Surgical Technique 2 (Baerveldt Glaucoma Implant)

Introduction

The Baerveldt glaucoma implant (Advanced Medical Optics, Inc., Santa Ana, CA) was first introduced in 1990. Its design is a modification of the earlier Molteno implant, with a nonvalved silicone tube (0.63 mm external diameter, 0.30 mm bore) attached to a medical-grade silicone plate. The implant is currently available in two sizes, 250 mm and 350 mm ( Fig. 112-1A and B ). The pars plana version is based on the 350 mm size plate, with the silicone tube attached to a small episcleral plate with an angled cannula to be inserted through a sclerotomy ( Fig. 112-1C ). In all designs, pressure reduction is achieved by redirecting aqueous fluid from within the eye through the silicone tube to an encapsulated space surrounding the plate, where it can diffuse into the fibrous walls of the bleb.

The glaucoma implant plate is barium-impregnated, which allows radiographic identification, as well as gamma-irradiated. It is also tumble polished to produce a low wetting angle. There is an anterior flange with two large suture holes and a 1-mm-high ridge with the tube opening on the posterior aspect of the ridge. The implant profile is 0.84 mm, a design that allows for a smooth, thin plate that maintains the flexibility and rigidity necessary for implantation. The plate itself is fenestrated with four holes to allow fibrous tissue growth between the anterior, conjunctival, and posterior scleral walls of the resulting fibrous capsule after implantation. This is thought to reduce the height of the bleb and thereby limit problems with ocular motility.

Indications for Use

The Baerveldt implant is useful in patients with medically uncontrolled glaucoma who are poor candidates for standard trabeculectomy surgery. This includes patients with neovascular glaucoma, penetrating keratoplasty with glaucoma, retinal detachment surgery with glaucoma, iridocorneal endothelial (ICE) syndrome, traumatic glaucoma, uveitic glaucoma, previous failed trabeculectomy, epithelial downgrowth, refractory infantile glaucoma, and contact lens wearers who need glaucoma filtration surgery.

Preoperative Considerations

The most difficult decision can be whether or not to proceed with the surgery, depending on the patient's overall health situation, potential for useful vision after surgery, and the likelihood of vision-threatening postoperative complications.

Having ascertained that proceeding with the Baerveldt implant surgery would be prudent, a thorough preoperative examination will aid in surgical planning and should be directed at identifying characteristics that would impede the success of the surgery. For instance, a very high preoperative intraocular pressure (IOP) increases the possibility of a delayed suprachoroidal hemorrhage, which may be mitigated by a stepwise decrease in IOP during the surgery. The condition of the conjunctiva and previous scarring will direct which quadrant to choose for the surgery. A sufficiently clear peripheral cornea, not obscured with arcus senilis, scarring, or sutures, will allow easier confirmation of intraocular tube position. A shallow chamber or one with vitreous strands may require a vitrectomy and pars plana implant.

The most appropriate site of implantation is the superotemporal quadrant, as this area offers the largest potential space between the globe and orbital wall, allowing excellent exposure and reducing the chance of postoperative motility problems. It also allows for the best cosmesis, as the graft is covered completely by the upper lid. In cases of a second implant or extenuating circumstances, the usual order for quadrant selection after superotemporal is superonasal, then inferonasal, then inferotemporal.

Anesthetic Considerations

A retrobulbar block is the method of choice for anesthesia of the globe. Intravenous sedation is often administered as an adjuvant. Topical or intracameral methods are inadequate for pain control, especially during manipulation of the rectus muscles. Some surgeons prefer a parabulbar block performed after topical anesthesia. A conjunctiva and Tenon's incision is made at the desired location as described below. Before extensive dissection is carried out, a blunt-tipped cannula is inserted under Tenon's and threaded posteriorly into the muscle cone, where the usual anesthetic mixture is injected. General anesthesia is only necessary when the patient is unable to comply with direction due to age, cognitive status, or history of inadequate pain control with local anesthesia.