By Richard P. Wilson
One way to relieve the dangerously high pressure in an eye with glaucoma is to make a new drain in the eye, a bypass for the blocked natural drain. This is called a trabeculectomy and is a surgical procedure. It takes the form of a “flap valve” on the top of the eye, the white part of the eye hidden under the upper eyelid. The eye pressure is relieved because fluid can now drain through the new valve. A trabeculectomy is usually done under local anesthesia. An anesthetist administers intravenous medication to relax the patient and reduce the discomfort of the local anesthetic injections. A shot of local anesthetic numbs the eye completely so that it will not move during surgery nor feel any pain. If preferred, the anesthesiologist can administer a general anesthetic, keeping the patient asleep for the whole operation. The surgery itself takes 35 minutes to an hour in most cases.
The advantages of having a local anesthetic are:
- less pain after surgery if a long-acting local anesthetic has been used
- no sore throat from the airway tube used in general anesthesia
- a quick return to normal alertness without the nausea and dull feeling often felt after general anesthesia
- less risk than a general anesthetic, especially in the elderly or those with health problems
General anesthetics are usually reserved for children, those with serious anxiety concerning the surgery, and patients with senility or a language problem that will prevent them from speaking with the anesthesia staff.
There are two major problems with a trabeculectomy. If the surgeon makes a full thickness hole in the eye to drain fluid, in the first few days after surgery too much fluid drains out and the pressure can drop to zero. This is difficult for the eye to adjust to and often results in problems. In order to prevent this, a flap of sclera (the outer white coat of the eye) is placed over the drainage hole limiting the amount of fluid getting out of the eye after surgery. This usually reduces the pressure within the eye in a controlled fashion and allows the eye to adjust to the lower pressure. However, the body responds to any cut in the same way. It tries to heal the cut. If the patient is too strong a healer and creates too much scar tissue, then the flap will seal down over the drainage hole and the pressure in the eye will again rise necessitating a return to drops and possibly pills to control the pressure. If the “flap valve” totally seals down and the new drain completely fails to work, then the patient is back where they started. The eye usually has not lost any vision but will unless the pressure is reduced. In that circumstance, a trabeculectomy is usually done again with the addition of 5-FU or mitomycin, medications used to slow down the healing process. If the surgeon feels that the patient may heal too rapidly because they are young, black, have intraocular inflammation, or have had previous eye surgery, then one of these medications is often administered with the first trabeculectomy. Results vary tremendously. However, as a general rule approximately 50% of trabeculectomy patients will have normal pressures and need no medications for one or more years postoperatively. If medications are added, the success rate of the procedure is over 90%.
The second most common problem is related to the health of the eye. Because the drain of the eye, the trabecular meshwork, was blocked, the eye had only to make a limited amount of fluid to keep the pressure high. Many of the medications used preoperatively also cut down the eye’s ability to make fluid. After the trabeculectomy, the part of the eye making fluid must adjust quickly to a now normal or slightly greater than normal size drain in the eye and increase its fluid output. In older patients, especially if the eye is not healthy, it may not be able to make this adjustment quickly. Then the balance between fluid made and fluid drained is lost: the new drain works well, but the eye is not making enough fluid to keep the front part full and it slowly begins to collapse. At this point, there are several options. If the eye seems to be making a moderate amount of fluid, generally watchful waiting will allow the eye to gradually make more fluid and refill the anterior chamber of the eye. If after several days this does not seem to be happening, fluid can be injected by the surgeon into the anterior chamber of the eye to refill it. Often fluid from between the layers of the eye must be drained in order to allow room for the front chamber of the eye to be filled. This reformation of the front chamber of the eye often stimulates the eye into a more normal fluid production.
The flow of fluid through the new drain is critical. The incision in the eye would behave like an incision in the leg or elsewhere and heal promptly if it were not for the pressure of the fluid pushing through the hole and keeping it open. Therefore, if several days go by without an adequate amount of flow, the drain will promptly heal. When fluid again is made at a normal rate, the drain will be too small or completely closed and intraocular pressure will rise.
After surgery, drops to relax the muscle in the eye, to prevent infection, and to retard healing are used. These are important in postoperative care and often can make a great deal of difference in the success of the procedure. The results of trabeculectomy surgery vary greatly, and usually depend as much upon the body’s response to the surgery, eg., inflammation leading to excessive healing or scarring, as it does the surgical technique.
A trabeculectomy, i.e., guarded filtration procedure, allows fluid from the anterior chamber of the eye (aqueous) to leak out gradually through a small hole in the wall of the eye (sclera) covered by a thin flap of the patient’s own tissue. The resultant pooling of fluid outside the sclera pushes up the thin, clear, outtermost layer of the eye (conjunctiva) is called a bleb. Aqueous leaks from this area into the veins and lymph vessels.