Glaucoma Filtering Surgery: Trabeculectomy
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 trabeculectomy, the most commonly performed surgical procedure used to treat most forms of glaucoma.
Also known as a “trab,” “filter,” or “bleb surgery,” this is the most commonly performed glaucoma surgery in the United States. In this procedure, a small canal is surgically created through the sclera, or white of the eye, in order to drain the aqueous humor, the intraocular fluid which fills the front of the eye and which generates the intraocular pressure. 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 trabeculectomy canal gives the fluid a low-resistance means of escape, leading to lower, better controlled pressure. The fluid which leaves the eye drains into a space beneath the conjunctiva, the clear membrane which covers the white of the eye, before being reabsorbed into your bloodstream. This fluid is different from tears and the eye’s natural external moisture, and will not drip onto the face.
Procedure in Detail
A review of ocular anatomy may be helpful to better understand this procedure.
Trabeculectomy 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 device known as a lid speculum is placed in order to hold open the eyelids during the procedure. The eye is rotated downward, exposing the conjunctiva and sclera above the cornea, which is where the surgical site will be located. A small incision is made in the conjunctiva, which is then carefully lifted and separated from the sclera. Trabeculectomy diagram A half-thickness flap of sclera is then dissected up to the edge of the cornea. This flap will later be sutured back into place, where it will function as a valve controlling the flow of fluid from the eye. At the base of the flap a small piece of the remaining sclera is removed, creating an opening into the anterior chamber of the eye. At this point a piece of the iris just below the opening is often removed. Called an iridectomy, this is done to prevent the iris from later blocking the opening. Once the surgeon determines that the opening is adequate, the scleral flap is sutured back into it original place. The sutures are tightened just enough to allow the right amount of aqueous humor to leak out of the eye. If the sutures are too tight then flow will be inadequate and the IOP will remain too high. Sutures tied too loosely can result in a very low pressure, which is also undesirable. Obtaining the right amount of flow is sometimes difficult during surgery, but fortunately adjustments can be made in the days and weeks following surgery, as described in the next section. Once the surgeon is happy with the flow of aqueous, the initial conjunctival incision is meticulously sutured to create a watertight seal. The end result is a small blister of fluid, known as a “bleb,” which collects around the surgical site, hidden from view by the upper eyelid. This fluid ultimately drains back into the bloodstream.
In a variation on standard trabeculectomy, a small device known as an ExPRESS® Mini glaucoma shunt may be implanted in the eye, just beneath the scleral flap. This tiny tube serves to create the opening into the anterior chamber, and is used in lieu of removing a small piece of scera. An iridectomy is usually not necessary. This shunt may be used in patients at high risk for bleeding or excessive inflammation, or in cases with other ocular abnormalities. The surgeon must evaluate each patient individually in order to determine if the ExPRESS® shunt would be beneficial. Learn more about the ExPRESS® shunt here
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, including the bleb. The shape and size of the bleb can tell your physician how well fluid is flowing from 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 occasionally 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.
The first two to three weeks following surgery are the most critical to achieving a successful outcome. Immediately following surgery, the eye will begin to heal. Our bodies were designed to heal themselves, to repair a wound by growth of new tissue and development of a scar. Yet we have purposely created a wound, an opening into the eye, which we do not want to heal. If the opening scars closed the surgery fails, and we will be back where we began, with high pressure. Risk factors associated with increased scarring include young age, dark pigmentation (African Americans, Latinos), ocular inflammations such as uveitis, and prior surgery involving the conjunctiva, such as previous glaucoma or retinal surgery.
A number of measures are taken to slow down and limit the healing process. First, medications such as mitomycin-C (MMC) or 5-fluorouracil (5-FU) are often used during surgery. These medications, known as antifibrotics, slow the growth of scar tissue on the surface of the eye. Following surgery, additional antifibrotic drugs are used in the form of eye drops to limit scarring and keep the fluid flowing. The primary medication used is a corticosteroid, and is commonly prescribed to be used as often as every two hours (about eight times per day). Corticosteroid drops are often continued in a tapering dosing regimen for 6 – 8 weeks following surgery. Antibiotic drops will also be prescribed during the first 1 – 2 weeks. Occasionally, other medications may also be required.
Postoperative visits are performed frequently during the first few weeks in order to assess the appearance of the bleb and the adequacy of aqueous flow and control of IOP. If healing is occurring too quickly, additional antifibrotic therapy may be provided in the form of 5-FU injections beneath the conjunctiva. The surface of the eye is numbed with drops and the medication is gently injected just under the conjunctiva near the bleb. If necessary, the tip of the needle can be used to very gently break up scar tissue under the conjunctiva. This procedure is well tolerated by most, with only minor discomfort or burning.
If the flow of aqueous fluid is too slow, and pressure therefore too high, the sutures placed on the scleral flap can be loosened or removed in order to allow more fluid to flow out of the eye. The means of adjusting these sutures depends upon how they were placed at the time of surgery. Some sutures are “releasable,” meaning that they can be easily grasped with forceps in the examination room of the office and gently removed from the eye. Again, this is done with eye drop anesthesia and causes no discomfort. Other sutures are “fixed” and must be cut in place using a laser in the doctor’s office. Regardless of how the sutures are placed, the result of removing or cutting them is usually an increase in aqueous flow out of the eye and a significant drop in the intraocular pressure.
Your surgeon may also ask you to massage the eye in the weeks following surgery. This is done by gently pushing with your finger against the lower part of the eye, just above the bony rim of the eye socket. By doing this, fluid is gently squeezed out of the eye and the scleral flap is stretched, improving the long-term flow of aqueous humor through the surgical site.
Months to Years after Surgery
Trabeculectomy is generally a very effective procedure for IOP reduction, with about an 85% chance of lowering pressure to the desired level. In approximately half of these successful cases no further use of glaucoma medications is necessary. The other half must continue at least one medication to maintain the desired level of IOP. While late failures can occur, most patients with IOP well controlled beyond the first six to eight postoperative weeks maintain low pressures for many years to decades. In approximately 10% to 15% of cases surgery fails to adequately control the IOP. Additional surgery may then be performed to reach the desired level of pressure within the eye. Another trabeculectomy may be an option, or a tube shunt procedure may be preferred (see section on Glaucoma Drainage Devices).
Patients who have undergone trabeculectomy have an 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 trabeculectomy is advised to contact their ophthalmologist immediately if the operated eye ever becomes significantly red or painful, or if vision suddenly decreases. For this reason, the use of contact lenses after trabeculectomy is discouraged.
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 trabeculectomy can be divided into early (days to weeks after surgery) or late (months to years after surgery) problems:
- 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
- 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 trabeculectomy, and may necessitate surgery for cataract removal in subsequent years.
- Drooping eyelid (ptosis)
- Bleb leak, due to breakdown of conjunctiva over the bleb, can cause hypotony (see above)
- Infection within the eye (endophthalmitis), risk increases with bleb leak
Alternatives to Trabeculectomy
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. Trabeculectomy, while the most common surgical procedure for glaucoma, 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 will soon be detailed in other sections of the library.