This article is one in a series looking at treatment options for glaucoma. Last month’s “Insights” (March 2014) looked at another type of shunt, the metal ExPRESS device, used in combination with trabeculectomy.
Many people with glaucoma are able to successfully control their symptoms—and prevent vision loss—for years and even decades through the use of prescribed medications. However, if eye drops and other medications prove ineffective, or if the patient can’t tolerate side effects from medications, then the doctor may recommend surgery.
One of the newer types of surgery, a tube shunt, is a flexible glaucoma drainage device that is implanted in the eye to divert aqueous humor (the fluid inside the eye) from the inside of the eye to an external reservoir. The shunt is shaped like a miniature computer mouse with a tube at the end of it. The tube portion enters the front of the eye, or anterior chamber, while the “mouse” or “plate” portion of the implant sits on the surface of the eyeball and is covered by the eyelid. The fluid that collects is then absorbed by the eye’s own veins and transported out of the eye cavity.
The tube shunt is made of silicone or polypropylene, a material that won’t break down in the body. The entire implant is covered with the eye’s own external covering. The procedure is done as outpatient surgery.
Traditionally, tube shunts were used to control eye pressure in patients in whom traditional eye surgery to relieve fluid pressure (trabeculectomy) had previously failed, or in patients who have had previous surgeries or trauma that caused substantial scarring of the conjunctiva. Tube shunts have also been successful in controlling eye pressure in other types of glaucoma, such as glaucoma associated with uveitis or inflammation, neovascular glaucoma (associated with diabetes or other vascular eye diseases), pediatric glaucoma, traumatic glaucoma, and others.
A multicenter randomized clinical trial (the Tube Versus Trabeculectomy or TVT Study) was conducted in patients who had previous surgery, including, trabeculectomy and/or cataract surgery. Those results showed that after five years, tube shunt surgery had a higher success rate compared to trabeculectomy, with similar reductions in eye pressure and the need for supplemental glaucoma medications.
In recent years, some surgeons are using tube shunts or glaucoma drainage devices as first-line surgery, and forgoing standard trabeculectomy surgery. The cost of tube shunt implantation is somewhat higher than for trabeculectomy; however, some would argue there are fewer risks. To weigh these factors, there is currently an ongoing trial comparing tube shunt versus trabeculectomy in patients who have had no prior ocular surgery. The tube shunt is covered by Medicare and most insurance plans.
Tube Shunts and Valves
You may hear the term “valve” used in reference to your tube shunt surgery. This refers to one of several different types of tube shunts available. Typically the device is selected based on the specific eye condition and surgeon preference, but the Ahmed glaucoma valve is the most commonly used type of shunt. The “valve” function refers to its ability to limit the flow of eye fluid in one direction, with a theoretical limit to how low the eye pressure can drop. Often supplemental glaucoma medications (eye drops) are still required after placement of an Ahmed valve.
Sometimes the tube shunt is not attached to a valve (e.g., the Baerveldt or Molteno shunts). In this case, some scarring has to take place initially before the tube opens, either by a special suture that dissolves over time, or another type of suture that the surgeon will remove approximately four weeks after surgery in the office. The risk of pressure falling too low, while rare, is somewhat higher with these devices than with the Ahmed valve, but non-valved shunts sometimes work better for attaining a lower eye pressure.
Because the various tube shunts have different sizes, features, and surgical techniques associated with them, the decision of which tube shunt to have implanted depends on your particular situation and your surgeon’s preference.
There have been two multicenter, randomized, clinical trials examining the Ahmed (valved) device versus the Baerveldt (non-valved) shunt. The Ahmed Baerveldt Comparison (ABC) study, after one year of follow-up, demonstrated that the average eye pressure was slightly higher with the Ahmed valve, yet there were fewer early and serious complications associated with its use, as compared with the Baerveldt.
The Ahmed Versus Baerveldt (AVB) study, an international, multicenter, randomized trial, demonstrated that both devices were effective in lowering eye pressure but the Baerveldt group had lower failure rate and required fewer glaucoma medications after three years. However, the Baerveldt group had more serious complications (similar to the findings in the ABC study).
Benefits, Risks, and Possible Complications
The vast majority of tube shunt procedures are successful and prevent the progression to blindness that can occur with glaucoma. Nonetheless, it is important for you to understand the risks and benefits before electing a tube shunt surgery. Any of the complications described below can occur even with the best surgical techniques.
Uncommon or rare complications include bleeding inside the eye; infection; fluid buildup behind the retina (choroidal detachment, which is not the same as retinal detachment); double vision with some types of implants; and tube-related complications. Bleeding inside the eye can be a very serious complication; for this reason, some surgeons will ask you to stop all blood thinner medications for at least one week before the surgery. Make sure you ask your general doctor if this is acceptable. If not, a discussion should occur between the ophthalmologist and the general doctor before the surgery.
Infection is another risk. As a precaution, ophthalmologists give antibiotics before, during, and after the surgery in addition to practicing sterile. However, on very rare occasions, infection inside the eye occurs anyway. This can be a very serious problem and may threaten vision. With tube shunt surgery, infection can occur months to years after the actual operation, sometimes requiring shunt removal. Your eye doctor can talk with you about how to prevent late infections like this. Have your ophthalmologist look at your eye immediately if there is any sign of infection.
Any tube shunt procedure may fail, with time, due to the natural healing tendencies of the eye. Scarring may be the eye’s attempt to “normalize” pressure, which in this case may mean it fails to achieve a lowered pressure. As a result, you may need to resume glaucoma medications down the road, or even right after surgery. In addition, sometimes a repeat surgery is required. Also, despite successful surgery, your vision may become worse from continuing degenerative changes in the eye. Cataracts may be accelerated by any of these operations. Fortunately, cataracts are relatively easy to fix surgically.
Tube-related complications can also occur, and some of these relate to placement of the tube inside the eye. If the tube is placed too close to the cornea, it can cause the cornea to swell. This is particularly a problem if you already have a corneal transplant. In some higher risk cases, the surgeon will elect to place the tube in the back of the eye; which means a retina surgeon may perform the procedure alongside your glaucoma surgeon. Another risk entails placing the tube too close to the iris, which can cause it to become clogged with iris tissue. Still another risk is of blood clots developing after the surgery.
Most of the above types of tube complications will resolve with time or with the help of an in-office laser procedure. There is one more type of complication to mention: over time, with or without infection, the tube can protrude through the delicate surface tissues of the eye, or conjunctiva, to where it’s exposed and visible on the eyeball. If that happens, it will require another trip to the operating room so the tube can be re-implanted.
Unfortunately, the damage that is caused by glaucoma is permanent and not yet reversible. Consequently, it is extremely rare for a tube shunt procedure to bring about any improvement in vision. Instead, the main benefit and reason for having the procedure is preventive; i.e., without it, vision may grow worse or, in rare cases, be totally lost. In most cases, doctors will not recommend a tube shunt unless they believe you to be at risk for vision loss. Therefore, for most patients, the benefits of the surgery tend to outweigh the risks, but this has to be evaluated separately for each patient.
Yvonne Ou, MD
Assistant Clinical Professor of Ophthalmology
Glaucoma and Cataract Surgery
UCSF Department of Ophthalmology
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