Tube Shunt Related Complications of the Cornea

Tube shunts can be placed in the anterior chamber, the ciliary sulcus, or the pars plana. However, if the eye is phakic, the choice is limited to the anterior chamber; ciliary sulcus placement is likely to result in cataract formation, and pars plana placement will likely complicate removal of the cataract that will likely develop. Most corneal complications of tube shunt surgery result from tubes that are too anterior. Loss of vision may result due to these complications. If the tube is inadvertently inserted too close to the cornea, a loss of endothelial cells will result in edema and require transplantation to restore vision. Reported rates of corneal complications range from 2% to 33% and consist mostly of corneal edema/decompensation and corneal graft failure. In a cohort of patients implanted with the Ahmed™ Glaucoma Valve (New World Medical, Inc., Rancho Cucamonga, California), postoperative corneal abrasions occurred in 5 of 60 (8%) eyes. Another study reported the rate of corneal drying/dellen later in the postoperative course (8 of 59 eyes; 13.6%). Contact between the tube and the cornea has been noted at a rate of up to 5%. As the rate of tube shunt implantation has increased, the incidence of corneal edema in patients with tube shunts has also increased. Some of these cases develop corneal opacification with decreased vision and may require corneal transplantation to clear the visual axis. One large study of patients with Ahmed tube shunts (159 eyes total) reported corneal graft failure resulting in repeat penetrating keratoplasty (PKP) in 11 of 31 (35%) eyes with corneal grafts. Improper anterior chamber tube entry may damage the cornea. If the entry angle is not parallel to the iris and aims anteriorly, the needle used to create the tunnel may tear or detach Descemet’s membrane. Entry through the cornea (rather than the sclera) may also predispose to epithelial downgrowth or tube extrusion. To avoid such a complication, fullthickness entry into the anterior chamber should be as far posterior as possible.