Many surgeons use Ahmed valves as a primary intervention in cases of neovascular, congenital, uveal or open-angle glaucoma, in the presence of perilymbal scarring or anterior synechiae. With these types of glaucoma, implantation of Ahmed valves increases the success of surgical treatment.

Valve devices are often used as the last treatment option after unsuccessful trabeculectomy, but this point of view is now changing. When compared with trabeculectomy, valve implants produce fewer late complications associated with seepage pads (for example, external filtration, phlebitis and endophthalmitis). A lower incidence of endophthalmitis may be associated with a more distant base from the limbus (8-10 mm), where the subconjunctival tissue is thicker and more pronounced than the fibroblastic reaction. These tissues differ from filtration pillows during trabeculectomy, often causing phlebitis and endophthalmitis, especially in cases of the use of mitomycin C and 5-fluorouracil.

Childhood glaucoma is a group of diseases potentially leading to blindness and often resistant to drug therapy, therefore in such situations the implantation of the Ahmed valve is often indicated. This gives a significantly more effective stabilization of the glaucomatous process during the first 2 years of life compared with trabeculectomy using mitomycin C. The advantages of the Ahmed FP7 silicone valve are the single-stage implantation, the intervention in only one quadrant, and the instantaneous control of the IEye pressure. This is especially important in cases where urgent combined intervention is necessary for corneal disease in severe congenital glaucoma.

Indications for implantation

The Ahmed S2 valve can be used in both children and adolescents. It is used in the absence of compensation. Eye pressure, despite the maximum drug therapy and laser intervention. This valve can be used as a primary intervention in cases of neovascular, postuveal and other complicated types of secondary glaucoma.

The second valveless model (model B1), if necessary, can be implanted later to improve the drainage of fluid through the primary filtration pad.

The tube extension (TE model) is used in the following situations:

with a pushed out or exposed tube;

when the surgeon wants to move the valve from the flat part of the ciliary body, inserting the tube into the anterior chamber;

when the tube is cut too short during implantation;

in all cases when the tube is accidentally cut.

Fastening in the area of ​​pars plana (model PC) with the introduction through it of the tube is advisable in patients after keratoplasty or with a modified cornea. Drainage is also used in patients with a very shallow anterior chamber, or if it is impossible to insert a tube into it because of the front synechias. Valve implantation in the pars plana area is often carried out with far advanced secondary glaucoma with an uncompensated level Eye pressure.

Operation technique

It is necessary to obtain the patient’s informed consent for the operation with the implantation of the Ahmed valve and strengthening the sclera using the previously prepared pericardium (Tutoplast), donor sclera, etc. If the introduction of the tube through the pars plana is planned, an examination of the retina is necessary. For indications, the implantation of the Ahmed valve can be combined with a vitrectomy through pars plana.

Course of operation

In addition to retro- or peribulbar anesthesia, tetracaine can be used topically. After applying the bridle suture (polyglactin thread 6/0), a special shield is placed on the cornea to protect it from bright light from the microscope. The Ahmed valve should be flushed with a balanced salt solution to ensure that air is completely removed and that the valve is functioning. To do this, a syringe with a needle 26G or a cannula 27G is inserted into the tube and abruptly press on the piston of the syringe with a balanced salt solution until the solution is visible behind the valve or on top of its base.

The preferred site for implantation is the upper outer quadrant. Do not implant the device in the upper inner quadrant, as this is associated with a high risk of developing a false Brown syndrome and damage to the tendon of the superior oblique muscle. Conjunctival flap base to the arch form between two adjacent straight muscles. For this purpose, convenient to use scissors Stevens.

The Ahmed valve should not be grabbed with a pair of tweezers for its body, since its damage will promote the growth of fibrovascular tissue. The device should be taken as a base next to a small hole, inserted into the formed subconjunctival space and hemmed with non-absorbable threads (for example, 9/0 nylon thread) to the sclera. When attaching the plateau to the sclera, it is necessary to ensure that the valve is located between the rectus muscles and the tube is directed radially in relation to the limbus. The front edge of the valve should be 8-10 mm from the limbus.

When implanting the Ahmed valve with two plateaus, the formation of a flap with the base to the arch is recommended. A traction seam on a tenon capsule with a 7/0 polypropylene thread or a 8/0 nylon thread may be useful. The connecting tube is placed above the upper straight muscle or below it, but under the connecting tube there should be no Tenon capsule tissue. The second plateau, which is also placed 8-10 mm from the limbus, should be attached to the sclera with a 9/0 nylon thread.

The length of the drainage tube is calculated from its location outside the cornea, taking into account that the length was sufficient for insertion into the anterior chamber. Then the drainage tube is cut obliquely so that the beveled end is directed toward the endothelium of the cornea. Then perform paracentesis outside the surgical area. If necessary, viscoelastic is introduced into the anterior chamber. Using a 23G needle, they enter the anterior chamber in the corneoscleral region parallel to the plane of the iris. A special forceps tube is held in the anterior chamber through a puncture made by a needle. The length of the tube in the anterior chamber should not be more than 2-3 mm and should not touch the cornea, iris or lens.

The inlet through which the tube was inserted is covered with a donor sclera or pericardium, which is hemmed to the sclera with a 9/0 polyglactin or nylon thread on a flat needle or on a TG needle. The conjunctiva is sutured with absorbable sutures (for example, 8/0 polyglactin thread). In 180o, antibiotics and corticosteroids are administered subconjunctivally from the site of intervention.

In some countries where donor tissue is not available, alternative methods of closing the exposed tube are used. One of them is the formation of a scleral flap by the base to the limb on a part of the thickness of the sclera. Then, a needle 23G forms a hole for insertion of the tube under the flap. The tube is covered with a flap, which is then sutured with a 9/0 nylon thread. After conjunctival closure, antibiotics and corticosteroids are administered subconjunctivally.

Implantation in the flat part of the ciliary body

After the formation of the conjunctival flap with the base to the arch, the Ahmed valve with fixation to the flat part of the ciliary body (purs plana) is flushed through the irrigation cannula of the PS. The valve is placed 6-8 mm from the limbus and fixed to the sclera with a 9/0 nylon thread. It is better and easier to fix the plateau of the valve to the sclera, while the eye is still tight.

If necessary, a vitrectomy can be performed through the pars plana, which will increase the hypotensive effect of drainage surgery, if the main part of the vitreous body is also removed from the area of ​​implantation of the drainage device. After performing a vitrectomy, it is possible to restore normal eye turgor using an infusion of a solution.

The valve drainage is then inserted through a puncture made by a 23G needle approximately 3.5 mm from the limbus. The tube can be cut so that its end reaches the edge of the pupil, which will allow to see it later for a slit lamp. Flexion of the tube is eliminated due to its passage through the plateau with fastening in the flat part of the ciliary body.

If a plateau with attachment for connecting with other drains is supplied separately (model PC), the surgeon should pull a tube through it with thin tweezers. Unlike other drainages, the advantage of the plateau with fastening in the area of ​​the flat part of the ciliary body is plateau mobility, allowing it to be located at any distance from the valve plateau.

After strengthening the sclera with donor material (sclera or pericardium), the port and conjunctiva are sutured.

Postoperative management

Antibiotics and corticosteroids are prescribed topically 4 times a day. Depending on the degree of postoperative inflammation, the mode of instillation is reduced during the month. Normal postoperative limitations are recommended.

results

Ahmed valves are used in all types of glaucoma. They showed a clear advantage over other valveless devices. The valve mechanism with the Venturi system allows you to quickly reduce intraglastic pressure and avoid hyperfiltration in the postoperative period. In in vitro studies tested the operation of the valve mechanism and its ability to control pressure by comparing the operation of the Krupin and Ahmed valves. Studies have shown that the Krupin valve exerts the same resistance to the outflow of moisture and also controls pressure, like a cannula. Only the Ahmed valve functioned as a valve, adjusting the pressure within the desired range, reducing or increasing the resistance to fluid flow.

Based on research conducted in the United States and in some other countries, it can be said with caution that early use of valves is warranted, especially in cases of neovascular, congenital, uveal and postkeratoplastic glaucoma.

Comparative studies of Wang have shown that Baerveldt drainage with an area of ​​350 mm2 and the Ahmed valve of the S2 model equally well control the Eye pressure in patients with refractory glaucoma, creating conditions for the preservation of visual functions, and have a small number of complications 1 year after surgery.

Over the past three years, the use of flexible plateaus (model FP7) has increased by 3 times compared with models S2 / S3. Although there are still no publications in the literature, many doctors suggest the formation of high filtration pads with a thinner wall after implantation of the FP7 model. As expected during the development, the FP7 model can provide a significant reduction in Eye pressure in comparison with the S2 model. A low filtration bag reduces complications such as diplopia and strabismus. Moreover, the use of silicone causes a smaller inflammatory reaction and reduces the thickness of the pseudo-capsule.

The new FP7 silicone valve has the advantage of single-stage implantation in one quadrant with a direct reduction of intra-eye pressure, which is very important for neovascular glaucoma. Also the implantation of the Ahmed valve is a safe and effective operation for uveal glaucoma in children, especially with a normal immune status and under close observation.

In patients with glaucoma requiring end-to-end keratoplasty, implantation of Ahmed valve drainage contributes to a more prolonged transparent engraftment of the graft due to a decrease in intraglastic pressure and a decrease in the number of installed drugs. Using drainage surgery, according to Kwon et al., Provides corneal transplant engraftment in 70 and 55% of cases when observed for 2 and 3 years, respectively, and allows to normalize the level of intra-eye pressure for 3 years in the majority (82%) of patients after end-to-end keratoplasty .

Complications

Hyperfiltration of aqueous humor is one of the early postoperative complications, leading to shallow anterior chamber, hypotension, and ciliochoroidal detachment. Ahmed valve drainage has a protective mechanism that minimizes the aqueous humor hyperfiltration in the early postoperative period. Drainage implants without this mechanism (for example, Baerveldt and Molteno drainage) are put in either 2 stages, or an additional suture is applied to prevent hypotension and ciliochoroidal detachment.

As a rule, the period of transient increase in intraglucose pressure is noted within one month after the operation. This period is called the phase of ophthalmic hypertension. However, this does not require mandatory additional intervention. Huang believe that the increase in intraglastic pressure is possible no earlier than 4 weeks after surgery.

In children, tube exposure is more common. This may be due to the fact that small children often rub their eyes. Since the exposure of the tube is associated with an increased risk of endophthalmitis, it is often necessary to have a surgical procedure to build up the tube, move the plateau of the valve or replace the implant.