| Glaucoma Valve |
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] 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, 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 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 encountered at the end of the plate. 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 . 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%). SURGICAL PROCEDURE The glucoma valve implantation is an outpatient surgery and is done 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. {Link without Title} ''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 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 IOP 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 patients IOP 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 IOP 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 . 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 FDA Trials , but hopefully the research will come soon. |
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