Tube-shunt surgery, or Seton tube shunt glaucoma surgery, is a surgical method to treat glaucoma. Glaucoma is a potentially blinding disease affecting 2–3% of the United States population. The major known cause of glaucoma is a relative increase in intraocular pressure, or IOP. The purpose of glaucoma treatment, whether medical or surgical, is to lower the IOP.
Aqueous fluid is made continuously, and circulates throughout the eye before draining though channels in the eye's anterior chamber. When too much fluid is made, or it is not drained sufficiently, the IOP rises. This fluid build-up can lead to glaucoma. Normal intraocular pressure is under 21 mm/Hg. Glaucoma develops at IOPs higher than 21mm/Hg. However, approximately 20% of glaucoma patients never have pressures higher than 21 mm/Hg.
Seton tube implants are also called glaucoma drainage tubes or implants. The Seton implant is comprised of two parts:
The function of the implant is to lower the intraocular pressure by filtering excess aqueous fluid out of the eye. During the first few weeks after surgery, a bleb of fibrous tissue and collagen forms around the plate of the implant. The formation of a filtration bleb is essential for filtering the excessive aqueous fluid. The thickness of the bleb, as well as the size or number of plates, determines the rate at which aqueous flows out of the anterior chamber of the eye. The excess aqueous fluid is shunted through the tubing of the implant, and passes through the space that develops between the bleb and the plate. By diffusion, the fluid flows into the capillaries where it exits the eye and enters general circulation. The IOP is lowered as a result of this decrease in fluid.
There are various types of implants used in glaucoma surgery. They fall into two categories: the non-valved (free flow implants) and valved (resisted-flow implants). One of the first free-flow implants was the Molteno implant, which consists of one or two polypropylene reservoirs connected to a silicone tube. The non-valved Baerveldt implant is larger than the Molteno, and is available in three sizes.
The restrictive implants, which include the Krupin and Ahmed implant, have valves that automatically close if the intraocular pressure is too low. This is important because in the first few weeks after surgery (before the bleb forms), the aqueous fluid can flow unimpeded through the implant. As a result, hypotony (low level of fluid in the eye) can develop.
Newer implants such as the Express shunt and the Gore-Tex tube shunt are in early stages of use.
Seton tube implants are employed to treat all forms of glaucoma, but are primarily used in patients with elevated IOP despite aggressive medical treatment. They are also used when other types of surgery, such as conventional filtration, or trabeculectomy , have not been successful, or would not be recommended. A trabeculectomy should not be performed on patients with neovascular glaucoma, as well as those who have ocular complications caused by previous glaucoma surgeries.
Implants are often placed in the eyes of patients with uveitic glaucoma (fluctuating IOP). The surgeon implants a tube with a ligature, and manipulates the ligature to control pressure. Seton tubes are also used in young patients with aniridia, who often develop glaucoma. These tubes should not be used for patients who have silicon oil implants for the treatment of retinal detachment.
A Seton implant is usually inserted under local anesthesia, but may be done under general anesthesia for an anxious patient or child. Since implantation may be painful for some children, drugs may be given intravenously during surgery.
After anesthesia is administered, the eye is draped and retractors are placed on the eye to hold it in place. An incision is made on the conjunctiva, a thin membrane layer that lies above the sclera (white of the eye). The implant plate is placed under the conjunctiva and sutured to the sclera, carefully avoiding damage to the recti muscles in the area. Incisions may be made in two quadrants of the eye if a double plate implant is inserted.
If the tubing is implanted into the anterior chamber, that portion of the eye is drained of excess fluid. If the tube is placed in the posterior chamber of the eye, all or part of the vitreous is removed. A needle puncture is made at the limbus where the cornea and the sclera meet, and the tubing is passed through this hole into one of the chambers of the eye. This opening is sealed with a donor scleral patch, which may be autologous (from the patient's own tissue). If a free-flow implant is used, the tubing is ligated with either a disposable suture, or the ligature is positioned such that it can be removed with a minor incision after a few weeks. As an alternative, the non-valved implant may be inserted in two stages. The plate is first implanted, and the tube is attached during a second surgery after the bleb has formed.
Prior to surgery, the patient's eye is examined with a slit-lamp biomicroscope. It is important that the conjunctiva in which the plate is placed is not scarred; that the cornea is clear; and that there are no attachments of the iris to the lens behind it or to the cornea in front of it. An ultrasound of pediatric patients is done to assess the size of the eye because not all implants are small enough to fit into a child's eye.
Antibiotic drops may be given for up to three days prior to surgery. The patient will continue most glaucoma medication until the day of surgery.
Informed consent must be given for the procedure. This includes consent for surgery and a list of risks for the Seton tube implant. It is important for the patient to understand that any vision loss acquired prior to surgery cannot be corrected.
For several weeks postoperatively, the patient is given topical antibiotics and steroids. In addition, oral steroids may be given to patients who had ocular inflammation prior to surgery. Some surgeons use atropine to maintain the eye in a temporary dilated state. Glaucoma medication may be continued for a few months due to possible IOP fluctuation during the early post-operative period. Follow-up visits are scheduled for one day after the surgery, weekly during the first month, twice a month during the second month, and again at three months. Patients can resume normal daily activities within a few days. The sutures may cause a foreign body sensation, which decreases as the stitches dissolve. This does not usually require treatment.
Aftercare in the surgeon's office involves monitoring for the signs of hypotony and lowered IOP. The treatment for post-operative hypotony is to tighten the tube of a non-valved implant. As the bleb forms, adjustments are made in the tubing ligature to increase flow through the ligature. If the pressure continues to rise, the tube may be blocked, and excess fluid may have to be tapped. Tube blockage may occasionally occur. Hypotony may also be caused by leakage from the conjunctival wound site.
This surgery has intraoperative and postoperative risks. During the procedure, an extraocular muscle can be severed. This is particularly true if the implant is placed in the inferior nasal section of the eye. Strabismus and double vision may follow. Also, the cornea may become scarred, hemorrhaging can occur within the eye, and the iris and lens can be damaged by the protruding tube.
Early post-operative complications include hyphema (blood clots in the anterior chamber of the eye), hypotony, tube obstruction, suture rupture with wound leakage, movement of the implanted plate, corneal edema, and detachment of the retina. Because of the position of the implant plate, retinal detachments are difficult to treat successfully if a Seton implant is present. Double vision during the early post-operative period may be due to swelling in the area, and often will resolve as the orbital edema decreases.
In the late post-surgical period, strabismus as well as orbital cellulitis, a condition that can spread to the central nervous system, can develop. Other long-term risks of glaucoma implant surgery include cataract formation, proptosis (bulging of the eye), and phthisis bulbi (a dangerous situation in which the eye is devoid of all fluid).
Surgical intervention is required for choroidal detachments, strabismus, and if tubing blocks or comes in contact with other structures of the eye, particularly the cornea. If the tube is blocked by blood clots, tissue plasminogen activator may dissolve them. A laser can cut strands of vitreous or iris that may clog the tubing. If bleb enlargement impinges on a muscle, causing strabismus, the implant may be removed and replaced with a smaller type. If the tubing continually rubs on the back or endothelium of the cornea, decomposition of the cornea is possible and a corneal transplant may be required if vision is comprised. In this case, the tubing will have to be relocated to the posterior chamber, and a vitrectomy performed.
Loss of vision is possible with this and all glaucoma surgery. For Seton tube implants, hypotony is the primary cause of vision loss. Other causes include retinal detachment, vitreous bleeding, and macular edema.
Usually the IOP is lower within two weeks of Seton tube placement. At two months, the pressure is stabilized at 16–18 mm/Hg. Glaucoma medication must still be taken. The IOP in 85% of patients with a non-valved implant is lower than 21 mm/Hg without additional medication intervention. Only 50% of patients with a Krupin valve implant have an IOP lower than 21 mm/Hg without added medical treatment.
For 70–90% of patients, the implant is functional one year after surgery. After three years, 60% remain functional. The failure rate for Seton implants is 4–8% per year, and differ for valved and non-valved implants. For the non-valved implants, the success rate is 90% at one year, but drops to 60% at two years. At least 66% of valved Seton tube implants are effective at one year, but this drops to 34% at six years. Choroidal detachment is a complication in one-third of these patients.
Strabismus is more common with the Krupin valve as opposed to the Ahmed valve, possibly because it is larger.
For high-risk glaucoma patients, the success rate for Seton tube surgery is approximately 50%. The rate of failure increases 10% with each year. High-risk patients include those who are aphakic (have no intraocular lens), have neovascular glaucoma (which develops from uncontrolled diabetes and hypertension), have congenital glaucoma, and who have had other unsuccessful glaucoma surgeries. Although the success rate for neovascular glaucoma is 56% at 18 months, eventually 31% of neovascular glaucoma patients will lose all vision except for light perception.
Trabeculectomy is another surgical filtration technique used to treat glaucoma. Trabeculectomy surgery is performed by making a flap in the sclera of the eye, which serves as an alternative drainage site for aqueous fluid. Patients who receive this treatment are not as high risk as those undergoing an implant procedure. Overall, they have a lower IOP, but may have more advanced glaucoma. If vascularization of the iris is present, as in neovascular glaucoma, a trabeculectomy is not performed. For patients who do not have neovascular glaucoma, the failure rate for trabeculotomy is similar to that of drainage tube implants.
Cyclodestruction is another alternative to Seton tube implants. Freezing temperatures or lasers are used to destroy the ciliary body, the part of the eye where the aqueous fluid is produced. When compared to the YAG laser cyclophotocoagulation, tube shunts are twice as successful.
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Martha Reilly, O.D.
Tube implants are performed by ophthalmologists as outpatient procedures in a hospital operating room . The implantation of a Seton tube takes about two hours. An ophthalmologist is a physician with advanced training in the treatment of eye disease. If general anesthesia or extra pain medication is administered, an anesthesiologist may be present during surgery.