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Glaucoma valves reduce the intraocular pressure (IOP) of the eye by directing the outflow of the aqueous humor through a shunt device. The IOP generally decreases from around 33 to 10 mmHg by removing on average 2.75 microliters/min.*The shunt device channels the aqueous humor in an alternate pathway through a small tube into an outlet chamber. The aqueous forms a bleb and is removed through intercellular spaces and the lymphatic system. These devices create a pathway that completely bypasses the body’s natural drainage device, the trabecular meshwork, for the treatment of glaucoma.
Overview of Glaucoma Valves
Types of Glaucoma Valves
The four main implants currently in use are the Ahmed glaucoma valve, the Baerveldt glaucoma implant, the Krupin slit valve, and the Molteno implant. These devices contain two main components, a long tube to shunt the aqueous humor, and an outflow device. The implants differ in their surface area, materials, and fluid resistance. However, all drainage devices are made out of materials to which fibroblast cannot adhere.Mechanics of Glaucoma Valves
The volume of the anterior chamber is 250 microliters, with a turnover of 1.5 - 2 hours, and a normal rate of humor formation is 2.0 - 3.0 microliters/min. Glaucoma valves aim to control the flow of aqueous humor out of the posterior humor to mirror this natural turnover, as opposed to shunts that simply provide a constantly open pathway for the drainage of the aqueous humor.The IOP of the patient is lowered by providing the aqueous humor with a new outflow pathway through the implant. The amount of outflow is dependant on the resistance of the valve. This resistance is dependant on the surface area of the implant encapsulation and the capsular thickness. Implants with a larger surface area and a thin capsule at the end of the plate will have a greater effect in lowering the patient’s IOP.
The ideal galucoma valve should be easy to implant, able to consistently reduce IOP, avoid complications, made out of inert, biocompatible biomaterials and be effective over patient’s lifetime. The surgical outcomes are described in the Surgical Outcomes section of this entry.
Cost of Glaucoma Valves
The cost of most implants is between $400 and $600. Since the surgery does not require general anesthesia and is performed on an outpatient basis, the surgical costs are relatively low.
The yearly costs of medication include: $336 for Xalatan, $329 for Alphagan, $223 for generic levobunothol, $208 for Ocupress, and $110 for generic timiolo maleate (.5%). These medication are supplementary to the surgery, and not alternatives for a glaucoma valve.
Surgical Procedure
The glucoma valve implantation is an outpatient surgery and is performed under local anesthesia. The procedure takes around 1 hour and is single-staged, as mentioned earlier. The surgeon proceeds similarly for all types of Ahmed Glaucoma Valves. *
Step 1: The implant should be examined and primed prior to implantation. Priming is accomplished by injecting 1cc balanced salt solution or sterile water though the drainage tube and valve, using a blunt 26 gauge cannula.
Step 2: A fornix-based incision is made through the conjunctiva and Tenon's capsule. A pocket is formed at the superior quadrant between the medial or lateral rectus muscles by blunt dissection of Tenon's capsule from the episclera.
Step 3: The valve body is inserted into the pocket between the rectus muscles and sutured to the episclera. The leading edge of the device should be at least 8-10mm from the limbus.
Step 4: The drainage tube is trimmed to permit a 2-3mm insertion of the tube into the anterior chamber (AC). The tube should be bevel cut to an anterior angle of 30° to facilitate insertion.
Step 5: A paracentesis is performed, and the AC is entered at the limbus with a sharp 23 gauge needle , parallel to the iris. Care must be taken to insure that the drainage tube does not contact the iris or corneal endothelium after insertion.
Step 6: The drainage tube is inserted into the AC approximately 2-3mm, through the needle track and parallel to the iris. The leading edge of the device should be 8-10mm from the limbus.
Step 7: The exposed drainage tube is covered with a small piece of preserved donor sclera or pericardium, which is sutured into place, and the conjunctiva is closed.
Alternative Step 7: As an alternative to Step 7 , a 2/3 thickness limbal-based scleral flap may be made. The tube is inserted into the AC through a 23 gauge needle puncture made under the flap and the flap is sutured closed
Post-Surgical Recovery
Recovery after surgery is usually marked by three postoperative phases.
Hypotensive Phase
The hypotensive phase occurs immediately after implantation and may last from one day to three to four weeks. This stage is characterized by a low intraocular pressure (IOP) (2 -3 mmHg to 10-12 mmHg), a disuse bleb, and minimally engorged blood vessels.
Hypertensive Phase
The hypertensive phase is characterized by a high intraocular pressure and an inflamed, dome – shaped bleb. This phase can last anywhere from 3-6 weeks after the operation to 4 – 6 months. The Ahmed valve has a higher hypertensive phase than compared to other valves. This might be due to the different surface area or biomaterials used in the implants.
Stable Phase
The stable phase is characterized by a stabilization of the patient’s IOP. A steady state is reached between the implant and surrounding bleb. Ideally, the patient's intraocular pressure will remain stable for the duration of their life.
Surgical Complications
Hypotony
Hypotony is the occurrence of a very low IOP (less than 5mmHg) and may be caused by over filtration, wound leak, or choroidal effusions (flat anterior chamber). The low intraocular pressure causes complications in an eye suffering from glaucoma because the drainage system is sickly. The damaged eye has trouble creating enough fluid to restore normal pressure. Non-valved implants initially had a higher occurrence of hypotony. Techniques are now in use to temporarily restrict the outflow of non valved devices to reduce the occurrence of hypotony.Hypotony is one of the most common surgical complications and can lead to more serious complications such as choroidal detachment, corneal decomposition, suprachoroidal hemorrhage, hyphema, and retinal detachment.
Diplopia
Diplopia, or double vision, occurs when the bleb interferes with the surround rectus muscles and the muscle sheath. This leads to an extra ocular muscle imbalance and double vision. Diplopia may also be cause by a high implant profile which pushes down onto the eye. Persistent diplopia may cause the removal of the implant device.Suprachoroidal Hemorrhage
Suprachoroidal hemorrhage, or bleeding above the choroid; between the retina and white of the eye, is a potentially serious, vision–threatening complication.Corneal Decomposition
Corneal decomposition may be caused by corneal–tube touch and low–grade inflammation around the shunt. If touch is occurring, the surgeon can go in and reposition the shunt so that it is no longer touching the cornea.Surgical Outcomes
Source: Glaucoma drainage implants: a critical comparison of types, Schwartz, Kenneth S. a ; Lee, Richard K. b ; Gedde, Steven J; Current Opinion in Ophtalmology, Volume 17(2), April 2006,pp 181-189
The study conducted by Schwartz KS at the Georgetown University Hospital , Glaucoma drainage implants: a critical comparison of types , compares the clinical outcomes (intra-ocular pressure) after surgery of the main types of glaucomas with the existing drainage devices. The Ahmed Glaucoma Valve (AGV) appears to have the best surgical results for glaucoma. There are results for the uveitic, neovascular, aphakic and pseudophakic glaucomas. The drainage implants, including their sizes and materials, considered in the study are summarized in Table 1.
Table 1
Table 2 compiles the overall success rates of the different drainage devices, taking into account the best success rate for the different types of glaucomas. The Ahmed Glaucoma Valve (AGV) has the best overall success rate for the neovascular, uveitic, aphakic and pseudophakic glaucomas, with an average of 85%. AVG is best at treating uveitic, aphakic and pseudophakic glaucomas, with success rates of 100% and 88% respectively. The Baerveldt and Krupin implants treat best neovascular glaucoma. The Bearveldt has an overall success rate of 82% and the Molteno 78%. There is no available date for the Krupin implant but for neovascular glaucoma.
Table 2
Neovascular glaucoma refers to angiogenesis in the angle of the eye, causing obstruction to aqueous humor outflow. According to Table 2, the clinical outcomes of neovascular glaucoma surgery are far from perfect. The Baerveldt and the Krupin implants have success rates of 78% and 77% respectively, which represent the best solution to treat neovascular glaucoma. The Ahmed Glaucoma Valve and the Molteno implants succeed in about 67% of the patients.
Uveitic glaucoma is due to an inflammation in the middle layer of the eye, the uvea. Several systemic diseases and masquerade syndromes can cause an inflammation of the uvea, or uveitis. The Ahmed Glaucoma Valve (AGV) is the best drainage device to treat uveitic glaucoma, with a success rate of 100%. Table 3 also reports a high success rate of 91-92% with the Baerveldt glaucoma implant.
Table 3
Aphakia is the absence of the lens in the eye and pseudophakia the medical condition of having an artificial lens in the eye. Aphakia usually results from aging, cataracts, ulcers, congenital anomalies and/or surgical removal. Pseudophakia logically follows one of the conditions causing Aphakia, in order to replace the lens's function. The AGV (Ahmed) treats both aphakia and pseudophakia best, with success rates of 88% in Table 4. The Baerveldt implant treats about 75% of eyes and the results range from 50% to 83% with the Molteno implant.
Table 4
Table 5 compiles the overall success rates of the different drainage devices, taking into account the best success rate for the different types of glaucomas. The Ahmed Glaucoma Valve (AGV) has the best overall success rate for the neovascular, uveitic, aphakic and pseudophakic glaucomas, with an average of 85%. AVG is best at treating uveitic, aphakic and pseudophakic glaucomas, with success rates of 100% and 88% respectively. The Baerveldt and Krupin implants treat best neovascular glaucoma. The Bearveldt has an overall success rate of 82% and the Molteno 78%. There is no available date for the Krupin implant but for neovascular glaucoma.
Table 5
Drug-eluting shunts
Since growth of scar tissue over the drainage device causes increased resistance of the aqueous through the device and a higher IOP. This scar tissue is a main cause of failure for the devices. One way to prevent such scarring might be to create a drug-eluting glaucoma valve.
Drug–eluting stents have revolutionized angioplasty by decreasing restenosis from 36% in a control group to 9% in the study group. Combining pharmaceuticals and devices in the field of aqueous shunts could also impose a dramatic impact on the market. As of today, there are no drug–eluting drainage devices in Food and Drug Administration (FDA) trials, but hopefully the research will come soon.
References
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Glaucoma Links
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