implants that better maintain the anterior chamber depth reduces this complication. Alternatively, placing the tube in pars plana helps to avoid corneal or graft decompensation.
Overhanging Bleb: If the patch graft is too thick or the plate is too anterior, an overhanging bleb may be created resulting in chronic Dellen formation and ocular irritation. This complication is best prevented by appropriate plate and patch graft placement during surgery.
Strabismus: Extraocular muscle imbalance with devastating diplopia may also occur and is particularly common in cases of inferior implant placement. This imbalance usually results from a mass effect of the plate and the surrounding bleb on adjacent extraocular muscles. Other possible causes include posterior fixation suture effect induced by scarring under the rectus muscles, entrapment of superior oblique muscle or fat fibrosis syndrome because of inadvertent manipulation of orbital fat, or a pseudo-Brown’s syndrome from supero-nasal insertion of drainage devices. Diplopia secondary to drainage devices is difficult to treat and various treatment options include prisms, muscle surgery, or even removal of the drainage implant.
Hypertensive Phase: Hypertensive phase is characterized by elevated IOP, typically in 30- to 50- mm Hg range, occurring anywhere between 1 and 6 weeks postoperatively. This condition has been reported after inserting all types of drainage devices, but more common with valved implants. In eyes with hypertensive phase, the tube does not appear to be occluded by any of the methods described above, and elevated IOP is presumably secondary to a thick-walled bleb over the plate of the implant, which decrease permeability of the aqueous humor through the conjunctiva. Treatment options for hypertensive phase include medical therapy, digital massage, bleb needling with or without 5-fluorouracil, and possible surgical excision of the bleb. When all these measures fail, a second drainage implant or cyclodestructive procedure may be considered.