Ahmed’s Glaucoma Valve

Seton; although its literal meaning is “thick and stiff bristle”, in glaucoma surgery it refers to synthetic and inert materials used to provide an open drainage fistula. For the first time in 1906, Rollet and Moreau formed the basic idea of future seton surgery by placing a “horsehair” in a paracentesis opening to lower intraocular pressure (IOP) in a case with absolute glaucoma. Zorab performed a similar procedure with a silk thread in 1912 and named it “aqueoplasty”. In subsequent years, gold, tantalum, platinum, cartilage, and silicone were used for this purpose, but their long-term results were unsuccessful. Molteno developed the first tube implant in 1969 by placing an acrylic plate connected to a tube inserted into the anterior chamber at the level of the limbus. Later, with his new implant design applied to the equatorial region in 1976, he formed the basis of the tube implants used today.

Implant Physiology The main purpose of implant physiology is the transportation of the aqueous humor to the episcleral plate surface in the post-equatorial region with the help of a tube placed in the anterior chamber. Since a capsule has not yet formed around the episcleral plate in the first 4-6 weeks, there is no resistance to fluid passage under the tenon. After this period, a fibrovascular capsule develops around the episcleral plate. There is no tight connection between the episcleral plate and the capsule; there is a filtration area between them where the aqueous humor circulates. Aqueous humor reaches the orbital capillaries and lymphatic vessels by passive diffusion through the space between the epithelial cells in the bleb capsule. It has been shown that latex molecules with a diameter of 0.2 μm pass through the capsule wall. The pressure inside the capsule is equal to the anterior chamber pressure. The most important factors affecting implant success are the bleb surface area and the permeability of the capsule wall. That is, a thin and wide capsule means lower IOP.

Indications The indication group for tube implants consists of resistant glaucoma cases where IOP control cannot be achieved despite maximally tolerated medical treatment and filtration surgery applied with antifibrotic agents. Although neovascular glaucoma is applied as the first choice for seton surgery, it is generally indicated in open-angle, closed-angle, and congenital glaucoma cases that cannot be controlled despite multiple filtration surgeries. Additionally, its use is also appropriate in glaucoma secondary to uveitis, pseudophakic glaucoma, iridocorneal endothelial syndromes, glaucoma secondary to penetrating keratoplasty, epithelial ingrowth, and glaucoma secondary to complicated retinal surgery. Since the complications that may occur after tube surgery are more problematic compared to glaucoma filtration surgery, it should not be considered in cases where primary filtering surgery can be successful.

Implant Types Implant designs are divided into two sections according to whether there is a system that restricts the fluid flow passing through the anterior chamber tube.

Non-Valved Implants There is no system to prevent the flow of fluid inside the tube from the anterior chamber to the episcleral region.

• Molteno Implant: It is the first applied implant. It consists of a silicone tube 16 mm in length (outer diameter 0.64 mm/inner diameter 0.30 mm) and a connected round polypropylene plate 13 mm in diameter and 1.65 mm in thickness. The end of the tube opens to the upper part of the episcleral plate. The plate area is 135 mm². Different variations of the Molteno implant are available. In the double-plate type, two plates of the same size are joined with a silicone tube to create a larger filtration area. Thus, the surface area increases to 270 mm². This type can be considered in neovascular glaucoma cases requiring a wider filtration area. In the Pediatric Molteno implant, the plate diameter is 8 mm. In recent years, a “Molteno Pressure Ridge” was developed to resolve the issues related to the implant not containing a valve system. It was thought that creating a triangular-shaped chamber on the upper surface of the implant without changing its dimensions, causing the fluid to first accumulate in a small 10.5 mm² reservoir and then pass to a larger area by overcoming the upper tenon capsule resistance, would create a certain resistance in fluid flow. Although it is the oldest manufactured implant and has long-term results in various glaucoma types, its lack of an effective valve system is its disadvantage. Today, the use of more flexible biomaterials instead of hard ones in the plate materials of seton implants has come to the forefront. The Molteno3 type, produced as the third generation, has a rectangular plate design made of silicone with a thickness of 0.7 mm and a surface area of 175 mm² or 230 mm². It is the thinnest available implant, its surface area has been increased compared to the older model, it has a certain slope for easy implantation, and it is made of a flexible material like silicone. The “pressure ridge” system is also present in this implant type, but it should still be considered a non-valved implant.

• Baerveldt Implant: It is an implant with a large surface area that can be placed in a single quadrant. It consists of a silicone tube with an inner diameter of 0.30 mm and an outer diameter of 0.64 mm, and a kidney-shaped silicone plate 0.84 mm high coated with barium. It is produced in 2 different sizes with surface areas of 250 mm² (BG-103-250) and 350 mm² (BG-101-350). Holes were added to the plate to prevent bleb elevation. Fibrous tissue advances through these holes, reducing bleb elevation. It has advantages such as easy implantation due to its large surface area and thin flexible structure, and also being visible radiologically. It can be implanted into the pars plana with the “Hoffmann elbow” attachment (BG-102-350). Although it has no valve structure, it has a wide area of use due to its large surface area and easy implantation. In comparative results with the Ahmed Glaucoma Valve, which is another frequently used implant today, although more successful results are obtained because it has a larger surface area, the hypotony that may be encountered in the early period due to the lack of a valve is its disadvantage.

• Schocket Implant: Also known as Anterior Chamber Tube Shunt to an Encircling Band (ACTSEB), this implant can be prepared by the surgeon by mounting a 30 mm long silastic tube (Storz N-5941-1; inner diameter 0.30 mm, outer diameter 0.64 mm) into the groove inside a No:20 (surface area 300 mm²) or No:220 (surface area 450 mm²) silicone cerclage band using a 10/0 nylon suture. The prepared band is placed 360° around the equator. Although it is the cheapest implant and provides a large filtration area, its use is limited because it requires 4-quadrant dissection. However, especially in cases where cerclage was previously performed due to detachment, the success rate related to the implant obtained by joining the existing band with a tube has been reported as 86% at 1 year.

Valved Implants They contain various systems (Valve, membrane, resistance matrix, etc.) that ensure fluid passage inside the tube within certain pressure values.

• Krupin Valve: The first prototype of this implant included a translimbal tube placed under the scleral flap 2-3 mm posterior to the limbus, forming an open-and-close valve system with horizontal and vertical slits at the tube end. Its latest model used today consists of an oval episcleral silicone disk (13×18 mm) and a connected tube with the same valve mechanism. Its thickness is 1.75 mm and its surface area is 180 mm². The operating pressure of the valve is between 9-11 mmHg.

• Joseph Valve: Similar to the Schocket implant, it consists of a 9 mm wide, 85 mm long, and 1 mm thick silicone band and a connected silicone tube (inner diameter 0.38 mm, outer diameter 0.58 mm). A long, thin slit on the upper surface of the silicone tube acts as a valve. The opening pressure of the tube is 4 mmHg. Although made in two different types (360°/surface area 765 mm² and 180°/surface area 383 mm²), it has no use today.

• White Glaucoma Pump Shunt: It is a single-piece silicone implant. It consists of an inner tube with an outer diameter of 0.64 mm and an inner diameter of 0.32 mm, suturable side wings, and an outer tube (outer diameter 1.4 mm, inner diameter 0.6 mm) connecting them with two one-way valves. Its surface area is 280 mm². The valve mechanism operates between 5-15 mmHg. It has no use today.

• Optimed Glaucoma Pressure Regulator: It is a modification of the first applied translimbal implants. Its surface area is 18 mm². It consists of a tube made of a polymethacrylate matrix and a connected silicone body. It has three models varying according to the number of capillary transit paths it contains. As the length of the transit paths increases, fluid flow decreases. It is not widely used today.

• Ahmed Glaucoma Valve: It consists of an oval pear-shaped (13×16 mm) polypropylene plate and a connected silicone tube (inner diameter 0.32 mm, outer diameter 0.64 mm). Its height is 1.9 mm, and its surface area is 184 mm² (Model S2). On the upper surface of the plate body, 2 thin silicone elastomer membranes are mounted tensely in front of the tube entry point. The aqueous humor inside the tube passes between these membranes, and the fluid flow encounters a certain resistance with the “venturi diaphragm” effect created by these tensely placed silicone leaves. The force between 8-12 mmHg created by the tensely stretched silicone leaves creates a valve effect, and the fluid flows towards the reservoir inside the valve. According to the “Bernoulli hydrodynamic principle,” the speed of a fluid passing from a wide pipe to a smaller outlet area increases. Accordingly, the reservoir pool is designed to progressively narrow. There are pediatric types with a surface area of 96 mm² (Model S3) and double-plate types with a surface area of 364 mm² (Model B1). Single (Model FP7), pediatric (Model FP8), and double-plate (Model FX1) types of these models made of silicone materials are replacing the old rigid polypropylene materials today. Special extenders to lengthen the tube and special attachments for pars plana application have also been produced. With the acquisition of early and advanced stage results, the Ahmed Glaucoma Valve is the most frequently applied tube implant today. Although its surface area is a disadvantage for the success of this implant, the ability to use silicone material in its new models, having a good valve system, and the ability to apply a double plate when necessary are its most important advantages.

Surgical Technique Anterior segment structures such as the conjunctiva, anterior chamber angle, and lens are evaluated by biomicroscopic examination. The area where the conjunctiva is most mobile is selected for implant placement. Care is taken to ensure that there is no peripheral anterior synechia (PAS) or neovascularization where the tube enters the anterior chamber, and attention is paid to corneal transparency. The conjunctival opening is related to the implant dimensions. In single-piece implants, the superotemporal quadrant is generally preferred to create the maximum surface area and to avoid contact with the oblique muscles. Implantation in the superonasal region should not be preferred as it can cause “acquired superior oblique syndrome”.

It is fixated to the sclera with a non-absorbable suture (5/0 polyester or nylon) through the holes on the episcleral plate 10-12 mm posterior to the limbus. For implants that do not contain a valve system, it is necessary to perform “temporary tube ligation” to prevent hypotony in the early postoperative period.

The Main Ones of These Techniques Are:

• Two-Stage Procedure: The episcleral plate is sutured to the sclera, but the tube is not placed in the anterior chamber. A trabeculectomy is performed from another quadrant. As the second stage, the tube is placed in the anterior chamber within 4-6 weeks.

• Vicryl Ligation Technique: The mouth of the tube is closed with a 6/0 vicryl suture just in front of the episcleral plate. As this suture dissolves spontaneously within 4-6 weeks, encapsulation develops on the plate during this time.

• Latina Suture: A 5/0 or 6/0 chromic catgut is passed through the tube. It is taken out from the part close to the episcleral plate and buried in the inferior fornix. When necessary, the suture is pulled out to open the tube lumen. Alternatively, an additional suture is placed around the tube.

• Tube Ligation in the Anterior Chamber: The tip of the tube that will enter the anterior chamber is closed with a 10/0 nylon suture. Later, at a desired time, suture lysis is performed with an Nd:YAG laser.

• Pneumatic Stent: Tube ligation can be achieved by utilizing the injection of expanding gases like perfluoropropane into the vitreous after vitrectomy for the tip of the tube implanted in the pars plana.

After one of these procedures is performed, or if a valved implant is being applied, the tip of the tube is directly placed into the anterior chamber so that it advances 2-3 mm from the iris surface and does not exceed the pupillary area. If there is vitreous in the anterior chamber, an anterior vitrectomy must be performed for cleaning. To insert the tube, the anterior chamber is entered 1-2 mm behind the limbus with a 22 G (0.72 mm) or 23 G (0.65 mm) needle parallel to the iris plane. If a pars plana vitrectomy was previously performed, the tube can be inserted into the pars plana 3.5 mm behind the limbus. Another modification is placing the tube in the scleral sulcus if there is dense PAS formation in pseudophakic cases. The entrance to the anterior chamber can be covered by preparing a 4×4 mm scleral flap, or the tube surface can be covered over the full-thickness sclera using tissue covering materials such as dehydrated human allograft dura mater, sclera, or pericardium. The tenon and conjunctiva are closed separately with 8/0 or 9/0 vicryl sutures.

Results

• Pediatric Glaucomas: Pediatric glaucomas, especially aphakic pediatric glaucomas, constitute an important group of tube implantation indications. In these cases, it is primarily important to determine the dimensions of the tube to be inserted. While it is appropriate to implant pediatric designs in eyes with an axial length below 21 mm, it is more appropriate to insert tubes made for adults in eyes with an axial length above this. In eyes with aphakic glaucoma, performing a detailed anterior vitrectomy in the presence of vitreous in the anterior chamber will also increase the chance of surgical success. While the success of the Ahmed glaucoma valve in pediatric glaucomas is 85% in the first year, this rate drops to 42% at the end of the fourth year. The most frequently encountered problems in the pediatric age group are tube erosion and endophthalmitis. Therefore, these cases should be closely monitored; if necessary, it is appropriate to repeat their examinations under general anesthesia. Again, the success of tube implants inserted in the pediatric age group decreases as the patient’s age decreases.

• Glaucomas Secondary to Penetrating Keratoplasty: Preserving graft transparency is as important as IOP control in glaucoma cases occurring after penetrating keratoplasty. The tube to be placed in the anterior chamber in these cases can mechanically cause corneal endothelial cell loss. Therefore, pars plana tube implantation during or after keratoplasty can be a suitable alternative. In a study by Ritterband et al., they achieved 59% graft transparency and 83% IOP control in a 2-year follow-up in cases where penetrating keratoplasty and pars plana tube placement were performed. Again, in pseudophakic cases, placing the tube in the ciliary sulcus is also beneficial in preventing chronic endothelial damage and ensuring corneal transparency.

• Uveitic Glaucomas: Uveitic glaucomas constitute a highly successful group within the tube implant group. In these cases, starting topical and, if necessary, systemic steroid treatment before the operation and intensively using medical anti-inflammatory treatment after the operation contribute significantly to surgical success. Papadaki et al. reported the 4-year success of the Ahmed glaucoma valve in uveitic glaucomas as 50%.

• Neovascular Glaucoma: Neovascular glaucomas are at the forefront of tube implantation indications. Because in these cases, the success of classical filtering surgery performed with antimetabolites is limited. Again, in the laser cyclophotocoagulation procedure to be performed, the necessity of repeating the procedure over time, as well as serious complications such as inflammation, vision loss, and hypotony, constitute disadvantages. In our study, we determined the surgical success of neovascular glaucoma cases as 63% and 25% for the Ahmed glaucoma valve at the end of 1 and 5 years of follow-up, and 37% and 30% for single-plate Molteno tube implantation cases. When all cases were evaluated, surgical success was lower in those whose preoperative visual acuity was below 0.1, diagnosis was diabetic retinopathy, and IOP was above 35 mm Hg. Despite all this, tube implants play an important role in the treatment of neovascular glaucoma. Completely performing the preoperative retinal ablation procedure and using anti-vascular endothelial growth factors in combination will also increase surgical success.

• Traumatic Glaucomas: In a study we conducted, it was found that glaucoma surgery was required at a rate of 17.6% after blunt or penetrating trauma, and tube implantation surgery was applied at a rate of 5%. Although it varies depending on the type and severity of the trauma, in one study, the surgical success in cases where a Molteno implant was applied was found to be 76% as a result of a 10-year follow-up.

Complications

• Excessive Filtration and Hypotony: In non-valved implants, if fluid passes from the anterior chamber before encapsulation forms around the episcleral plate, shallow anterior chamber, hypotony, and choroidal detachment may develop due to excessive filtration. In addition, sudden hypotony following very high IOP values can lead to diffuse retinal hemorrhages called decompression retinopathy, especially in young patients. To prevent this, tube ligation must definitely be performed for the first 4 weeks in implants without a valve system.

• Low Filtration and IOP Increase: If the tube was ligated, an IOP increase in the early period is an expected situation. Furthermore, a temporary “hypertensive phase” may be encountered in the 6-8 week period because the encapsulation bleb has not yet vascularized. This situation occurs because while the capsule forms over the plate in the early period, the vascular structures inside the capsule have not yet developed and the fluid cannot be expelled outside the capsule. The use of glaucoma medications may be required during this period, and it will pass once capsule development is complete. Additionally, proximal or distal tube obstruction should be investigated in this case. Obstruction of the proximal end of the tube may occur due to the iris, fibrin, blood, or vitreous. This situation can be easily understood in a biomicroscopic examination, and tube irrigation can be performed or the obstruction can be opened with a laser. If the obstruction is distal, that is, in the area close to the episcleral plate, bleb needling or opening of the encapsulation area can be performed.

• Wound Dehiscence and Tube Erosion: Since tube implantation is performed in eyes with severe conjunctival scarring and contraction, separation between the wound edges is more frequent. These openings can cause epithelial ingrowth and fistula formation. Therefore, attention should be paid to closing the tenon and conjunctiva separately in primary suturing, and if there is a wound opening in the early period, it should be repaired. In a study by Lankaranian et al., while no conjunctival erosion was detected in an 8-year follow-up with double-layer pericardium closure, they detected conjunctival erosion at a rate of 16% with single-layer pericardium covering. In cases where single-layer pericardium covering was applied, the average time of erosion onset was 9 months. Again, in cases of scleromalacia where the sclera has thinned and glaucoma, double-layer pericardium application in the form of a sandwich under and over the tube can also be performed. In cases of tube erosion where the conjunctiva is also severely damaged, covering the open parts with an amniotic membrane can also be applied as an alternative.

• Bleb Encapsulation: Bleb encapsulation occurring in the long term is one of the most important causes of implant failure. In one study, bleb encapsulation after the Ahmed glaucoma valve was reported as 23%. Although antimetabolite application during implantation was suggested to prevent this, controlled studies have observed that antimetabolite application in tube implants does not increase surgical success. Since the tenon tissue is the most important factor in bleb encapsulation, in recent years, it has been thought that placing the plate part on it after the tenon tissue is thoroughly separated from the conjunctiva will create a healthier capsule and it has been proposed that it will reduce the risk of bleb encapsulation.

• Corneal Decompensation and Graft Rejection: Corneal decompensation is difficult to occur unless there is direct tube-endothelium contact. However, endothelial loss chronically continues after implantation. In a study by Kim et al., this rate was determined to be 10.5% in one year. While endothelial loss is maximum where the tube is placed, this rate is lower in the central cornea.

• Strabismus and Diplopia: This complication especially occurs in large implants that contact the extraocular muscles. While this rate was 77% in the first and large Baerveldt implant, it dropped to 6% with the opening of fenestrations. The causes of strabismus and diplopia are the mass effect due to a very large bleb, stretching of the muscles, fat tissue herniation, the Faden effect developing due to scarring under the rectus muscles, and “acquired superior oblique syndrome”.

• Cataract: It occurs due to tube-lens contact. The tube should be placed in the anterior chamber in a way that does not exceed the iris surface. If a cataract has developed, cataract surgery can be performed safely in the later period.

• Tube Migration: It occurs as a result of wound contraction occurring around the plate during wound healing and the plate not being sutured well. If the episcleral plate contracts backward, the tip of the tube may disappear from the anterior chamber, or the tube may move and cause endothelial loss. Therefore, the episcleral plate must be sutured to the sclera with non-absorbable suture material, and at the same time, the tube part must be fixated with a suture over the sclera.

• Endophthalmitis: Endophthalmitis is a rare complication in tube implants. It can generally occur in the late period due to conjunctival erosion around the tube and plate. Sometimes it may be necessary to remove the tube to control the infection. It should be distinguished from sterile endophthalmitis. This condition may respond to steroid treatment.

• Optic Nerve Trauma: Today, the size differences of newly produced implants have brought up the risk of trauma to the optic nerve posteriorly. The critical distance to avoid pressure on the optic nerve is 2 mm. The implants that can particularly exceed this limit are the Ahmed Glaucoma valve S2, FP7, and Baerveldt 350 mm² implants. The placement of these implants, especially in eyes with axial lengths below 20 mm and in the superonasal quadrant, seriously poses a risk of optic nerve trauma. The implant surface area is an important parameter in terms of the bleb morphology it will create; however, studies have observed that implants with very large surface areas (such as the 500 mm² Baerveldt implant) do not change surgical success very much. There is no significant difference in surgical success if implants that cannot be placed in the superotemporal quadrant are placed in the inferior quadrant.