In 1907, Rollet made a dimensional attempt to reduce intraocular pressure by diverting aqueous humor and subconjunctival space through the limbus by implanting horsehair. After this, many attempts were made, most often unsuccessful, to create drainage devices of various materials. All of these operations failed due to excessive scarring in the area of intervention.
A breakthrough in this area occurred in 1969, when Dr. Anthony Molteno suggested using a large surface of the eyeball near the limbus for the distribution of aqueous humor. Most of the operations after 3-6 months were unsuccessful due to ulceration of the conjunctiva over drainage, plate exposure and scarring processes. The efficiency of operations increased in 1973, when Molteno began to divert the aqueous humor to an area 8–10 mm from the limbus. To date, all antiglauco drainage is based on the Molteno concept.
In the first anti-glaucomatous drains, any resistance to the outflow of aqueous humor from the anterior chamber was not taken into account. Among such valveless drains, the most frequently used are: Molteno drainage, proposed by Anthony Molteno in 1969, and Baerveldt drainage, proposed by George Baerveldt in 1992.
In the anti-glaucomatous drains, which appeared later, they began to take into account the resistance to the outflow of aqueous humor: a valve mechanism was created. Significant progress in this direction was made by Theodore Krupin, who in 1979 created the Krupin valve; In 1993, Abdul Mateen Ahmed introduced the Ahmed valve into widespread practice.
Currently, tubular implants are divided into two types: valveless (for example Molteno, Baerveldt, Schocket) and valve (for example, Krupin, Ahmed).
Basic requirements for anti-glaucomatous drainage
possibility of removing aqueous humor from the anterior chamber without developing hypotension
the duration of the hypotensive effect with minimal side effects on the surrounding tissue
In valveless implants, such as Molteno and Baerveldt drains, outflow resistance is not taken into account, which can lead to hypotension, the formation of a shallow anterior chamber and ciliochoroidal detachment. In order to avoid these complications, several modifications of drainage have been proposed: imposition of an occlusive ligature on the tube for a certain time; plugging the tube with a material that slowly absorbs intraocular fluid; introduction to the tube of the thread, which can later be removed.
The Krupin drain valve consists of a slit-shaped silicone valve attached to a silicon plateau. As the intraocular pressure rises, the valve opens, allowing the aqueous humor to leak. The length and width of the gap and the elasticity of the tube help to control the outflow of aqueous humor. This is a valve of obstructive type, i.e. to enhance the outflow, it is necessary to open the gap in the tube.
Model S2, the prototype valve Ahmed, is a silicone membrane, folded together and elongated at the top. The membranes are based on polypropylene plateaus with additional ridges and grooves that stretch the membranes. When the pressure becomes higher than 8-10 mm Hg. Art., silicone membranes open to drain fluid. The resistance of the valve is self-regulated by the level of Eye pressure. With a high initial pressure in the front chamber, the valve automatically opens completely. As soon as the pressure decreases, the membranes decrease in size, reducing the outflow of fluid. Since the valve starts to work only at a pressure above 8-10 mm Hg. Art., it prevents the development of hypotension.
In contrast to valveless devices, the Ahmed valve is implanted in one stage, and immediately after the operation, the Eye pressure decreases. Among all tubular implants, the Ahmed valve has the lowest postoperative level of hypotension and a small number of complications. Internal resistance in the valve system is provided by the Venturi mechanism. The valve inlet area is larger than the outlet area.
The Ahmed valve is a non-obstructive type device. Since the lateral dimension of the valves is larger than the circumference of the narrow tube, each particle passing through this tube also passes through the valve mechanism.
Now Ahmed valves of various design, a form, from different material and with a various surface area are developed. They are also produced with useful supplements that can be used in both valve and valveless drains. A brief story about the evolution of Ahmed valves describes the main concepts that have had a great influence on the development of antiglaucoma drainage devices, and also highlights important issues for doctors, such as choosing the right model for a particular clinical situation.
Ahmed Valve Prototype: Model S2
The first Ahmed valve was the S2 model. The device consists of a polypropylene plateau on which a valve mechanism with an area of 184 mm2 is attached. After the formation of a filtering pad above the device, fluid drainage mainly occurs through the vascularized tissue of the capsule. Silicone tube length – 25 mm, internal diameter – 0.635 mm; the width of the device is 13 mm, length – 16 mm, thickness – 1.9 mm. Thin silicone membranes are 8 mm long and 7 mm wide.
Various biomaterials and scar tissue formation: model FP7
In the original antiglauco valve model S2 used polypropylene plateau. Ayyala found that polypropylene is less biocompatible compared to silicone. The use of silicone can reduce the degree of inflammation and scarring during the encapsulation process of the device, and the severity of encapsulation affects the duration and extent of the hypertension phase.
The result of these studies was the release of the firm New World Medical silicone implants, including the valve Ahmed on an elastic plateau (model FP7).
These implants are more flexible, with a smaller thickness of the edges (60% less than the model S2). According to Sarkisian et al., The thin lower edge facilitates the closure of the conjunctiva, reduces the frequency of its perforations, promotes better healing and better formation of the filtration bag. The thin implant profile also avoids the need for posterior laxative cuts or the use of conjunctival autograft.
On the elastic plateau there are three holes located behind the valve and outside or between the two supporting elements. The holes are made so that the fluid drains through them to the upper part of the implant, thereby increasing the efficiency of aqueous humor. Moreover, fibrous tissue, germinating through these holes, helps to reduce the height of the filtration pad. Thanks to these innovations, the Ahmed FP7 valve is able to provide better control of Eye pressure.
Drainages: B1 and B4 models, FX1 and FX4
Long-term results of studies have shown a direct dependence of the reduction of intraglycemic pressure on the size of the implant, but only if its area is sufficient. The Baerveldt implants with an area of 250, 350 and 500 mm2 are located partly under the rectus muscles, only slightly protruding, and act as a moisture distribution tank.
Britt in a prospective study, comparing Baerveldt implants of 350 mm2 and 500 mm2, did not find a statistically significant difference in the reduction of intraglastic pressure for 18 months. observations after their application.
Similar results were obtained by Ayyala when comparing the Molteno dual-board drainage with the Ahmed single chamber valve. These data confirm the view that an increase in the surface area of the implant increases the hypotensive effectiveness only to a certain size, and a further increase in the area does not provide an additional advantage.
To increase the area, New World Medical has launched an implant with two plateaus (model B1). The area of the entire surface of such an implant was 364 mm2.
The second plateau (model B4) can also be attached to the valve to the right and to the left of it, if it is required to reduce the Eye pressure by increasing the surface area. Model B4 consists of a silicone drainage tube 14.8 mm long, attached to a polypropylene plateau 12.2 mm wide and with a surface area of 180 mm2.
The second plateau can be stitched in a quadrant near the base of the valve, after which the wall of the filtration cushion is punctured with a tube that does not have a valve, and the plateau is fixed. Thus, the filtration cushion is connected via a tube to the second valveless plateau. Recently, these devices were made of silicone (models FX1 and FX4).
Ahmed Valves for pediatric practice: S3, FP8 models
The Ahmed S3 valve for children has a reduced size and silicone plateau (model FP8) with a base area of 96 mm2, a length of 10 mm and a width of 9.6 mm.
Tube Extension: TE Model
The tube extension is a simple device made of silicone, which can be used for all existing drains (Molteno, Baerveldt, Krupin and Ahmed). In situations where it is necessary to change the length of the valve tube, an extension cord is the ideal solution. It consists of three parts.
The first part connects the valve tube with the extension tube.
In the second part there are two holes for fixing the tube extension to the sclera after the valve tube is tightly connected to the extension tube.
The third part of the extension is the same size as the valve tube.
The dimensions of the extension tube: height – 1.14 mm, width – 3.05 mm, length – 24 mm. It can be stitched to the sclera with 8/0 or 9/0 vicryl threads. The tube extension is a useful device in particularly difficult situations.
Fastening in the flat part of the ciliary body: PC, PS2 and PS3, PC7 and PC8 models
Glaucoma-implanted tubes, inserted at an acute angle into the posterior chamber of the eye, tend to bend. Given this, the PC model with fastening in the flat part of the ciliary body in the center has an opening for guiding the tube through the pars plana.
The mount for the flat part of the ciliary body is made of silicone. Its dimensions: width – 4.6 mm, length – 5.6 mm. For filing there are two holes with a diameter of 0.025 inches and a central hole with an internal diameter of 0.635 mm. Fasteners are designed so that they can be placed at any distance from the valve. Fastenings are available both separately (PC model) and in conjunction with a polypropylene plate (PS2, PS3 models) or silicone platea (PC7 and PC8 models).
TI Tweezer Tweezer
To capture the tube extension and facilitate its insertion into the anterior chamber, special tweezers have been created, which can be used for all implants.
Clinical indications and contraindications
Until recently, Ahmed valves were widely used in refractory glaucoma, when previously performed trabeculectomy was unsuccessful, or when the probability of its effectiveness with or without anti-metabolic agents was low. Ahmed valves can be used for advanced congenital, aphakic or artifacial open-angle glaucoma, for angle-closure glaucoma, for keratoprosthetic induced glaucoma, penetrating keratoplasty, trauma, for aniridia, Sturge-Weber syndrome and other types of secondary glaucoma.