Glaucoma implant allows easy placement, good coverage

August 1, 2005

Indianapolis—The Baerveldt Glaucoma Implant (AMO) overcomes the first hurdle with glaucoma drainage devices by allowing easy placement in the eye in a single quadrant and covers a sufficiently large surface area to obtain safe IOP levels—a good combination in this type of device, according to Louis Cantor, MD.

"I have used other devices in the past and have had good results. But, for example, the Molteno Implant (Molteno Ophthalmic Ltd.) requires placement in two quadrants of the eye to achieve adequate drainage," Dr. Cantor said. He is the Jay C. and Lucile L. Kahn Professor of Glaucoma Research and Education, and director, Glaucoma Service, Indiana University School of Medicine, Indianapolis.

"Implantation of that type of drainage device requires more surgery and dissection compared with what is required for single-quadrant devices such as the Baerveldt Glaucoma Implant," Dr. Cantor added.

"It is important to achieve a certain surface area to have adequate long-term IOP control. In most cases, I use the smaller of the two Baerveldt models," he explained. In comparison, each of the plates in the Molteno implant covers 135 mm2 (270 mm2 total).

"One of the advantages of the Baerveldt implant is the ability to achieve a surface area that was found to be important with the Molteno implant in a single-quadrant device," Dr. Cantor said. "In early studies of the Molteno implant, the double-plate implant was found to be approximately twice as effective as the single-plate implant. Those results are very similar to the success rates reported in the literature with the Baerveldt 250-mm2 implant."

A possible disadvantage of the Baerveldt implant in the early postoperative period is the absence of a built-in flow restrictor, such as that in the Ahmed Glaucoma Valve Implant (New World Medical Inc.). The device, which is inside the drainage tube, restricts some of the flow early after implantation to avoid overfiltration and consequent hypotony, Dr. Cantor explained.

Some surgeons have gotten around the absence of a valve in the Baerveldt implant with some effective procedures: placing a temporary ligature on the tube to avoid excessive initial drainage, using a high-molecular-weight viscoelastic agent to obstruct the tube temporarily, or using collagen plugs. Once the healing process starts around the drainage plate on the eye wall, a capsule forms that provides natural resistance to excessive outflow during the early postoperative period, Dr. Cantor explained.

Although extra maneuvers have to be performed to restrict outflow initially, Dr. Cantor pointed out that the absence of a valve may be beneficial in the long term because it is one less device that can malfunction in patients who may live with the drainage device for 20 or 30 years.

Despite the absence of a valve, Dr. Cantor prefers the Baerveldt implant because he achieves better IOP control. The Ahmed implant, also a single-plate device, is smaller (the FP-7 model is 184 mm2) than the Baerveldt implant. This may account for some poorer long-term outcomes compared with the Baerveldt implant that Dr. Cantor has observed in his surgeries.

"The surface area of the Ahmed implant does not seem large enough to obtain the IOP reductions needed," he said.

A recent modification to the latest generation of the Baerveldt implant has reduced the risk of ocular motility disturbances in some patients. With earlier generations of the device, the bleb infringed on the adjacent ocular muscles, causing motility disturbances. Fenestrations were incorporated into the plate of the drain that allowed for fibrovascular ingrowth that limits the size of the bleb that forms around the plate, Dr. Cantor explained. Ocular motility disturbances can still occur, but the current Baerveldt model has reduced the risk.