Risks Of Glaucoma Drainage Devices
It is not clear exactly where glaucoma drainage devices sit in terms of overall risk. Multiple studies have suggested that they are safer than trabeculectomy with Mitomycin-C (MMC). However, at least one study has shown that the risk of “severe” complications and loss of vision could be greater with use of Glaucoma Drainage Devices than with trabeculectomy. This may be due to what is called “selection bias” in that traditionally Glaucoma Drainage Devices were offered only to patients who had already failed trabeculectomy or were at a high risk of trabeculectomy failure.
Many of the risks of glaucoma drainage devices are shared with trabeculectomy. Some, however, are unique to these implants. Following is a list of the more worrisome or more common risks. Note that this list is not exhaustive as other less common or less severe complications may be encountered.
Although glaucoma surgery is done to lower the IOP, there is a limit to how low the IOP can safely be lowered. When the IOP is below about 5mmHg we call this “hypotony”. Eyes with such low IOPs are at greater risk of loss of vision. The rate of early hypotony after placement of a glaucoma drainage device is in the range of 12-26%. It’s so common after placement of a glaucoma drainage device that it has a name, the “hypotensive phase”. This phase generally lasts up to four weeks after surgery. As a general rule, the bigger the implant plate the greater the risk of hypotony.
In theory, valved glaucoma drainage devices should have a lower risk of hypotony. It’s surprising, then, that immediate post-operative hypotony has been reported in essentially all studies of valved aqueous shunts. Indeed, since most non-valved tubes are now temporarily tied off at the time of implantation there seems to be little difference in rates of hypotony in the early post-operative period between valved and non-valved glaucoma drainage devices.
Late post-operative hypotony can also occur. This is commonly caused by “overfiltration” (i.e., the aqueous shunt is working too well). If the plate is made of silicon this can be addressed by going back to the operating room and removing part of the plate. This is not possible with Molteno™ or older Ahmed glaucoma drainage devices as they are made of a rigid polypropylene material that cannot be easily cut.
Post-operative IOP Elevation
This is so common it’s almost expected. Called the “ocular hypertensive phase”, it’s thought to be due to the body’s healing response. This phase begins about three to six weeks after placement of a Glaucoma Drainage Device and can last up to six months. It is more commonly seen after placement of an Ahmed valve than with the Baerveldt® or Molteno™ shunts.
Most of the time the IOP can be controlled during this phase with temporary use of steroid and glaucoma eye drops. At times it may even be necessary to “massage” the eye at home in an attempt to force fluid out of the eye and into the bleb. The presence of this hypertensive phase may be related to a higher risk of eventual failure of the aqueous shunt.
Another cause of elevated IOP is clogging of the tube. There are a number of things that could clog the tube: iris, inflammatory debris, and vitreous are three of the more common things to block the flow of aqueous out of the eye.
Double vision (“diplopia”) is more common with the Baerveldt® Glaucoma Drainage Device. This is thought to be due to the implant “wings” sitting beneath the eye muscles. An initial study noted diplopia in over three quarters of patients who had this device implanted! More recent studies, however, have noted much lower (though still significant) incidences of double vision ranging from 6% to 18% with use of the Baerveldt® implant. Other aqueous shunts appear to have an incidence of diplopia in the range of 2-7%.
Exposure or Movement of the Glaucoma Drainage Device
Any time something is placed in the body there is a risk that it will move out of position. In the case of glaucoma drainage devices, there are three main problems noted:
Exposure of the Tube
Most surgeons cover the tube with a tissue graft made of donor sclera, dura (the tissue which surrounds the brain), pericardium (the tissue which surrounds the heart), or fascia lata (the tissue which surrounds the thigh muscle). This will often protect the overlying conjunctiva and prevent it from breaking down.
If the conjunctiva does break down then the tube will be exposed to the tear film which naturally has some bacteria floating in it. These bacteria could then cause an infection that could track back to the plate requiring surgical removal of the glaucoma drainage device. Many times this can be treated by surgical repositioning. Occasionally, however, the glaucoma drainage device must be surgically removed to prevent infection and permanent damage to the eye.
As long as the plate is securely sutured to the sclera it should not move out of position. However, as the sclera is a thin tissue, suture erosion may occur even with good suturing technique. This is especially true after blunt trauma to the eye. Movement of the plate can result in eye muscle irritation causing double vision or pain. It can also result in tube retraction from the eye. Treatment would require returning to the operating room.
Migration of the Tube
Tubes have been reported to migrate from their original position in the anterior chamber. If the tip of the tube moves against the corneal endothelium or out of the anterior chamber then surgical revision is often needed to protect the eye from further loss of vision.
Damage to the Cornea
The underside of the cornea is very delicate. It is covered in a single layer of “endothelial” cells that, if damaged, cannot be replaced by the body. Corneal endothelial cells have a critical job – keeping your cornea clear. If these cells are damaged then the cornea swells up and loses its transparency.
If the tube is not inserted through the cornea correctly or if it moves within the eye over time its tip may touch the underside of the cornea. Over time enough endothelial cells may be damaged by this contact to cause swelling of the cornea. Swelling of the cornea can result in loss of vision requiring corneal transplantation. If this tube-cornea “touch” is noted early enough loss of vision can be avoided by surgically repositioning the tube.
Unfortunately, endothelial cell loss has been observed after placement of glaucoma drainage devices even if the tube is not touching the cornea. It is not entirely understood why this would happen if there is no actual contact between the tube and the underside of the cornea. Nevertheless, swelling of the cornea (”decompensation”) may occur in up to 30% of patients who have had a glaucoma drainage device placed in the eye.
Infection is rarely seen after placement of a glaucoma drainage device. This is one area in which aqueous shunts have a significant advantage over trabeculectomy (especially if Mitomycin-C is used during trabeculectomy). However, if an infection involves the space around the aqueous shunt then the implant will likely have to be surgically removed.
Unlike with trabeculectomy, a well-placed glaucoma drainage device should not cause early cataract formation. However, if the tube touches the lens then a cataract can form due to the microtrauma.
The tube of a glaucoma drainage device may contact the iris as it enters the anterior chamber. This may result in distortion of the pupil. This is a purely cosmetic issue and does not affect the IOP so long as the iris is not blocking the tip of the tube.
Although not really a complication, risk of failure is important to know as you would probably not choose to undergo a surgery that was likely to stop working right away. The likelihood that a glaucoma drainage device will fail is similar to that of trabeculectomy: about 10%-15% will “fail” each year after surgery. As with trabeculectomy, the most common cause of glaucoma drainage device failure is scar formation resulting in failure of the bleb. By five years out from surgery approximately 50% of aqueous shunts can be expected to have failed.
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- Glaucoma Drainage Devices
- Types of Glaucoma Drainage Devices
- How Well Do Glaucoma Drainage Devices Work?
- Does It Matter Which Glaucoma Drainage Device Is Implanted?
- When Should Glaucoma Drainage Devices Be Considered?
- When Should Surgical Treatments Other Than Glaucoma Drainage Devices Be Considered?