When medications or laser procedures fail to adequately control intraocular pressure (IOP), or if side effects prevent the continued use of medications, surgery becomes necessary to achieve the desired level of IOP. Surgery for glaucoma lowers IOP by either increasing the flow of fluid out of the eye or by decreasing the production of intraocular fluid.
It must be remembered that the optic nerve damage and subsequent loss of vision caused by glaucoma cannot be reversed. Thus, the point of surgery is not to improve vision, but rather to prevent further vision loss. Currently there is no glaucoma treatment that can restore vision already lost to the disease. For this reason, early diagnosis and treatment are vital. We recommend reading the section about glaucoma before continuing here.
This section describes glaucoma drainage devices.
Known as a “glaucoma drainage device,” “tube shunt,” “glaucoma valve,” or “Seton,” this procedure is often performed after other forms of glaucoma filtering surgery, most commonly trabeculectomy, have failed to adequately control intraocular pressure (IOP). Occasionally, it will be performed as an initial filtering procedure, usually in cases considered high risk for trabeculectomy failure, such as neovascular or inflammatory glaucomas. A drainage device consists of a small tube which is inserted into the eye, usually into the anterior chamber near the edge of the cornea, and a plastic plate which is placed outside the eye beneath the conjunctiva, the clear membrane which covers the eye. The plate serves a bit like a reservoir, creating a space for aqueous humor to collect, much like the bleb in trabeculectomy.
For simplicity, think of glaucoma as being caused by a clogged drain. Aqueous humor is constantly being created but cannot escape quickly enough, building up pressure and damaging the optic nerve. The drainage device gives the fluid a low-resistance means of escape, leading to lower, better controlled pressure. The fluid which leaves the eye via the tube drains into the space around the plate, beneath the conjunctiva, before being reabsorbed into the bloodstream.
Ahmed glaucoma valve
A number of drainage devices are available from various manufacturers, varying in overall shape and size. The most significant difference between devices is the presence or absence of a flow-restricting valve. In some devices, a small valve serves to limit the amount of fluid that can flow out of the eye. The valve is meant to close if the IOP drops too low in order to avoid hypotony, or overly low pressure which can damage the eye. Drainage devices without valves must be temporarily obstructed at the time of implantation in order to avoid hypotony immediately following the procedure, as will be discussed in greater detail below. The type of device used is dependent upon surgeon preference, with numerous factors taken into consideration. There have been no good studies proving that any shunt is superior to another in terms of successful lowering of IOP or development of complications. To the right is an animated image of an Ahmed glaucoma valve.
Procedure in Detail
A review of ocular anatomy may be helpful to better understand this procedure.
Drainage device surgery is generally performed under local anesthesia- you will be awake but relaxed, and will experience no pain. After the skin around the eye is cleansed, a surgical drape is placed over the eye to maintain sterility. A small instrument known as a lid speculum is placed in order to hold open the eyelids during the procedure. The eye is then rotated into the necessary position to expose the area of conjunctiva where the plate will be placed. An incision is made in the conjunctiva, usually at the edge of the cornea (limbus), and the conjunctiva is carefully lifted away from the sclera, creating a pocket for the plate. The plate is then gently guided into the pocket and sutured to the sclera approximately one centimeter back from the edge of the cornea, which prevents movement of the shunt. The tube is then measured and cut to the proper length. If the drainage device does not have a valve, the tube must be adjusted in order to avoid excessive early flow. This is done by tying sutures around the tube in order to constrict it. With time, these sutures dissolve and allow fluid to flow freely through the tube. In some cases a ‘stent’ suture may be threaded within the tube itself, placed such that it can be removed in the office after several weeks of healing, if necessary. Shunts with valves require no additional preparation. The tube is then inserted into the eye through a small opening made with a needle. In most cases the tube is placed in the anterior chamber, entering the eye just beyond the edge of the cornea. Occasionally, a tube may be placed into back portion of the eye, such as in cases with extensive anterior chamber scarring from prior inflammation or surgery, or in eyes that have undergone corneal transplantation. Once the tube is in good position in the eye, a small, thin piece of tissue known as a patch graft is sutured over the exposed external portion of the tube in order to protect it. The conjunctiva is then brought back to its original position and sutured to create a watertight closure.