Fig. 16.1
The Ahmed Valve. The valved device developed by Ahmed consists of a silicone drainage tub connected to a plate in silicone, polypropylene or polyethylene, in a range of dimensions (from 85 to 184 mm2) depending on the model (Fig. 16.1). The aqueous humor drains through the tube to the subconjunctival bleb above the plate. Inside its lumen, the Ahmed valve is fitted with a unidirectional flow limiter based on a Venturi system. This limiter theoretically restricts the aqueous flow when the IOP drops below 8–12 mmHg. The system contributes to a reduction in the risk of ocular hypertension, particularly in the post-operative period when the bleb has still not been encapsulated
Fig. 16.2
The Baerveldt Implant. The Baerveldt glaucoma draining implant is a non-valved implant. It consists of a silicone tube attached to a round silicone plate impregnated with barium (Fig. 16.2). The terminal plate has a surface area of 250 mm2 or 350 mm2, depending on the model. The fenestrations in the plate promote the growth of fibrous bands that will contribute to reducing the profile of the bleb. During the implantation procedure, the Baerveldt Implant requires an external suture around the tube to prevent an excessive outflow of fluid in the early postoperative period. Once the bleb has been encapsulated, the external suture can be removed or it may reabsorb spontaneously. With this temporary suture in place, fluctuations in the IOP may be observed (ocular hypotonia or hypertonia). Nevertheless, this delayed drainage of the aqueous humor may be beneficial, given that the early fluid outflow is often associated with fibrocapsular thickening of the bleb capsule
Implantation, Insertion, Position
The position of the implant must be determined following clinical examination and the results of any previous surgery. Generally-speaking, the drainage devices are implanted in the supero-temporal quadrant, given that the blebs located in the inferior segments are associated with a higher rate of infection. Nevertheless, previous surgery or scar tissue may preclude positioning the device in the superior quadrant. A description of the supero-temporal approach follows: the surgical approaches used to implant the Ahmed Valve and the Baerveldt have many steps in common. The surgical steps for the implantation procedure for both devices is described below, and the differences between the two devices are highlighted. A traction suture in 7.0 vicryl may be passed through the cornea in the superior or supero-nasal sectors, at approximately 1 mm from the limbus. Alternatively, the suture may be passed through the sclera close to the limbus, following the peritomy.
Fig. 16.3
Adjusting the tension of the suture in infero-nasal direction with consequent exposure of the supero-temporal quadrant of the eye. The traction suture serves to position the eye in an infero-nasal direction (Fig. 16.3). This guarantees adequate surgical exposure of the supero-temporal quadrant. A screen to block light must be positioned on the cornea to avoid retinal phototoxicity during surgery
Fig. 16.4
Conjunctival peritomy with fornix based flap. The surgeon must create a conjunctival peritomy with a hinge at the fornix; it must be created at the limbus (and extend for 3 h), using ring forceps and Westcott scissors. The radial incisions of this peritomy must be created posteriorly on the temporal and superior meridians. The anterior edge of the peritomy must lie at the limbus
Fig. 16.5
Intermuscular membranes cut under the conjunctival flap. The creation of the conjunctival peritomy with a limbus-based flap is more difficult and is associated with an increased risk of post-operative infections of the bleb. Additional anesthetic agent must be injected into the subconjunctival space below the peritomy to boost the local anesthesia. The conjunctival flap must be extended posteriorly from each edge of the peritomy. Blunt Westcott scissors can be used to mobilize the conjunctiva and cut the intramuscular membrane, if necessary. This step provides adequate access to the posterior scleral surface for the successive positioning of the implant’s plate. When a Baerveldt implant is used, the conjunctiva located in the infero-temporal sector must be cut delicately to create an internal pouch that will house the suture at the end of the procedure. Bipolar cauterization can be used to achieve hemostasis of the episcleral vessels beneath the conjunctival flap. The following steps vary depending on the device implanted
The Baerveldt Implant
The Baerveldt implant must be extracted from its container without being contaminated in any way.
Fig. 16.6
The blue suture is inserted in the device tube at the tube-plate junction. A length of 4.0 nylon thread is positioned inside the tube’s lumen (of diameter 0.3 mm) just a few millimetres beyond the tube-plate junction
The rectus muscles adjacent to the quadrant chosen for the implant are identified and isolated using muscle hooks. In general, the muscle isolated are the superior rectus and the lateral rectus.
Fig. 16.7
The muscle hooks are used to isolate the superior and lateral rectus and Bearveldt implant is positioned under the muscles. The hooks can be used to position the wings of the implant plate below the rectus muscles
Using forceps, the plate is positioned 9–10 mm posterior to the limbus. Two suture threads in 7.0 Vicryl or 6.0 Prolene are used to anchor the implant to the sclera.
Fig. 16.8
Final position of the device. Fixating sutures are inserted in the two fixating holes on the device (near the tube-plate junction) The proximal portion of the implant’s plate has two large holes for fixing to the sclera
The suture knots must be fixed firmly to avoid micromovements of the device following implantation as this may lead to excessive endocapsulation or trauma to the surrounding tissues. Once the implant has been fixed to the sclera, the free ends of the suture must be shortened. While the surgeon is stabilizing the plate with toothed forceps, the guides for the needle can be used to rotate the suture knots into the implant’s fixing holes to prevent the exposure of the knots or erosion of the conjunctiva in the post-operative period. Before proceeding, the surgeon should mobilize the conjunctival flap to ensure that it has not been trapped behind the implant. At this point, the surgeon can release the traction suture and allow the eye to return to its original position.
Fig. 16.9
The tube is in the primary position and the surgeon places the free suture end in the conjunctival pouch in the infero-temporal quadrant of the eye bulb. Only 3 mm of the suture should be visible in the tube lumen. A length of nylon 4.0 thread must be pulled so that approximately 3.0 mm are still visible in the distal section of the tube
This thread can now be cut and safely positioned in the conjunctival pouch that was created previously in the adjacent quadrant—normally the infero-temporal quadrant—at an adequate distance from the limbus.
Fig. 16.10
The binding of the suture that forms a knot in the inferior surface of the tube is highlighted. The suture should be tightened enough in order to create an indentation in the tube. A 7.0 Vicryl thread ties the tube above the visible suture zone to guarantee good closure. This biodegradable ligature is used to prevent the unpredictable aqueous flow in the post-operative period and avoid complications associated with hypotonia. The knot must be rotated and positioned underneath the tube. In this way, only the loop of the suture is left on the surface of the tube. This will facilitate the laser lysis of the suture following surgery. High magnification visualization of the ligature can be used to confirm the indentation in the silicone tube created by the suture. The water-tightness can be controlled by injecting BSS into the tube and observing any liquid flow over the plate; liquid flow over the plate indicates that the closure is not watertight
The Ahmed Implant
The Ahmed flexible valved device should be removed from its packing taking care to avoid any contamination. Firstly, the patency of the device must be controlled by injecting BSS using a blunt cannula. The flow through the valve mechanism must be controlled by direct vision. Similar to the Baerveldt implant, the superior and lateral rectus muscles must be identified.
