Given that glaucoma drainage devices have been around for almost fifty years, there has been surprisingly little innovation. All modern implants have two components: a tube that is placed inside the eye; and a plate onto which the fluid drains. Some glaucoma drainage devices also have a valve that regulates flow out of the eye. Other than the presence or absence of a valve, the main difference among implants are size and material used.
Types of Glaucoma Drainage Devices
Non-Valved Glaucoma Drainage Devices
The original glaucoma drainage device is made of polypropylene. Still being used by glaucoma surgeons today, it is now available with either a single plate (130mm2) or double plate (270mm2) reservoir. The additional plate adds surface area which is believed to further lower the intraocular pressure (IOP).
This barium-impregnated silicon implant was introduced in 1992 by Dr. George Baerveldt. Unlike the Molteno™ implant which is round, the Baerveldt® implant has a wing-like appearance. These “wings” fit underneath the eye muscles allowing for a larger surface area onto which aqueous fluid can flow. In fact, the Baerveldt® implants are the largest available, ranging from 250mm2to 425mm2
Valved Glaucoma Drainage Devices
Valved implants were created to restrict the flow of fluid out of the eye. Why would you want to do that? Because without some resistance to flow the aqueous humor would simply flood the subconjunctival space resulting in an IOP of zero. Eventually the body creates a capsule of scar tissue around the implant which acts to limit flow. Until that happens, however, something must be done to restrict the amount of fluid leaving the eye.
Surgeons will often tie the tubes of non-valved implants. These ties will either dissolve over time or have to be removed later. If the knot is undone too soon then a sudden drop in IOP will occur. Until the tube is allowed to open, however, the IOP will remain high and there will be little effect from the surgery.
Tying off the tube is not necessary when a valve is present. By design a valve should remain closed below a threshold IOP. In theory this would prevent hypotony (IOP <5mmHg) while allowing some reduction of IOP in the immediate post-operative period. There are two valved glaucoma drainage devices in common use today:
Introduced in 1976, the first valved implant is still being manufactured today. Made of silicon and shaped like a disk, it has a surface area of 184mm2. The valve is a simple slit that is designed to open at an IOP of 11mmHg and close at 9mmHg. When it works as intended hypotony should not occur.
Introduced by Marteen Ahmed™ in 1993,this polypropylene or silicon valved glaucoma drainage device is frequently used in the United States. Like the Krupin, it is designed to limit flow out of the eye under a given IOP. In the case of the Ahmed™ it has been engineered to open at an IOP of 8mmHg. It also has a similar surface area: 185mm2. A second plate can be attached to the first to add another 180mm2 of surface area for a total of 365mm2.
There is even a pediatric version with a surface area of 96mm2. Unlike the Krupin implant which is shaped like a disc, the Ahmed™ implant has a very unique shape. I think it looks a bit like a trilobite.
How to Choose?
With all the options available how do you (or your surgeon) choose the best glaucoma drainage device? Simply put, it’s a complex decision requiring the skill and nuanced understanding of glaucoma that only years of experience can provide. Some general guidelines, however, are as follows. Non-valved implants with large surface areas are preferred when the target IOP is in the low teens. Valved implants are often preferred if the IOP must be brought under control quickly or in those patients who are at greatest risk of complications from hypotony (such as those who are nearsighted).
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