Tube shunts are another type of glaucoma surgery that can be used when medical therapy has proved to be inadequate. The procedure involves implanting a small silicone tube into the anterior chamber of the eye. A tiny plate is also placed far back in the eye, under the muscles that control eye movement. The fluid that drains from the tube collects in a pocket of scar tissue that forms around the plate and is eventually re-absorbed by the tissues surrounding the eye. The tube and plate are hidden by the eyelids and are not easily visible. Furthermore, once the healing process is complete patients do not feel the presence of the tube.
The decision of whether to do a trabeculectomy or a tube will depend on what is thought to be best for a given patient’s type of glaucoma and particular circumstances. Furthermore, there is more than one type of tube and the specific kind that is chosen is also based upon the goals of the surgery. It is not possible to explain all of the details that will be weighed when making the decision of trabeculectomy versus tube surgery but some of the important factors are discussed below.
Certain types of glaucoma have a low success rate with trabeculectomies and the preferred initial surgical option is to use a tube implant. In general, any glaucoma that is being caused by inflammatory conditions within the eye, or abnormal tissue growing over and blocking the drainage system, will do better with a tube than with a trabeculectomy. Some of these conditions are:
- Uveitis or Iritis – In this condition there is inflammation within the eye that can cause the drainage system to lose its function or even to become sealed shut when the iris adheres to it. Exposure of the bleb formed by a trabeculectomy to the inflammation in the eye often leads to a vigorous healing response in the bleb which ultimately will scar it closed. The advantage of the tube implant is that it is much harder for the opening of the tube within the eye and the pocket of fluid around the plate to become closed. For this reason it is the preferred procedure.
- Neovascular Glaucoma – This form of glaucoma typically affects individuals with diabetes, or eyes that have problems with the circulation supplying the eye. Neovascularization refers to the new growth of abnormal blood vessels in response to the retina being starved for oxygen due to the compromised blood flow in these conditions. The new blood vessels can grow over and block the drainage system of the eye leading to high eye pressures. There is a high risk of the blood vessels continuing to grow after a trabeculectomy and closing off the opening of the drain created on the inside of the eye. The tube opening could also become blocked by these blood vessels but it is far less likely.
- Iridocorneal Endothielal Syndrome (ICE Syndrome) – A condition where the endothelial layer (the most inner layer) of the cornea grows over the trabecular meshwork and across the surface of the iris. These abnormal endothelial cells form adhesions that attach the iris to the cornea and block off the drainage system. Tubes are the most effective surgery for this condition.
These are only some of the instances where a tube is preferred. In patients that have already had a trabeculectomy that did not work often times a tube implant is the next surgical intervention. Also, if there is scarring present on the surface of the eye a tube may be thought to have a higher chance of success at controlling the pressure. Each individual decision is tailored to a patient’s given condition in order to maximize the chance of success.
Potential side effects of drainage implants include:
- Hypotony (very low eye pressure) – This is a serious complication, but advances in the surgical techniques and implants have reduced the risk of hypotony.
- Motility – Muscle imbalances such as strabismus (mis-alignment of the eyes or cross-eyes) and diplopia (double vision) can occur as a result of the implants.
- Blockage – The tube can become blocked, which may lead to additional surgery.
- Cataracts – As with trabeculectomies, the incidence of cataracts after a tube implant surgery is high. Eventually, everyone develops a cataract, it is just that the process is often accelerated after glaucoma surgery. Removing a cataract prior to a tube implant can maximize the chances of long-term success.
- Retinal Detachment, bleeding and breakdown of the cornea are also unlikely but possible side effects of drainage implant surgery.
Both trabeculectomies and tube implants are good procedures but there is a chance that our pressure goal may not be achieved. The reason for these surgeries to not have a 100% success rate is because a significant portion of their efficacy depends on how the eye heals. The extent of healing is only partially under our control. Too little healing and the drain can work too well. This can lead to very low pressures which can cause it’s own set of problems. Too much healing and scar tissue can close the drain off. For these reasons, once we decide that a surgical procedure is necessary it is important to keep in mind that you may need to continue your eye drops. It may even mean that an additional surgery is necessary at some point in the near or distant future. This is why surgery is often performed only after any less invasive measures have been exhausted. You can feel confident that we will try to avoid surgery if possible and that no matter what the outcome, we will be there for you to make sure we do everything possible to care for your eyes.