Glaucoma valve

A glaucoma valve is a medical shunt used in the treatment of glaucoma to reduce the eye's intraocular pressure (IOP).

Mechanism

The device works by bypassing the trabecular meshwork and redirecting the outflow of aqueous humor through a small tube into an outlet chamber or bleb. The IOP generally decreases from around 33 to 10 mmHg by removing aqueous on average 2.75 microliters/min. [Brubaker RF. "Flow of aqueous humor in humans [The Friedenwald Lecture] ." "Invest Ophthalmol Vis Sci." 1991 Dec;32(13):3145-66. PMID 1748546.]

Types

There are also several different glaucoma drainage implants. These include the original Molteno implant (1966), the Baerveldt tube shunt, or the valved implants, such as the Ahmed glaucoma valve implant and the later generation pressure ridge Molteno implants. These are indicated for glaucoma patients not responding to maximal medical therapy, with previous failed guarded filtering surgery (trabeculectomy). The flow tube is inserted into the anterior chamber of the eye and the plate is implanted underneath the conjunctiva to allow flow of aqueous fluid out of the eye into a chamber called a bleb.

The ExPress Mini Shunt is a newer, non-valved device that was originally designed to provide a direct conduit from the anterior chamber to the sub-conjunctival space or bleb. In this position it was unstable and tended to erode through the conjunctiva. Now the more common use is as a modificition of the trabeculectomy procedure, placed under a scleral flap, replacing the slerostomy step. (see trabeculectomy).

Indications

The glaucoma valve implant is indicated for glaucoma patients not responding to maximal medical therapy, with previous failed guarded filtering surgery (trabeculectomy) or in cases where conventional drainage surgery is unlikely to succeed. Common situations where the use of a glaucoma implant as a primary procedure is indicated include

* Neovascular glaucoma -- glaucoma associated with vascular disease of the eye (often diabetes).
* Cases of Uveitis -- acute or chronic inflammation of the eye.
* Traumatic glaucoma -- glaucoma associated with injury to the eye.
* Silicone glaucoma -- glaucoma due to Silicone used to repair a detached retina.
* Infantile/Juvenile glaucoma -- often associated with developmental defects of the eye.

urgical technique

The flow tube is inserted into the anterior chamber of the eye and the plate is implanted underneath the conjunctiva to allow flow of aqueous fluid out of the eye.

* The first-generation Molteno and other non-valved implants sometimes require the ligation of the tube until the bleb formed is mildly fibrosed and water-tightMolteno AC, Polkinghorne PJ, Bowbyes JA. The vicryl tie technique for inserting a draining implant in the treatment of secondary glaucoma. Aust N Z J Ophthalmol. 1986 Nov;14(4):343-54 [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=3814422&dopt=Citation.] ] This is done to reduce postoperative hypotony -- sudden drops in postoperative intraocular pressure (IOP).
* Valved implants such as the Ahmed glaucoma valve attempt to control postoperative hypotony by using a mechanical valve. Studies show that in severe cases of glaucoma, double plate Molteno implants are associated with lower mean IOP in the long term compared to the Ahmed glaucoma valveAyyala RS, Zurakowski D et al. Comparison of double-plate Molteno and Ahmed glaucoma valve in patients with advanced uncontrolled glaucoma.Ophthalmic Surg Lasers. 2002 Mar-Apr;33(2):94-101. [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=11942556&dopt=Citation] .]
* Second and third generation Molteno implants incorporate a biological valve and studies show considerable improvement in postoperative outcome over the older style Ahmed and Molteno implants.

Complications

The ongoing scarring over the conjunctival dissipation segment of the shunt may become too thick for the aqueous humor to filter through. This may require preventive measures using anti-fibrotic medication like 5-fluorouracil (5-FU) or mitomycin-C (during the procedure), or creating a necessity for additional surgery.

References

ee also

*Eye surgery
*Glaucoma surgery

External links

* [http://biomed.brown.edu/Courses/BI108/2006-108websites/group02glaucoma/index.html Glaucoma Valves website at Brown University]