Trabeculectomy, XEN gel stent, and Tube shunt
Posted by: Delray Eye Associates
Revised on July 12, 2019
The initial treatment for glaucoma typically includes eye drops and laser, which are generally associated with minimal risk. Although most glaucoma can be controlled in this way, a significant minority will require surgery for glaucoma. There are newer, “minimally invasive” procedures for treating early or mild glaucoma, involving enhancing the natural drainage systems. Once glaucoma is severe or advanced and uncontrolled, typically a drainage or filtration surgery is required to construct a new drainage system for the eye.
After surgery, the fluid drains to a pocket under the surface of the eye, called a “bleb”, then returns to the blood stream. This often allows for the possibility of lower pressures, but there is some risk of the pressure becoming too low. Low pressure and other problems sometimes require further surgery. These types of surgery involve sutures on the eye surface and frequent eye drops to control inflammation, all of which can cause irritation and blurry vision for several weeks. Frequent visits may be necessary to monitor and alter the healing process to maximize results and prevent sight-threatening complications. Scarring around the drainage site must be just enough to ensure the pressure ends up in a safe range once the eye heals. Excess scarring can result in pressure too high, and not enough scarring can lead to low pressure problems, which can affect vision. Most of the healing seems to occur around the first few months after surgery. Different eyes tend to heal differently, but darkly pigmented races, younger age, inflammation and prior surgery are risk factors for scar formation.
The traditional, and still most common, drainage operation is known as a trabeculectomy. This involves creating a flap under which the fluid drains out of the eye. Potent anti-scarring medication is usually applied at the time of surgery to prevent excess scarring. The pressure may be adjusted during surgery using sutures that may be cut later using a laser in the office. Advantages of trabeculectomy may include more control over the amount of fluid drainage, and more options for further surgery if needed later. Drawbacks include long term infection risk through the bleb, which sometimes develops a thin and leaky wall that can allow entry of bacteria. Also, postoperative care is often critical to achieving a good long term result. Sometimes the trabeculectomy can be revised later to improve drainage, called a “needling”.
The XEN gel stent, approved by the FDA in November 2016, offers a less invasive alternative to trabeculectomy, limiting some of the downsides and risks of traditional surgery. There is perhaps less pressure lowering with this device, and not all patients are good candidates. There is also limited ability to titrate the pressure. Postoperative care including close monitoring and needling procedures in the office is important to optimize results, as with the other glaucoma procedures.
Tube shunt surgery has steadily increased in popularity in recent years. The surgery involves placement of a long plastic tube connecting to a plastic plate implanted way back under the eyelids. The bleb forms over this plastic plate, and tends to be thicker and not prone to infection. The tube is covered by a patch of biologic material for protection. The surgery is inherently more standardized than trabeculectomy, but results may be no more predictable, since healing varies significantly between different eyes and patients. The Ahmed tube comes equipped with a valve to prevent excessive drainage, allowing for immediate drainage through the tube. Nonvalved implants such as the Baerveldt or Molteno require the tube to be tied off for several weeks, often with a dissolvable suture, to allow for a capsule to form around the plate to prevent low pressure.
These larger implants can offer better long term pressure control. Different implants could be better for individual patients. Tubes may have specific advantages for certain types of glaucoma, and with issues of scarring, and may be a good choice once trabeculectomy has failed. Disadvantages include a small risk of double vision, cornea problems, and lack of ability to titrate the flow through the tube.
All of these types of surgeries have a similarly good track record of success, with long term success rates in the 70-80% range. A recent large well-conducted study showed higher 5-year success rates in patients with the Baerveldt tube shunt compared with trabeculectomy in a large group of patients with history of prior eye surgeries, many who had significant scarring or failed trabeculectomy. An ongoing study compares trabeculectomy with the Baerveldt as a first-time glaucoma operation.
Many factors determine which procedure is best for a given patient, including other eye diseases, ability to follow up, age and general health. Different surgeons may get better results with one surgery than another under specific circumstances. No one surgery is clearly superior to another for all patients.