By Richard P. Wilson
Glaucoma is a disease in which the drainage mechanism of the eye has become blocked. Since an eye normally produces a watery fluid called aqueous throughout life, this fluid has nowhere to go and backs up. This causes a build-up of pressure within the eye which injures the optic nerve. The safest and simplest type of surgery to reduce intraocular pressure is a trabeculectomy, a procedure which makes a flap valve on top of the eye. This allows the aqueous to seep out under this flap valve and be absorbed under the conjunctiva, the clear top layer of the eye, and into the bloodstream. There is little chance that this procedure will work if there is existing inflammation, excessive scarring from previous surgeries, or unusual healing is expected. In these cases, the next step is an aqueous shunt, a tiny plastic tube from the anterior chamber of the eye to a reservoir that is placed halfway back around the eye. The reservoir is a plate that prevents the top layer of the eye from sealing to the wall of the eye and preventing drainage. Aqueous fluid is drained through the tube to the top of these plates and then is absorbed into the lymph and blood vessels around the eye.
The surgical procedure is much more extensive than a trabeculectomy. It usually lasts from 45 minutes to one and a half hours or more if the vitreous jelly from inside the back of the eye has to be removed or excessive scarring from previous operations is encountered. One of the nice aspects of the procedure is that the vast majority of the work is done outside of the eye. The only intraocular part of the procedure is a small incision made by a needle where the tube is to be inserted. The tube is then placed through this incision and a small piece of donor sclera (the white wall of the eye) or fascia (the tough material that holds muscles together) is sewn over the entrance of the tube into the eye. This adds to the safety of the procedure.
The major complications seen with aqueous shunts are caused by the sudden drop in intraocular pressure in eyes which are used to a high pressure. Before the operation there was very little way for fluid to get out of the eye. Therefore, the eye made less than a normal amount of fluid and was able to keep a high pressure in the eye. Suddenly, a new drain aimed at creating a normal or even somewhat greater than normal outflow is made. Many eyes, especially those that are sickly from glaucoma and other diseases or have had their fluid making ability beaten down by medications over the years, have difficulty changing gears and making more fluid. This may result in a period where the eye stops making fluid and allows a buildup of fluid between the layers of the eye. In many instances, this goes away on its own and the eye gradually returns to making a normal amount of fluid. In some instances, however, the fluid between the layers of the eye will need to be drained and the inside of the eye filled to a normal pressure. This priming of the pump often results in normal aqueous production almost immediately. In order to prevent the problems associated with the sudden drop in pressure, an absorbable suture can be used to cut off the flow through the tube. Slits in the side of the tube between the anterior chamber of the eye and the suture tying it off control the intraocular pressure for the first week or so after surgery. Then the body seals down around the slits and medication is required to control the pressure until the tie is absorbed or removed and the shunt starts to work properly.
After the operation heals, the tube is almost impossible to see without a microscope. The plates are placed well back and may be visible if the eye is turned completely down and the lid lifted quite high. Otherwise, the reservoir too is invisible under the lid. Because the procedure is fairly extensive on the outside of the eye, there is mild to moderate discomfort during the early postoperative period but this quickly diminishes.
Potential complications include contact of the tube with the cornea. If this is over a small area, then only localized damage results and no further action is needed. If there is contact of the entire tube to the lining of the cornea, then the tube may have to be repositioned.
The tube may also come in contact with the lens in the eye. Small localized cataracts can result from this. We have not seen a generalized cataract that needs to be removed from any of our shunt procedures.
Very rarely, the tube may erode through the top, clear layer of the eye covering it. This requires surgical repair. Infections and bleeding are possible, but have not been a problem in our series to date.
If there is no barrier to the vitreous jelly from the back of the eye coming forward and becoming caught in the tube that shunts fluid to the reservoirs, then this will need to be removed. This is done with a small needle that cuts and then sucks out the jelly replacing it with a watery fluid. Removing the vitreous jelly increases the potential for bleeding between the layers of the eye, or a tear in the retina if the jelly pulls on the retina possibly leading to a retinal detachment. This procedure often presents more serious complications than the shunt procedure by itself.
Aqueous shunts are usually quite successful, considering the desperate nature of the eyes that are operated on. Approximately 15% of the procedures have had to be revised if the buildup of scar tissue around the posterior reservoir is too thick for the aqueous to pass through. This is a fairly simple procedure and not at all like the original procedure. A small incision is made over the plate and the scar tissue removed. The incision is then sewn up. If this is performed, the success rate of the procedure is at least 75% in the difficult glaucomas and much higher in some of the less difficult glaucomas.
In summary, possible complications with this procedure include but are not limited to:
- loss of vision
- the necessity for further surgery: a. to stimulate the flow of fluid into the eye, b. to remove the jelly or lens from the eye to promote normal circulation of fluid inside the eye, c. to remove the silicone tube if the pressure in the eye remains too low or the tube erodes through the surface causing problems
- persistent discomfort and pain which has been encountered in one patient but did not require removal of the shunt
- ocular deformity or altered appearance due to surgery
- catastrophic hemorrhage or deterioration that could require removal of the eye
As mentioned earlier, minor complications such as too low a pressure for a short term after surgery have been common with this procedure, but major complications have been unusual. These complications have to be looked at in light of the alternatives. The only other viable alternative is a destructive procedure which aims to destroy part of the eye that makes the fluid. This destruction of the ciliary body cuts down on the amount of aqueous production in the eye so that it will hopefully match the amount of fluid leaving the eye. The intraocular pressure could then be controlled with medication. As would be expected with this kind of destructive procedure, a substantial amount of inflammation with decreased vision for a time postoperatively and a greater chance for decreased vision permanently is encountered. There is also a chance that the eye will not make enough fluid, collapsing like a balloon without enough air in it. Although this is not painful, visual acuity is poor and the lid droops. The final alternative, of course, is to do nothing. This generally results in a gradual, or occasionally more rapid, loss of vision and in some cases, pain that necessitates removal of the eye.
Understanding the alternatives places the tube shunt procedure as the surgery of choice. It is difficult surgery, prone to minor complications with major complications a possibility. In many cases of complicated glaucoma, it is the safest alternative with the greatest chance of controlling the glaucoma and preserving vision.