Glaucoma Drainage Implants (Shunt Tubes)
A glaucoma drainage implant or device, also known as a shunt tube, is implanted to maintain an artificial drainage pathway to control intraocular pressure for patients with glaucoma. Intraocular pressure is lowered when aqueous humor flows from inside the eye through the tube into the space between the plate that rests on the scleral surface and surrounding fibrous capsule. The implantation of a glaucoma drainage device is used to treat glaucoma that is refractory to medical and standard surgical therapy. These are usually cases where standard drainage procedures have been unsuccessful or have a poor prognosis including failed trabeculectomy, juvenile glaucoma, neovascular glaucoma and glaucoma secondary to uveitis, traumatic glaucoma, cataract with glaucoma and high risk cases of primary glaucoma.
Importantly, for certain glaucoma drainage implants, radiographic findings may suggest the diagnosis of an orbital foreign body if the ophthalmic history is unknown, as reported by Ceballos and Parrish (2002). In this case report, a patient was denied an MRI examination for fear of dislodging an apparent “metallic foreign body.” In fact, the patient had a Baerveldt glaucoma drainage implant that was mistakenly identified as an orbital metallic object based on its radiographic characteristics (i.e. due to the presence of barium-impregnated silicone).
At least one glaucoma drainage implant, the Ex-PRESS miniature glaucoma shunt (Optonol Ltd., Neve Ilan, Israel) is made from 316L stainless steel which, according to De Feo, et al. (2009), may affect MRI examinations of the optic nerve. Geffen, et al. (2010) reported that the Ex-PRESS glaucoma shunt is acceptable for patients undergoing MRI at 3-Tesla or less.
Many other glaucoma drainage implants are made from nonmetallic materials and are safe for patients undergoing MRI procedures. Commonly used devices that do not contain metal include, the following:
- Baerveldt glaucoma drainage implant (Pharmacia Co., Kalamazoo, MI)
- Krupin-Denver eye valve to disc implant (E. Benson Hood Laboratories, Pembroke, MA)
- Ahmed glaucoma valve (New World Medical, Rancho Cucamonga, CA)
- Molteno drainage device (Molteno Ophthalmic Ltd., Dunedin, New Zealand)
- Joseph valve (Valve Implants Limited, Hertford, England)
Allemann R, et al. Ultra high-field magnetic resonance imaging of a glaucoma microstent. Curr Eye Res 2011;36:719-26.
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De Feo F, et al. Magnetic resonance imaging in patients implanted with Ex-PRESS stainless steel glaucoma drainage microdevice. Am J Ophthalmol. 2009;147:907-11.
Geffen N, et al. Is the Ex-PRESS Glaucoma Shunt ,magnetic resonance imaging safe? J Glaucoma. 2010;19:116-118.
Hong CH, et al. Glaucoma drainage devices: A systematic literature review and current controversies. Surv Ophthalmol 2005;50:48-60.
Jeon TY, et al. MR imaging features of giant reservoir formation in the orbit: An unusual complication of Ahmed glaucoma valve implantation. Am J Neuroradiol 2007;28:1565-1566.
Mabray MC, et al. Ex-PRESS glaucoma filter: An MRI compatible metallic orbital foreign body imaged at 1.5 and 3T. Clin Radiol 2015;70:e28-34.
Seibold LK, et al. MRI of the Ex-PRESS stainless steel glaucoma drainage device. Br J Ophthalmol 2011;95:251-254.
Sidoti PA, Baerveldt G. Glaucoma drainage implants. Curr Opin Ophthalmol 1994;5:85-98.
Traverso CE, et al. Long term effect on IOP of a stainless steel glaucoma drainage implant (Ex-PRESS) in combined surgery with phacoemulsification. British Journal of Ophthalmology 2005;89:425-429.