The device works by bypassing the trabecular meshwork and redirecting the outflow of aqueous humour 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.
Professor Anthony Molteno developed the first glaucoma drainage implant, in Cape Town in 1966. Following on the success of the Molteno implant, several varieties of device have been developed from the original, 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 modification of the trabeculectomy procedure, placed under a scleral flap, replacing the sclerostomy step (see trabeculectomy).
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.
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-tight. 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 valve.
- 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.
- Baervedlt shunts (non-valved) have been shown to have lower rates of surgical failure than Ahmed shunts (valved), this have may be related to the high rate of valve failure, or to the larger plate surface of the Baerveldt shunt.
The majority of complications occur shortly after surgery. These are generally related to high pressure (due to inflammation following surgery) or low pressure (too much aqueous flow through the tube). Periods of low pressure which are more associated with non-valved shunts, can cause retinal detachments, maculopathy or haemorrhages. Periods of high pressures, which are more associated with valved shunts, are detrimental to the optic nerve.
Long term complications of this surgery include diplopia and corneal oedema.
There are also device related complications, which will require surgical revision e.g. erosion, where the conjuctiva dissipates over the shunt leaving it exposed, the condition of which may be revised or prevented in advance by the use of amniotic membrane, donor patch grafts, or collagen matrix implant; blockage, where a particle becomes logged in the tube line blocking flow; retraction, where the tube line slips out of correct position such that flow is inhibited or halted; valve failure, where the valve stops working blocking flow completely – this will require the device to be replaced.
Surgical failure occurs due to 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 antifibrotic medication like 5-fluorouracil (5FU) or Mitomycin-C (during the procedure), or creating a necessity for revision surgery with the sole or combinative use of biodegradable spacer or collagen matrix implant.
- Bruboker, Richard F. (1991-12-01). "Flow of Aqueous Humor in Humans [The Friedenwald Lecture]". Investigative Ophthalmology & Visual Science 32 (13): 3145–66. PMID 1748546.
- http://www.glaucoma.org.nz/memberFiles/Eyelights11-05.pdf[full citation needed]
- Oscar, Albis-Donado; Gil-Carrasco, Gil-Carrasco; Romero-Quijada, Romero-Quijada; Thomas, Thomas (2010). "Evaluation of ahmed glaucoma valve implantation through a needle-generated scleral tunnel in Mexican children with glaucoma". Indian Journal of Ophthalmology 58 (5): 365–73. doi:10.4103/0301-4738.67039. PMC 2992909. PMID 20689189.
- Molteno, AC; Polkinghorne, PJ; Bowbyes, JA (1986). "The vicryl tie technique for inserting a draining implant in the treatment of secondary glaucoma". Australian and New Zealand journal of ophthalmology 14 (4): 343–54. doi:10.1111/j.1442-9071.1986.tb00470.x. PMID 3814422.
- Ayyala, RS; Zurakowski, D; Monshizadeh, R; Hong, CH; Richards, D; Layden, WE; Hutchinson, BT; Bellows, AR (2002). "Comparison of double-plate Molteno and Ahmed glaucoma valve in patients with advanced uncontrolled glaucoma". Ophthalmic surgery and lasers 33 (2): 94–101. PMID 11942556.
- Budenz, DL; Barton, K; Feuer, WJ; Schiffman, J; Costa, VP; Godfrey, DG; Buys, YM; Ahmed Baerveldt Comparison Study Group (2011). "Treatment outcomes in the Ahmed Baerveldt Comparison Study after 1 year of follow-up". Ophthalmology 118 (3): 443–52. doi:10.1016/j.ophtha.2010.07.016. PMC 3020266. PMID 20932583.
- Christakis, PG; Kalenak, JW; Zurakowski, D; Tsai, JC; Kammer, JA; Harasymowycz, PJ; Ahmed, II (2011). "The Ahmed Versus Baerveldt study: One-year treatment outcomes". Ophthalmology 118 (11): 2180–9. doi:10.1016/j.ophtha.2011.05.004. PMID 21889801.
- Oana, Stirbu; Jorge Vila (Dec 2012). Shaarawy, Tarek, ed. "Tube Exposure Repair". Journal of Current Glaucoma Practice 6 (3): 139–142. doi:10.5005/jp-journals-10008-1121. Retrieved Dec 2012.
- Rosentreter, A; Schild AM; Dinslage S; Dietlein TS (Feb 2012). "Biodegradable implant for tissue repair after glaucoma drainage device surgery". J Glaucoma 21 (2): 76–8. doi:10.1097/IJG.0b013e3182027ab0. PMID 21278584.
- Rosentreter, A; Mellein AC; Konen WW; Dietlein TS (2010). "Capsule excision and ologen implantation for revision after glaucoma drainage device surgery". Graefes Arch Clin Exp Ophthalmol 248 (9): 1319–24. doi:10.1007/s00417-010-1385-y. PMID 20405139.