2.8\ Glaucoma Surgery

2.8.1\ Discussion

Glaucoma shunts and valves are surgically implanted devices that reduce intraocular pressure by decompression of aqueous humor. Several types of implants are commercially available, including the Ahmed, Baerveldt, Krupin, and Molteno. The Molteno and Baerveldt devices are non-valved devices (Fig. 2.27), while the Krupin and Ahmed devices include valves (Fig. 2.28). The basic design of a valved shunt consists of a tube drain, valve, and footplate (Fig. 2.29). The one-way valve closes below a certain intraocular pressure, thereby preventing hypotonia of the globe. End plates with larger surface area have greater ability to dissipate the aqueous humor. The devices are usually implanted superotemporal to the globe, with end plate positioned against the scleral surface and the fine tube drain inserted into the anterior chamber. However, the device can also function in the inferotemporal or superomedial quadrant and can drain into the paranasal sinuses (Fig. 2.30). A fibrous capsule forms around the end plate, eventually forming a reservoir or bleb. The fluid is normally resorbed by the surrounding tissues, such that there is no significant accumulation. Glaucoma valve shunts often contain radiopaque barium-impregnated silicone. Alternatively, these devices can be composed of polypropylene, which is of intermediate attenuation on CT. Glaucoma valve implants are MRI compatible and appear as low signal on both T1and T2-weighted sequences surrounded by a small amount of fluid in the reservoir. Complications include hypotonia, malposition, tube obstruction, and giant bleb formation (Fig. 2.31), secondary to adhesions between Tenon’s capsule and the episcleral space, infection (Fig. 2.32), and choroidal detachment (Fig. 2.33). Newer non-tube implants, such as the Ex-PRESS shunt, do not require iridotomy and result in less postoperative inflammation. The Ex-PRESS shunt is a non-­valved stainless steel implant that is inserted under a scleral flap in a paralimbal site (Fig. 2.34).

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Fig. 2.27  Baerveldt shunt. Axial (a) and coronal (b) CT images demonstrate the hyperattenuating device (arrows) positioned superolateral to the globe. Several radiolucent Ahmed valves are also present within the bilateral orbits

Fig. 2.28  Ahmed valve. Axial CT image shows the valved device (arrow) positioned alongside the left globe

Fig. 2.29  Photo of a glaucoma valve device during surgery. The components include the valve on the footplate (arrowhead) and tube (arrow) (Courtesy of Fatoumata Yanoga MD)

Fig. 2.30  Glaucoma valve drainage into maxillary sinus. Coronal CT image shows a radiopaque Ahmed valve positioned inferior to the left globe (arrow), where it drains into the maxillary sinus. There are also superolateral and superomedial radiolucent Ahmed valves (arrowheads)

Fig. 2.31  Glaucoma tube shunt-related blebs. Coronal (a) CT image shows a large fluid collection (arrow) around the radiolucent inferolateral Ahmed valve. Coronal T2-weighted MRI (b) shows bilateral linear low-signal

Ahmed valves surrounded by minimal fluid on the right and a larger amount fluid on the left (arrow), which indents the globe

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Fig. 2.32  Orbital cellulitis after Ahmed valve implantation. Axial (a) and coronal (b) CT images show preand postseptal inflammatory changes surrounding the device in the superolateral left orbit

Fig. 2.33  Hemorrhagic suprachoroidal detachments following glaucoma valve implantation. Axial CT image shows a suprachoroidal hemorrhage within the left globe. A radiolucent Ahmed valve is present temporally and there is preseptal edema