Glaucoma Filtering Surgery: Glaucoma Drainage Devices
When medications or laser procedures fail to adequately control intraocular pressure (IOP), or if side effects prevent the continued use of medications, surgery becomes necessary to achieve the desired level of IOP. Surgery for glaucoma lowers IOP by either increasing the flow of fluid out of the eye or by decreasing the production of intraocular fluid.
It must be remembered that the optic nerve damage and subsequent loss of vision caused by glaucoma cannot be reversed. Thus, the point of surgery is not to improve vision, but rather to prevent further vision loss. Currently there is no glaucoma treatment that can restore vision already lost to the disease. For this reason, early diagnosis and treatment are vital. We recommend reading the section about glaucoma before continuing here.
This section describes glaucoma drainage devices.
Known as a “glaucoma drainage device,” “tube shunt,” “glaucoma valve,” or “Seton,” this procedure is often performed after other forms of glaucoma filtering surgery, most commonly trabeculectomy, have failed to adequately control intraocular pressure (IOP). Occasionally, it will be performed as an initial filtering procedure, usually in cases considered high risk for trabeculectomy failure, such as neovascular or inflammatory glaucomas. A drainage device consists of a small tube which is inserted into the eye, usually into the anterior chamber near the edge of the cornea, and a plastic plate which is placed outside the eye beneath the conjunctiva, the clear membrane which covers the eye. The plate serves a bit like a reservoir, creating a space for aqueous humor to collect, much like the bleb in trabeculectomy.
For simplicity, think of glaucoma as being caused by a clogged drain. Aqueous humor is constantly being created but cannot escape quickly enough, building up pressure and damaging the optic nerve. The drainage device gives the fluid a low-resistance means of escape, leading to lower, better controlled pressure. The fluid which leaves the eye via the tube drains into the space around the plate, beneath the conjunctiva, before being reabsorbed into the bloodstream.
Ahmed glaucoma valve
A number of drainage devices are available from various manufacturers, varying in overall shape and size. The most significant difference between devices is the presence or absence of a flow-restricting valve. In some devices, a small valve serves to limit the amount of fluid that can flow out of the eye. The valve is meant to close if the IOP drops too low in order to avoid hypotony, or overly low pressure which can damage the eye. Drainage devices without valves must be temporarily obstructed at the time of implantation in order to avoid hypotony immediately following the procedure, as will be discussed in greater detail below. The type of device used is dependent upon surgeon preference, with numerous factors taken into consideration. There have been no good studies proving that any shunt is superior to another in terms of successful lowering of IOP or development of complications. To the right is an animated image of an Ahmed glaucoma valve.
Procedure in Detail
A review of ocular anatomy may be helpful to better understand this procedure.
Drainage device surgery is generally performed under local anesthesia- you will be awake but relaxed, and will experience no pain. After the skin around the eye is cleansed, a surgical drape is placed over the eye to maintain sterility. A small instrument known as a lid speculum is placed in order to hold open the eyelids during the procedure. The eye is then rotated into the necessary position to expose the area of conjunctiva where the plate will be placed. An incision is made in the conjunctiva, usually at the edge of the cornea (limbus), and the conjunctiva is carefully lifted away from the sclera, creating a pocket for the plate. The plate is then gently guided into the pocket and sutured to the sclera approximately one centimeter back from the edge of the cornea, which prevents movement of the shunt. The tube is then measured and cut to the proper length. If the drainage device does not have a valve, the tube must be adjusted in order to avoid excessive early flow. This is done by tying sutures around the tube in order to constrict it. With time, these sutures dissolve and allow fluid to flow freely through the tube. In some cases a ‘stent’ suture may be threaded within the tube itself, placed such that it can be removed in the office after several weeks of healing, if necessary. Shunts with valves require no additional preparation. The tube is then inserted into the eye through a small opening made with a needle. In most cases the tube is placed in the anterior chamber, entering the eye just beyond the edge of the cornea. Occasionally, a tube may be placed into back portion of the eye, such as in cases with extensive anterior chamber scarring from prior inflammation or surgery, or in eyes that have undergone corneal transplantation. Once the tube is in good position in the eye, a small, thin piece of tissue known as a patch graft is sutured over the exposed external portion of the tube in order to protect it. The conjunctiva is then brought back to its original position and sutured to create a watertight closure.
The Day after Surgery
Once surgery is completed, a patch is usually placed over the operated eye. This patch is often left in place overnight, though some patients may need to remove it and begin using eye drops later that day. Pain or discomfort is generally mild. The first postoperative visit is performed the following day. The surgeon will check your vision, IOP, and the appearance of the eye. At this point instructions regarding acceptable levels of activity and use of postoperative eye drops, including antibiotics, will be given. Degree of activity, including ability to drive in the days following surgery, depends on each patient’s unique situation and must be judged individually. In general, strenuous activity, heavy lifting, and bending over should be avoided.
The First Few Weeks after Surgery
Discomfort is generally mild in the first weeks after surgery. The eye may feel a bit sore, and often there is a scratchy, foreign body sensation due to stitches. Vision is quite variable in these early days, ranging from almost normal to quite blurred. Don’t be alarmed if vision is poor initially, as acuity generally returns to preoperative levels after a few weeks.
During the 4-8 weeks following surgery the eye heals around the plate, creating a space somewhat like a small reservoir for fluid to collect, similar to the bleb in trabeculectomy. Pressure may begin to rise approximately four to six weeks after surgery, during what is known as the “hypertensive phase.” This is due to scar tissue forming around the plate. When this occurs, additional glaucoma medications may be required to maintain an adequate level of intraocular pressure. Over time, this tissue often thins out, with an associated drop in pressure. When this occurs, some IOP-controlling medications may be discontinued. In general, however, glaucoma drainage devices are more likely than trabeculectomy to require continued use of medications for long-term IOP control.
Unlike trabeculectomy, in which a number of postoperative manipulations may be required to maintain IOP during the early postoperative period (see section on trabeculectomy), drainage device surgery is a bit more straight-forward. There are no sutures to be removed, no ocular massage to be performed. Occasionally, injections of antifibrotic medications such as 5-FU may be given to modify the healing process. Postoperative visits are usually less frequent than after trabeculectomy.
Months to Years after Surgery
Glaucoma drainage devices vary in their long-term effectiveness at IOP control, primarily based upon the condition of the eye being treated. Results will depend upon the type of glaucoma, degree of inflammation in the eye, and amount of scarring or tissue damage from prior surgery or trauma, as well as other factors. Most studies indicate rates of success between 50% and 85%. Most patients will need to continue at least one IOP-lowering medication to maintain the desired level of pressure within the eye.
Patients who have undergone drainage device surgery have a small increased lifetime risk of developing an infection inside the eye. Known as endophthalmitis, this kind of infection can seriously harm the eye and lead to loss of vision. The increased risk is due to the opening made in the eye; just as fluid can more easily escape the eye, bacteria can more easily enter. While this risk is generally quite low, anyone who has undergone glaucoma filtering surgery of any kind is advised to contact their ophthalmologist immediately if the operated eye ever becomes significantly red or painful, or if vision suddenly decreases.
Risks of Surgery
All surgical procedures carry some risk of complications. Some risks are common to all procedures and patients, and others are more specific to certain types of surgeries or to patients with particular conditions. A thorough explanation of complications will be provided with a surgical consent, should you choose to have surgery, and your physician will review the specific issues you may face based upon your unique circumstances.
Complications of tube shunts can be divided into early (days to weeks after surgery) or late (months to years after surgery) problems. Some possible surgical complications are noted here:
- Failure to control IOP, as described in detail above
- Bleeding within the eye (hyphema), usually resolves within one week
- Low intraocular pressure (hypotony) resulting in retinal damage (maculopathy), fluid or blood accumulation within the layers of the eye (suprachoroidal fluid or hemorrhage), or shallow anterior chamber
- Wound leak, may require additional sutures
- Double vision, usually resolves after several weeks
- Infection within the eye (endophthalmitis)
- Loss of central vision, rarely severe loss
- Cataract (clouding of the lens of the eye)- development or progression of cataract is fairly common after tube shunt surgery, and may necessitate surgery for cataract removal in subsequent years
- Drooping eyelid (ptosis)
- Exposed tube or plate, due to breakdown of conjunctiva over the shunt, can cause hypotony (see above)
- Infection within the eye (endophthalmitis), risk increases with exposed shunt
- Corneal clouding, possibly requiring cornea surgery/transplant
Alternatives to Glaucoma Drainage Device Surgery
As with most diseases, there are a number of treatment options for glaucoma. As previously noted, surgery is usually considered when medications and laser trabeculoplasty have failed to adequately control IOP. Drainage device surgery is not the only option available. Other surgical procedures may be considered based upon the type of glaucoma, condition of the eye, and level of IOP required. Your surgeon will discuss alternatives for your particular situation. Some of these procedures are detailed in other sections of the library.