Eye Photo credit: Advanced Neovascular Glaucoma with Florid Rubeosis Iridis” by Christopher.Shen via http://eyewiki.aao.org/.
Before the turn of the millennium the answer to this question was easy: when all other attempts to control the intraocular pressure (IOP) had failed. Since the publication of the Tube Versus Trabeculectomy (TVT) study, however, much has changed in how glaucoma drainage devices are used. This study, which was limited to those patients who had already had eye surgery, provided evidence that aqueous shunts could lower IOP just as effectively as trabeculectomy. Additionally, with a safety profile on par with trabeculectomy, tubes could be considered earlier in the course of surgical treatment options.
When Should Glaucoma Drainage Devices Be Considered?
Your target IOP is in the mid- to upper-teens
A systematic review of the higher quality studies of glaucoma drainage devices was published in 2005. This review showed that the average IOP two years out from aqueous shunt surgery ranged from 14-17mmHg. More recently, an “Ophthalmic Technology Assessment” of aqueous shunts was conducted by the American Academy of Ophthalmology. In it the authors mention that after placement of a glaucoma drainage device the IOP can be expected to settle around 18mmHg.
You Are at High Risk for Trabeculectomy Failure
A number of conditions exist that could increase the risk of trabeculectomy failure. These include black race, young age, history of prior eye surgery, ocular inflammation, long history of drop use, neovascular glaucoma, etc. A glaucoma drainage device may be a better option than trabeculectomy for those with one of more of these conditions whose target IOP is in the mid- to upper-teens.
You’ve Already Had Eye Surgery
With certain types of prior eye surgery, a glaucoma drainage device is really the only good option. This is the case with those who have already had a corneal transplant. Most eye surgery, however, involves making an incision through the conjunctiva. Cutting any tissue in the body results in inflammation. Inflammation of the conjunctiva increases the risk of trabeculectomy bleb failure. So, it should come as no surprise that most eye surgeries would reduce the success rate of trabeculectomy.
Even prior cataract surgery decreases the chances of trabeculectomy success. That is surprising as modern cataract surgery does not require making an incision in the conjunctiva. Even more surprising is that at least in young patients prior laser therapy such as Argon Laser Trabeculoplasty (ALT) or Laser Peripheral Iridotomy (LPI) can increase the risk of surgical failure.
Thus, a glaucoma drainage device may be a better option than trabeculectomy for anyone that has had prior eye surgery and has a target IOP is in the mid- to upper-teens.
(This will take you to Dr. David Richardson’s website)
You’re Eye Has Been Seriously Injured by Trauma in the Past
Surgery is sometimes described as “controlled trauma.” If controlled trauma can increase the risk of trabeculectomy failure, then it should come as no surprise that uncontrolled trauma to the eye can also increase the risk of trabeculectomy failure. Additionally, ocular trauma can result in scarring and/or abnormal anatomy that can make trabeculectomy difficult to perform. As such, placement of a glaucoma drainage device may be one of the few options available to control IOP after significant trauma to the eye.
Your Eye is Chronically Inflamed (a condition called “Uveitis”)
Inflammation is the enemy of a trabeculectomy bleb. Eyes that are chronically inflamed are simply more likely to result in trabeculectomy failure. Yes, inflammatory debris can also clog a tube. Overall, however, glaucoma drainage devices are more resistant to failure from inflammation than is trabeculectomy.
You Have “Neovascular” Glaucoma
Conditions such as diabetes mellitus and “strokes” of the eye (called retinal vascular occlusions) can result in growth of blood vessels within the eye. Unlike normal blood vessels these new (“neo”) vessels (”vascular”) are delicate and prone to bleeding. They may also grow like vines over the trabecular meshwork. This growth can result in permanent scarring and closure of the drainage system. If trabeculectomy has been performed these vessels can obstruct the fistula causing surgical failure.
You Have Congenital Glaucoma
Congenital glaucoma is often treated with removal of the trabecular meshwork using techniques called goniotomy or trabeculotomy. If the glaucoma does not respond to these treatments then a glaucoma drainage device may be the best next option.
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