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Editors Selection IGR 13-3

Glaucoma Drainage Devices: Corneal endothelial cell count after Ahmed GDD

Comment by Josh Stein on:

24132 Changes in corneal endothelial cells after Ahmed glaucoma valve implantation: 2-year follow-up, Lee EK; Yun YJ; Lee JE et al., American Journal of Ophthalmology, 2009; 148: 361-367

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Several studies, including the Tube versus Trabeculectomy (TVT) study, have demonstrated that glaucoma drainage device (GDD) surgery is safe and effective. However, this surgical procedure is not without risks, one of which is corneal decompensation. Lee et al. (1296) prospectively followed 41 eyes undergoing Ahmed GDD surgery for refractory glaucoma, using specular microscopy to study the potential effect of the surgery on the corneal endothelial cellular count (ECC). ECC was also measured in the nonsurgical contralateral eye, used as controls. The researchers found that corneal ECC declined steadily in the postsurgical eyes, with an average postoperative ECC decrease of 15.3% at 12 months and 18.6% at 24 months. At each follow-up visit, the mean ECC was lower in the postoperative eyes than in the control eyes, and ECC measurements were lower in the superotemporal region of the cornea, closest to the site of the tube, than in other corneal regions. Lee et al. offer clues to why some patients experience corneal edema or decompensation after GDD surgery. However, the study findings should be interpreted with some caution, as it is difficult to ascertain whether the observed decrease in ECC can be attributed to undergoing GDD implantation, simply undergoing incisional intraocular surgery of any kind, or having had a preoperatively elevated IOP. It is well-known that in the absence of surgery, elevated IOP alone can cause corneal decompensation. Furthermore, a decrease in ECC has been observed after other surgeries, such as cataract surgery or trabeculectomy. While the authors matched the study eyes with contralateral control eyes, the preoperative IOP was considerably higher in the study eyes (37 mmHg) than in the control eyes (19 mmHg). To reduce confounding, it would have been helpful to instead match the study eyes to a group of eyes of similarly aged patients with elevated IOP who were undergoing trabeculectomy surgery. The authors conclude that ECC is reduced by GDD surgery. What the report does not point out, however, is that despite a reduction of ECC, none of the study eyes experienced overt corneal edema or a decline in best-corrected visual acuity. Therefore, while the decrease in ECC is statistically significant, it is unclear whether it is clinically significant. The study underscores the importance of limiting the potential for damage to the corneal endothelium ‐ for example, by checking carefully that the placement of the tube in the anterior chamber is not situated close to the corneal endothelium and, whenever possible, placing the tube in the posterior chamber or pars plana.

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